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Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu–Bonin–Mariana Arc: Syntectonic granitic crust formation during back-arc rifting
Abstract
A widely held hypothesis is that modern continental crust of an intermediate (i.e. andesitic) bulk composition forms at intra-oceanic arcs through subduction zone magmatism. However, there is a critical paradox in this hypothesis: to date, the dominant granitic rocks discovered in these arcs are tonalite, rocks that are significantly depleted in incompatible (i.e. magma-preferred) elements and do not geochemically and petrographically represent those of the continents. Here we describe the discovery of a submarine knoll, the Daisan–West Sumisu Knoll, situated in the rear-arc region of the intra-oceanic Izu–Bonin–Mariana Arc. Remotely-operated vehicle surveys reveal that this knoll is made up entirely of a 2.6 million year old porphyritic to equigranular granodiorite intrusion with a geochemical signature typical of continental crust. We present a model of granodiorite magma formation that involves partial remelting of enriched mafic rear-arc crust during the initial phase of back-arc rifting, which is supported by the preservation of relic cores inherited from initial rear-arc source rocks within magmatic zircon crystals. The strong extensional tectonic regime at the time of intrusion may have allowed the granodioritic magma to be emplaced at an extremely shallow level, with later erosion of sediment and volcanic covers exposing the internal plutonic body. These findings suggest that rear-arc regions could be the potential sites of continental crust formation in intra-oceanic convergent margins.
... The following arcs contain TTGs that formed in subductiongenerated island arcs and continental arcs: Ordovician-Eocene Sikhote-Alin-Sakhalin Table 2; Elburg and Foden, 1999;Ishihara, 2001;Haraguchi et al., 2003;Kuritani et al., 2005;Hanzl et al., 2008;Tikhomirov et al., 2008;Dhuime et al., 2009;Fiorentini and Garwin, 2010;Johnson et al., 2014aJohnson et al., , 2014bTani et al., 2015;Wang et al., 2015;Ichiyama et al., 2017;Liao et al., 2018;Chen et al., 2019;Osozawa et al., 2019;Xu et al., 2019;. There are also TTGs in the Cambrian Zhaheba (China), Jurassic Eldivan (Turkey), Cretaceous Muslim Bagh (Pakistan) and Cretaceous-Paleocene Waziristan (Pakistan) ophiolites, which formed in suprasubduction zones and intra-oceanic back-arc basins (Supplementary ...
... Eoarchean (3867-3650 Ma) TTGs from the Itsaq Gneiss Complex in the Greenland Craton have more fractionated N-MORB-normalised trace element patterns than those of the Izu-Bonin-Mariana arc of Japan; however, the former are replete with the pronounced positive LILE and negative HFSE anomalies associated with subduction zone processes ( Fig. 7a; Kamber et al., 2002;Haraguchi et al., 2003;Hoffmann et al., 2011;Johnson et al., 2014aJohnson et al., , 2014bTani et al., 2015;Ichiyama et al., 2017). The respective N-MORB-normalised trace element patterns of the TTGs from the Itsaq Gneiss Complex and Izu-Bonin-Mariana arc have been interpreted to indicate that they formed in a subduction zone and an intra-oceanic arc, respectively ( Fig. 7a; Kamber et al., 2002;Hoffmann et al., 2011;Johnson et al., 2014aJohnson et al., , 2014b. ...
... The respective N-MORB-normalised trace element patterns of the TTGs from the Itsaq Gneiss Complex and Izu-Bonin-Mariana arc have been interpreted to indicate that they formed in a subduction zone and an intra-oceanic arc, respectively ( Fig. 7a; Kamber et al., 2002;Hoffmann et al., 2011;Johnson et al., 2014aJohnson et al., , 2014b. The more primitive nature of the TTGs from the Izu-Bonin-Mariana arc is reflective of their intra-oceanic arc origin; however, these TTGs do exhibit more pronounced positive Pb anomalies than those of the Itsaq Gneiss Complex ( Fig. 7a; Haraguchi et al., 2003;Johnson et al., 2014aJohnson et al., , 2014bTani et al., 2015;Ichiyama et al., 2017). Volcanic arc (Nutman et al., 2007), proto-arc (Nutman et al., 2013), and island arc (Nutman and Bridgwater, 1986;Nutman et al., 2013) origins have been proposed for the TTGs of the Itsaq Gneiss Complex, suggesting that they may have formed in a similar environment to those of the Izu-Bonin-Mariana arc. ...
In this study, we applied classification and tectonic setting diagrams, N-MORB-normalised trace element, and incompatible element temporal variation diagrams, Nb/Nb*, Pb/Pb*, La/Smn, La/Nbn, Th/Nbn and Pb/Cen ratios and field-structural relationships to Archean tonalites, trondhjemites and granodiorites (TTGs) and Phanerozoic arc-generated TTGs. Geochemical analyses were compiled from the literature to elucidate how Archean continental crust formed, whether plate tectonics operated in the Archean, if this Archean form of plate tectonics resembled modern-style plate tectonics, and when plate tectonics commenced in the early Earth. Archean TTGs used for this study were not noticeably influenced by alteration or crustal contamination; therefore, their geochemistry is indicative of their original juvenile sources and the tectonic environments in which they were generated. These rocks are predominantly calcic to calc-alkalic and magnesian granitoids derived from low-K mafic/tholeiitic sources. Most Archean TTGs have high La/Yb(cn) and Sr/Y ratios and low Yb(n) values and Y contents characteristics of adakites and formed by partial melting of subducting oceanic crust or the lower arc crust. However, some Archean TTGs resemble modern arc andesites, dacites and rhyolites that have low La/Yb(cn) and Sr/Y ratios and high Yb(cn) and Y contents and formed by fractional crystallisation of basaltic magmas derived from partial melting of the sub-arc mantle wedge. Archean TTGs overlap with their Phanerozoic counterparts and mainly plot in the arc fields of tectonic setting discrimination diagrams. The N-MORB-normalised trace element patterns of Archean TTGs from well-studied cratons bear striking resemblance to those of TTGs from modern active arcs (e.g., Izu-Bonin-Mariana, Andean) and from older Phanerozoic arcs (e.g., Sierra Nevada, Gangdese), consistent with their formation in arcs. The temporal variations in the trace element geochemistry of Archean TTGs underwent a noticeable change on a global scale at ca. 3500–3200 Ma. The vast majority (99%) of Archean TTGs have Nb/Nb* and Pb/Pb* anomaly ratios of <1 and > 1, respectively, and are interpreted to have formed in arc settings; the remainder had non-arc origins. The La/Smn, La/Nbn, Th/Nbn and Pb/Cen ratios of these TTGs suggest that 99% formed in supra-subduction zone settings, namely arcs, forearcs and back-arcs, the remainder being derived from mantle plumes and mid-ocean ridges. This study suggests that throughout the Archean, TTGs were generated in subduction zones by modern-style plate tectonic processes, beginning in the Hadean (>4000 Ma). TTGs began to form in intra-oceanic arcs at ca. 4020 Ma prior to a global-scale switch to Andean-style continental arc magmatism at ca. 3500–3200 Ma, which led to the genesis of TTGs in continental arcs worldwide in the Paleoarchean. Modern-style plate tectonic processes predominantly contributed to the formation of Archean continental crust. As such, global-scale vertical tectonic processes did not play a significant role in the formation of Archean continental crust. Most plutons are emplaced vertically into the crust, as exemplified by modern arcs. Vertical tectonic processes are locally observed in Archean cratons, leading many researchers to erroneously conclude that vertical tectonic processes were predominant in the early Earth.
... Given their intermediate silica content, low to moderate K 2 O (0.63%-1.46%) and K 2 O/ Na 2 O ratios (0.15-0.35), and enrichment of LILEs over HFSEs, the WLA tonalites and HET quartz diorites bear a close resemblance to the well-documented felsic magmatic suites (SiO 2 > 63%) in a few classic intraoceanic island arcs, such as the tonalite-granodiorite plutons from the Izu-Bonin-Mariana (IBM) arc system in the Western Pacific (Suzuki et al., 2015;Tani et al., 2015) and the felsic volcanicplutonic suites in the Riwanchaka arc of central Tibet . Generation of such felsic magmas can be usually attributed to either fractional crystallization of basaltic magmas (Brophy, 2008;Barker et al., 2013) or partial melting of lower to middle crust (Shukuno et al., 2006;Brophy, 2008;Tamura et al., 2009;Suzu-ki et al., 2015). ...
... Given their unradiogenic I Sr (t) ratios, such highδ 18 O supracrustal components might be confined to low-maturity volcanoclastic deposits from forearc basins. With such dual source affinities as in the case of the granodiorites in the IBM system (Tani et al., 2015), the HET quartz diorites can be ascribed to dehydration melting of K-rich mafic to intermediate juvenile underplates with minor high δ 18 O supracrustal inputs from a forearc basin. For the Late Ordovician HES hornblende diorites, their mafic composition (Fig. 6A), enriched concentrations of Na 2 O (Na 2 O/K 2 O = 5-9), TiO 2 and Nb ((Nb/Th) PM = 1.2-2.1, ...
... While the Late Cambrian WLA tonalites are geochemically equivalent to the Tanzawa tonalitic pluton (Suzuki et al., 2015) and imply the presence of ancient oceanic crustal remnants in current island arc lower crust (Figs. 13A and 13B), the Early Ordovician HET granodiorites with a continental crustal affinity resemble the Pliocene granodioritic knoll in the rear-arc region of the IBM arc (Tani et al., 2015) and thus argue for an analogous partial remelting scenario during the initial phase of backarc rifting. On a regional scale, sporadic Late Cambrian-Early Ordovician magmatic records include ca. ...
Updating magmatic profile in crucial constituent terranes across the Central Asian Orogenic Belt presents a key to chronicling the timeline of prolonged accretionary processes and termination of the Paleo-Asian Ocean in the northern China−southern Mongolia tract. Here we performed a systematic geochronological and geochemical study on a spectrum of Paleozoic intrusions from the Erenhot region in central Inner Mongolia, North China, within the hinterland of the tract, with four distinct magmatic episodes unraveled. Combining these episodes with the previously documented events from contiguous regions defines two major tectono-magmatic cycles. The early Paleozoic cycle (500−450 Ma) evolved from initial fluid-fluxed tholeiitic and calc-alkaline granitoids to melt-fertilized mafic-intermediate magmatism. It appears to experience the initiation and maturation of a Western Pacific-type intra-oceanic arc system that culminated in ridge subduction. The late Paleozoic cycle expanded in magmatic expression from sporadic Late Devonian (373−365 Ma) calc-alkaline intermediate-felsic pulses through Early-Middle Carboniferous (356−320 Ma) medium to high-K calc-alkaline flare-up to a Late Carboniferous−Early Permian (310−277 Ma) province of diverse lithologies. These magmatic episodes seem to encompass a complete active continental arc−back-arc system that spanned from resuming oceanic plate subduction through slab rollback and backarc rifting to ridge-trench collision and backarc basin closure. Featuring a Rodinia-aged terrane affinity and a representative Paleozoic magmatic profile, the Erenhot region provides an optimal site for correlating the evolution of mosaic terranes in southern Mongolia and northern China, and for evaluating the coupled evolution of shifting tectonic regimes and plural crustal generation mechanisms within a retreating accretionary orogen.
... ca. 315 Ma Granodiorite Given their intermediate to felsic silica content, tholeiitic to calcalkaline character, low Mg # of 37-55 and enrichment of LILEs over HFSEs, the HTGL granodiorites seem to closely resemble a few welldocumented felsic magmatic suites (> 63%) in classic island arcs of various ages (e.g., Bosch et al., 2011;Rioux et al., 2007;Tani et al., 2015). Such felsic magmas can be usually attributed to either fractional crystallization of basaltic parental magmas (Erdmann and Koepke, 2016) or partial fusion of mafic protoliths at lower to middle crust (Brophy, 2008;Suzuki et al., 2015). ...
... With their low-K felsic character and magmatic heritage from previous evolved mafic to intermediate crustal underplates, ca. 315 Ma HTGL grandodiorites bear a close resemblance to the Pliocene porphyritic granodiorites in the rear-arc region of the IBM system (Tani et al., 2015). Given the generation of the latter from partial remelting of enriched mafic rear-arc crust during the initial phase of (Beard and Lofgren, 1991). ...
... In (f), the percentages of Pb and U that must be added by fluid to produce high Pb/Ce and U/Nb ratios are shown (Regelous et al., 2014). back-arc rifting (Tani et al., 2015), the likelihood is that the HTGL grandodiorites might have formed under a similar geodynamic scenario but tend to reflect crustal protolith inheritance from a Mesoproterozoic remnant arc terrane. Such initial back-arc rifting is not only evidenced by the extrusion of coeval (ca. ...
Characterizing island arc and back arc basin system through detailed magmatic profile remains a pivotal task in any regional paleotectonic and supercontinental reconstructions along ancient convergent plate margins, as is of paramount importance for the northern China–southern Mongolia (NCSM) tract along the eastern Central Asian Orogenic belt (CAOB). Here we document episodic Late Carboniferous intermediate to felsic intrusive suites in the West Ujimqin region of central Inner Mongolia, spanning from ca. 315 Ma granodiorites through ca. 310 Ma hornblende diorites to ca. 305–301 Ma diorites. The earlier granodiorites exhibit dominant magnesian-tholeiitic affinity, enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), and possess whole-rock ISr(t) ratios of 0.707387–0.708799, negative ƐNd(t) of −2.3 to −2.4 and zircon ƐHf(t) values of −0.69 to +4.87 and δ¹⁸O of +6.33 to +8.40‰. These features are consistent with the magmatic derivation from dehydration melting of tholeiitic mafic to intermediate lower crustal underplates with obvious high δ¹⁸O supracrustal inputs. The ca. 310 Ma pod-like hornblende diorites display high-Ca boninite-like elemental traits and MORB-like isotopic compositions (ISr(t) = 0.704190 to 0.704203, ƐNd(t) = +7.9 to +8.3, zircon ƐHf(t) = +14.7 to +17.5 and δ¹⁸O = +4.88 to +6.16‰), indicative of a prior melt-depleted then fluid-enriched mantle source. By contrast, subsequent (301–305 Ma) diorites show arc-like elemental hallmarks, with notable enriched LILEs and depleted HFSEs as well as variable isotopic values (ISr(t) = 0.703690–0.704065, ƐNd(t) = +3.5 to +8.4, zircon ƐHf(t) = +7.10 to +16.4 and δ¹⁸O = +3.75 to +6.83‰). They are interpreted to represent the evolved products of hornblende-dominated fractional crystallization from depleted basaltic parental magmas. Given their coincidence with regional back-arc-basin type ophiolitic stratigraphy and detrital zircon age spectra, these intermediate to felsic intrusive suites tend to document the timeline of a back arc basin formation from initial rifting to maturation. In conjunction with Early Carboniferous forearc-type ophiolitic complexes and forearc magmatic records at West Ujimqin and the adjacent region, a Carboniferous island arc and back arc system can be reconstructed from Xilinhot to West Ujimqin. The characterization of this northward (present day coordinates) subduction system in terms of its temporal evolution not only provides overwhelming evidence that modern Pacific-like, archipelago-type arc configuration had prevailed in central Inner Mongolia during the Carboniferous, but also offers vivid insights into the diversity of crustal generation mechanism in a subduction factory that ushered in seminal continental crustal construction in the eastern CAOB.
