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Metamorphic zircon formation by solid‐state recrystallization of protolith igneous zircon
Abstract
Protolith zircon in high-grade metagranitoids from Queensland, Australia, partially recrystallized during granulite-grade metamorphism. We describe the zircon in detail using integrated cathodoluminescence, U–Pb isotope, trace element and electron backscatter diffraction pattern (EBSP) analyses. Primary igneous oscillatory zoning is partially modified or obliterated in areas within single crystals, but is well preserved in other areas. A variety of secondary internal structures are observed, with large areas of transgressive recrystallized zircon usually dominant. Associated with these areas are recrystallization margins, interpreted to be recrystallization fronts, that have conformable boundaries with transgressive recrystallized areas, but contrasting cathodoluminescence and trace element chemistry. Trace element analyses of primary and secondary structures provide compelling evidence for closed-system solid-state recrystallization. By this process, trace elements in the protolith zircon are purged during recrystallization and partitioned between the enriched recrystallization front and depleted recrystallized areas. However, recrystallization is not always efficient, often leaving a ‘memory’ of the protolith trace element and isotopic composition. This results in the measurement of ‘mixed’ U–Pb isotope ages. Nonetheless, the age of metamorphism has been determined. A correlation between apparent age and Th/U ratio is indicative of incomplete re-setting by partial recrystallization. Recrystallization is shown to probably not significantly affect Lu–Hf ages. Recrystallization has been determined by textural and trace element analysis and EBSP data not to have proceeded by sub-grain rotation or local dissolution/re-precipitation, but probably by grain-boundary migration and defect diffusion. The formation of metamorphic zircon by solid-state recrystallization is probably common to high-grade terranes worldwide. The recognition of this process of formation is essential for correct interpretation of zircon-derived U–Pb ages and subsequent tectonic models.
... This type of zoning is common in H 2 O-undersaturated subsolidus melting, whereas oscillatory zoning indicates crystallization directly from the melt (e.g., Schaltegger et al., 1999). During metamorphism, oscillatory zoning might be progressively replaced by unzoned textures due to solid-state recrystallization (Hoskin and Black, 2000;Pidgeon, 1992). ...
... Younger rims of Paleoproterozoic and Neoproterozoic ages are also found (Stages IV and V). They are mostly unzoned and feature consistently lower values of U, Th and Th/U ratios, probably related to solidstate recrystallization (e.g., Hoskin and Black, 2000;Pidgeon, 1992). ...
Supracrustal rocks offer a window into tectonic processes of the early Earth, since they are common in the Archean lithosphere. However, these rocks are usually affected by several episodes of metamorphism that can compromise their Usingle bondPb systematics, leading to equivocal interpretations of depositional ages and sources. In northeast Brazil, supracrustal rocks are frequent within the Archean basement of the São José do Campestre Massif. These metapelitic migmatites show a high-temperature mineral assemblage, with garnet + sillimanite ± spinel and retrograde cordierite, with abundant anatectic melt migration at conditions of upper amphibolite to granulite facies. Zircon Usingle bondPb dating coupled to trace elements analysis through LA-ICP-MS, as well as zircon internal zoning patterns suggest a maximum depositional age of 3305 ± 16 Ma followed by high-temperature metamorphism in the Mesoarchean, Paleoproterozoic and Neoproterozoic. Mesoarchean high temperature metamorphism occurred between 3084 ± 4 and 3006 ± 6 Ma and generated a wide range of textures that could be grouped in, at least, three stages of zircon growth. The first, during prograde heating, led to dissolution of detrital cores through the process of Ostwald Ripening and reprecipitation in oscillatory zoned rims. The second, probably at peak conditions, occurred above the stability of monazite, as evidenced by high Th/U ratios within zircon grains, rounded shape and sector-zoned cores. The third, during retrograde cooling, is mostly driven by garnet breakdown, and resulted in the crystallization of convoluted rims. Ti-in-zircon temperatures indicate minimum temperatures of 712 ± 21 °C for prograde/retrograde stages and 881 ± 50 °C for peak conditions. The Paleoarchean sedimentation and Mesoarchean metamorphism are coeval with similar events in Kaapvaal and Dharwar Cratons (South Africa and South India, respectively), but show no correlation to any Archean domains in South America to date. Solid-state recrystallization during the Paleoproterozoic (ca. 2.0 Ga) and the Neoproterozoic (ca. 0.6 Ga) correlates with orogenic events in both the Borborema Province and São Francisco Craton, suggesting a common evolution since the Rhyacian.
... Here we expand on our earlier work to explore the potential of zircon chemistry to quantify the oxidation state of subsolidus systems (Trail and McCollom, 2023). Zircon chemistry has been shown to provide a chemical record of a variety of high T non-igneous conditions (e.g., Hoskin and Black, 2000;Hoskin, 2005;Rubatto et al., 2011;Rubatto, 2017;Walsh and Spandler, 2023). Moreover, zircon chemical information can be contextualized with absolute age information via the U-Pb geochronology system (Schmitt, 2011;Schoene, 2014) and crystallization temperature (Ferry and Watson, 2007). ...
... The opposite trend is found in the Boggy Plain Zoned Pluton in eastern Australia in which the Ce anomaly is less pronounced in the lower T hydrothermal system, implying a lower oxygen fugacity than the igneous system (Hoskin, 2005). Additionally, our results could be utilized to investigate the fluid oxygen fugacity during fluid-rich crustal metamorphism or subduction zone processes (e.g., Hoskin and Black, 2000;Harley et al., 2007;Rubatto, 2017). ...
... Ghost zoning in some zircon crystals is evidence for recrystallization (Pidgeon 1992), observed at the edge or inside of some zircon crystals in the studied samples (Supplementary Figure S3c, d). Ghost zoning in some zircon crystals left a partial memory of previous trace element contents; the isotope data on these areas could yield an age older than the recrystallization process and younger than the protolith igneous zircon (Hoskin and Black 2000). Although the U, Th, and Pb contents changed through the different degrees of recrystallization in most of the zircons (Hoskin and Black 2000), it is possible that the unconformable overgrowths could cause an isochemical re-homogenization in the older zoned zircon (Pidgeon et al. 1998). ...
... Ghost zoning in some zircon crystals left a partial memory of previous trace element contents; the isotope data on these areas could yield an age older than the recrystallization process and younger than the protolith igneous zircon (Hoskin and Black 2000). Although the U, Th, and Pb contents changed through the different degrees of recrystallization in most of the zircons (Hoskin and Black 2000), it is possible that the unconformable overgrowths could cause an isochemical re-homogenization in the older zoned zircon (Pidgeon et al. 1998). Based on Pidgeon et al. (1998), recrystallization in zircon crystals can happen without losing elements such as U, Th, and Pb. ...
... Most of the domain I is partly replaced to the CL-gray domain II, in which former oscillatory zones, now 'ghost-like' or 'bleached,' are still visible but blurred or obliterated in areas/chaotic textures within single crystals ( Fig. 9b-9e). The domain II presumably represents product of in situ replacement and partial modification of the domain I (Hoskin and Black, 2000). They contain distinctive UHP eclogite-facies minerals relics of Dia-Arg-Mgs-Rt-Ph I-Grt I (Figs. ...
... Ubiquitous post-peak new zircon growth on relict grains during eclogite facies metamorphism and during subsequent retrogression, but not at (U)HP conditions also been recognized in quartzo-feldspathic gneisses from the Papua New Guinea UHP terrane (Zirakparvar et al., 2014) and eclogites from the Kokchetav UHP-HP Massif (Kazakhstan) (Skuzovatov et al., 2021). Therefore, the second clusters of zircons U-Pb may be affected by incomplete replacement and recrystallization (Hoskin and Black, 2000). ...
