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Similar range of La N /Yb N ratios for TTG and GGM. Euanomalies vary independently of the La N /Yb N ratio with a clear difference between TTG (high) to GGM (low) giving evidence of unrelated garnet/amphibole and plagioclase fractionation
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A new model for Archaean granitoid magmatism is presented which reconciles the most important geochemical similarities and differences between tonalite-trondhjemite-granodiorite (TTG) and potassic granitoids. Trace element abundances reveal a strong arc magmatism signature in all studied granitoids from Barberton Mountain Land. Characteristic featu...
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... steeper patterns (i.e. higher La N /Yb N ratios) than GGM (Con- die 1993), the patterns of both groups require stronger HREE removal (or retention) than is seen in many post- Archaean granitoids. Indeed, comparison of TTG and from BML reveals that there exists no discernible difference in REE fractionation. On a plot of La N /Yb N vs Eu/Eu* (Fig. 4) it is clearly visible that TTG and GGM of the BML display the same range of LREE/ HREE ratios (La N /Yb N for TTG and GGM: 5.3-56.4 and 6.2-64.5, ...
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... Negative Eu anomalies are a prominent feature of granites of all ages (Condie 1993), and thus imply that plagioclase fractionation was an essential process of granite formation throughout Earth's history. Impor- tantly, plagioclase fractionation is unrelated to that of garnet/amphibole as there is no correlation between La N /Yb N vs Eu/Eu* (Fig. 4). In our model, this implies that both the TTG and GGM series initially experienced garnet/amphibole±pyroxene fractionation. Fractiona- tion of these minerals in the absence of plagioclase causes a positive Eu anomaly, a feature observed neither in TTG nor GGM of the BML. Hence, it appears that once a certain extent of Si-and ...
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... Another possibility is that there has been a change in the δ 186 W values of intra-oceanic arc lavas over time, such that subduction zones magmas in the Archean are characterized by UCC-like δ 186 W values, while modern arc magmas are characterized by more elevated δ 186 W values. While the nature of TTG genesis is highly debated (e.g., Moyen and Martin, 2012), the TTGs from the Wyoming craton analyzed here, and the TTGs from the Barberton Greenstone Belt have both been suggested to be the products of subduction zone magmatism (Mueller et al., 2010;Kleinhanns et al., 2003). Both of these locations have UCC-like δ 186 W values, and thus could reflect lighter W isotope compositions of arc magmas in the Archean. ...
... Though, there is today a consensus that TTG magmas are sourced from the partial melting and/or crystallization of hydrated basalticmelts with residual amphibole and garnet (Martin et al., 2005), the tectonic setting related to their formation remains, controversial. Proposed geodynamic models include partial melting of over-thickened mafic arc crust (Hastie et al., 2016;Nagel et al., 2012), subducted oceanic crust , subducted oceanic plateau (Martin et al., 2005), fractional crystallization of hydrous arc basalts (Jagoutz et al., 2013;Kleinhanns et al., 2003) and/or prolonged differentiation of thickened basaltic plateaus, triggered by underplating of long-lasting mantle upwellings (Bédard, 2018;Johnson et al., 2017). This controversy is linked to the large geochemical diversity of TTGs, suggesting that the melting of mafic rocks occurred at different depths, either at "low pressure"(LP-) TTG (<0.6 Gpa), "medium pressure"(MP-) TTG ...