... However, the origin of and processes controlling formation of these felsic magmas are contentious. The main end-member hypotheses include the traditional model, extreme fractionation of mafic liquids (Hochstaedter et al., 2000;Wade et al., 2005), and more recent models that invoke widespread melting of preexisting crust of variable age (Shukuno et al., 2006;Tamura et al., 2009;Tamura & Tatsumi, 2002;Tani et al., 2015). Both models have been applied to explain Izu-Bonin rhyolites, and this lack of a consensus is reflected in many arcs (Leat & Larter, 2003). ...
... This study does not consider RASC rocks older than 7 Ma, since the paucity of dated rhyolitic dredges older than 7 Ma (Ishizuka et al., 2003) makes interpreting mafic-felsic relationships difficult. During the time span 7-1 Ma, some volcanic rocks in both the back-arc knolls and RASC regions have similar compositions regardless of edifice size and age (Hochstaedter et al., 2000;Ishizuka et al., 1998Ishizuka et al., , 2003Tani et al., 2015). We term this the RASC-type geochemical signature. ...
... Therefore, an important result of this study is that RASC-type rhyolites were a significant eruptive product in the RASC and back-arc knolls from 4.4 to 1.1 Ma. Although Ishizuka et al. (2003) reported few felsic rocks younger than 2.8 Ma, other studies have shown indirect evidence for widespread felsic volcanism during this period Haraguchi et al., 2017;Tani et al., 2015). The Site U1437 core documents rear-arc-derived felsic tephras continuously throughout the entire 4.4-1.1 Ma record (Figure 3). ...
Felsic magmas from the Izu‐Bonin rear arc have compositions that resemble average continental crust in some respects. In order to understand their origin, we studied 1.1–4.4 Ma tephras in a rear‐arc drill core from International Ocean Discovery Program Expedition 350, Site U1437. They provide a well‐dated record of changing magmatic compositions during the early stages of the most recent episode of Izu‐Bonin arc rifting. Based on our comprehensive recontextualization of published analyses of <7 Ma regional dredged rocks across the arc, basalts to rhyolites are shown to vary in coherent chronological and spatial trends and can be classified into three series: light rare earth element‐depleted volcanic front series; a rift‐related series with nearly flat rare earth element patterns; and light rare earth element‐enriched rear‐arc seamount chain‐type (RASC‐type) series, which are abundant in the studied Site U1437 tephra record. All three series erupted simultaneously between 4.4 and 1.1 Ma, including the RASC‐type rhyolites which erupted until 1.1 Ma in significant quantities. Remarkably, trace element and radiogenic isotope ratios are similar between rhyolites and basalts from the same region. This recontextualization of rhyolite affinity represents a significant departure from existing frameworks. Geochemical modeling shows that fractional crystallization can largely explain <4.4 Ma RASC‐type rhyolites with some additional open system processing evident in rhyolites with >73% SiO2. However, trace element and Hf isotope ratios preclude rear‐arc rhyolite derivation by partial melting of the Oligocene‐Eocene arc basement. Thus, we favor a model where fractional crystallization is more important than crustal melting in producing intra‐oceanic arc rhyolites in this region.
... K, LREE, Zr) compared to the highly depleted arc front magmas (Hochstaedter et al. 2000(Hochstaedter et al. , 2001Machida and Ishii 2003;Ishizuka et al. 2003a;Machida et al. 2008;Tollstrup et al. 2010). In addition to volcanic samples, plutonic rocks have been recovered on the rear-arc Manji seamount (Ishizuka et al. 2002) and from the back-arc Daisan West Sumisu Knoll in the extensional zone at the eastern termination of the Manji seamount chain (Tani et al. 2015). These rocks match the calcalkaline trend and incompatible element enrichment patterns of the volcanic rocks from rear-arc seamount chains. ...
... These compositionally resemble Cordilleran I-type magmas, and yielded zircon whose ca. 8-7 Ma interior ages reveal recycling of pre-existing Miocene rear-arc crust during a remelting episode at ca. 2.6 Ma indicated by rim ages (Tani et al. 2015). Remelting and reprocessing of the mafic arc crust has been related to initiation of extension and rifting behind the arc (Tani et al. 2015). ...
... 8-7 Ma interior ages reveal recycling of pre-existing Miocene rear-arc crust during a remelting episode at ca. 2.6 Ma indicated by rim ages (Tani et al. 2015). Remelting and reprocessing of the mafic arc crust has been related to initiation of extension and rifting behind the arc (Tani et al. 2015). ...
Long-lived intra-oceanic arcs of Izu-Bonin-Marianas (IBM)-type are built on thick, granodioritic crust formed in the absence of pre-existing continental crust. International Ocean Discovery Program Expedition 350, Site U1437, explored the IBM rear arc to better understand continental crust formation in arcs. Detailed petrochronological (U–Pb geochronology combined with trace elements, oxygen and hafnium isotopes) characterizations of zircon from Site U1437 were carried out, taking care to exclude potential contaminants by (1) comparison of zircon ages with ship-board palaeomagnetic and biostratigraphic ages and ⁴⁰Ar/³⁹Ar geochronology, (2) analysing zircon from drill muds for comparison, (3) selectively carrying out in situ analysis in petrographic thin sections, and (4) minimizing potential laboratory contamination through using pristine equipment during mineral separation. The youngest zircon ages in Site U1437 are consistent with ⁴⁰Ar/³⁹Ar and shipboard ages to a depth of ~1390 m below sea floor (mbsf) where Igneous Unit Ig 1 yielded an ⁴⁰Ar/³⁹Ar age of 12.9 ± 0.3 Ma (all errors 2σ). One single zircon (age 15.4 ± 1.0 Ma) was recovered from the deepest lithostratigraphic unit drilled, Unit VII (1459.80–1806.5 mbsf). Site U1437 zircon trace element compositions are distinct from those of oceanic and continental arc environments and differ from those generated in thick oceanic crust (Iceland-type) where low-δ¹⁸O evolved melts are produced via re-melting of hydrothermally altered mafic rocks. Ti-in-zircon model temperatures are lower than for mid-ocean ridge rocks, in agreement with low zircon saturation temperatures, suggestive of low-temperature, hydrous melt sources. Zircon oxygen (δ¹⁸O = 3.3–6.0‰) and hafnium (εHf = + 10–+16) isotopic compositions indicate asthenospheric mantle sources. Trace element and isotopic differences between zircon from Site U1437 rear-arc rocks and the Hadean detrital zircon population suggest that preserved Hadean zircon crystals were probably generated in an environment different from modern oceanic convergent margins underlain by depleted mantle.
... Mizuno et al. (1978) and Okino and Kato (1992) mentioned that the Daito and Oki-Daito ridges (10 in Table 1 and Fig. 3) consisted of pre-Eocene basement rocks overlain by middle Eocene sedimentary rocks. More recently, andesites dredged on the Daito Ridge (8 and 9 in Table 1 Tani et al. (2012) revealed after analyzing samples collected during a diving cruise conducted in 2010 that the ADO region dominantly exposed deep crustal sections of gabbroic, granitic, and metamorphic rocks with possible continental affinities. Jurassic to Cretaceous zircon U-Pb ages have been obtained from the plutonic rocks (7 in Table 1 and Fig. 3). ...
... He proposed a zonation from Late Cretaceous amphibolites with quartz-albite metamorphic soles distributed mainly in the eastern belt, then Early to Late Cretaceous dismembered ophiolites with mostly tectonic melanges in the central belt and finally Late Cretaceous to Oligocene ophiolitic complexes in the western belt. Such a zonation is discussed by Tani et al. (2015), who obtained new zircon ages from the northern ophiolite belts. They showed these to be Eocene in age and thus probably genetically associated with the WPB opening. ...
... As for the southern ophiolites, in Cebu, Lagonoy and Dinagat, gabbroic and leucocratic rocks associated with the ophiolitic complexes are Jurassic to Late Cretaceous in age (∼90-200 Ma). Tani et al. (2015) then concluded that these Mesozoic ophiolites from the southern Philippines may potentially be correlated with the Mesozoic arc and ophiolitic rocks of the ADO region before the WPB opening. ...
We compiled the most relevant data acquired throughout the Philippine Sea Plate (PSP) from the early expeditions to the most recent. We also analyzed the various explanatory models in light of this updated dataset. The following main conclusions are discussed in this study. (1) The Izanagi slab detachment beneath the East Asia margin around 60–55 Ma likely triggered the Oki-Daito plume occurrence, Mesozoic proto-PSP splitting, shortening and then failure across the paleo-transform boundary between the proto-PSP and the Pacific Plate, Izu-Bonin-Mariana subduction initiation and ultimately PSP inception. (2) The initial splitting phase of the composite proto-PSP under the plume influence at ∼54–48 Ma led to the formation of the long-lived West Philippine Basin and short-lived oceanic basins, part of whose crust has been ambiguously called “fore-arc basalts” (FABs). (3) Shortening across the paleo-transform boundary evolved into thrusting within the Pacific Plate at ∼52–50 Ma, allowing it to subduct beneath the newly formed PSP, which was composed of an alternance of thick Mesozoic terranes and thin oceanic lithosphere. (4) The first magmas rising from the shallow mantle corner, after being hydrated by the subducting Pacific crust beneath the young oceanic crust near the upper plate spreading centers at ∼49–48 Ma were boninites. Both the so-called FABs and the boninites formed at a significant distance from the incipient trench, not in a fore-arc position as previously claimed. The magmas erupted for 15 m.y. in some places, probably near the intersections between back-arc spreading centers and the arc. (5) As the Pacific crust reached greater depths and the oceanic basins cooled and thickened at ∼44–45 Ma, the composition of the lavas evolved into high-Mg andesites and then arc tholeiites and calc-alkaline andesites. (6) Tectonic erosion processes removed about 150–200 km of frontal margin during the Neogene, consuming most or all of the Pacific ophiolite initially accreted to the PSP. The result was exposure of the FABs, boninites, and early volcanics that are near the trench today. (7) Serpentinite mud volcanoes observed in the Mariana fore-arc may have formed above the remnants of the paleo-transform boundary between the proto-PSP and the Pacific Plate.
... However, the volume of felsic melt produced by fractional crystallization is relatively small. Larger volumes of felsic magmas can be formed by partial melting of amphibolitic middle to lower crusts (e.g., Tamura and Tatsumi, 2002;Tani et al., 2015;Haraguchi et al., 2017). Several studies reported that partial melting of amphibole-rich mafic rocks would lead to the preferential retention of the MREE and thus in concave REE patterns and decreasing HREE concentrations (Brophy, 2008;Davidson et al., 2013). ...
Viti Levu, Fiji, provides one of the best exposed Phanerozoic analogues for the formation of juvenile continental crust in an intra-oceanic setting. Tonalites and trondhjemites are present in several large (75 – 150 km²) adjacent, mid-Cenozoic plutons. We report major and trace element data including rare-earth element (REE) and high-precision high field strength element (HFSE) compositions, new Hf-Nd-Sr-Pb isotope data, and zircon U/Pb-ages, O-Hf isotopes, and trace elements, from five different plutons. The Eocene Yavuna pluton and the Miocene Colo plutons are mainly composed of tonalites and trondhjemites and represent the exposed middle crust of the former Vitiaz island arc. The plutons can be divided into three suites. One suite is light REE (LREE) depleted with some trace element ratios lower than average normal mid-ocean ridge basalts (N-MORB). A second suite has flat REE patterns similar to local island-arc basalts. Both suites occur near the coast of Viti Levu, include a wide compositional spectrum from gabbro to tonalite, and can be produced mostly by fractional crystallization of mafic precursor melts. The third suite is characterized by LREE-enrichments with higher LaN/YbN (2.3 – 4.9), higher Zr/Y (4.3 – 7.1), and lower Nb/Ta (9.6 – 12.4). They occur closer to the center of the island and are bimodal trondhjemite-gabbro intrusions. These characteristics are consistent with formation mostly by partial melting of mafic crust. Trace element modeling shows that the trace element ratios of the third suite can be produced by 10 – 20 % melting of the mafic crust in the presence of residual amphibole, resulting in the retention of the medium rare earth elements (MREE) and diagnostic trace element ratios including low Nb/Ta and high Zr/Y. Geochemical similarities of the LREE-enriched suite to typical “low”-pressure Archean tonalites-trondhjemites-granodiorites (TTGs) imply a common petrogenetic origin and similar mechanisms for the generation of juvenile Archean and modern differentiated crust by partial melting of mafic crust with residual amphibole. In modern oceanic arcs, genetically unrelated felsic plutonic as well as volcanic rocks co-exist, and in this regard, the Fijian plutons accompany major tectonic disruptions to arc processes.
... In fact, a back-arc basin generally forms c. 30-50 Ma after initial subduction (e.g. Xia et al. 2012;Tani et al. 2015;Hickey-Vargas et al. 2018;Magni, 2019). The IAB-like protoliths of metamorphic rocks from BDHG in the North Qilian Orogenic Belt therefore represent the arc magmatism, which should be earlier than the back-arc basin magmatic activity (c. ...