In the Dabie orogen, central China, the ultrahigh pressure metamorphism (UHPM) was best recorded from meta-mafic and ultramafic rocks occurring as lenses within voluminous amphibolite-facies metafelsic rocks. However, whether or not these metafelsic rocks experienced ultra-deep subduction (>5 GPa) has been rarely quantified because of the rare preservation of peak diagnostic assemblages. In this study, we reconstruct the P-T-t paths for paragneisses and metagranites from the Dabie region to better
elucidate their metamorphic evolution based on detailed petrological, mineralogical and Raman
micro-spectrometer analysis of zircon inclusion combined with conventional geothermobarometry,
phase equilibrium modelling and U-Pb dating. At least three stages of metamorphic evolution are identified for these lithologies. The UHPM is represented by the assemblages of coesite + garnet Ia/Ib + jadeite + phengite + K-feldspar + aragonite for paragneiss and diamond + aragonite + magnesite + phengite + rutile + garnet for metagranite, respectively. The paragneiss experienced peak conditions of 4.5–6.0 GPa/690–720°C, followed by a ecompression heating to 3.5–4.6 GPa/750–850 °C. Also, the diamond + aragonite + magnesite inclusions in zircon from the metagranite provide the first direct mineralogical evidences for ultra-deep subduction. Furthermore, the minimum peak pressure was defined at 5.4 GPa based
on pseudosection for metagranite. As a result, this study provides the first mineralogical and modelling evidences of ultra-deep subduction for the metafelsic rocks in the Dabie orogen. The dating results suggest that the UHPM, anatexis and amphibolite-facies overprint occurred at 236 ± 3 Ma, 226 ± 1 Ma and 195 ± 4 Ma, respectively. The estimated average exhumation rate is 5.8–7.2 km/Ma from peak UHP to amphibolite-facies stages. Moreover, a comparison of different collisional orogens suggests that the preservation of UHP minerals in metafelsic rocks strongly depends on exhumation rate.
... In anatectic melts or fluids, newly grown or overgrown zircon domains exhibit regular growth textures similar to those observed in primary magmatic domains. However, zircons formed through subsolidus metamorphic reactions or replacement (also referred to as ''recrystallization") of existing domains generally appear featureless under CL, sometimes showing faint sector zoning, convoluted zoning, and blurred primary zones (Hoskin and Black, 2000), as observed in UL26-1 and UL26-2 zircons. As depicted in Fig. 2, inward-penetrating, featureless, or weakly zoned bright-CL domains partially or completely replace the dark-CL cores, which typically contain numerous inclusions of britholitelike phases and pyrochlore. ...
... The luminescent secondary domains of UL26-1 zircon may represent MSiO 4 -poor zircon resulting from the dissolution-reprecipitation reaction (Geisler et al., 2007). The distinct boundary between primary and secondary zircon domains could be considered the ''recrystallization front" at an inwardmoving interface (Hoskin and Black, 2000;Geisler et al., 2007). It is worth noting that zircon reactivity depends on its composition and inertness related to prior exposure to thermal and metasomatic overprinting (Vonlanthen et al., 2012;Cho et al., 2021). ...
... Eighteen zircons of sample Dy-44 Table 3), yielding U and Th concentrations of 94-441 and 50-185 ppm, respectively, with Th/U ratios of 0.28-0.56, indicative of a magmatic origin (Hoskin and Black, 2000). These zircons yield a weighted mean age of 803.7 ± 3.4 Ma (MSWD = 0.85; Fig. 9a), interpreted as the crystallization age of the Liyangdou porphyritic biotite granodiorite. ...
... Fourteen analyzed zircons from sample Dg-1 have U and Th concentrations of 156-601 and 71-301 ppm (Supplementary Table 3), respectively, with Th/U ratios of 0.15-0.77, indicating that these zircons are magmatic (Hoskin and Black, 2000). These zircons yield a weighted mean age of 814.2 ± 3.9 Ma (MSWD = 0.41; Fig. 9b), interpreted as the crystallization age of the Donggang granite. ...
... In more details, the emplacement age at 2038 ± 12 Ma of the protolith of the Port-Béni orthogneiss is provided by the analyses of the zircon inner cores characterized by Th/U ratios above 0.1, a feature usually associated with a magmatic origin (Hoskin and Black, 2000). This age is confirmed by the upper intercept age obtained from all the measurements at 2039.5 ± 23 Ma. ...
By re-examining the historical outcrops of Port-Béni located in the Trégor unit of the North Armorican Cadomian belt, the present work delivers four new ages that provide additional constraints on the Proterozoic history of northern Brittany. It is established that granitic, porphyritic rocks crystallized at the end of the Rhyacian (Paleoproterozoic), 2038 ± 12 Ma ago, before being transformed into orthogneisses at a late Neoproterozoic (Ediacaran) age of 621 ± 2 Ma, which is a minimum age, given the retrograde alteration these rocks underwent. The age of ca. 1.8 Ga previously proposed for the protolith of the Port-Béni orthogneiss should be discarded, and these two new ages are consistent with most of those yielded so far by the other Icartian (i.e., Eburnean) basement relics from the Armorican Massif. The gneissic basement was then intruded and disrupted into xenoliths by a granodioritic magma that crystallized 604.5 ± 2.0 Ma ago. This age, slightly younger than previously thought, corresponds to the emplacement age of one of the main units of the North Trégor batholith − the Pleubian-Talbert unit −, part of the Trégor volcano-plutonic complex, which may have built up over a longer period than that indicated by the uncertainty associated with this age. Caution should be exercised in extrapolating this age to that of the whole complex. Finally, doleritic dykes, possibly resulting in two swarms previously thought to be Paleozoic in age, have crosscut this complex. One of the latest yielded an age of 597 ± 15 Ma, indicating that the Trégor doleritic dyking episodes also occurred during the late Neoproterozoic, in between ca. 605 Ma and ca. 580 Ma. As the doleritic dykes are of tholeiitic composition, which distinguishes them from the earlier calc-alkaline magmas, they suggest that the intra-arc extension, documented in the southern, adjacent Saint-Brieuc unit of the belt, also affected the Trégor unit. They may likely have fed northern equivalents of the lava flows from the Paimpol Formation (exposed in between the Saint-Brieuc and the Trégor units), when magma production became moderately influenced by the Cadomian (i.e., Pan-African) subduction and mostly dominated by extension, possibly as a result of a steepening of a north-dipping subduction slab. Indeed, a re-examination of the available geochemical and geochronological data in the light of our new results documents that arc-magma production moved progressively from north (Trégor unit) to south (Saint-Brieuc unit) over time, in the interval 605-580 Ma.
... All zircons have subhedral to euhedral crystal shapes with clear oscillatory zoning ( Figure S1). Th/U ratios are systematically higher than 0.1, indicating that these zircons are of magmatic origin (Hoskin & Black, 2000). ...