Archean granitoids played key roles in the generation and differentiation of the Archean continental crust and provide clues to understand crustal processes in the early Earth. Abundant Mesoarchean granitoids were emplaced in the Nyabessane granite-greenstone terrane (NGB), part of the Ntem Complex of the Northwest Congo Craton. They include charnockites, tonalite-trondhjemite-granodiorite (TTG), granitic and monzogranitic gneisses. Here, we present a geochemical and geochronological (zircon LA-ICP-SF-MS Usingle bondPb) study of these granitoids to determine their petrogenesis and to better constrain the crustal evolution of the Ntem Complex. Field and petrographic observations indicate that most of these granitoids underwent extensive metamorphism and deformations, associated with anatexis. Zircon Usingle bondPb dating results suggest that the charnockite, TTGs and granitic gneisses, and monzogranites have emplacement ages of 2910 ± 11 Ma, 2870–2865 Ma and 2852 ± 31 Ma, respectively. The charnockites have low SiO2 (55–58 wt%) and high Al2O3 (16–18 wt%), CaO (7–8 wt%) and MgO (~ 4.5 wt%) content with Mg# ~ 54, and exhibit magnesian, metaluminous characteristics of the Cordilleran granitoid-type formed in magmatic arc. The 2.87–2.86 Ga TTG gneisses are silica-rich (55–58 wt% SiO2), sodic (3–5 wt% Na2O, Na2O/K2O = 1–3), with HREE-depleted, and display the typical Archean medium- to low pressure TTG geochemical features groups. Their chemical compositions are characteristics of TTG-like melts derived from the partial melting of hydrated low- to high-K metabasic/thickened lower crust at various depths followed by magmatic differentiation during ascent. When compared to the TTG gneisses in the NGB, the ~2.87 Ga granitic gneisses are K-rich, and high Gd/Yb, Th/Yb, Th/Nb, Sr/Y and La/Yb ratios but lower MgO, Yb, V, Y, Cr, Ni and Sr contents, matching typical Archean hybrid and potassic granitic rocks. We propose that the granitic gneisses were derived from the intra-crustal melting of a pre-existing felsic crust. The granitic and the TTG gneisses were generated contemporaneously through the same magmatic event at ~2.87–2.86 Ga. The ~2852 Ma monzogranitic gneisses are ferroan and metaluminous rocks, and show LREE enrichment with strongly fractionated REE patterns and positive Eu anomalies. Geochemical features, together with the presence of ~2.9 Ga-old inherited zircon grains are consistent with the remelting of Mesoarchean granitoids. Considering the petrogenetic, regional geological and geochronological data, the Mesoarchean granitoid magmatism of the Ntem Complex was likely generated via complex transitional geodynamic regimes involving subduction and accretion processes.
... The crystallization temperature of zircons depends on the aluminum saturation index (ASI) (Boehnke et al., 2013;Watson & Harrison, 1983 Figure S4a in Supporting Information S1), indicating that there is no significant fractionation of minerals with various ASI values. In contrast to most Phanerozoic igneous rocks, Archean TTG could have been derived from the differentiation of hydrous basaltic magma under water-saturated conditions (e.g., Kleinhanns et al., 2003;Liou & Guo, 2019;Müntener et al., 2001), in which garnet incorporated with light Si isotopes (Méheut & Schauble, 2014;Yu et al., 2018) became part of the cumulate residue. In this scenario, δ 30 Si melt values would increase, accompanied by the strong fractionation of light rare earth elements (e.g., La) relative to heavy rare earth elements (e.g., Yb). ...
The tonalite–trondhjemite–granodiorite suites (TTGs) are key components of the Archean continental crust and therefore crucial to the understanding of the evolution of the early Earth. Here, we present in situ zircon Si–O isotope data of TTGs from Barberton. Results show that the 3.45–3.42 Ga (Group 1) and 3.24–3.23 Ga TTGs (Group 2) have elevated δ³⁰Simelt values but mantle‐like δ¹⁸Ozrc values, whereas the 3.23–3.22 Ga TTGs (Group 3) have coupled elevated δ³⁰Simelt and δ¹⁸Ozrc values relative to mantle‐derived rocks. We suggest that the Group 1 and 2 TTGs had a silicified source that was affected by low‐δ¹⁸O fluid released from the komatiitic rocks. The low‐δ¹⁸O fluid decreased the δ¹⁸Ozrc values of Group 1 and 2 TTGs but had negligible influence on their δ³⁰Simelt values. The Group 3 TTGs were generated solely from the silicified source, as the low‐δ¹⁸O fluid had become exhausted at that time.
... We stress that the metavolcanic rocks investigated in this study should not be classified as boninites on geochemical grounds (Fig. 9a-d), which is also supported by the occurrence of spinifex textured lava flow units, a hallmark feature of komatiites and related high-MgO basaltic Barker (1979); b) incompatible element abundance patterns of Buffalo River granitoids normalised to the primitive mantle values recommended by Palme and O'Neill (2014). Data for tonalite-trondhjemite-granodiorite rocks from Barberton are from Kleinhanns et al. (2003). ...