The North Qilian Orogenic Belt is surrounded by the Tarim Craton to the NW and the North China Craton to the NE. The Precambrian continental crust remnants that are distributed in the North Qilian Orogenic Belt are termed the North Qilian Block (NQB), and their tectonic evolution has profound implications for the evolution of the Columbia Supercontinent. Here we present major- and trace-element and Sr–Nd–Hf isotope data for (meta-) basalts from the Beidahe Group (BDHG) and Zhulongguan Group (ZLGG) in the western North Qilian Orogenic Belt, to investigate the tectonic evolution of the NQB during the Proterozoic Eon. The protoliths of Palaeoproterozoic amphibole gneisses and plagioclase amphibolites from the BDHG are calc-alkaline series basalts. These metabasalts show island-arc-basalt affinities with variable Nd and Hf isotopes ( ϵ Nd ( t ) = −5.0–0.6 and 2.7–4.3; ϵ Hf ( t ) = −14.2–2.0 and 6.9–8.8) and were generated by partial melting of the asthenospheric mantle that was metasomatized by aqueous fluid and sediment melt in a continental-arc setting. The early Mesoproterozoic ZLGG basalts show features of shoshonite-series basalts and are geochemically similar to ocean-island basalts. These basalts show variable ( ⁸⁷ Sr/ ⁸⁶ Sr) i , ϵ Nd ( t ) and ϵ Hf ( t ) values of 0.70464–0.70699, −1–2.6 and −1.5–5.7, and are products of mantle plume magmatism that participated with subducted oceanic crust in an intracontinental rift setting. This study suggests that the NQB underwent tectonic evolution from palaeo-oceanic subduction to intracontinental rifting during the Palaeoproterozoic–Mesoproterozoic eras. Furthermore, the above tectonomagmatic events were in response to convergence–splitting events of the Columbia Supercontinent during the Palaeoproterozoic–Mesoproterozoic eras.
... The existence of the coeval Kogarasu and Shimobe plutons also can be explained as the syncollisional rapid granitic magma formation in the arc-arc collision zone. Intrusion of felsic intrusive rocks continued until the late Pliocene in the rear-arc seamount chain of the IBM arc (Tani et al., 2015) and in the Izu peninsula (Tani et al., 2011). ...
The Japanese archipelago underwent two arc–arc collisions during the Neogene. SW Honshu arc collided with the Izu‐Bonin‐Mariana arc and the NE Honshu arc collided with the Chishima arc. The complicated geological structure of the South Fossa Magna region has been attributed to the collision between the Izu‐Bonin‐Mariana arc and the SW Honshu arc. Understanding the geotectonic evolution of this tectonically active region is crucial for delineating the Neogene tectonics of the Japanese archipelago. Many intrusive granitoids occur around the Kofu basin, in the South Fossa Magna region. Although the igneous ages of these granitoids have been estimated through biotite and hornblende K–Ar dating, here, we perform U–Pb dating of zircon to determine the igneous ages more precisely.
In most cases, the secondary post‐magmatic overprint on the zircon U–Pb system was minor. Based on our results, we identify four groups of U–Pb ages: ca. 15.5 Ma, ca. 13 Ma, ca. 10.5 Ma, and ca. 4 Ma. The Tsuburai pluton belongs to the first group, and its age suggests that the granite formation within the Izu‐Bonin‐Mariana arc dates back to at least 15.5 Ma. The granitoids of the second group intruded into the boundary between the Honshu arc and the ancient Izu‐Bonin‐Mariana arc, suggesting that the arc–arc collision started by ca. 13 Ma. As in the case of the Kaikomagatake pluton, the Chino pluton likely corresponds to a granodiorite formed in a rear‐arc setting in parallel with the other granodiorites of the third group. The U–Pb age of the Kogarasu pluton, which belongs to the fourth group, is the same as those of the Tanzawa tonalitic plutons. This might support a syncollisional rapid granitic magma formation in the South Fossa Magna region.
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... The occurrence of these bodies necessitates a better understanding of what the pPSP was. It had been suggested that the pPSP consisted of Mesozoic terranes of various origins including island arcs as those trapped in the Philippines, and the ADO ridges (Hall, 2002;Hickey-Vargas et al., 2008;Tani et al., 2015;Ishizuka et al., 2018). However, Philippine ophiolites are relatively poorly preserved, highly dismembered, and in some places, accessibility issues have prevented thorough sampling. ...
The proto-Philippine Sea Plate (pPSP) has been proposed by several authors to account for the origin of the Mesozoic supra-subduction ophiolites along the Philippine archipelago. In this paper, a comprehensive review of the ophiolites in the eastern portion of the Philippines is undertaken. Available data on the geology, ages and geochemical signatures of the oceanic lithospheric fragments in Luzon (Isabela, Lagonoy in Camarines Norte, and Rapu-Rapu island), Central Philippines (Samar, Tacloban, Malitbog and Southeast Bohol), and eastern Mindanao (Dinagat and Pujada) are presented. Characteristics of the Halmahera Ophiolite to the south of the Philippines are also reviewed for comparison. Nearly all of the crust-mantle sequences preserved along the eastern Philippines share Early to Late Cretaceous ages. The geochemical signatures of mantle and crustal sections reflect both mid-oceanic ridge and supra-subduction signatures. Although paleomagnetic information is currently limited to the Samar Ophiolite, results indicate a near-equatorial Mesozoic supra-subduction zone origin. In general, correlation of the crust-mantle sequences along the eastern edge of the Philippines reveal that they likely are fragments of the Mesozoic pPSP. © 2019 China University of Geosciences (Beijing) and Peking University
... However, a key characteristic of many oceanic arcs is the observed increase, inboard from the volcanic front, of incompatible and especially lithophile element abundances (e.g., Gill, 1981). For example, low K basalts and andesites along the Izu-Bonin volcanic front in the western Pacific have significantly lower U, Th, Th/U and are depleted in rare earth elements (REE) compared to medium and high-K rear arc lavas, the latter being comparable to average granodioritic continental crust (e.g., Hochstaedter et al., 2001;Ishizuka et al., 2003Ishizuka et al., , 2006bKimura et al., 2010;Tamura et al., 2010;Tani et al., 2015). Therefore, for a full accounting of the genesis of juvenile continental crust, we need to understand spatial and temporal variations within such geochemically asymmetrical oceanic arcs that lead to generation of these more enriched silicic magmas in oceanic arc settings. ...
A 1.2 kilometer thick Paleogene volcaniclastic section at International Ocean Discovery Program Site 351-U1438 preserves the deep-marine, proximal record of Izu-Bonin oceanic arc initiation and volcano evolution along the Kyushu-Palau Ridge (KPR). Pb/U ages and trace element compositions of zircons recovered from volcaniclastic sandstones preserve a remarkable temporal record of juvenile island arc evolution. Pb/U ages ranging from 43 to 27 Ma are compatible with provenance in one or more active arc edifices of the northern KPR. The abundances of selected trace elements with high concentrations provide insight into the genesis of U1438 detrital zircon host melts, and represent useful indicators of both short and long-term variations in melt compositions in arc settings. The Site U1438 zircons span the compositional range between zircons from mid-ocean ridge gabbros and zircons from relatively enriched continental arcs, as predicted for melts in a primitive oceanic arc setting derived from a highly depleted mantle source. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic melts that evolved toward more Th and U-rich compositions with time. Th, U and light rare earth element enrichments beginning about 35 Ma are consistent with detrital zircons recording development of regional arc asymmetry and selective trace element-enriched rear arc silicic melts as the juvenile Izu-Bonin arc evolved.
... Recently, Saito and Tani (2017) proposed that two processes are required for the transformation of Izu-Bonin-Mariana arc crust into mature continental crust; (1) collision with mature arc and formation incompatible elements-enriched hybrid lower crust (basalt + metasediments), and (2) subsequent anatexis to form incompatible elements-enriched granitoids with composition similar to mature continental crust. Alternatively, melting of enriched basalt either in the subduction zone (Gazel et al., 2015) or in the rear-arc crust (Tani et al., 2015) may produce enriched primary and granitic magmas with enriched composition, respectively. ...
The igneous rocks of the Wadi Hamad area are exposed in the northernmost segment of the Arabian-Nubian Shield (ANS). These rocks represent part of crustal section of Neoproterozoic continental island arc which is intruded by late to post-collisional alkali feldspar granites. The subduction-related intrusives comprise earlier gabbro-diorites and later granodiorites-granites. Subduction setting of these intrusives is indicated by medium- to high-K calc-alkaline affinity, Ta-Nb troughs on the spider diagrams and pyroxene and biotite compositions similar to those crystallized from arc magmas. The collisional alkali feldspar granites have high-K highly fractionated calc-alkaline nature and their spider diagrams almost devoid of Ta-Nb troughs. The earlier subduction gabbro-diorites have lower alkalis, LREE, Nb, Zr and Hf values compared with the later subduction granodiorites-granites, which display more LILE-enriched spider diagrams with shallower Ta-Nb troughs, reflecting variation of magma composition with arc evolution. The later subduction granitoids were generated by lower degree of partial melting of mantle wedge and contain higher arc crustal component compared with the earlier subduction gabbro-diorites. The highly silicic alkali feldspar granites represent extensively evolved melts derived from partial melting of intermediate arc crustal sources during the collisional stage. Re-melting of arc crustal sources during the collisional stage results in geochemical differentiation of the continental arc crust and the silicic collisional plutonism drives the composition of its upper part towards that of mature continental crust.
Lead isotope values for volcanogenic massive sulfide (VMS) deposits provide important insights into metal sources and the nature of pre-accretionary tectonostratigraphic terranes and underlying basements. Deposits of this type in New England formed in diverse tectonic settings including volcanic arcs and backarcs, a supra–subduction zone arc, a rifted forearc foreland basin, and a rifted continental margin. Following VMS mineralization on or near the seafloor, components of the tectonostratigraphic assemblages—volcanic ± sedimentary rocks, coeval intrusions, sulfide deposits, and underlying basements—were diachronously accreted to the Laurentian margin during the Paleozoic. Lead isotope data for galena show relatively large ranges for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb. Evaluation of potential lead sources, using for comparison Pb-isotope data from modern and ancient settings, suggests that principal sources include the mantle, volcanic ± sedimentary rocks, and deeper basement rocks. Integration of the Pb-isotope values with published data such as Nd isotopes for the volcanic rocks and from deep seismic reflection profiles points to the involvement of several basements, including those of Grenvillian, Ganderian, Avalonian, and West African (and (or) Amazonian) affinity. Clustering of Pb-isotope data for VMS deposits within individual Cambrian and Ordovician volcanic and volcanosedimentary settings, delineated by differences in ²⁰⁶Pb/²⁰⁴Pb and µ (²³⁸U/²⁰⁴Pb) values, are consistent with lead derivation from at least four and possibly five different tectonostratigraphic assemblages with isotopically distinct basements. Collectively, our Pb-isotope data for New England VMS deposits provide a novel window into the nature of subarc basement rocks during pre-accretionary sulfide mineralization outboard of Laurentia during early Paleozoic time.
Oceanic arcs are known as the critical sites to generate new continental crust. However, additional processes are needed to transform mafic oceanic crust to mature continental crust. Ascertaining the petrogenesis of granitoids therein is vital to understand such a maturation process. In this study, we carried out a combined study of in-situ zircon U-Pb dating and Hf-O isotopic analyses, as well as whole-rock geochemical and Sr-Nd isotopic compositions for the Changwan granitoids in the Erlangping oceanic arc unit of the Tongbai orogen. This intrusion mainly consists of biotite granite with subordinate trondhjemite. LA-ICPMS and SIMS zircon U-Pb dating yielded identical U-Pb ages of ca. 460 Ma, representing the intrusion ages. Whole-rock analyses demonstrated that the Changwan granitoids have relatively high SiO2 (72.71–77.14 wt%) and Al2O3 (12.61–14.24 wt%), but low MgO (0.31–0.74 wt%) contents and Mg# (32.0–40.2). The relatively high K2O (2.81–3.63 wt%) contents and K2O/Na2O values (0.76–1.03) of the granitic samples suggest they belong to middle-K weakly peraluminous I-type granite, while the trondhjemitic samples with low K2O (1.06–1.96 wt%) and K2O/Na2O values (0.23–0.42) are tholeiitic to middle-K series. In both cases, the samples show arc-type trace element distribution patterns. They are characterized by depleted Sr-Nd-Hf isotopic compositions with (⁸⁷Sr/⁸⁶Sr)i of 0.7038–0.7050, εNd(t) of +0.52 ~ +2.85, zircon εHf(t) of +3.4 ~ +9.6, and relatively low zircon δ¹⁸O values of 4.71 ± 0.17–4.97 ± 0.18‰. These isotopic fingerprints suggest the Changwan granitoids were derived from a relatively depleted source that has been hydrothermally altered under high temperature conditions. The most candidate is the accreted Erlangping lower oceanic crust which might have been heterogeneously modified in a supra-subduction zone. This is further verified by the trace element modeling on potential mafic source rocks enriched in incompatible elements in the Erlangping unit. We suggest that the high K2O/Na2O granitic samples were derived from K-rich segment of the Erlangping lower oceanic crust whilst the low K2O/Na2O trondhjemites from K-poor part. Therefore, our results show that partial melting of enriched oceanic crust in supra-subduction zone plays an important role for crust maturation in accretionary orogens.
Magmatic rocks from intra-oceanic arcs are critical for understanding the formation of continental crust and tectonic evolution. The early tectonic evolution of the Neo-Tethyan Ocean before the final Indo-Asia collision remains mysterious, and the geodynamic processes that triggered the Cretaceous magmatism in central Myanmar is still debated. The Cretaceous magmatic complex in the Banmauk-Kawlin area (BKC), west Myanmar terrane (WMT) is composed of the Kanza Chaung granitoid batholith, the Mawgyi Volcanic rocks, and the Pinhinga plutonic complex. Zircon U-Pb dating results of various rocks from the Kanza Chaung batholith suggest magmatism lasted from ca. 110 to ca. 94 Ma, roughly overlapping with new geochronological data for the Mawgyi Volcanics. Mafic rocks, including basalts from the Mawgyi Volcanics and gabbros from the Kanza Chaung Batholith, have geochemical features resembling intra-oceanic arc magmas, characterized by high large-ion-lithophile elements (LILEs) and low high-field-strength elements (HFSEs) and flat trace element patterns. They have depleted Sr (initial ⁸⁷Sr/⁸⁶Sr = 0.7035–0.7054) and Nd (εNd(t) = 0.39–6.71) isotopic compositions, with zircon εHf(t) values ranging from +5.8 to +16.1, probably derived from partial melting of the mantle wedge. Diorites formed by differentiation of basaltic magma have similar trace element patterns and Sr-Nd isotopes. The granitic rocks were likely originated from partial melting of juvenile arc lower-crust, indicated by their high SiO2 (>65.0 wt%), low MgO (<2.50 wt%) and depleted Nd and zircon Hf isotopes. The εNd(t) values of the BKC shift markedly (from ~ + 7 to 0) from 105 to 94 Ma, which correlates with a temporal increase of Th/Nb, La/Ta, and La/Sm. Given the juvenile characteristics of the WMT crust, this can be explained by exotic isotopically enriched crustal components subducted into the mantle source, rather than steady-state sediment subduction and crustal contamination. Given the Albian unconformity in the WMT and recent paleomagnetic data, such continent crustal components were likely introduced by collision followed by subduction of Greater India-derived continental sliver beneath the WMT. Thus crust with an Indian continent affinity was possibly accreted to an intra-oceanic arc (WMT) during the mid-Cretaceous.