Aridification of Central Asia in the Late Mesozoic led to drastic environmental changes characterized by widespread aeolian deposits. We systematically investigated fluvial‐aeolian deposits in the Middle Jurassic Toutunhe Formation, Upper Jurassic Kalazha Formation, and Lower Cretaceous Tugulu Group in the Junggar Basin to the north of the Tianshan Orogenic Belt via unmanned aerial vehicle‐based photogrammetry, scanning electron microscope, grain‐size analysis, and detrital zircon geochronology. Paludal and deltaic environments transitioned to a fluvial‐aeolian environment from the late Middle Jurassic to the Late Jurassic. Fan delta and incisive braided river deposits accumulated in the earliest Cretaceous and evolved into a lacustrine environment with aeolian deposits in the lakeshore. Aeolian deposits are characterized by moderate‐ to well‐sorted and subangular to subround sandstones with large‐scale, high‐dip cross‐bedding, inversely graded lamination, dominant saltation grains, crescent‐shaped, and dish‐shaped impact structures. Aeolian deposits contain heavy minerals including more ilmenite, zircon, garnet, and, tourmaline and less magnetite and epidote than the fluvial deposits. The preserved aeolian sediments of the Kalazha Formation extend west–east for more than 100 km, suggesting a wide desert area during the latest Jurassic. The detrital zircon age patterns indicate that the provenance of the aeolian deposits was similar to that of coeval fluvial deposits. The cooccurrence of fluvial and aeolian deposits and the similar provenances but orthogonal flow directions indicate that the aeolian deposits were mainly sourced from the nearby fluvial material within the basin. The evolution of the fluvial‐aeolian system responded to a complete base‐level cycle controlled by the aridification and tectonics. Due to decreased sediment supply caused by aridification, the base level rose, leading to the change from braided rivers to meandering rivers, along with the deposition of aeolian sediments. Due to the tectonic reactivation in the Late Jurassic, the base level fell, causing the occurrence of alluvial fans and the expansion of the aeolian sediments. Previous studies revealed that the Tianshan in the Jurassic exhibited low relief. The fluvial‐aeolian system played an important role in maintaining the limited relief in southern Central Asia.
... Most of the zircon grains are prismatic, transparent to sub-transparent and colorless, and 100 to 250 μm in size with aspect ratios between 1 : 1 and 3 : 1. Zircon grains commonly display oscillatory and planar zoning in CL images; these features are typical of magmatic zircons (Fig. 4). Uranium concentrations are (Hoskin and Black, 2000). Most of the results plot close to the concordia curve (Fig. 4). ...
The East Qinling Molybdenum Belt (EQMB), which is located on the southern margin of the North China Craton (NCC), is the largest Mo province in the world. This belt hosts a significant number of Mesozoic magmatic-hydrothermal Mo deposits and a small portion of pre-Mesozoic Mo deposits. Understanding the mineralization timing and mechanism of the unique pre-Mesozoic Mo deposits is essential to comprehend the evolution of the EQMB, the pre-Mesozoic Mo enrichment, and the Mesozoic Mo miner-alization event. The recently discovered Zhaiwa deposit is a porphyry Mo deposit located in the Xiong'er Ter-rane of the EQMB. In this study, five molybdenite samples from the Mo-bearing quartz veins were analyzed for Re-Os isotopes composition. These samples yield an isochron age of 1794 ± 45 Ma, which represents the age of mineralization. The mineralization is mostly hosted within the biotite-amphibole plagiogneiss and granite porphyry. LA-ICP-MS U-Pb data of zircons constrain the crystallization age of the granite porphyry to be at 1791 ± 16 Ma. The close spatial and temporal association suggests that the granite porphyry is the causative rocks of the Mo mineralization. The δ 34 S values of pyrite vary from 5.3 to 6.8‰, suggesting that the S was mainly derived from magmatic source. The intrusion of magmas and associated Mo mineralization are contemporane-ous to the regional Xiong'er volcanism that occurred during the late Paleoproterozoic. The Xiong'er volcanism was triggered by partial melting of lithospheric mantle in an extensional setting. The results of our study provide robust evidence for a late Paleoproterozoic Mo metallogenic event along the southern margin of the NCC. Future exploration should also consider the potential of late Paleoproterozoic porphyry Mo mineralization existing in the EQMB, which is closely associated with the Xiong'er volcanism.
... Apart from this general texture, further processes are evident from the grains. Blurred (Z57), ghost (Z15) and convolute zoning (Z60) as features typical of solid-state recrystallisation (Hoskin and Black 2000) are seen in some grains. A few of them (Z7, Z8) suffered magmatic dissolution. ...
The Mongolian Altai Domain of the Central Asian Orogenic Belt is formed by a giant Lower Palaeozoic accretionary wedge that was later thrust over the northerly Central Mongolian Microcontinent. This accretionary complex mainly consists of late Cambrian–Ordovician volcano-sedimentary rocks represented by various formations within the Tugrug Group which were deformed, metamorphosed and intruded by numerous plutons during the Devonian–Carboniferous orogenic events. In this work, we report new U–Pb zircon ages of two felsic igneous rocks indicating an existence
of the so far neglected late Ordovician magmatic event affecting the Mongolian Altai accretionary wedge. The felsic
volcanic sheet inside the upper part of the Tugrug Group in the western Gobi Altai Zone (eastern part of the Mongolian
Altai Domain) yields an age of 457±2 Ma and nearby granite pluton intruding the entire volcano-sedimentary sequence gives an age of 445±1 Ma. Both rocks are high-K calc-alkaline, peraluminous, with similar geochemical patterns characterised by enrichment in mobile lithophile elements over Nb, Ti, P and Sr and nearly identical REE trends. All together, these features point to an analogous volcanic-arc-related magma source. This magmatism reflecting terminal stages of the accretionary wedge formation in the Mongolian Altai Domain may be related to the recently proposed late Ordovician orogenic event.
... The bright-CL cores of zircon grains show oscillatory growth zoning with variable Th/U ratios (0.59-1.01; Table 5), indicating magmatic origin for these grains (Williams and Claesson, 1987;Hoskin and Black, 2000;Rubatto, 2002). Morphology, internal zoning patterns, and chemical compositions suggest that the zircon grains crystallized during the early to late magmatic stage. ...
... Trace element migration is found to be intimately associated with the defect density in materials (Watanabe, 1985). The segregation of trace elements which are not inherently part of the host mineral to the grain boundary is more energetically favourable to reduce the lattice stress principally caused by the differences in ionic size between the host grain sites and the trace element (Hoskin and Black, 2000;Reddy et al., 2016). The enrichment of trace elements in dislocations and both low and highangle boundaries has been documented in minerals from different geological settings (Suzuki, 1987;Hiraga et al., 2003;Reddy et al., 2016;Dubosq et al., 2018;Fougerouse et al., 2019;Tacchetto et al., 2021Tacchetto et al., , 2022. ...
... Our study presents that nine spots of the youngest detrital zircon grains from the Songshan quartzite (18SS-1) lying on the discordant line yield an upper intercept age of 2439 ± 25 Ma (MSWD = 0.78) (Fig. 6d). These zircons show clear oscillatory zonings on the CL image (Fig. 6a) and have Th/U ratios higher than 0.4, REE patterns normalized to C1chondrite are characterized by a steeply-rising slope from the LREE to the HREE with a positive Ce anomaly and negative Eu anomaly (Fig. 6g), suggesting a magmatic origin (Vavra et al., 1999;Hoskin and Black, 2000;Corfu et al., 2003;Hoskin and Schaltegger, 2003). Therefore, this age represents the crystallization age of these zircons and can place further and reliable constrains at the maximum depositional age for the sedimentary protoliths of the Songshan Group in the Dengfeng area. ...
The responses of oceanic chemistry and redox state to the oxygenation of atmosphere at the period (2.45-2.10 Ga) of the global Great Oxidation Event remain poorly constrained due to the lacking of coeval marine chemical precipitates, e.g., Iron Formation. Here, we report a Granular Iron Formation (GIF) in the Songshan Group of the North China Craton and its depositional age was constrained at 2.44-2.32 Ga at the first time. In the GIF, he-matite is the primary iron oxide and its occurrence indicates that the shallow seawater at 2.44-2.32 Ga was oxygenated, which is in agreement with the positive δ 13 C excursion of the coeval carbonates up and below the Songshan GIF. No significant correlations between redox-sensitive trace elements (Cr, V, Mo and U) and Al 2 O 3 contents exclude terrigenous clastic origin, suggesting that these elements are enriched during the deposition and have been adsorbed by the Fe-rich chemical precipitates. The high Cr (up to 2000 ppm) and B (up to 300 ppm) contents of the GIF and the authigenic hematites further indicate an increase of chemical weathering and subsequent input into the seawater. Combined with high V (up to 1500 ppm) and P (up to 7000 ppm) contents in the GIF, it can be inferred that continental oxidative weathering was intensive during 2.44-2.32 Ga. The enrichments of Cr contents and Cr/Ti ratios of the Songshan GIF and its authigenic hematites suggest that Cr was transported to the oceans as soluble Cr (VI) species by rivers because of the oxidative Cr(III) in Cr-rich accessory minerals in soil or crust. The enrichment processes of Cr and V in the hematites are likely to be that soluble Cr (VI) and V (V) supplying by the downwelling surface waters transform into Cr (III) and V (III) in the suboxic-anoxic bottom waters and then removed from seawater via authigenic burial of hematite because trivalent cations can substitute in the hematite structure. Based on above, we therefore suggest a new depositional scenario for the Songshan GIF, in which precipitation occurred due to upwelling of deep, anoxic, reduced Cr-V-rich ferruginous waters into an oxygenated, high productivity shallow-water setting.