... The Ancient Gneiss Complex in Swaziland comprises Eoarchean-Mesoarchean TTG gneisses that have undergone multiple deformation events and subordinate remnants of supracrustal greenstone belts and interlayered amphibolites, which probably originated as mafic dykes ( Fig. 1 Hunter et al., 1978Hunter et al., , 1984Kröner et al., 1989Kröner et al., , 2014Hoffmann et al., 2016). The TTG protoliths in the Ancient Gneiss Complex formed in continental arc and plume-related intraplate settings (Supplementary Table 1; Kröner et al., 2014;Hoffmann et al., 2016 , 1987b;Yearron, 2003;Kleinhanns et al., 2003;Moyen et al., 2007Moyen et al., , 2019Matsumura, 2014;Huber and Byerly, 2018). These TTG occurrences formed in convergent margins, subduction zones, intra-oceanic subduction zones, island arcs, synkinematic/slab break-off structures, and oceanic plateaux (Supplementary Table 1). ...
... The Paleoarchean (3230 Ma) Nelshoogte TTG pluton in the Kaapvaal Province of South Africa has less evolved and more primitive N-MORB-normalised trace element patterns than those of the Andean arc of South America; the latter has more pronounced positive LILE and negative HFSE anomalies than the former (Fig. 8a). The TTGs from the Kaapvaal Province also exhibit these anomalies, with the former interpreted to have formed in a subduction zone and the Andean arc TTGs in a continental arc (Kleinhanns et al., 2003;Mamani et al., 2010). That the Andean arc TTGs have more fractionated N-MORB-normalised trace element patterns relative to those of the Nelshoogte pluton are a manifestation of the continental arc origin of the former, which suggests that the latter may have formed in either a juvenile continental arc or perhaps an intra-oceanic arc (Fig. 8a). ...
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.
... Although it is widely accepted that TTG magmas were produced by partial melting of hydrated basaltic sources (i.e. amphibolite, Barker and Arth, 1976), the geodynamic setting(s) in which they formed is debated (Kleinhanns et al., 2003;Bédard, 2006;Moyen, 2011;Johnson et al., 2017;Pourteau et al., 2020;Smithies et al., 2021). ...
... However, around 20% of TTG have elevated Sr, Al 2 O 3 and Na 2 O contents, which some think reflects partial melting of deeply-subducted oceanic crust at pressures exceeding plagioclase stability (>2.5 GPa, Moyen and Stevens, 2006). By contrast, others have argued that these 'high-P ' signatures may indicate either hydrous-fluid-fluxed ('wet') melting of amphibolite (Pourteau et al., 2020) levels (Kleinhanns et al., 2003;Smithies et al., 2021;Liou et al., 2022) Regardless of the tectonic setting, a critical role for fluids is implicated in nearly all models of TTG formation (André et al., 2019(André et al., , 2022Smithies et al., 2021). Seawater is an obvious source of this water, particularly given that a global ocean likely covered most of Earth's surface in the early Archaean (Bindeman et al., 2018). ...
Water is an essential ingredient in transforming primitive mantle-derived (mafic) rocks into buoyant (felsic) continental crust, thereby driving the irreversible differentiation of Earth's lithosphere. The occurrence in Archaean cratons of sodic granites of the tonalite–trondhjemite–granodiorite (TTG) series, high-MgO variolitic basalts, high-Mg diorites (sanukitoids) and diamonds with harzburgitic inclusion assemblages, all require the presence of hydrous fluids in Earth's deep crust and upper (lithospheric) mantle since at least the Paleoarchaean (3.6–3.2 billion years ago). However, despite its importance, where and how water was stored in Archaean crust, and how some water was transported into the upper mantle, are poorly understood. Here, we investigate Archaean crustal fluid budgets through calculated phase equilibria for three protolith compositions — a low-MgO mafic (basaltic) composition, a high-MgO (picritic) composition and an ultrahigh-MgO ultramafic (komatiitic) composition — that are representative of mafic to ultramafic magmatic rocks in Archaean greenstone belts. We show that the mode and stability of hydrous minerals, in particular chlorite, is positively correlated with protolith MgO content, such that high-MgO basalts can store up to twice the amount of crystal-bound H2O than low-MgO basalts. Importantly, ultrahigh-MgO rocks such as komatiite can store four times as much H2O, most of which is retained until temperatures exceeding 700 °C. Warmer geotherms in the early Archaean favoured dehydration of hydrated high-MgO and ultramafic rocks in the deep crust, leading to hydration and/or fluid-fluxed melting of overlying basaltic rocks to produce ‘high-pressure’ TTG magmas. Burial of Archaean mafic–ultramafic crust along cooler geotherms resulted in dehydration of ultramafic material within the lithospheric mantle, providing the source of enriched Archaean basalt that was parental to large volumes of ancient TTG-dominated continental crust.