The opening of the Japan Sea separated southwest Japan from the Eurasian continent during the early to middle Miocene. Since then, diverse igneous activities have occurred in relation to the subduction of the Philippine Sea Plate beneath southwest Japan. The Okinawa Trough formed in the back‐arc region of the Ryukyu Arc since the late Miocene. In the Koshikijima Islands, off the west coast of Kyushu and near the northern end of the Okinawa Trough, felsic to intermediate igneous rocks with middle to late Miocene radiometric ages occur as granitic intrusions and dikes. We obtained zircon U–Pb ages and whole‐rock major‐ and trace‐element compositions of Koshikijima granitic rocks to elucidate their magmagenesis. The U–Pb ages of granitic rocks in Kamikoshikijima and Shimokoshikijima and a dacite dike are about 10 Ma, suggesting that most magmatism on the Koshikijima Islands was coeval with early rifting in the Okinawa Trough. We infer that magmagenesis occurred via melting of lower crustal mafic rocks related to rifting in the Okinawa Trough based on the arc‐like trace‐element compositions of these I‐type granites. Andesitic dikes preceded felsic igneous activity on the Koshikijima Islands, and their ages and petrochemistry will help elucidate the magmatism and tectonics in this area throughout the Miocene.
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The Nishiyama volcano is a Quaternary stratovolcano consisting of the northwestern part of the Hachijo-jima island, located in a volcanic front of the Izu-Bonin arc. The Holocene activity of the Nishiyama volcano began at~10-13 ka and has mainly produced basaltic lava flows and scoria fall deposits. While gabbroic and doleritic enclaves are generally found in scoriaceous pyroclasts, we discovered anorthositic enclaves in the lava flows. Here, we report the petrographical and petrological characteristics of the anorthositic enclaves. In the basaltic lava flows, the amount of plagioclase phenocrysts positively correlated with the whole-rock content of Al 2 O 3 , CaO, and Sr with these elements preferentially contained in the feldspar. In addition, an ideal anorthite content obtained from the whole-rock chemistry of the basaltic lava flows (An 88) was largely consistent with the measured anorthite content obtained from anorthosite (An 84). These results suggest that the plagioclase fractio-nation and/or accumulation controls the whole-rock chemical composition of basaltic lava flows and that the accumulated plagioclases represent part of anorthositic enclaves. Anorthosite was divided into three types of textures: 1) Comb texture, 2) adcumulate texture, and 3) radial texture. These textures (from the comb texture to the radial texture) reflected the change in the undercooling state of the magma. Based on the petrography of the anorthosite and lava flows, the plagioclase was in a liquidus phase in the Nishiyama basaltic magma. Additionally , the anorthosite was formed by the effects of adiabatic ascent and the degassing of near-H 2 O-saturated magma at the shallow magma reservoir (~5 km in depth) beneath the Nishiyama volcano.
According to the theory of plate tectonics, plate boundary is the locus of the various geological processes shaping our Earth, and thus the focus of the modern geology. It is generally accepted that the plate boundaries can be divided into three types, i. e., divergent (mid-oceanic ridge), convergent (subduction and collision zones), and strike-slip (transform fault). As for the convergent boundary, it can be further subdivided into intra-oceanic arc by oceanic-oceanic subduction, Andean continental arc by oceanic-continental subduction, and collisional belt by continental-continental collision. The intra-oceanic arc is relatively less studied compared to other types of convergent boundary. Traditionally, the Japanese islands were thought as an intra-oceanic arc, but this point of view is not hold anymore since it was found that they are fragments from the Asian continental margin. According to the available investigations, intra-oceanic arcs are mostly located along the western Pacific Ocean, represented by the Izu-Bonin-Mariana arc between the Pacific and Philippine plates, and the southwestern (SW) Pacific arc between the Pacific and Australian plates. The most important topic for the study of the intra-oceanic arc is that how subduction was initiated between oceanic-oceanic plates. It is mostly thought that the transform fault can make contact of the different aged oceanic plates, and subsequently in this case that the older and dense lithosphere can underthrust beneath the younger and less-dense plate, which is also ascribed as "Subduction Initial Rule-SIR". However, no actual example was found to date on the Earth for the situation described above, and moreover, it is hard to understand that the intra-oceanic arcs are extensively developed in SW Pacific, although the transform faults occurs everywhere in the seafloor. Data compilation indicates that the above intra-oceanic arcs are in fact continental relics rifted from the Australian continent. The subduction of the old and dense Pacific plate resulted in formation of the continental arc and associated back-arc basin, and the spreading within the back-arc basin then rifted the arc and made it evolved toward oceanic lithosphere. Therefore, the oceanic-oceanic subduction is not valid theoretically, and not documented in the geological records either. However, in the Caribbean, Scotia and Aleutian areas, the intra-oceanic arcs were formed by subduction jump and polarity reverse due to subduction of oceanic plateau. Whatever, the spontaneous initial subduction model needs further geological data to be verified.
Before the India–Asia collision, Neotethyan subduction gave rise to an Andean-type convergent margin on the southern margin of Asia. To investigate the spatial and temporal distribution of the subduction-related magmatism, we undertook a combined determination of zircon U–Pb ages and Hf isotopes of Mesozoic to Paleogene intrusive and volcanic rocks from southern Tibet to Myanmar to characterize the two parallel magmatic belts that have previously been considered separately. One belt extends from the Gangdese Batholith in the Southern Lhasa sub-terrane to the Lohit Batholith, the Sodon Pluton and the Popa–Loimye Arc in the West Burma Block, and the other from the Central Lhasa Plutonic Belt to the Bomi–Chayu Batholith, the Dianxi Batholith and the Shan Scarps Batholith in central Myanmar. The Gangdese belt, as the main arc component, consists typically of I-type granitoids that contain magmatic zircons showing positive ε Hf ( t ) values. In contrast, the Central Lhasa Plutonic Belt belt is dominated by S-type granites in which most zircons show negative ε Hf ( t ) values suggesting the involvement of older continental crust in their petrogenesis. The distinct geochemical characteristics indicate not only distinct tectonic settings of their genesis but also the diverse nature of the crust forming the exotic continental ribbons amalgamated to Asia.
Supplementary material: Details of sample locations and analytical results are available at: https://doi.org/10.6084/m9.figshare.c.4311485
All the granitoids in the Japanese Islands are Phanerozoic and of arc-type. They are part of the Late Mesozoic Circum-Pacific granite superprovince. Most of the Japanese granitoids were formed when the Japanese Islands belonged to the Eurasian continent, as the growing front of the continent. They are mostly of I-type, and S-type granitoids are very small in amount. The origin of these granitoids is mostly partial melt of mantle-derived mafic lower crust of arc without an involvement of ancient cratons or their derivatives.
The granitic magmatism was quite episodic. 80% of their surface exposure area is occupied with 50-130Ma, Paleogene to Cretaceous granitoids. In Southwest Japan, they constitute three arc-parallel granitic provinces called Ryoke, San-yo and San-in belts. A transect from the Ryoke to San-yo belt represents the hypothetical crustal cross section of the Cretaceous Eurasian continental margin. The Hidaka belt in Hokkaido is another example of a crustal cross section, exposing the deep Kuril arc at Miocene. On the fore-arc side of the Southwest Japan, 13-15Ma, Middle Miocene granitic rocks are exposed sporadically but widely. The magmatism was very short-lived, supposed to be generated in an unusual tectonic setting related to back-arc opening and incipient subduction of the Philippine Sea plate. Middle Miocene and still younger granitoids are exposed in the Izu Collision Zone. The Quaternary granitoids of ~1Ma are exposed at the Central Highlands in central Japan. Jurassic and Triassic granitoids occur in the Hida belt, which is the most back-arc side unit of the Japanese Islands. Paleozoic granitoids are rare. They are exposed as geologically isolated small bodies or tectonic blocks.
Granitic rocks (sensu . lato) are major constituents of upper continental crust. Recent reviews reveal that the average composition of Phanerozoic upper continental crust is granodioritic. Although oceanic arcs are regarded as a site producing continental crust material in an oceanic setting, intermediate to felsic igneous rocks occurring in modern oceanic arcs are dominantly tonalitic to trondhjemitic in composition and have lower incompatible element contents than the average upper continental crust. Therefore, juvenile oceanic arcs require additional processes in order to get transformed into mature continental crust enriched in incompatible elements.Neogene granitoid plutons are widely exposed in the Izu Collision Zone in central Japan, where the northern end of the Izu-Bonin-Mariana (IBM) arc (juvenile oceanic arc) has been colliding with the Honshu arc (mature island arc) since Middle Miocene. The plutons in this area are composed of various types of granitoids ranging from tonalite to trondhjemite, granodiorite, monzogranite and granite. Three main granitoid plutons are distributed in this area: Tanzawa plutonic complex, Kofu granitic complex, and Kaikomagatake granitoid pluton. Tanzawa plutonic complex is dominantly composed of tonalite and trondhjemite and characterized by low concentration of incompatible elements and shows geochemical similarity with modern juvenile oceanic arcs. In contrast, Kofu granitic complex and Kaikomagatake granitoid pluton consists mainly of granodiorite, monzogranite and granite and their incompatible element abundances are comparable to the average upper continental crust. Previous petrogenetic studies on these plutons suggested that (1) the Tanzawa plutonic complex formed by lower crustal anatexis of juvenile basaltic rocks occurring in the IBM arc, (2) the Kofu granitic complex formed by anatexis of 'hybrid lower crust' comprising of both basaltic rocks of the IBM arc and metasedimentary rocks of the Honshu arc, and (3) the Kaikomagatake granitoid pluton formed by anatexis of 'hybrid lower crust' consisting of K-rich rear-arc crust of the IBM arc and metasedimentary rocks of the Honshu arc. These studies collectively suggest that the chemical diversity within the Izu Collision Zone granitoid plutons reflects the chemical variation of basaltic sources (i.e., across-arc chemical variation in the IBM arc) as well as variable contribution of the metasedimentary component in the source region. The petrogenetic models of the Izu Collision Zone granitoid plutons suggest that collision with another mature arc/continent, hybrid lower crust formation and subsequent hybrid source anatexis are required for juvenile oceanic arcs to produce granitoid magmas with enriched compositions. The Izu Collision Zone granitoid plutons provide an exceptional example of the collision-induced transformation from a juvenile oceanic arc to the mature continental crust.
SUMMARY: Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs.
In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (≤1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (≤2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by
The Izu-Bonin-Mariana (IBM) arc system extends 2800km from near Tokyo,
Japan to Guam and is an outstanding example of an intraoceanic
convergent margin (IOCM). Inputs from sub-arc crust are minimized at
IOCMs and output fluxes from the Subduction Factory can be more
confidently assessed than for arcs built on continental crust. The
history of the IBM IOCM since subduction began about 43 Ma may be better
understood than for any other convergent margin. IBM subducts the oldest
seafloor on the planet and is under strong extension. The stratigraphy
of the western Pacific plate being subducted beneath IBM varies simply
parallel to the arc, with abundant off-ridge volcanics and
volcaniclastics in the south which diminish northward, and this seafloor
is completely subducted. The Wadati-Benioff Zone varies simply along
strike, from dipping gently and failing to penetrate the 660 km
discontinuity in the north to plunging vertically into the deep mantle
in the south. The northern IBM arc is about 22km thick, with a felsic
middle crust; this middle crust is exposed in the collision zone at the
northern end of the IBM IOCM. There are four Subduction Factory outputs
across the IBM IOCM: (1) serpentinite mud volcanoes in the forearc, and
as lavas erupted from along (2) the volcanic front of the arc and (3)
back-arc basin and (4) from arc cross-chains. This contribution
summarizes our present understanding of matter fed into and produced by
the IBM Subduction Factory, with the intention of motivating scientific
efforts to understand this outstanding example of one of earth's most
dynamic, mysterious, and important geosystems.
Intra-oceanic arc systems (IOASs) represent the oceanic endmember of arc-trench systems and have been the most important sites of juvenile continental crust formation for as long as plate tectonics has operated. IOASs' crustal profiles are wedge-shaped, with crust up to 20-35 km thick; a more useful definition is that IOASs occur as chains of small islands, generally just the tops of the largest volcanoes. A very small fraction of IOASs lie above sea level, but advancing marine technologies allow their most important features to be defined. Modern IOASs subduct old, dense oceanic lithosphere and so tend to be under extension. They consist of four parallel components: trench, forearc, volcanic-magmatic arc, and back-arc, occupying a ≥200 km zone along the leading edge of the overriding plate. These components form as a result of hydrous melting of the mantle and reflect the strongly asymmetric nature of subduction processes. Forearcs preserve infant arc lithosphere whereas magmatism in mature IOASs is concentrated along the volcanic-magmatic front. Mature IOASs often have minor rear-arc volcanism and, because most IOASs are strongly extensional, sea-floor spreading often forms back-arc basins. Sub-IOAS mantle is also asymmetric, with serpentinized harzburgite beneath the forearc, pyroxene-rich low-V p mantle beneath the magmatic front, and lherzolite-harzburgite beneath back-arc basins. Because most IOASs are far removed from continents, they subduct oceanic lithosphere with thin sediments and have naked forearcs subject to tectonic erosion. IOASs evolve from broad zones of very high degrees of melting and sea-floor spreading during their first 5-10 Ma, with the volcanic-magmatic front retreating to its ultimate position c. 200 km from the trench.