... Majority of the grains exhibit magmatic characteristics, displaying distinct crystal zoning and high Th/U ratios (ranging from 0.27 to 2.74). Only one zircon grain (KCQ71-1-29; 929 Ma) is identified as having metamorphic origin, indicated by its weak internal zoning and low Th/U ratio (0.18) (Hoskin and Black, 2000). ...
The Tianshan range, a Paleozoic orogenic belt in Central Asia, has undergone multiple phases of tectonic activities characterized by the N‐S compression after the early Mesozoic, including the far‐field effects of the Cenozoic Indian‐Asian collision. However, there are limited reports on the tectonic deformation and initiation of Triassic intracontinental deformation in the Tianshan range. Understanding this structural context is crucial for interpreting the early intracontinental deformation history of the Eurasian continent during the early Mesozoic. Growth strata and syn‐tectonic sediments provide a rich source of information on tectonic activities and have been extensively used in the studies of orogenic belts. Based on detail fieldwork conducted in this study, the middle‐late Triassic Kelamayi Formation of the northern Kuqa Depression in the southern Tianshan fold‐thrust belt has been identified as the typical syn‐tectonic growth strata. The youngest detrital zircon component in two lithic sandstone samples from the bottom and top of the Kelamayi growth strata yielded U‐Pb ages of 223.4 ± 3.1 and 215.5 ± 2.9 Ma, respectively, indicating that the maximum depositional age of the bottom and top of the Kelamayi growth strata is 226‐220 and 218‐212 Ma. The geochronological distribution of detrital samples from the Early‐Middle Triassic and Late Triassic revealed abrupt changes, suggesting a new source supply resulting from tectonic activation in the Tianshan range. The coupling relationship between the syn‐tectonic sedimentation of the Kelamayi Formation and the South Tianshan fold‐thrust system provides robust evidence that the Triassic intracontinental deformation of the South Tianshan range began at approximately 226‐220 Ma (during the Late Triassic) and ended at approximately 218‐212 Ma. These findings provide crucial constraints for understanding the intraplate deformation in the Tianshan range during the Triassic.
... Some zircon surfaces were re-formed by magmatism in the later period, and the boundary between the core and the zone was obvious. Therefore, in this experiment, some zircons with obvious magmatic zircon characteristics were selected for testing to obtain their formation age [30]. ...
Owing to the paucity of research on synchronous mafic rocks in the Tarim Basin, the late Paleozoic–early Mesozoic tectonic development of this region is not well defined. The Halaqi region is situated on Tarim’s northwest edge, and numerous mafic dikes can be found cross-cutting the Permian strata. The whole-rock geochemistry, zircon U–Pb age, and Sr–Nd isotopic signature of these mafic rocks have never been reported before, and this contribution can offer geochronological and petrogenetic investigations that provide fresh insight into the geodynamic development of the area. Major oxide contents in the Halaqi mafic rocks vary, including SiO2 (45.74–50.31 wt.%), Al2O3 (13.28–14.8 wt.%), FeOT (16.48–19.19 wt.%), MgO (7.58–10.32 wt.%), CaO (7.19–12.39 wt.%), Na2O (2.97–4.50 wt.%), K2O (0.24–0.63 wt.%), TiO2 (1.11–1.29 wt.%), MnO (0.14–0.16 wt.%), and P2O5 (0.13–0.17 wt.%). The mafic rocks are enriched in high-field-strength elements (e.g., Zr and Hf) and large-ion lithophile elements (e.g., Sr, Th, and U) but depleted in Nb, Ta, and P. The total REEs in the rocks are lower (ΣREE = 72.80–86.85 ppm), and HREEs are somewhat depleted in comparison to LREEs, with positive Eu anomalies (Eu/Eu* = 1.05–1.17) but weak negative Ce anomalies (Ce/Ce* = 0.91–0.93). Zircon U–Pb ages of 201–247 Ma were obtained from a total of 18 magmatic zircon grains found in the mafic rocks that were studied. These results point to a middle-to-late Triassic emplacement. The mafic dikes exhibit somewhat enriched Nd isotopic compositions (εNd(t) = –1.6~–0.2) and an older Nd model age (TDM = 1.24–1.37 Ga). The Halaqi middle–late Triassic mafic dikes are thought to have originated from the same tectonic background as the Permian Tarim Large Igneous Province, along with similar geochemical and isotopic compositions. This suggests that they are all products of the interaction between asthenospheric and lithospheric mantles in an intraplate extensional environment. Research indicates that the Triassic mafic magmatism in northwest Tarim could be the product of the continuous thermal pulse of the Tarim mantle plume and be a part of the Tarim LIP.
... They are transparent, pale dark, and exhibit oscillatory zoning characteristic of magmatic zircon ( Figure 3a). All zircons from yield varying uranium (50~241 ppm) and thorium (49~202 ppm) contents, with Th/U ratios ranging from 0.83 to 1.24 (Table 1, Figure 3a), which are consistent with the magmatic origin (Hoskin, & Black, 2000). ...
The Wutai-Hengshan area located in the Trans-North China Orogen (TNCO) is a classic example of the geological evolution in the North China Craton (NCC) and carries varied Mesozoic magmatism associated with lithospheric destruction. In this paper, we provide zircon U-Pb geochronology, whole-rock chemistry, and zircon Lu-Hf isotopic data of the Xiachehe granite porphyries to discuss their petrogenesis and tectonic implications. According to the results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), Xiachehe granite porphyries yield zircon U-Pb ages of 135.1 ± 0.8 Ma, representing age in the Early Cretaceous. The Xiachehe granite porphyries are high-K calc-alkaline and peraluminous. These rocks have REE and trace element patterns, characterized by enrichment in large-ion lithophile elements (LILE) and light rare-earth elements (LREE), and depletion in high-field-strength elements (HFSE), heavy rare-earth elements (HREE), with negative Eu anomalies. Therefore, They are classified as I-type granites with adakitic features. The Xiachehe granite porphyries have homogeneous zircon Hf isotopic compositions. Their zircons have negative ε Hf (t) values (-20.7 to -19.3) and two-stage Hf model ages of 2417–2501 Ma, indicating that the studied rocks were derived from partial melting of Neoarchean-Paleoproterozoic crustal materials with mantle-derived inputs. Therefore, it is suggested that the Xiachehe granite porphyries formed in a back-arc extensional setting likely associated with the ancient Pacific Plate subduction and retreat beneath the Eurasian continent. The TNCO was influenced by the subduction and retreat of the ancient Pacific Plate in the Early Cretaceous.
... Hence, a magmatic origin can be inferred for these zircons (cf. Hoskin & Black, 2000). Thirteen zircons among 22 dated grains yielded a concordant age of 55.16 ± 0.95 Ma and a weighted mean 206 Pb/ 238 U age of 55.88 ± 0.69 (MSWD = 1.3) (Figures 10b and 10c; Table S1 in Supporting Information S1). 13 of 22 ...