... 3.5 Ga, which is consistent with the observation that the >3.5 Ga TTG pluton (i.e., Steynsdorp pluton) in the BGGT has whole-rock [La/Yb] N ratios (<10) lower than those (>10) of the 3.45-3.20 Ga TTG plutons (e.g., the 3.45 Ga Stolzburg and Theespruit plutons, the 3.23 Ga Nelshoogte, Kaap Valley, Honingklip and Usutu plutons) (e.g., Kleinhanns et al., 2003;Wang et al., 2020). Turner et al. (2020) derived a linear empirical relationship between whole-rock Th/Y ratios and SiO 2 contents (i.e., Th/Y = 0.0269 Â SiO 2 -1.3169 with an R 2 value of 0.9812) through statistical analysis of a large dataset (n = 41186) and inferred an arc-like andesitic source (SiO 2 contents = 59 ± 6 wt.%; Th/Nb = 2.7 ± 1.9) for the Jack Hills detrital zircons by inverting zircon trace element contents to melt composition via partition coefficients; this led them to conclude that plate tectonics operated on Earth since Hadean times. ...
It is intriguing to speculate on how continental crust grew and differentiated during the Archean and which tectonic regimes and magmatic mechanisms were responsible for such a process. The continental nucleus of the Kaapvaal Craton, which includes the Barberton granitoid-greenstone terrane (BGGT) in South Africa and the Ancient Gneiss Complex (AGC) in Swaziland, exhibits the best preserved geological record from 3.6 to 3.0 Ga and thus provides the optimal location to address this issue. In this study, combined U-Pb, O and Lu-Hf isotope and trace element analyses were carried out on detrital zircon grains from two Moodies Group sandstones and three modern river sand samples in the Barberton and Swaziland regions, southern Africa. The results, in conjunction with previously published data, show that the detrital zircons in the Moodies sandstones on both sides of the Inyoka Fault, had two similar U-Pb age clusters of 3.25–3.30 Ga and 3.40–3.47 Ga and the oldest age up to 3.57 Ga. This suggests that they were derived from older felsic volcanic rocks of the greenstone belt and the granitoid gneisses in the BGGT and AGC. Using the approach created for Earth’s oldest zircons (i.e., Jack Hills detrital zircons), we inferred that the source magmas of the studied detrital zircons had average arc-like andesitic compositions (SiO2 = 58.0 ± 5.6 wt. %; Th/Nb = 5.0 ± 3.2), similar to those of the Jack Hills detrital zircons. However, these inferred compositions conflict with the source rocks of the detrital zircons, namely, the felsic volcanic rocks and the granitoid gneisses in the BGGT and AGC, which have high SiO2 contents (average = 70 ± 4.3 wt. %) and moderate Th/Nb ratios (average = 0.76 ± 0.44). These findings suggest that the inferred average andesitic compositions for the Earth’s oldest zircons should be reevaluated. The U-Pb age spectra of detrital zircons in modern river sands suggest that the exposed rocks in southwestern Swaziland are dominantly ca. 3.45 Ga tonalitic, trondhjemitic and granodioritic (TTG) gneisses without Eoarchean crust. The concordant zircon U-Pb age data reveal four major magmatic episodes for the eastern Kaapvaal Craton at 3.52, 3.46, 3.26 and 3.10 Ga. The first magmatic episode is suggested to dictate crustal growth, whereas the other three magmatic episodes are predominated by crustal reworking. This process is manifested by major positive changes in εHf(t) and εNd(t) values starting at ∼3.5 Ga, gradual decreases in εHf(t) and εNd(t) values afterwards, and increases in δ¹⁸O values at ∼3.25 Ga. Combined with other geological observations, we suggest that modern-style plate tectonic processes, such as subduction, may not have been a requirement for crustal growth and reworking in the eastern Kaapvaal Craton during the Paleoarchean (3.6-3.2 Ga). Instead, they could be ascribed to episodic partial convective overturns at different crustal depths.