Trace elements, including rare earth elements (REE), exhibit systematic variations in plutonic rocks from the Captains Bay pluton which is zoned from a narrow gabbroic rim to a core of quartz monzodiorite and granodiorite. The chemical variations parallel those in the associated Aleutian calcalkaline volcanic suite. Concentrations of Rb, Y, Zr and Ba increase as Sr and Ti decrease with progressive differentiation. Intermediate plutonic rocks are slightly enriched in light REE (La/Yb=3.45–9.22), and show increasing light REE fractionation and negative Eu anomalies (Eu/Eu*=1.03–0.584). Two border-zone gabbros have similar REE patterns but are relatively depleted in total REE and have positive Eu anomalies; indicative of their cumulate nature. Initial 87Sr/86Sr ratios in 8 samples (0.70299 to 0.70377) are comparable to those of volcanic rocks throughout the arc and suggest a mantle source for the magmas. Oxygen isotopic ratios indicate that many of the intermediate plutonic rocks have undergone oxygen isotopic exchange with large volumes of meteoric water during the late stages of crystallization; however no trace element or Sr isotopic alteration is evident.
Major and trace element variations are consistent with a model of inward fractional crystallization of a parental high-alumina basaltic magma at low pressures (〈6 kb). Least-squares approximations and trace element fractionation calculations suggest that differentiation in the plutonic suite was initially controlled by the removal of calcic plagioclase, lesser pyroxene, olivine and Fe-Ti oxides but that with increasing differentiation and water fugacity the removal of sub-equal amounts of sodic plagioclase and hornblende with lesser Fe-Ti oxides effectively drove residual liquids toward dacitic compositions. Major and trace element compositions of aplites which intrude the pluton are not adequately explained by fractional crystallization. They may represent partial melts derived from the island arc crust. Similarities in Sr isotopes, chemical compositions and differentiation trends between the plutonic series and some Aleutian volcanic suites indicates that shallow-level fractional crystallization is a viable mechanism for generating the Aleutian calcalkaline rock series.
The collision of the Luzon Arc with southern China represents the best example of arc-continent collision in the modern oceans, and compares closely with the Early Ordovician accretion of the Lough Nafooey arc of Connemara, Ireland, to the passive margin of Laurentia. We propose a general model for steady-state arc-continent collision in which arc crust is progressively added to a passive margin during a process of compression, metamorphism and magmatism lasting 3-10 Ma at any one location on the margin. Depending on the obliquity of the angle of collision, the timing of active collision may be diachronous and long-lived along the margin. Magmatism accompanying accretion can be more enriched in incompatible trace elements than average continental crust, contrasting with more depleted magmatism prior to collision. Accretion of a mixture of depleted and enriched arc lithologies to the continental margin allows the continental crust to grow through time by arc-passive margin collision events. During the collision the upper and middle arc crust are detached from the depleted ultramafic lower crust, which is subducted along with the mantle lithosphere on which the arc was founded. Rapid (2-3 Ma) exhumation and gravitational collapse of the collisional orogen forms the Okinawa and South Mayo Troughs in Taiwan and western Ireland, respectively. These basins are filled by detritus eroded from the adjacent collision zone. During subsequent subduction polarity reversal, continuous tearing and retreat of the oceanic lithosphere along the former continent-ocean transition provides space for the new subducting oceanic plate to descend without need for breaking of the original slab.
The Tanzawa plutonic complex (TPC), central Japan, is a suite of tonalitic-gabbroic plutons exposed in a globally unique arc-arc collision zone, where an active intraoceanic Izu-Bonin-Mariana (IBM) arc is colliding against the Honshu arc. The TPC has been widely accepted as an exposed middle crust section of the IBM arc, chiefl y because of geochemical similarities between the TPC and IBM rocks and previously reported precollisional Miocene K-Ar ages. However, new zircon U-Pb ages show that the main pulse of TPC magmatism was syncollisional and that plutons were emplaced rapidly and cooled soon after Pliocene collision. Trace element compositions of TPC zircon show distinctively elevated Th/Nb ratios compared to zircon from other noncollisional IBM silicic plutonic rocks, indicating the involvement of continental sediments from the Honshu arc in their magma genesis.
Across-arc geochemical trends can constrain subduction zone dynamics by providing clues to the source composition and melting systematics of subduction-related magmas over a wide portion of the mantle wedge. The Izu-Bonin arc contains an active volcanic front, an active extensional zone, and a series of 3-9 Ma southwest trending across-arc seamount chains. The volcanic front (VF) contains one of the most depleted suites of any volcanic arc, with basalt containing 0.2-0.7 ppm Nb, 25-50 ppm Zr, Nb/Zr
The melting relations of five metamorphosed basalts and andesites (greenstones and amphibolites), collected from the late Jurassic Smartville arc complex of California, were investigated experimentally at 800-1000° C and 1, 3, and 6. 9 kb. Dehydration-melting (no water added) experiments contained only the water structurally bound in metamorphic minerals (largely amphiboles). They yielded mildly peraluminous to metaluminous granodioritic to trondhjemitic melts (Na/K is a function of starting composition) similar in major element composition to silicic rocks in modern oceanic arcs. The dehydration melts are water-undersaturated, with, and coexist with the anhydrous residual solid (restite) assemblage plagioclase + orthopyroxene + clinopyroxene + magnetite ± ilmen-ite, with plagioclase constituting ∼ 50% of the restite mode. In the dehydration-melting experiments at 3 kb the onset of melting occurred between 850 and 900 ° C, as amphibole and quartz broke down to yield pyroxenes plus melt. Total pressure is greater than in the dehydration-melting experiments and has little effect on melt composition or phase relations.In the water-saturated (water added, so that experiments, melts formed at 3 kb and above are strongly peraluminous, rich in Ca and poor in Fe, Mg, Ti, and K. Their compositions are unlike those of most silicic igneous rocks. These melts coexist with the amphibole-rich, plagioclase-poor restite assemblage amphibole + magnetite ± clinopyroxene ± plagioclase ± ilmenite. The highly aluminous nature of the melts and the plagioclase-poor nature of the restite both reflect the substantial contribution of plagioclase (along with quartz) to melts in high-pressure water-saturated systems. Water pressure equals Ptoul in the water-saturated experiments and has a profound effect on both melt composition and phase relations. At 1 kb, the water-saturated experiments yielded melt and mineral products with some characteristics of the dehydration-melting experiments (no amphibole at high T), and some characteristics of the 3-kb, water-saturated experiments (amphibole plus melt coexisting at lower T, elevated Al, lowered Fe). As pressure is increased from 3 to 6. 9 kb, the stability fields of both plagioclase and clinopyroxene decrease relative to amphibole and the Al contents of the melts increase.These experiments have important implications for the petrogenesis of low-K silicic rocks in arcs. First, dehydration melting is a viable mechanism for the formation of these rocks; water-saturated melting is not. Second, because of the influence of rock composition on melt composition, low-grade metamorphic and hydrothermal processes that alter the alkali contents and Na/ K in arc basement terranes may have a direct impact on the petrogenesis of silicic magmas in arcs, particularly the formation of extremely low-K trondhjemites. Third, the experiments predict that anhydrous, pyroxene- and plagioclase-rich 'granulitic' restite assemblages should develop as a result of partial melting in arc terranes. Such assemblages occur in at least two deeply eroded arc complexes.
The majority of felsic rocks from composite centers in the southernmost Cascades have geochemical and Sr, Nd and Pb isotopic
ratios that suggest derivation by partial melting of lower crust that is compositionally similar to calc-alkaline basalts
observed in the region. Only a few felsic rocks have δ18O and Pb isotopic compositions that indicate interaction with the upper crust. Mineralogical and geochemical differences among
the felsic magmas result primarily from melting under variable f(H2O) and temperature conditions. Partial melting under low f(H2O) and high temperature conditions leaves an amphibole-poor residuum, and produces magmas that have orthopyroxene as the most
abundant ferromagnesian phenocryst, relatively low silica contents, and straight rare earth element patterns. Partial melting
under higher f(H2O) and lower temperature conditions leaves an amphibole-rich residuum, and produces magmas that have amphibole ± biotite phenocrysts,
relatively high silica contents, and pronounced middle rare earth element depletions. These conclusions are consistent with
published thermal models that suggest that reasonable volumes of basaltic magma emplaced beneath large composite centers in
the southernmost Cascades can serve as the heat source for melting of the lower crust. Melting of the lower crust under variable
f(H2O) conditions is likely to result from differences in the H2O contents of these basaltic magmas.
We present major, trace element, and Pb-Sr-Nd-Hf isotope data for Quaternary basalt and basaltic andesite lavas from cross-chain volcanoes in the northern Izu (N-Izu) arc. Lavas from Izu-Oshima, Toshima, Udonejima, and Niijima islands show consistent chemical changes with depth to the Wadati-Benioff zone, from 120 km beneath Izu-Oshima to 180 km beneath Niijima. Lavas from Izu-Oshima at the volcanic front (VF) have elevated concentrations of large ion lithophile elements (LILEs), whereas rear-arc (RA) lavas are rich in light rare earth elements (LREEs) and high field strength elements (HFSEs). VF lavas also have more radiogenic Pb, Nd, Sr, and Hf isotopic compositions. We have used the Arc Basalt Simulator version 3 (ABS3) to examine the mass balance of slab dehydration and melting and slab fluid/melt-fluxed mantle melting and to quantitatively evaluate magma genesis beneath N-Izu. The results suggest that the slab-derived fluids/melts are derived from ∼20% sediment and ∼80% altered oceanic crust, the slab fluid is generated by slab dehydration for the VF magmas at 3.3–3.5 GPa/660°C–700°C, and slab melt for RA magmas is supplied at 3.4–4.4 GPa/830°C–890°C. The degree of fluxed melting of the mantle wedge varies between 17% and 28% (VF) and 6% and 22% (RA), with a slab flux fraction of 2%–4.5% (VF fluid) to 1%–1.5% (RA melt), and at melting depths 1–2.5 GPa (VF) and 2.4–2.8 GPa (RA). These conditions are consistent with a model whereby shallow, relatively low temperature slab fluids contribute to VF basalt genesis, whereas deeper and hotter slab melts control formation of RA basalts. The low-temperature slab dehydration is the cause of elevated Ba/Th in VF basalt due mainly to breakdown of lawsonite, whereas deeper breakdown of phengite by slab melting is the cause of elevated K and Rb in RA basalts. Melting in the garnet stability field, and at lower degrees of partial melting, is required for the elevated LILEs, LREEs, and HFSEs observed in the RA basalts. Less radiogenic Sr, Nd, Hf, and Pb in RA basalts are all attributable to lesser slab flux additions. The low H2O predicted for RA basalt magmas (
The Earth is an unusual planet in our solar system in having a bimodal
topography that reflects the two distinct types of crust found on our
planet. The low-lying oceanic crust is thin (˜7 km on average),
composed of relatively dense rock types such as basalt and is young
(≤200 Ma old) (see Chapter 3.13). In contrast, the high-standing
continental crust is thick (˜40 km on average), is composed of
highly diverse lithologies (virtually every rock type known on Earth)
that yield an average intermediate or "andesitic" bulk composition
(Taylor and McLennan (1985) and references therein), and contains the
oldest rocks and minerals yet observed on Earth (currently the 4.0 Ga
Acasta gneisses (Bowring and Williams, 1999) and 4.4 Ga detrital zircons
from the Yilgarn Block, Western Australia (Wilde et al., 2001)),
respectively. Thus, the continents preserve a rich geological history of
our planet's evolution and understanding their origin is critical for
understanding the origin and differentiation of the Earth.The origin of
the continents has received wide attention within the geological
community, with hundreds of papers and several books devoted to the
topic (the reader is referred to the following general references for
further reading: Taylor and McLennan (1985), Windley (1995), and Condie
(1997). Knowledge of the age and composition of the continental crust is
essential for understanding its origin. Patchett and Samson (Chapter
3.10) review the present-day age distribution of the continental crust
and Kemp and Hawkesworth (Chapter 3.11) review secular evolution of
crust composition. Moreover, to understand fully the origin and
evolution of continents requires an understanding of not only the crust,
but also the mantle lithosphere that formed more-or-less
contemporaneously with the crust and translates with it as the
continents move across the Earth's surface. The latter topic is reviewed
in Chapter 2.05.This chapter reviews the present-day composition of the
continental crust, the methods employed to derive these estimates, and
the implications of the continental crust composition for the formation
of the continents, Earth differentiation, and its geochemical
inventories.
The Early to Middle Jurassic Talkeetna Arc section exposed in the Chugach Mountains of south–central Alaska is 5–18 km wide and extends for over 150 km. This accreted island arc includes exposures of upper mantle to volcanic upper crust. The section comprises six lithological units, in order of decreasing depth: (1) residual upper mantle harzburgite (with lesser proportions of dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lower crustal gabbronorite; (5) mid-crustal plutonic rocks; (6) volcanic rocks. The pyroxenites overlie residual mantle peridotite, with some interfingering of the two along the contact. The basal gabbronorite overlies pyroxenite, again with some interfingering of the two units along their contact. Lower crustal gabbronorite (≤10 km thick) includes abundant rocks with well-developed modal layering. The mid-crustal plutonic rocks include a heterogeneous assemblage of gabbroic rocks, dioritic to tonalitic rocks (30–40% area), and concentrations of mafic dikes and chilled mafic inclusions. The volcanic rocks (∼7 km thick) range from basalt to rhyolite. Many of the evolved volcanic compositions are a result of fractional crystallization processes whose cumulate products are directly observable in the lower crustal gabbronorites. For example, Ti and Eu enrichments in lower crustal gabbronorites are mirrored by Ti and Eu depletions in evolved volcanic rocks. In addition, calculated parental liquids from ion microprobe analyses of clinopyroxene in lower crustal gabbronorites indicate that the clinopyroxenes crystallized in equilibrium with liquids whose compositions were the same as those of the volcanic rocks. The compositional variation of the main series of volcanic and chilled mafic rocks can be modeled through fractionation of observed phase compositions and phase proportions in lower crustal gabbronorite (i.e. cumulates). Primary, mantle-derived melts in the Talkeetna Arc underwent fractionation of pyroxenite at the base of the crust. Our calculations suggest that more than 25 wt % of the primary melts crystallized as pyroxenites at the base of the crust. The discrepancy between the observed proportion of pyroxenites (less than 5% of the arc section) and the proportion required by crystal fractionation modeling (more than 25%) may be best understood as the result of gravitational instability, with dense ultramafic cumulates, probably together with dense garnet granulites, foundering into the underlying mantle during the time when the Talkeetna Arc was magmatically active, or in the initial phases of slow cooling (and sub-solidus garnet growth) immediately after the cessation of arc activity.
Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≃ Rb ≃ (≃ Tl) ≃ Ba(≃ W) > Th > U ≃ Nb = Ta ≃ K > La > Ce ≃ Pb > Pr(≃ Mo) ≃ Sr > P ≃ Nd (> F) > Zr = Hf ≃ Sm > Eu ≃ Sn (≃ Sb) ≃ Ti > Dy ≃ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the istopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. -from Authors
We propose that across-arc differences in the geochemistry of Izu-Bonin arc magmas are controlled by the addition of fertile-slab fluids to depleted mantle at the volcanic front, and residual-slab fluids to fertile mantle in the back arc without slab melting or contemporaneous back arc spreading. The arc consists of a volcanic front, an extensional zone, and seamount chains (the Western Seamounts) that trend into the Shikoku Basin. Each province produces a distinct suite of arc-like volcanic rocks that have relative Nb depletions and high ratios of fluid-mobile elements to high field strength elements. The volcanic front has the lowest concentrations of incompatible elements and the strongest relative enrichments of fluid-mobile elements (high U/Nb, Ba/Nb, Pb/Zr, Th/Nb, 206 Pb/ 204 Pb, e Nd , and 87 Sr/ 86 Sr). A fluid derived from both sediment and altered oceanic crust explains most of the slab-related characteristics of the volcanic front. The Western Seamounts and some of the extensional zone rocks have lower e Nd , 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, Ba/Th, and U/Th; moderate Ba/Nb and U/Nb; and similar or higher Th/Nb and Th/Nd. Although the lower e Nd and higher Th/Nd tempt a sediment melt explanation, a lack of correlation between the strongest sediment proxies, such as e Nd , Th/Nb, and Ce/ Ce*, precludes sediment melts. The subduction component for the Western Seamounts is probably a fluid dehydrated from a residual slab that was depleted in fluid-mobile elements beneath (as well as trenchward of) the volcanic front. This depleted fluid is added to elementally and isotopically more enriched mantle beneath the Western Seamounts. Index terms: Minor and trace element composition; mineralogy and petrology; physics and chemistry of magma bodies., 2001. Across-arc geochemical trends in the Izu-Bonin arc: Contributions from the subducting slab, Geochem. Geophys. Geosyst., vol. 2, Paper number 2000GC000105 [12,776 words, 15 figures, 3 tables]. Published July 2, 2001.
Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an FeNi core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements.The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth.The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at ∼ 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga).The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an FeNi alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence (∼ 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition.The Earth has a bulk of ∼ 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth (∼ 14%) and the core (∼ 86%). Chondritic meteorites display a range of ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.
High-Si dacites comprise one mode in each of two bimodal volcanic rock suites in Fiji. Both suites are low-K throughout but differ in level of iron-enrichment and light REE-enrichment. Sparsely-phyric dacite lava and tephra cover 102-103 km2 in each suite, apparently having erupted in shallow marine environments. Tephra predominates. Typical phenocrysts are plagioclase >>quartz >>Fe-Ti oxides and augite; hypersthene and hornblende are rare and biotite is absent. Most samples have >72% SiO2, <15% Al2O3, and <0.8% K2O. Normatively, most have 6% Or and An x 100/(Ab + An) ratios of 20 over a wide range in Ab/Qz ratios; thus they cross the feldspar-quartz cotectic at a high angle. Their compositions are similar to those of 20 to 40% partial melts of basalt at PH2O = 5 kb. Tonal ite-trondhjemite and gabbro plutons also form a bimodal suite. Trondhjemite and dacite mineralogy and compositions are analogous although trondhjemites contain primary hornblende. REE patterns vary considerably in the three otherwise similar suites, ranging from light REE-depletion with Lae.f. =8, to middle REE-enrichment with Nde.f. = 360, to middle-REE depletion with Dye.f = 7 (Fig. 5). The latter characterizes all trondhjemites analyzed and is compatible with fusion of lower Fijian crust.
This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.
Field observations and structural analyses of the orientations of ductile flow fabrics and brittle fractures are used to infer diapiric rise of the Yakushima pluton into epizonal Paleogene sediments of an accretionary prism in Miocene times. Inside the pluton, preferred orientations of orthoclase megacrysts define a weak primary flow fabric. Shape and intensity of fabric ellipsoids derived from patterns in the alignment of the megacrysts reveal a circulation cell within the domal structure of the Yakushima granite. The asymmetry of this circulation pattern suggests that the pluton rose obliquely upward to the southeast, toward the Ryukyu trench. Fold axes in the ductile strain aureole of the pluton are distorted into conformity with the pluton’s shape in the direction of magma ascent Late aphte sheets, whose orientations record the local paleostress pattern, were emplaced into a concentric fracture system in and around the pluton. The aphtes indicate the lifting, and tangential spreading with concentrically disposed intermediate stress axes, of a brittle roof above a buoyant ellipsoidal body of residual magma. Comparable with strain patterns in theoretical models of a viscous sphere rising in ductile surroundings, the deformation pattern in and around the pluton is attributed to the final upsurge of oblique diapiric emplacement.
A series of water-deficient partial melting experiments on a low-K tholeiite were carried out under lower crustal P-T-H2O conditions (900-1200°C, 0.7-1.5 GPa, 2 and 5 wt% H2O added) using a piston-cylinder apparatus. With increasing temperature at 1.0 GPa, supersolidus mineral assemblages vary from amphibolitic to pyroxenitic. Garnet crystallizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low-K calc-alkalic trends, and are classified as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu-Bonin-Mariana (IBM) arc, for example Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) characteristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust (~50% melting at 1050°C and below 1.2 GPa). Estimated densities of pyroxenitic restites (~3.9 g/cm3) after extraction of andesitic melts are higher than that of mantle peridotite beneath the island arc (3.3 g/cm3). The high density of the restite could cause delamination of the IBM arc lower crust. Rhyolitic magmas in the IBM arc (e.g. Niijima) could be formed by low degrees of partial melting of the amphibolitic crust at a temperature just above the solidus (10% melting at or below 900°C).
The bulk composition of the continental crust throughout geological history is thought by most previous workers to be andesitic. This assumption of an andesitic bulk composition led to an early hypothesis by Taylor (1967) that the continental crust was created by are magmatism. This hypothesis for the origin of continental crust was challenged by several authors because: (i) the mean rate of are crust addition obtained by Reymer and Schubert (1984) is too small to account for some certain phases of rapid crustal growth; and (ii) the bulk composition of ocean island arcs, the main contributor to the Archean and early Proterozoic crust, is basaltic rather than andesitic (Arculus 1981; Pearce et al. 1992). New data fi om the Northern Izu-Bonin are are presented here which support the Taylor (1967) hypothesis for the origin of the continental crust by andesitic arc magma. A geological interpretation of P. wave crustal structure obtained fi om the Northern Izu-Bonin are by Suyehiro et al. (1996) indicates that the are crust has four distinctive lithologic layers: fi om top to bottom: (i) a 0.5-2-km-thick layer of basic to intel mediate volcaniclastic, lava and hemipelagite (layer A); (ii) a 2-5-km-thick basic to intermediate volcaniclastics, lavas and intrusive layer (layer B); (iii) a 2-7-km-thick layer of felsic (tonalitic) rocks (layer C); and (iv) a 4-7-km-thick layer of mafic igneous rocks (layer D). The chemical composition of the upper and middle part of the northern Izu-Bonin are is estimated to be similar to the average continental crust by Taylor and McLennan (1985). The rate of igneous addition of the Northern Izu-Bonin are since its initial 45-Ma magmatism was calculated as 80 km(3)/km per million years. This rate of addition is considered to be a reasonable estimate for all arcs in the western Pacific. Using this rate, the global rate of crustal growth is estimated to be 2.96 km(3)/year which exceeds the average rate of crustal growth since the formation of the Earth (1.76 km(3)/year). Based on this estimate of continental growth and the previously documented sediment subduction and tectonic erosion rate (1.8 km(3)/year, von Heune & Scholl 1991), several examples of growth curves of the continental crust are presented here. These growth curves suggest that at least 50% of the present volume of the continental crust can be explained by are magmatism. This conclusion indicates that are magmatism is the most important contributor to the formation of continental crust, especially at the upper crustal level.
Echelon seamount chains trending ENE-WSW exist in the backarc region of the northern Izu-Ogasawara Arc. The Kan'ei, Manji, Enpo and Genroku seamount chains (from north to south) constitute four especially well documented and investigated example. These seamount chains formed between 17 Ma and 3 Ma (Ishizuka, 1999). The eastern sections of each seamount chain are over-printed by many small knolls formed by intra arc rifting volcanism after 2.5 Ma (e.g. Hochsteadter et al., 2000; Morita, 1999). We report results of detail petrological analysis of basalts from two different seamounts and andesites from four different seamounts of the Enpo seamount chain. The Ar-Ar age of these volcanic rocks range from 5.8 Ma to 3.9 Ma (Ishizuka, 1999). Bulk chemistry of trace element and composition of chrome spinel included in olivine phenocrysts indicate that there are two kinds of primitive basalts in the Enpo seamount chain. These were produced from different mantle sources each other. One type of basalt has 'enriched' composition similar to enriched mid-ocean ridge basalt (E-MORB). Trace element signatures indicate that the other type of basalt is produced by 'subduction-related' magmatism. It is defined that there are two mantle souses, enriched (E-MORB-like) and subduction related, for the volcanic rocks constituting the Enpo seamount chain. Enriched basalts exhibit reverse zoning and resorptive rims of olivine crystals, which may indicate magma mixing. We use mineralogical and geochemical studies, to conclude that most andesites from this region are produced mainly by fractionation of 'subduction related' basalts, and that fractionation occurs along with magma mixing and/or interaction with crustal materials. Andesites, which cannot be explained by fractionation, require either different mechanisms for magma genesis and/or the presence of a different primary basalt with a different petrological characteristics. Volcanism on the Enpo seamount chain is characterized by complex relationships between enriched (E-MORB-like) basalt, subduction-related basalt, fractionation-related andesites, and other apparently unrelated andesites.
Every geologist is acquainted with the principle of "uniformitarianism," which holds that present-day processes are the key to those that operated in the past. But the extent this applies to the processes driving the growth and differentiation of the Earth's continental crust remains a matter of debate. Unlike its dense oceanic counterpart, which is recycled back into the mantle by subduction within 200 Ma (see Chapter 3.13), the continental crust comprises buoyant quartzofeldspathic materials and is difficult to destroy by subduction. The continental crust is, therefore, the principal record of how conditions on the Earth have changed, and how processes of crust generation have evolved through geological time. It preserves evidence of secular variation in crustal compositions, and thus the way in which the crust has formed throughout Earth's history. Exploring the nature and origin of these variations is the focus of this chapter.Continental rocks are highly differentiated, and so the crust is enriched in incompatible components compared to the primeval chondritic composition (see Chapter 3.01). Of these, water is perhaps the most relevant, both for the origin and evolution of life, and also for many models of crust generation and differentiation. Similarly, the mass of continental crust is just 0.57% of the silicate Earth, and yet it contains ˜35% of the potassium (using the crustal composition estimates in Table 1). Continental rocks comprise the buoyant shell that was once thought to float on a basaltic substratum, inferred from the wide distribution of chemically similar continental flood basalts (von Cotta, 1858). The links with the adjacent oceans were perhaps unclear, "the greatest mountains confront the widest oceans" ( Dana, 1873). Yet, it has long been argued that the rock that has the most similar composition to the average continental crust, andesite, may be generated by fractional crystallization of basalt ( Daly (1914) and Bowen (1928); but see the contrary arguments of Kelemen (1995) and Chapter 3.18). The average age of the continental crust is old, almost 2 Ga, the processes of crust generation may have changed with time, and the early crust may have been generated and destroyed more rapidly than in more recent times (Armstrong, 1991; Bowring and Housh, 1995).
The laser-heating ⁴⁰ Ar/ ³⁹ Ar dating method was applied to volcanic rocks systematically collected from the back-arc region of the central part of the Izu-Bonin arc. Dating results combined with whole-rock chemistry and other geological information reveal the volcanic history of the back-arc region of the Izu-Bonin arc. In the back-arc seamount chains area, andesitic-basaltic volcanism initiated at c. 17 Ma, slightly before the Shikoku Basin ceased spreading, and continued until c. 3 Ma. Relatively old volcanism (>8 Ma) has been found only from the western part of the seamount chains, and younger volcanism mainly occurs in the eastern part of the chains, indicating the western margin of the active volcanic zone of the Izu-Bonin arc has migrated eastward with time. At around 2.8 Ma, volcanism initiated in the western part of the back-arc knolls zone. This volcanism is characterized by eruption of clinopyroxene-olivine basalt. In the first stage of rifting, this type of basalt erupted from N-S-trending fissures and/or vents aligned in this direction and formed N-S-trending ridges. Between 2.5 and 1 Ma, many small knolls were formed by eruption of basalt and minor felsic rocks. Volcanism younger than 1 Ma occurred only in the currently active rift zone and its adjacent area.
The active volcanic zone in the back-arc seamount chains area converged to the volcanic front with time from 17 to 3 Ma. Active rifting and rifting-related volcanism also migrated or converged eastward after 1 Ma. The observed temporal variation of locus of volcanism may be explained by rapid retreat of the Philippine Sea Plate relative to the Pacific Plate and resulting steepening of the subducting slab.