The poor understanding of the structural history of the Late Triassic flysch in the northeastern Tethyan Himalaya has caused many disputes regarding India‐Asia collisional tectonics. Here, we conducted an integrated study including tectonostratigraphic analysis, structural analysis, and zircon U‒Pb and muscovite ⁴⁰Ar/³⁹Ar dating to unravel the structural evolution of the Late Triassic flysch. Field geological mapping revealed that the flysch can be subdivided into three structural units, including the Gyaca mélange, Qiongjie‐Dengmu shear zone and Langjiexue fold‐thrust belt. The three units all contain voluminous siliciclastic rocks deposited in an abyssal submarine fan environment at the Indian passive continental margin. The Gyaca mélange shows a two‐stage deformation process that generated a collisional mélange resulting from the subduction of the Indian passive continental margin. ⁴⁰Ar/³⁹Ar dating of top‐to‐the‐south shear bands and block‐in‐matrix structure indicate that the India‐Asia collision happened no later than the Selandian (ca. 60 Ma). The divergent imbricated thrusting and folding of the Gyaca mélange and Langjiexue fold‐thrust belt formed a positive flower structure. The zircon U‒Pb and ⁴⁰Ar/³⁹Ar dating of syntectonic dikes and sericite flakes within the Gyaca mélange yield ages of ca. 56–55 Ma for the genesis of the divergent structures. Moreover, the Qiongjie‐Dengmu dextral shear zone yields a sericite ⁴⁰Ar/³⁹Ar age of ca. 35 Ma and thus indicates a transient strike‐slip stress regime. Generally, the early India‐Asia collision tectonics in the northeastern Tethyan Himalaya showed episodic evolution with changing structural styles from mélange formation to imbricate fold‐thrust belt development and finally strike‐slip shear generation.
... They were lighter in color than the hornblende-bearing granite porphyry and showed clear magmatic oscillatory zoning, part of which had a darker core (Figure 5b). The Th/U ratio ranged from 0.12 to 0.99, with a mean value of 0.59, indicating a magmatic origin [50,51]. All zircon test spots were located on or near the concordant curve, and the concordant 206 Pb/ 238 U apparent age range was 126-153 Ma (Figure 6b). ...
The Huayangchuan U-polymetallic deposit in the Qinling Orogen is a newly verified carbonatite-hosted deposit on the southern margin of the North China Craton (NCC) in Central China. Granitic magmatism is extensively developed in the Huayangchuan deposit area and is lacking analysis on the reasons for these situations; however, its ages, petrogenesis, and relationship with uranium mineralization are not well constrained. Zircon U–Pb ages for the hornblende-bearing granite porphyry and medium-fine-grained biotite granites in close proximity to carbonatite rocks are 229.8 ± 1.1 and 135.3 ± 0.6 Ma, respectively. High-K calc-alkaline series and weakly peraluminous Triassic hornblende-bearing granite porphyry are slightly enriched in light rare earth elements (LREE) with flat heavy rare earth element (HREE) patterns, enriched in Ba and Sr, and depleted in Nb, Ta, P, and Ti, i.e., geochemical characteristics similar to those of adakite-like rocks. The Early Cretaceous medium-fine-grained biotite granites are characterized by LREE enrichment and flat HREE patterns, which belong to high-K calc-alkaline series, and metaluminous belong to weakly peraluminous I-type granite, with U and large ion lithophile element (LILE) enrichment and high field strength element depletion. The high initial 87Sr/86Sr ratios and enriched Nd (εNd(t) = −10.7 to −9.5 and −19.9 to −18.9, respectively) and Hf (εHf(t) = −21.8 to −13.0 and −30.5 to −19.0, respectively) isotopes revealed that both granitic rocks from the Huayangchuan deposit mainly originated from lower crustal materials, generated by partial melting of the ancient basement materials of the Taihua Group. Triassic hornblende-bearing granite porphyry is significantly different from the mantle origin of the contemporaneous U-mineralization carbonatite. In combination with tectonic evolution, we argue that the Qinling Orogenic Belt was affected by the subduction of the North Mianlian Ocean during the Late Triassic. The ongoing northward subduction of the Yangtze Craton resulted in crustal thickening, forming large-scale Indosinian carbonatites, U-polymetallic mineralization, and contemporaneous intermediate-acid magmatism. Additionally, due to the tectonic system transformation caused by Paleo-Pacific Plate subduction, intracontinental lithosphere extension and lithospheric thinning occurred along the southern NCC margin in the Early Cretaceous. Intense magma underplating of the post-orogeny created a large number of magmatic rocks. The tremendous heat could have provided a thermal source and dynamic mechanism for the Yanshanian large-scale U-polymetallic mineralization events.
... A few zircons are weakly zoned or non-zoned (Fig. S1). The results show that 87 % of the total zircons have Th/U ratio > 0.4, and only 1 % have Th/U ratio < 0.1 (Text S4), indicating that most of the detrital zircons in Wuwei Basin are magmatic and only a small amount may be Metamorphic (Hoskin and Black, 2000), which is consistent with the internal structure of zircons shown by cathodoluminescence images. ...
... In the case of zircons from garnet mafic granulite (14LH05-1), they predominantly exhibit homogeneous CL images with no zoning, and they show slight retention of inherited regions (Fig. 10A). These characteristics suggest a recrystallized origin during metamorphism (Hoskin and Black, 2000;Taylor et al., 2016). Thus, the two-group concordia ages (1951 ± 14 Ma and 1799 ± 12 Ma) may record the timing of metamorphic evolution. ...
High-pressure (HP) and ultrahigh-temperature (UHT) granulites with a high geothermal gradient (greater than 500 °C/GPa) are prominent features of Paleoproterozoic orogenic belts and may represent paired metamorphic belts present during the early stages of plate tectonics. Understanding their pressure−temperature−time (P-T-t) paths and metamorphic evolutionary relationships could provide valuable constraints on the tectonic processes of Paleoproterozoic orogenic belts. Here, we describe garnet mafic and clinopyroxene-orthopyroxene (Cpx-Opx) granulites from the Diebusige area of the Alxa Block in the western part of the Khondalite Belt, North China Craton. Through detailed petrographic, phase equilibrium modeling, and Ti-in-amphibole thermometric studies, we obtained the preserved peak mineral assemblages of two types of mafic granulites: garnet + clinopyroxene + amphibole + plagioclase + quartz + ilmenite, and clinopyroxene + orthopyroxene + plagioclase + amphibole + garnet (rare) + ilmenite. The preserved peak P-T conditions were determined to be 850−890 °C/11.4−13.2 kbar (HP granulite-facies) and 950−970 °C/8.2−9.2 kbar (UHT conditions), with thermal gradients of ∼70 °C/kbar (moderate differential temperature/differential pressure, dT/dP) and ∼110 °C/kbar (high dT/dP), respectively. Using sensitive high-resolution ion microprobe U-Pb dating and rare earth element analysis of zircons, we found that the garnet mafic granulite recorded an HP granulite-facies metamorphic age of ca. 1.95 Ga and a retrograde cooling age of ca. 1.8 Ga, while the Cpx-Opx granulite recorded a consistent retrograde cooling age of ca. 1.8 Ga. By combining these results with the metamorphic evolution and timing (ca. 1.92−1.91 Ga) of UHT rocks from the Khondalite Belt, we suggest that the garnet (HP) mafic and Cpx-Opx (UHT) granulites may represent different stages of the same metamorphic event, shedding light on the processes involved in the collision and subsequent exhumation of Paleoproterozoic orogenic belts.