... The search for modern analogues that mimic Archean TTG generation is the focus of many studies addressing Archean crustal evolution. Four different models have been proposed for the origin and the growth of Archean conti-nental crust (Moyen et al., 2017): (1) fractional crystallization of arc-like basaltic melts to form tonalites (e.g., Kleinhanns et al., 2003;Hawkesworth et al., 2010;Jagoutz, 2013); (2) partial melting of hydrous metabasalts in a hot subducted slab, also described as the ''adakitic model" (Martin, 1986); (3) partial melting in the deeper regions of thickened oceanic crust, oceanic plateaus (Bolhar and Van Kranendonk, 2007;Hoffmann et al., 2011) or oceanic island arcs (Polat, 2012) by magmatic underplating, and (4) a combination of fractional crystallization of incoming melts, partial melting of pre-existing basaltic crust and incorporating of crustal material (Arndt, 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.
... However, experimental petrology suggests that the melting product of sodic TTGs has insufficient K contents to form K-rich granitoids (Watkins et al., 2007). Although K-rich granitoids can be explained by crystal fractionation of sodic TTG melts (Kleinhanns et al., 2003;Wang et al., 2020a), field observations show that K-rich granitoids were often emplaced after sodic TTG rocks. It is also noteworthy that K-rich granitoids show highly variable trace element compositions, which cannot be explained either by their origin from the same magma source or by crustal anataxis or magma differentiation under the same physicochemical conditions. ...
... If the K-rich granitoids were produced by partial melting of the pre-existing felsic rocks, their composition would be responsible for a high degree of partial melting. Alternatively, some studies have argued that K-rich granitoids were produced by crystal fractionation of sodic TTG magmas (Kleinhanns et al., 2003;Wang et al., 2020b). Although the negative Eu anamolies in the ca. ...
The Earth’s lithosphere changed dramatically in many aspects during the Neoarchean era. In this period, the crust became more felsic and shows significant diversification in the composition of granitoids. Although this change has been noticed for many years, the mechanism that drove the change remains unclear. Here we report a geochemical study of Neoarchean granitoids from the Taihua Complex in the North China Craton. Zircon U-Pb dating reveals two episodes of granitoid magmatisms at ca. 2.65 Ga and ca. 2.55–2.50 Ga. Sodic TTGs at ca. 2.65 Ga are characterized by low K2O/Na2O ratios of < 0.6, high Sr/Y and (La/Yb)N ratios, positive Eu anomalies, positive zircon ɛHf(t) values of 0.9 to 5.2, and zircon δ¹⁸O values of 6.3–6.5‰, suggesting that they were derived from partial melting of the thickened oceanic crust. In contrast, K-rich granitoids at ca. 2.65 Ga have higher K2O/Na2O ratios, low Sr/Y and (La/Yb)N ratios, negative Eu anomalies, mostly positive zircon ɛHf(t) values of −0.4 to 8.0, and high zircon δ¹⁸O values of 7.0–8.4‰. The K-rich granitoids at 2.55–2.50 Ga show similarly high Sr/Y and (La/Yb)N ratios to the sodic TTGs at ca. 2.65 Ga. All these geochemical evidences suggest that these Neoarchean K-rich granitoids were derived from partial melting of the low-T seawater-hydrothermally altered oceanic basalt at different depths. Therefore, the sodic TTG and K-rich granitoids are linked to each other through both similarity and difference in their source compositions rather than magmatic processes such as partial melting and crystal fractionation. On the other hand, high-K granites at ca. 2.55–2.50 Ga are highly siliceous, have very high K2O/Na2O ratios of 4.0–7.9, and contain many relict zircons with U-Pb ages of ca. 3.49–2.68 Ga, indicating their formation by reworking of the pre-existing continental crust. Using a filtered global geochemical database, we find that the Neoarchean is the most important period for the growth of both sodic TTGs and K-rich granitoids. The Sr/Y and (La/Yb)N ratios of these granitoids also reach the maximum during this period, suggesting global-scale thickening of the oceanic crust caused by warm plate convergence. Once the orogenic lithospheric mantle was foundered for thinning, the thickened oceanic crust experienced partial melting at different depths. This gave rise to the different types of granitoid for growth of the continental crust along convergent plate boundaries. Therefore, partial melting of the thickened crust at different depths is a viable mechanism for the compositional transition of continental crust in the Neoarchean.