New Sr, Nd, Hf, and Pb isotope and trace element data are presented for
basalts erupted in the Izu back arc. We propose that across-arc
differences in the geochemistry of Izu-Bonin arc basalts are controlled
by the addition of aqueous slab fluids to the volcanic front and hydrous
partial melt of the slab to the back arc. The volcanic front has the
lowest concentrations of incompatible elements, the strongest relative
enrichments of fluid-mobile elements, and the most radiogenic Sr, Nd,
Hf, and Pb, suggesting the volcanic front is the result of high degrees
of partial melting of a previously depleted mantle source caused by an
aqueous fluid flux from the slab. Relative to the volcanic front, the
back arc has higher concentrations of incompatible elements and elevated
La/Yb and Nb/Zr, suggesting lower degrees of partial melting of a less
depleted or even enriched mantle source. Positive linear correlations
between fluid-immobile element concentrations and the estimated degree
of mantle melting suggest the slab contribution added to the mantle
wedge in the Izu back arc is a supercritical melt. Pb, Nd, and Hf
isotopes and Th/La systematics of back-arc basalts are consistent with a
slab melt composed of >90% altered oceanic crust and <10%
sediment; that is, altered oceanic crust, not subducted sediment,
dominates the slab contribution. High field strength element systematics
require supercritical melts to be in equilibrium with residual rutile
and zircon.
Detailed petrological and geochemical analyses of volcanic rocks from back-arc en echelon seamounts, consisting of four cross-chains in the Izu-Bonin back arc, provide insights into the origin of complex magma types for back-arc volcanism associated with back-arc spreading in an oceanic arc environment. The sampled volcanic rocks are classified into three distinct rock suites on the basis of trace element characteristics: an ``Enriched Suite'' with enriched trace element compositions and high Nb/Zr (0.075-0.10) a ``Less Enriched Suite'' with depleted high-field-strength elements (HFSEs), lower Nb/Zr (0.03-0.075), and enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs); and a ``Depleted Suite'' with very depleted HFSEs, the lowest Nb/Zr (2sigma depleted mid-ocean ridge basalt mantle (D-DMM). (3) Source mantle for Enriched Suite rocks is distributed independently beneath the Izu-Bonin back-arc region. (4) Less Enriched Suite magmas erupted in all seamount chains, indicating that these magmas constitute the major volcanic component of back-arc cross-chain volcanism in the northern Izu-Bonin arc. We conclude that three distinct mantle sources with various contributions from a deeply subducted slab exist beneath the cross-chains in the Izu-Bonin back arc.
We report results of detailed petrological analyses of volcanic rocks from the Enpo seamount chain that reveal the characteristics of volcanism in a backarc en echelon seamount chain in an oceanic island arc setting. We identified two types volcanic rock suites in the Enpo seamount chain based on bulk rock trace element chemistry and compositions of chrome spinels included in olivine phenocrysts of primitive basalts. ``More Enriched Suites'' (MES) have enriched HFSE compositions (such as Nb, Ta), higher Nb/Zr values, and low Cr# in spinel. ``Less Enriched Suites'' (LES) have depleted HFSE compositions, lower Nb/Zr values, and high Cr# in spinel. These results require distinct mantle sources with different for the volcanic rocks constituting the Enpo seamount chain. Mineralogical and geochemical analyses show that petrological variations within LES lavas are explained by fractionation with open system magma mixing. In contrast, variations between MES lavas require different mechanisms of magma genesis because petrological variation within this suite cannot be explained by fractionation. MES basalts also exhibit mineralogical evidence for magma mixing. Volcanism on the Enpo seamount chain is characterized by complex spatial and temporal relationships between MES lavas consisting of subalkaline basalt, alkaline andesite, and hornblende (water rich) andesite, and LES lavas consisting of subalkaline basalt and fractionated andesite. We suggest that these features are characteristic of variability within magma systems that feed backarc seamount chains in oceanic island arcs.
Evolution of arc crust and subarc mantle in the Izu-Bonin-Mariana (IBM) intraoceanic arc-trench system is examined by petrological modeling of arc magma generation and differentiation. Characteristic seismic structural features of the IBM arc highlighted in this modeling include the presence of (1) a middle crust with a P wave velocity (Vp) of 6.0-6.5 km/s, (2) a 6.5-6.8 km/s Vp layer at the top of the lower crust, (3) a high-velocity (Vp = 6.8-7.2 km/s) lower crust, and (4) an uppermost mantle exhibiting rather low velocities (Vp = 7.4-7.7 km/s). The formation of the middle crust, which is considered to have an intermediate composition, is examined by (1) the mantle-derived basalt model including anatexis of the initial mafic lower crust or mixing of mantle-melting-derived basaltic magma with crust-melting-derived felsic magma and (2) the mantle-derived andesite model including differentiation of boninitic magma. The Vp calculated for the inferred compositions for each layer of the IBM crust on the basis of the basalt model is consistent with the observed values, whereas the andesite model cannot account for the characteristic Vp of the middle crust, the uppermost lower crust, and the uppermost mantle. The results further suggest that the volume of mafic restite and cumulates that are ``crustal residues'' resulting from the evolution of middle and upper arc crust is, at least, 3 to 9 times greater than that of the seismically defined IBM lower crust. One possible explanation to overcome this apparent dilemma is that the mafic to ultramafic crustal components are transformed to subarc mantle. During this process the subarc Moho is chemically transparent and permeable to the crustal components. This crust-mantle transformation could play the major role in the creation of mature arc crust with intermediate compositions similar to continental crust.
The processes that create continental crust in an intraoceanic arc setting are a matter of debate. To address this issue, we conducted an active source wide-angle seismic study to examine along-arc structural variations of the Izu intraoceanic arc. The data used were acquired over a 550-km-long profile along the volcanic front from Sagami Bay to Tori-shima. The obtained structural model showed the existence of felsic to intermediate composition middle crust with a P wave velocity (Vp) of 6.0-6.5 km s-1 in its upper part and 6.5-6.8 km s-1 in its lower part. The thickness of the middle crust varied markedly from 3 to 13 km. The underlying lower crust also consisted of two layers (Vp of 6.8-7.2 km s-1 in the upper part and Vp of 7.2-7.6 km s-1 in the lower part). The upper of these layers was interpreted to consist of plutonic gabbro, and the lower layer was interpreted to be mafic to ultramafic cumulates. Average crustal velocities calculated from our model showed remarkable lateral variation, which correlated well with arc volcanism. Low average crustal seismic velocities (~6.7 km s-1), due to thick middle crust, were obtained beneath basaltic volcanoes (e.g., O-shima, Miyake-jima, Hachijo-jima, Aoga-shima), while higher average velocities (~7.1 km s-1) were obtained beneath rhyolitic volcanoes (e.g., Nii-jima, Kurose, South Hachijo caldera, Myoji knoll, and South Sumisu caldera). We concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the Izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution, or remelting of preexisting continental crust, or both.
The Barrington Tops Batholith (BTB) is a shallow level intrusive complex of Permian age (262 Ma) in Zone A of the New England Fold Belt, northern New South Wales. It comprises a comagmatic suite of early pyroxene+plagioclase-phyric quartz diorite stocks and dykes (10% by area), augite-hypersthene granodiorite (75%), and aplite. Less voluminous hornblende-biotite granodiorite (15%) appears to be related to the augite-hypersthene granodiorite by the addition of minor amounts of an incompatible element rich component. The augite-hypersthene granodiorite plutons formed by fractional crystallisation of a dry (< 2 wt.% H20), high temperature (> 1000°C) liquid. Major element modelling, using analysed phenocryst compositions, closely reproduces the geochemical differentiation trends observed. Combined petrographic and mineral chemistry evidence indicates that parental liquids were of quartz dioritic and more mafic compositions, probably mantle-derived. Low initial 87Sr/86Sr (0.7036–0.7039) and high ∈Nd values (+ 5.6 to + 7.8), suggest these were produced from a geochemically depleted mantle source.
The BTB may be regarded as an end-member of the spectrum of calc-alkaline granitic rocks that are formed by the mixing of variably fractionated melts of upper mantle origin with lower continental crust material. It is suggested that lower crustal mafic and ultramafic xenoliths found m Mesozoic to Cenozoic basalts represent cumulates complementary to the more evolved mantle-derived component of calc-alkaline magmatism. Crystal-liquid fractionation, aided by mixing of evolved mantle-derived magma with crustal material, appears to be the most important process giving rise to the diverse geochemical and isotopic continuum displayed by calc-alkaline magmatism m orogemc terrains.
We carried out a reflection/refraction seismic survey across the southern Izu-Ogasawara arc–back arc system, covering three arcs with different crustal ages. The oldest Eocene arc has middle and lower crust with high velocities of 6.4–6.6 and 6.8–7.4 km/s, respectively, suggesting denser crustal materials. The current volcanic arc has middle and lower crust with lower velocities of 5.7–6.5 and 6.7–7.1 km/s, suggesting advanced crustal differentiation. The crust-mantle transition layer, with a velocity of 7.5–8.0 km/s, is distributed beneath the current volcanic arc and the rear arc, suggesting a pool of dense materials emanating from the crust through the crustal growth. These structural differences between the Eocene arc and current arc indicate a difference of crustal growth based on basaltic and andesitic magmas according to known petrologic studies. Commonly, rifted crusts have lower crusts with high velocities of over 7.0 km/s, and the arc–back arc transition zone also has a thinner more reflective crust that may have been affected by postrifting magmatism.
A detailed structural model of the crust, subducting slab, and underlying upper mantle across the northern Izu-Ogasawara (Bonin)
island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent
forward modeling combined with tomographic inversion. The model indicates that the crust is thickest beneath the presently
active rift zone and a granitic crust may have formed in the mid-crust. A highly attenuative mantle (that is, one with low
quality Q) seems to be confined mainly beneath the presently active rift zone. In contrast, high P-wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin, suggesting a large-scale
magma input responsible for the arc formation.
This chapter has four main aims. Provide a comprehensive picture of the composition of volcanic rocks from subduction-related magmatic arcs. Review evidence in favor of the existence of andesitic, as well as basaltic primary magmas in arcs. Present new data on the composition of arc lower crust, based mainly on our work on the Talkeetna arc section in southcentral Alaska. Summarize evidence from arc lower crustal sections that a substantial proportion of the dense, lower crustal pyroxenites and garnet granulites produced by crystal fractionation are missing.
The Koloula Igneous Complex, on the island of Guadalcanal, consists of a low-K calc-alkaline sequence of 26 different intrusive phases. The major intrusions are characterized by K/Rb>400, Rb/Sr<0.06, δ18O of 5.7 to 7.2 and uniform 87Sr/86Sr of 0.70372. This article presents the first data describing oxygen and strontium isotopic behaviour within a plutonic suite that formed by crystal fractionation.
A series of water-deficient partial melting experiments on a low-K tholeiite were carried out under lower crustal P–T–H2O conditions (900–1200 °C, 0.7–1.5 GPa, 2 and 5 wt% H2O added) using a piston-cylinder apparatus. With increasing temperature at 1.0 GPa, supersolidus mineral assemblages vary from amphibolitic to pyroxenitic. Garnet crystallizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low-K calc-alkalic trends, and are classified as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu–Bonin – Mariana (IBM) arc, for example Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) characteristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust (∼ 50% melting at 1050 °C and below 1.2 GPa). Estimated densities of pyroxenitic restites (∼ 3.9 g/cm3) after extraction of andesitic melts are higher than that of mantle peridotite beneath the island arc (3.3 g/cm3). The high density of the restite could cause delamination of the IBM arc lower crust. Rhyolitic magmas in the IBM arc (e.g. Niijima) could be formed by low degrees of partial melting of the amphibolitic crust at a temperature just above the solidus (10% melting at or below 900 °C).
The bulk composition of the continental crust throughout geological history is thought by most previous workers to be andesitic. This assumption of an andesitic bulk composition led to an early hypothesis by Taylor (1967) that the continental crust was created by arc magmatism. This hypothesis for the origin of continental crust was challenged by several authors because: (i) the mean rate of arc crust addition obtained by Reymer and Schubert (1984) is too small to account for some certain phases of rapid crustal growth; and (ii) the bulk composition of ocean island arcs, the main contributor to the Archean and early Proterozoic crust, is basaltic rather than andesitic (Arculus 1981; Pearce et al. 1992). New data from the Northern Izu–Bonin arc are presented here which support the Taylor (1967) hypothesis for the origin of the continental crust by andesitic arc magma. A geological interpretation of P wave crustal structure obtained from the Northern Izu–Bonin arc by Suyehiro et al. (1996) indicates that the arc crust has four distinctive lithologic layers: from top to bottom: (i) a 0.5–2-km-thick layer of basic to intermediate volcaniclastic, lava and hemipelagite (layer A); (ii) a 2–5-km-thick basic to intermediate volcaniclastics, lavas and intrusive layer (layer B); (iii) a 2–7-km-thick layer of felsic (tonalitic) rocks (layer C); and (iv) a 4–7-km-thick layer of mafic igneous rocks (layer D). The chemical composition of the upper and middle part of the northern Izu–Bonin arc is estimated to be similar to the average continental crust by Taylor and McLennan (1985). The rate of igneous addition of the Northern Izu–Bonin arc since its initial 45-Ma magmatism was calculated as 80 km3/km per million years. This rate of addition is considered to be a reasonable estimate for all arcs in the western Pacific. Using this rate, the global rate of crustal growth is estimated to be 2.96 km3/year which exceeds the average rate of crustal growth since the formation of the Earth (1.76 km3/year). Based on this estimate of continental growth and the previously documented sediment subduction and tectonic erosion rate (1.8 km3/year, von Heune & Scholl 1991), several examples of growth curves of the continental crust are presented here. These growth curves suggest that at least 50% of the present volume of the continental crust can be explained by arc magmatism. This conclusion indicates that arc magmatism is the most important contributor to the formation of continental crust, especially at the upper crustal level.
Amongst island arcs, Izu–Bonin is remarkable as it has widespread, voluminous and long-lived volcanism behind the volcanic front. In the central part of the arc this volcanism is represented by a series of seamount chains which extend nearly 300 km into the back-arc from the volcanic front. These back-arc seamount chains were active between 17 and 3 Ma, which is the period between the cessation of spreading in the Shikoku Basin and the initiation of currently active rifting just behind the Quaternary volcanic front. In this paper we present new age, chemical and isotopic data from the hitherto unexplored seamounts which formed furthest from the active volcanic front. Some of the samples come from volcanoes at the western limit of the back-arc seamount chains. Others are collected from seamounts of various sizes which lie on the Shikoku Basin crust (East Shikoku Basin seamounts). The westernmost magmatism we have sampled is manifested as a series of volcanic edifices that trace the extinct spreading centre of the Shikoku Basin known as the Kinan Seamount Chain (KSC).