... The analyzed zircon grains exhibit highly variable U (70-1582 ppm) and Th (36-798 ppm) contents, with Th/U ratios of 0.3~1.6 (Table S2). These characteristics suggest a magmatic origin [46]. The 23 grains from sample D024 yield a diverse age distribution spanning from 240 to 1100 Ma, with the youngest age cluster yielding a weighted mean 206 Pb/ 238 U age of 242.6 ± 2.9 Ma (n = 8, mean square weighted deviation (MSWD) = 1.4). ...
The assemblage of oceanic islands and seamounts, arising from the widespread presence of mature oceans, plays a crucial role in reconstructing the evolutionary history of the paleoocean. Oceanic islands or seamounts within the Longmuco-Shuanghu metamorphic complex, a remnant of the Paleo-Tethys Ocean in the central Tibetan Plateau, have seldom been reported due to their remoteness. This study has identified an oceanic island-seamount in the Maoershan area, situated to the west of the Longmuco-Shuanghu metamorphic complex, composed of basalt, diabase, limestone, and siliceous rocks. Based on field observations, petrology, zircon U-Pb dating, whole-rock geochemistry, and Sr-Nd isotopes analyses, we have identified a suite of mafic rocks with OIB affinity. The youngest zircon U-Pb age cluster was concentrated at ~243–241 Ma. The geochemical characteristics of the siliceous rocks indicate a mixture of terrigenous material, suggesting that they formed in a continental margin. In combination with regional geological data, we conclude that the Longmuco-Shuanghu Paleo-Tethys Ocean remained open during the Middle Triassic. Furthermore, a fraction of the oceanic island-seamounts underwent scraping and transformed into a metamorphic complex, while other segments experienced deep subduction, resulting in the formation of high-pressure metamorphic rocks. Collectively, these processes gave rise to the distinctive high-pressure metamorphic complex within the central Qiangtang terrane.
... Most of the zircon grains displayed clear or weak oscillatory zonings, whereas some zircons showed irregular zoning in CL images. Some zircon grains showed core-rim structures and preserved primary zonings, indicating that they possibly have been modified (Hoskin and Black, 2000). Most of the rims were too narrow and not analyzed. ...
There are multiple Precambrian microcontinents within the Central Asian Orogenic Belt (CAOB), and the paleogeographic locations of those in the southwest CAOB (e.g. Central Tianshan Block, Yili Block, Aktau-Mointy terrane, Erementau-Niyaz terrane and Kokchetav terrane) remain controversial. The Central Tianshan Block is one of the representative Precambrian microcontinents in the southwest CAOB. It can be divided into the western Central Tianshan Block and eastern Central Tianshan Block. To investigate their relationships and paleogeographic locations, we collected metasedimentary rocks from the Kekesu Group in western Central Tianshan Block. The results of zircon LA–ICP–MS U-Pb dating, combining with previously obtained magmatic ages, suggest that the Kekesu Group was deposited during the latest Mesoproterozoic to earliest Neoproterozoic (ca. 1010–969 Ma). Detrital zircon populations of Kekesu Group were dominated by 2.0–0.9 Ga grains with an age peak at 1474 Ma. The Archean to end-Paleoproterozoic zircon grains had variable εHf(t) values from +9.7 to −8.7, and most of the Mesoproterozoic zircons had positive εHf(t) values (+13.9 to +2.4). The Kekesu Group in western Central Tianshan Block has similar zircon age populations and εHf(t) values to those in eastern Central Tianshan Block. In combination with magmatism, these data suggest that western and eastern Central Tianshan Block had similar tectonic affinities and locations during the earliest Neoproterozoic. We compiled 4566 concordant detrital zircon U-Pb age data of Mesoproterozoic to early Neoproterozoic strata from microcontinents in the southwest CAOB and potential sources. The zircon age probability density plots, kernel density estimates and multidimensional scaling plot suggest that the Central Tianshan and Yili blocks had similar sources mainly derived from the Musgrave Province in central Australia in the latest Mesoproterozoic to early Neoproterozoic, and were possibly located near Australia during assembly of Rodinia. The sources for the Erementau-Niyaz and Kokchetav terranes were probably mainly within the northern Laurentia in the latest Mesoproterozoic to earliest Neoproterozoic.
... (average = 1.97). The high Th/U ratios may be due to the loss of U during their formation via solid- state recrystallization processes (Vavra et al., 1996;Hoskin and Black, 2000;Wu and Zheng, 2004;Yakymchuk et al., 2018). These metamorphic zircon domains show clustered 207 Pb/ 206 Pb ages of 1776-1802 Ma, with a weighted average age of 1789 ± 43 Ma (MSWD = 0.04), which is interpreted as the age of metamorphism (Fig. 4a). ...
The Archean-Paleoproterozoic basement rocks, exposed in the Xingdi region of the Quruqtagh Block, provide a great opportunity to characterize the early crustal evolution of the Tarim Craton and reconstruct the Columbia supercontinent. Here we report zircon U-Pb ages, Lu-Hf isotope compositions, and bulk-rock geochemical data of orthogneisses and granitic veins from the Xingdi region of the Quruqtagh Block in the northeastern margin of the Tarim Craton. The orthogneisses formed in the late Neoarchean to early Paleoproterozoic (2504 ± 33 Ma, 2419 ± 24 Ma and 2450 ± 27 Ma), and were metamorphosed in the late Paleoproterozoic (1789 ± 43 Ma and 1748 ± 35 Ma). The orthogneisses have relatively high Sr (312-710 ppm) contents, and low Y (5.3-13.8 ppm) and Yb (0.46-1.10 ppm) contents, resulting in high Sr/Y (32-118) and La/Yb (17-64) ratios that are characteristics of adakites. The relatively high Mg # (49-55) and depleted zircon Hf isotope characteristics (positive εHf (t), with T DM2 of ca. 2.5-2.8 Ga) indicate that the orthogneisses crystallized from partial melting of garnet-bearing amphibolite in a subduction environment. The granitic veins that intrude the orthogneisses formed during the late Paleoproterozoic (1760 ± 25 Ma), which is roughly contemporaneous with the metamorphic ages of the orthogneisses; the granitic veins are characterized by extremely high SiO 2 (>80 %), enrichments in Rb, Ba and U, and depletions in Nb and Ta. It is inferred that the granitic veins formed by in situ anatexis of the orthogneisses during the late Paleoproterozoic. Based on the zircon ages with Hf isotope compositions, it is evident that the continental crust in the Xingdi region of the Tarim Craton largely formed between 2.5 and 2.8 Ga and was reworked at ca. 1.8 Ga; this evolutionary history is similar to that of the Dunhuang Block. Comparison of the Paleoproterozoic tectono-thermal events in the different terranes of the Tarim Craton, it is suggested that the Tarim Craton is composed of discrete terranes that detached from different ancient cratonic nuclei prior to the late Paleoproterozoic, and assembled within the Columbia supercontinent respectively.
Early Paleozoic magmatism in the West Kunlun Orogenic Belt (WKOB) preserves important information about the tectonic evolution of the Proto-Tethys Ocean. This paper reports whole-rock compositions, zircon and apatite U-Pb dating, and zircon Hf isotopes for the Qiaerlong Pluton (QEL) at the northwestern margin of WKOB, with the aim of elucidating the petrogenesis of the pluton and shedding insights into the subduction-collision process of this oceanic slab. The QEL is mainly composed of Ordovician quartz monzodiorite (479 ± 3 Ma), quartz monzonite (467–472 Ma), and syenogranite (463 ± 4 Ma), and is intruded by Middle Silurian peraluminous granite (429 ± 20 Ma) and diabase (421 ± 4 Ma). Zircon εHf(t) values reveal that quartz monzodiorites (+2.1 to +9.9) and quartz monzonites (+0.6 to +6.8) were derived from a mixed source of juvenile crust and older lower crust, and syenogranites (−5.6 to +4.5) and peraluminous granites (−2.9 to +2.0) were generated from a mixed source of lower crust and upper crust; diabases had zircon εHf(t) values ranging from −0.3 to +4.1, and contained 463 ± 5 Ma captured zircon and 1 048 ± 39 Ma inherited zircon, indicating they originated from enriched lithospheric mantle and were contaminated by crustal materials. The Ordovician granitoids are enriched in LILEs and light rare-earth elements, and depleted in HFSEs with negative Nb, Ta, P, and Ti anomalies, suggesting that they formed in a subduction environment. Middle Silurian peraluminous granites have the characteristics of leucogranites with high SiO2 contents (74.92 wt.%–75.88 wt.%) and distinctly negative Eu anomalies (δEu = 0.03–0.14), indicating that they belong to highly fractionated granite and were formed in a post-collision extension setting. Comparative analysis of these results with other Early Paleozoic magmas reveals that the Proto-Tethys ocean closed before the Middle Silurian and its southward subduction resulted in the formation of QEL.