... Estas rochas pertencem à série cálcio-alcalina, e apresentam alta razão K/Na (geralmente >1), alto Rb (150-250 ppm) e Sr (150-600 ppm). Em relação aos padrões de fracionamento de elementos terras raras, essas litologias têm padrão ETR variado e razão (La/Yb)N [(LaN/YbN = 38(±6) ou 80(±24)); (LaN/ YbN =10-60); (LaN/YbN = 6-64)] (SYLVESTER, 1994;FROST et al., 1998;KLEINHANNS;KRAMERS;KAMBER, 2003). Existem muitas hipóteses a respeito da origem dos granitos potássicos do final do período Arqueano, algumas das quais incluem a fusão parcial de tonalitos pré-existentes (CONDIE; BOWLING; ALLEN, 1986;MARTIN;QUERRÉ, 1984), metassomatismo alcalino (WEAVER, 1980;CON-DIE;NARAYANA, 1982), cristalização fracionada de magmas graníticos ou TTG (RIDLEY; VEARNCOMBE; JELSMA, 1997), derivação do manto (KLEINHANNS; KRA-MERS;KAMBER, 2003), e mistura entre magmas derivados da migmatização de complexos TTG (JAYANANDA et al., 2000;LÓPEZ;FERNÁNDEZ;CASTRO, 2006). ...
... Estas rochas pertencem à série cálcio-alcalina, e apresentam alta razão K/Na (geralmente >1), alto Rb (150-250 ppm) e Sr (150-600 ppm). Em relação aos padrões de fracionamento de elementos terras raras, essas litologias têm padrão ETR variado e razão (La/Yb)N [(LaN/YbN = 38(±6) ou 80(±24)); (LaN/ YbN =10-60); (LaN/YbN = 6-64)] (SYLVESTER, 1994;FROST et al., 1998;KLEINHANNS;KRAMERS;KAMBER, 2003). Existem muitas hipóteses a respeito da origem dos granitos potássicos do final do período Arqueano, algumas das quais incluem a fusão parcial de tonalitos pré-existentes (CONDIE; BOWLING; ALLEN, 1986;MARTIN;QUERRÉ, 1984), metassomatismo alcalino (WEAVER, 1980;CON-DIE;NARAYANA, 1982), cristalização fracionada de magmas graníticos ou TTG (RIDLEY; VEARNCOMBE; JELSMA, 1997), derivação do manto (KLEINHANNS; KRA-MERS;KAMBER, 2003), e mistura entre magmas derivados da migmatização de complexos TTG (JAYANANDA et al., 2000;LÓPEZ;FERNÁNDEZ;CASTRO, 2006). ...
... Em relação aos padrões de fracionamento de elementos terras raras, essas litologias têm padrão ETR variado e razão (La/Yb)N [(LaN/YbN = 38(±6) ou 80(±24)); (LaN/ YbN =10-60); (LaN/YbN = 6-64)] (SYLVESTER, 1994;FROST et al., 1998;KLEINHANNS;KRAMERS;KAMBER, 2003). Existem muitas hipóteses a respeito da origem dos granitos potássicos do final do período Arqueano, algumas das quais incluem a fusão parcial de tonalitos pré-existentes (CONDIE; BOWLING; ALLEN, 1986;MARTIN;QUERRÉ, 1984), metassomatismo alcalino (WEAVER, 1980;CON-DIE;NARAYANA, 1982), cristalização fracionada de magmas graníticos ou TTG (RIDLEY; VEARNCOMBE; JELSMA, 1997), derivação do manto (KLEINHANNS; KRA-MERS;KAMBER, 2003), e mistura entre magmas derivados da migmatização de complexos TTG (JAYANANDA et al., 2000;LÓPEZ;FERNÁNDEZ;CASTRO, 2006). Além disso, como a crosta continental começou a emergir nessa época (exposta à superfície), os processos de erosão e intemperismo começaram a atuar nessas rochas de uma maneira até então inédita, liberando diversos elementos químicos em seus sedimentos e impactando na composição da atmosfera (KRAMERS, 2002;ALLEN, 2008;KUMP, 2008;LEE et al., 2016 ) e dos oceanos (VEIZER, 1989;SHIELDS;VEIZER, 2002;SHIELDS, 2007), o que também contribuiu para a geração e proliferação de novas formas de vida no planeta. ...
This atlas presents, in a didactic way and always preceded by a technical explanation, a collection of geophysical maps of magnetometry and gamma-ray spectrometry on a regional scale, referring to the state of Ceara, Northeastern Brazil.