In the Izu–Bonin Arc, hydrothermal activities have been reported from volcanoes along present-day volcanic front, a rear arc volcano and a back-arc rift basin as well as a remnant arc structure now isolated from the Quaternary arc. It is widely known that characteristics of hydrothermal activity (mineralogy, chemistry of fluid etc.) vary depending upon its tectonic setting. The Izu–Bonin Arc has experienced repeated back-arc or intra-arc rifting and spreading and resumption of arc volcanism. These characteristics make this arc system a suitable place to study the tectonic control on hydrothermal activity. The purpose of the present paper is, therefore, to summarize volcanotectonic setting and history of the Izu–Bonin Arc in relation to the hydrothermal activity. The volcanotectonic history of the Izu–Bonin Arc can be divided into five stages: (i) first arc volcanism (boninite, high-Mg andesite), 48–46 Ma; (ii) second arc volcanism (tholeiitic, calc-alkaline), 44–29 Ma; (iii) first spreading of back-arc basin (Shikoku Basin), 25–15 Ma; (iv) third arc volcanism (tholeiitic, calc-alkaline), 13–3 Ma; and (v) rifting in the back-arc and tholeiitic volcanism along the volcanic front, 3–0 Ma. Magmas erupted in each stage of arc evolution show different chemical characteristics from each other, mainly due to the change in composition of slab-derived component and possibly mantle depletion caused by melt extraction during back-arc spreading and prolonged arc volcanism. In the volcanotectonic context summarized here, hydrothermal activity recognized in the Izu–Bonin Arc can be classified into four groups: (i) present-day hydrothermal activity at the volcanic front; (ii) active hydrothermal activity in the back arc; (iii) fossil hydrothermal activity in the back-arc volcanoes; and (iv) fossil hydrothermal activity in the remnant arc. Currently hydrothermal activities occur in three different settings: submarine caldera and stratocones along the volcanic front; a back-arc rift basin; and a rear arc caldera. In contrast, hydrothermal activities found in the back-arc seamount chains were associated with rear arc volcanism in Neogene after cessation of back-arc spreading of the Shikoku Basin. Finally, sulfide mineralization associated with boninitic volcanism in the Eocene presumably took place during forearc spreading in the initial stage of the arc. This type of activity appears to be limited during this stage of arc evolution.
The climactic eruption of Mount Mazama has long been recognized as a classic example of rapid eruption of a substantial fraction of a zoned magma body. Increased knowledge of eruptive history and new chemical analyses of ∼350 wholerock and glass samples of the climactic ejecta, preclimactic rhyodacite flows and their inclusions, postcaldera lavas, and lavas of nearby monogenetic vents are used here to infer processes of chemical evolution of this late Pleistocene — Holocene magmatic system. The 6845±50 BP climactic eruption vented ∼50 km3 of magma to form: (1) rhyodacite fall deposit; (2) welded rhyodacite ignimbrite; and (3) lithic breccia and zoned ignimbrite, these during collapse of Crater Lake caldera. Climactic ejecta were dominantly homogeneous rhyodacite (70.4±0.3% SiO2), followed by subordinate andesite and cumulate scoriae (48–61% SiO2). The gap in wholerock composition reflects mainly a step in crystal content because glass compositions are virtually continuous. Two types of scoriae are distinguished by different LREE, Rb, Th, and Zr, but principally by a twofold contrast in Sr content: High-Sr (HSr) and low-Sr (LSr) scoriae. HSr scoriae were erupted first. Trace element abundances indicate that HSr and LSr scoriae had different calcalkaline andesite parents; basalt was parental to some mafic cumulate scoriae. Parental magma compositions reconstructed from scoria wholerock and glass data are similar to those of inclusions in preclimactic rhyodacites and of aphyric lavas of nearby monogenetic vents.
Preclimactic rhyodacite flows and their magmatic inclusions give insight into evolution of the climactic chamber. Evolved rhyodacite flows containing LSr andesite inclusions were emplaced between ∼30000 and ∼25000 BP. At 7015±45 BP, the Llao Rock vent produced a zoned rhyodacite pumice fall, then rhyodacite lava with HSr andesite inclusions. The Cleetwood rhyodacite flow, emplaced immediately before the climactic eruption and compositionally identical to climactic rhyodacite (volatile-free), contains different HSr inclusions from Llao Rock. The change from LSr to HSr inclusions indicates replenishment of the chamber with andesite magma, perhaps several times, in the latest Pleistocene to early Holocene.
Modeling calculations and wholerock-glass relations suggest than: (1) magmas were derived mainly by crystallization differentiation of andesite liquid; (2) evolved preclimactic rhyodacite probably was derived from LSr andesite; (3) rhyodacites contain a minor component of partial melt from wall rocks, and (4) climactic and compositionally similar rhyodacites probably formed by mixing of evolved rhyodacite with HSr derivative liquid(s) after replenishment of the chamber with HSr andesite magma. Density considerations permit a model for growth and evolution of the chamber in which andesite recharge magma ponded repeatedly between cumulates and rhyodacite magma. Convective cooling of this andesite resulted in rapid crystallization and upward escape of buoyant derivative liquid which mixed with overlying, convecting rhyodacite. The evolved rhyodacites were erupted early in the chamber's history and(or) near its margins. Postcaldera andesite lavas may be hybrids composed of LSr cumulates mixed with remnant climactic rhyodacite. Younger postcaldera rhyodacite probably formed by fractionation of similar andesite and assimilation of partial melts of wallrocks.
Uniformity of climactic rhyodacite suggests homogeneous silicic ejecta from other volcanoes resulted from similar replenishment-driven convective mixing. Calcalkaline pluton compositions and their internal zonation can be interpreted in terms of the Mazama system frozen at various times in its history.
Mineral chemistry, major and trace elements, and 87Sr/86Sr ratios are presented for 29 igneous rocks dredged from the northern portion of the Izu-Ogasawara arc. These rocks are compositionally bimodal. Basement gabbro and trondhjemite from the arc are extremely poor in K2O (0.05–0.19%) and Rb (0.48–0.62 ppm), and their REE patterns and Sr isotope ratios indicate that there are island arc tholeiites. Quaternary volcanic rocks from the present volcanic front (Shichito Ridge; active arc), back-arc seamounts (east side; inactive arc) and Torishima knoll between the two back-arc depressions (incipient back-arc basins) behind the active arc have the same geochemical characteristics as the above plutonic rocks though they are not as depleted in K and Rb. Rhyolite pumice from the backarc depression is also the depleted island arc tholeiite, whereas basalts from the depression have compositions that are transitional between MORB and island arc tholeiites in trace element (Ti, Ni, Cr, V, Y and Zr) and mineral chemistries. The back-arc depression basalts have relatively high BaN/CeN(0.66–1.24), Cen/YbN(1.1–1.9) and K/Ba(45–105) and low 87Sr/86Sr (0.70302–0.70332) and Ba/Sr (0.1–0.2), which are similar to other back-arc basin basalts and E-type MORB, but are quite unlike the depleted island arc tholeiites. The diverse trace element and Sr isotope compositions of basalt-andesite from the back-arc depressions imply the interplay between E-type MORB and island arc tholeiite. These chemical characteristics and the relationships of (Ce/Yb)N vs (Ba/Ce)N and (Ce/Yb)N vs 87Sr/86Sr suggest that the back-arc depression magmas are generated by mixing of E-type MORB and depleted island arc tholeiite magmas. Geochemical characters of the associated rhyolite from the depression are compatible with partial melting of lower crust.
Tonalitic rocks dredged from the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge are classified as biotite-hornblende tonalites and hornblende tonalites. These rocks have radiometric ages of 37-38 Ma, indicating that felsic plutonic activity occurred during the early stages of Izu-Ogasawara (Bonin)-Mariana (IBM) arc volcanism. Therefore, this tonalite complex has great importance for understanding the initial processes of island arc and continental crust formation. These tonalitic rocks exhibit the following petrological and geochemical characteristics: (1) common lamellar twins and oscillatory zoning patterns in plagioclase phenocrysts throughout the compositional range; (2) hornblende tonalite shows parallel REE patterns and increasing total REE content with increasing SiO2, except for an increasingly strong negative Eu anomaly at higher SiO2 levels; and (3) isotopic composition remains constant over a wide silica variation. We compare this tonalite with younger tonalities of the same arc from the Tanzawa Complex (10-5 Ma), central Japan, considered to represent the lower-middle crust of the IBM arc, and find the following differences: (1) cumulate textures found in Tanzawa tonalites are not observed in samples from the Komahashi-Daini Seamount; and (2) Komahashi-Daini Seamount tonalites, unlike those from Tanzawa, exhibit linear variations of Zr and REEs vs. SiO2 plots. These data and other observations support the interpretation that tonalite in the Komahashi-Daini Seamount was produced by crystal fractionation from basaltic magma. We suggest that fractional crystallization operated during the early stage of oceanic island arc formation to produce tonalite, whereas tonalities in later stages formed largely by partial melting of basaltic lower crust, as represented by the tonalites in the Tanzawa Complex.
Granitic—rhyolitic liquids were produced experimentally from moderately hydrous (1.7–2.3wt% H2O) medium-to-high K basaltic compositions at 700MPa and fO2 controlled from Ni-NiO –1.3 to +4. Amount and composition of evolved liquids and coexisting mineral assemblages vary with fO2 and temperature, with melt being more evolved at higher fO2s, where coexisting mineral assemblages are more plagioclase- and Fe–Ti oxide-rich and amphibole-poor. At fO2 of Ni–NiO +1, typical for many silicic magmas, the samples produce 12–25wt% granitic–rhyolitic liquid, amounts varying with bulk composition. Medium-to-high K basalts are common in subduction-related magmatic arcs, and near-solidus true granite or rhyolite liquids can form widely, and in geologically significant quantities, by advanced crystallization–differentiation or by low-degree partial remelting of mantle-derived basaltic sources. Previously differentiated or weathered materials may be involved in generating specific felsic magmas, but are not required for such magmas to be voluminous or to have the K-rich granitic compositions typical of the upper continental crust.
The back-arc region of the Izu-Bonin arc has complex bathymetric and structural features, which, due to repeated back-arc rifting and resumption of arc volcanism, have prevented us from understanding the volcano-tectonic history of the arc after 15 Ma. The laser-heating 40Ar/39Ar dating technique combined with high density sampling of volcanic rocks from the back-arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back-arc rifting. Based on the new 40Ar/39Ar dating results: (1) Back-arc rifting initiated at around 2.8 Ma in the middle part of the Izu-Bonin arc (30°30′N–32°30′N). Volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north–south-trending fissures and/or lines of vents. (2) Following this earliest stage of volcanism, at ca. 2.5 Ma, compositionally bimodal volcanism occurred and formed small cones in the wide area. This volcanism and rifting continued until about 1 Ma in the region west of the currently active rift zone. (3) After 1 Ma, active volcanism ceased in the area west of the currently active rift zone, and volcanism and rifting were confined to the currently active rift zone. The volcano-tectonic history of the back-arc region of the Izu-Bonin arc is an example of the earliest stage of back-arc rifting in the oceanic island arc. Age data on volcanics clearly indicate that volcanism changed its mode of activity, composition and locus along with a progress of rifting.
Ultrasonic compressional wave velocities (Vp) and shear wave velocities (Vs) were measured with varying pressure up to 1.0 GPa in a temperature range from 25 to 400 °C for a suite of tonalitic–gabbroic rocks of the Miocene Tanzawa plutonic complex, central Japan, which has been interpreted as uplifted and exposed deep crust of the northern Izu–Bonin–Mariana (IBM) arc. The Vp values of the tonalitic–gabbroic rocks increase rapidly at low pressures from 0.1 to 0.4 GPa, and then become nearly constant at higher pressures above 0.4 GPa. The Vp values at 1.0 GPa and 25 °C are 6.3–6.6 km/s for tonalites (56.4–71.1 wt.% SiO2), 6.8 km/s for a quartz gabbro (53.8 wt.% SiO2), and 7.1–7.3 km/s for a hornblende gabbro (43.2–47.7 wt.% SiO2). Combining the present data with the P wave velocity profile of the northern IBM arc, we infer that 6-km-thick tonalitic crust exists at mid-crustal depth (6.1–6.3 km/s Vp) overlying 2-km-thick hornblende gabbroic crust (6.8 km/s Vp). Our model shows large differences in acoustic impedance between the tonalite and hornblende gabbro layers, being consistent with the strong reflector observed at 12-km-depth in the IBM arc. The measured Vp of Tanzawa hornblende-bearing gabbroic rocks (7.1–7.3 km/s) is significantly lower than that Vp modeled for the lowermost crustal layer of the northern IBM arc (7.3–7.7 km/s at 15–22 km depth). We propose that the IBM arc consists of a thick tonalitic middle crust and a mafic lower crust.
The bulk compositions of two exposed sections of intra-oceanic island arc crust (the Talkeetna volcanics and Border Ranges ultramafic-mafic complex in southeastern Alaska, and the Canyon Mountain Complex in northeastern Oregon) represent basalt (MgO = 11%) and probably basaltic andesite (MgO = 8%), respectively. Both have REE abundances approximately 10 times chondrite, with no LREE-enrichment. These compositions, determined by mass balance based on the relative thicknesses of lithologic units in the exposed sections, are reasonable parental magma compositions based on Fe/O*MgO and Ni contents of olivine in the basal crustal cumulate rocks and the lack of significant Eu anomalies. Accretion of arcs such as these is inappropriate as a sole mechanism for generating post-Archean continental crust, since the latter is significantly LREE-enriched and is generally considered to be andesitic. Thus, if these arcs represent a major ingredient in Proterozoic and younger continental crust, their bulk compositions need to be modified. A combination of processes may be invoked, including (a) delamination of basal cumulates to make the sections less mafic, (b) addition of an alkalic component to enrich the crust in LREE and other lithophile elements, and (c) partial melting of the lower crust combined with delamination of the residue to do both. The basal parts of these crustal sections represent potential mantle heterogeneities.
Use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series. Coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams, viz.: (1) TAS - total alkalies (Na2O+K2O) vs. SiO2; (2) K2O vs. SiO2; (3) AFM; (4) Jensen; (5) KTP - K2OTiO2P2O5; (6) vs. SiO2; and (7) vs. . Different versions of these boundaries are collated to indicate their variable position, and it is demonstrated that inter-laboratory analytical precision suffices to account for almost all of their spread on the TAS and K2O vs. SiO2 diagrams.