The crustal evolution of the Angolan Shield (AS) remains poorly constrained. To address this, we analysed U-Pb and Lu-Hf isotopes in detrital and igneous zircons to investigate the age and provenance of extensive sedimentary strata in southwestern Angola and use it as a proxy to gain insight into the Archean to
Mesoproterozoic evolution of the region. Mesoproterozoic maximum depositional ages for the Iona (<1323 ± 13 Ma), Ompupa (<1215 ± 13 Ma), and Cahama (<1184 ± 23 Ma) siliciclastics challenge previous correlations with the Paleoproterozoic Chela Group. Provenance analysis reveals that the Mesoproterozoic strata were derived internally from the AS.
Our combined dataset indicates that the widespread Eburnean magmatism (~2.05–1.93 Ga) resulted
from reworking of Archean crust, possibly in collision orogens. A major increase in the eHf(i) and eNd(i)
values at ~1.87–1.73 Ga indicates a change in geodynamics, with magmatism of the Epupa–Namibe Metamorphic Complex (ENMC) generated in an extensional accretionary orogen at the southern margin of the Eburnean–Archean crustal block. Magmatism resumed in the Mesoproterozoic (~1.56–1.50 Ga), with suprachondritic eHf(i) values indicating significant juvenile addition. The Kunene Complex (KC: ~1.50–1.36 Ga) anorthosite-mangerite-charnockite-granite magmatism displays variable eHf(i) and eNd(i) values, consistent with mixing between reworked ENMC-crust and juvenile melts in a long-lived
accretionary orogen back-arc region. Post-KC (~1.36–1.30 Ga) magmatism shows an increased juvenile contribution, potentially linked to partial melting of ENMC and ~1.56–1.50 Ga juvenile crust during an orogenic event, or alternatively, related to renewed slab retreat and back-arc extension. The Hf isotopic compositions of ~1.29–1.18 Ga zircons are compatible with a renewed input from the depleted mantle and/or reworking of the earlier ~1.56–1.50 Ga juvenile crust. Emplacement of ~1.13–1.10 Ga mafic dikes/sills marks the end of Mesoproterozoic magmatism in the AS. Our new data enhance our understanding of the Archean to Mesoproterozoic crustal evolution of the AS.
Background
The intricate textural patterns commonly observed in metamorphosed and recrystallized zircon (ZrSiO 4 ) underscore the crucial necessity of understanding the underlying mechanisms governing their formation to ensure accurate interpretation of the chemical and isotope data they contain. This study employed a combination of microanalytical techniques, including electron backscattered diffraction (EBSD) analysis, electron microprobe (EMP) mapping, and scanning electron microscope (SEM) imaging, to investigate the processes of formation and modification of zircon in a late Pleistocene (~ 35 ka) syenite enclosed within the Nari Tephra Formation on Ulleung Island in South Korea.
Findings
Under cathodoluminescence (CL), zircons within the syenite reveal dark, featureless, or oscillatory-zoned cores containing numerous inclusions of britholite. These cores are partially or entirely replaced by inward-penetrating bright-CL domains that exhibit minimal inclusion presence. Despite these changes, the external morphologies of the zircons remain largely unchanged, and the faded oscillatory zoning is preserved in the replaced regions. EMP mapping discloses amoebiform micro-domains with high Y, U, and Th concentrations within the dark-CL cores, while the bright-CL domains are relatively deficient in these trace elements. Microstructural analysis of the zircons using EBSD mapping indicates no significant misorientation between the dark-CL cores and the bright-CL rims. Deformation-related low-angle boundaries by lattice distortion are clearly observed in certain grains, cutting across the discrete SEM and EMP domains, and often aligned along submicron pore trails.
Conclusions
Microstructural and microchemical analyses carried out in this study establish that the zircons within the Ulleung syenite have undergone subsolidus recrystallization, a process likely influenced by the presence of fresh melts or fluids. This recrystallization process could be attributed to either coupled dissolution and reprecipitation or thermoactivated particle and defect volume diffusion due to inherent lattice strain. The subsequent deformation observed in the zircons might be a result of increased stress within the magma system after the recrystallization.
The Aqishan-Caixiashan polymetallic ore cluster, located in the eastern Tianshan Orogenic Belt, is part of the southern Central Asian Orogenic Belt (CAOB) and experienced extensive tectono-magmatic events with polymetallic mineralization during the Carboniferous. However, the Carboniferous tectonic affinity and magmatic evolution of the ore cluster are still controversial, which limits further understanding of the tectonic evolution and regional metallogeny. With the aim of solving these problems, we have carried out geochronological and geochemical studies on Carboniferous intrusive rocks and volcanic rocks from the ore cluster. The samples of the Carboniferous magmatic rocks are enriched in LREEs and LILEs but depleted in HFSEs, which attest to the I-type affinity and are similar to those formed in a continental arc. The early Carboniferous granodiorites and granites (350–343 Ma) in the Central Tianshan Block (CTB), with high Mg# values of 37–66, low Y values of 2.3–17.2 ppm, and high Sr/Y ratios (34–85), show characteristics of adakites that are formed by partial melting of delaminated oceanic crust and/or lower crust. Additionally, they have a wide range of εHf(t) values (5.4–15.9), high εNd (t) values (1.78–6.31) and low ISr values (0.705–0.706), indicating a mixed source of depleted mantle and lower crust for its origin. The Late Carboniferous volcanic rocks (314–306 Ma) in the Aqishan-Yamansu Tectonic Belt (AYTB) north of the CTB have obviously negative Nb–Ta-Ti anomalies, as well as high Y contents (12–20 ppm) and low Sr/Y ratios (5.6–29.9) compared to primitive mantle, which has a relatively flat chondrite-normalized REE pattern, resembling arc-related magmas. Additionaly, they have positive values of εHf(t) values (8.7–14.7), high εNd(t) values (3.04–4.45) and low ISr ratios (0.704–0.707), showing a depleted mantle source, which is weakly affected by the contamination of ancient crustal material. Combining this study and previous works, we suggest that the continuous southwards subduction of the North Tianshan Ocean during the Carboniferous generated the continental arc and forearc basin in the northern part of the CTB. The expansion of the Aqishan-Yamansu forearc basin formed intrusions and volcanic formations along the arc belt. The early Carboniferous intermediate-felsic mantle-derived adakitic magmas in the CTB are probably related to regional Zn-Pb enrichment and mineralization, and the Late Carboniferous intrusions and volcanic formations in the AYTB are related to large-scale Fe-Cu-Zn-Pb-Ag mineralization.
The mineral zircon is used widely to constrain the age of rocks and the processes that formed them. Although zircon is robust to a range of physical and chemical processes, it may show evidence for rapid re-equilibration that is generally considered to reflect interaction with hydrous fluids. Here, we show that zircon grains that crystallized from melt produced during the catastrophic meltdown of the Chernobyl nuclear reactor exhibit re-equilibration textures that occurred in an environment without free water. The process of re-equilibration involved a melt-mediated interface-coupled dissolution-reprecipitation that took place over a few days to produce textures that are commonly observed in igneous and anatectic systems. Thus, the composition of zircon can be modified even in the absence of hydrous fluids in a short time frame. Through this process, zircon crystals may track the timing of the last silicate melt they interacted with.
Geodynamic processes impart characteristic compositional signatures to minerals; petrochronology is the science of deciphering these signatures with respect to time. This chapter provides a critical appraisal of the thermodynamic and kinetic processes that lead to commonly observed patterns of intracrystalline isotopic dates and trace-element concentrations. The now routine ability to characterize multi-scale variations in isotopic dates and mineral compositions – from regional to (sub)micron scales – heralds a new era in petrochronology. We emphasize the potential for such information to constrain histories of mineral growth, temperature, deformation and fluid-rock interaction over sub-million year timescales, affording fresh insight into the geodynamics of Earth’s lithosphere.
The Triassic granitoids in Central Tianshan play a key role in determining their petrogenesis and tectonic evolution on the southern margin of the Central Asian Orogenic Belt. In this study, we have presented SHRIMP zircon U–Pb ages, Hf isotopic, and geochemical findings on the Xingxingxia biotite granite, amazonite granite, and granitic pegmatite in Central Tianshan, NW China. Zircon U–Pb dating yielded the formation ages of 242 Ma for the biotite granite and 240 Ma for the amazonite granite. These granitoid rocks have high K 2 O and low MgO and CaO contents. They are enriched in Nb, Ta, Hf, and Y, and depleted in Ba and Sr, showing flat HREE patterns and negative Eu anomalies. They have typical A‐type granite geochemical signatures with high Ga/Al (8–13) and FeO T /(FeO T +MgO) ratios, showing an A2 affinity for biotite granite and an A1 affinity for amazonite granite and granitic pegmatite. Zircon ɛ Hf ( t ) values of the granitoids are 0.45–2.66 with Hf model ages of 0.99–1.17 Ga. This suggest that these A‐type granites originated from partial melting of the lower crust. We proposed that Xingxingxia Triassic A‐type granites formed under lithospheric extension from post‐orogenic to anorogenic intraplate settings and NE‐trending regional strike‐slip fault‐controlled magma emplacement in upper crust.
The Fe-rich deposits from Hakimpur (Bangladesh) are associated with a broad range of rock types including BIF, biotite schist, quartzite and amphibolite. Geochemical analysis coupled with petrographic studies indicates the BIF and BIF-hosted rocks were derived from sedimentary protoliths. In the sediments, clay minerals were altered at a certain thermobarometric condition during metamorphism: Clay (illite) → chlorite → biotite → amphibole, which is evident from silicification, chloritization, biotitization, saussuritization and epidotization in petrographic results. Monazite chemistry U-Th-Pb geochronological evidence indicates the deposit was affected by a single, medium-grade metamorphic event that occurred at 1728±28 Ma during the Basement rock’s magmatic event activated. The metamorphic alteration was took place by the hydrothermal action or orogenic activity due to magmatism of Palaeoproterozoic basement rock occurred surrounding the areas. The temperatures condition and oxygen fugacities obtained from zircon and coexisting magnetite and ilmenite assemblages [677–692°C and 522–809°C (at 10-23.9 to 10-11.9 fo2), respectively] are partly compatible with known crystallization conditions of the dioritic basement rock in Bangladesh. Bangladesh J. Sci. Ind. Res. 58(4), 241-264, 2023
Integrated petrological, geochemical, isotopic and thermobarometric study of metasedimentary rocks from the amphibolite‐ to granulite‐facies transition zone of the Eastern Dharwar Craton (EDC), South India, has provided new insight into the evolution of the lower continental crust in this region. Phase equilibrium modelling of metapelites and metagreywackes suggests that they reached peak metamorphism at ~800–850°C and 6–7 kbar (corresponding to a paleodepth of ~20 km), with minor retrograde change occurring at ~700°C and 3–5 kbar during exhumation. U–Pb ages of conventionally separated zircon from metapelite samples range from 2.5 to 3.4 Ga, whereas garnet‐hosted zircons yield younger ages of 2.5–2.7 Ga. Zircon Th/U ratios and Hf isotopes reveal several significant pulses of zircon growth at 3.0, 2.95 and 2.7 Ga. Hf isotope data suggest the evolution of juvenile magma at around 3.2 Ga, while Hf model ages show that the crust building process also involved the recycling of pre‐existing Mesoarchean crust. Our study confirms the presence of a Paleoarchean component in the EDC lower crust, as well as older metamorphic events in the terrain and the gradational distribution of the metamorphic rocks.
Corona textures, characterized by specific mineral assemblages as layers between existing phases, are frequently observed in high-grade metamorphic rocks. The mechanisms of corona formation have been extensively investigated, however direct dating of corona phases has rarely to be attempted. In order to decipher the timing and duration of corona formation and its implications for metamorphic processes, an integrated garnet Lu–Hf and Sm–Nd and zircon U–Pb petrochronological study was conducted on a corona-bearing granulite from the Miyun region in the North China Craton. The Miyun region, located at the joint area of the Archean Eastern Block and the Paleoproterozoic Trans-North China Orogen, offers a distinctive opportunity to study polymetamorphic processes within the North China Craton. The sample shows a corona texture, primarily composed of garnet, minor clinopyroxene and quartz, that formed at the boundary between plagioclase and orthopyroxene. Pseudosection modelling for the effective bulk composition of reactive domains and conventional thermobarometric calculations indicate that the anhydrous corona assemblage (comprising clinopyroxene, garnet and quartz) formed at ~ 0.9 GPa and ~ 740 ℃. The subsequent hydrous retrogression took place at ~ 0.5 GPa and ~ 670 ℃ and led to the growth of amphibole and biotite. A Lu–Hf garnet date of 1864 ± 3 Ma is significantly younger than the U–Pb date of ca. 2454 Ma for metamorphic zircons in the sample, consistent with the disequilibrium partitioning of rare earth elements between the two phases. The well-preserved Lu zoning in garnet supports the interpretation that the Lu–Hf date represents the age of garnet growth and approximates the timing of formation of the anhydrous corona assemblage. Thus, the U–Pb age reflects a distinct Archean metamorphic event that has been widely documented in high-grade rocks across the Eastern Block of the North China Craton. The garnet Sm–Nd date of 1817 ± 6 Ma is indistinguishable from an amphibole-biotite Lu–Hf date of 1819 ± 16 Ma and ~ 47 Myr younger than the corresponding garnet Lu–Hf date. The younger Sm–Nd age represents the timing of cooling and post-dates retrogression. The Paleoproterozoic and Archean ages in the Miyun granulite align with the major tectonic events in the Trans-North China Orogen and the Eastern Block, respectively. This consistency suggests that the Miyun region likely experienced concurrent metamorphic events in both the Trans-North China Orogen and the Eastern Block. This texture-based Lu–Hf and Sm–Nd dating approach has wide applicability for rocks with explicit textures, facilitating precise extraction of temporal intervals in the metamorphic process.
The granulite facies Scourian rocks in the central region of the Lewisian complex of northwest Scotland play an important role in the understanding of the development of mid-Archean high-grade gneiss complexes. Whereas the later, largely Proterozoic history of the Scourian has been clarified in a detailed U-Pb study, unraveling the Archean ages of the protoliths using conventional isotope techniques has proved impossible. In order to advance our understanding of the deep crustal processes responsible for these granulites, it is essential to establish the age(s) of accretion of the protoliths and their subsequent tectonothermal history. A combined cathodoluminescence (CL) and sensitive high-mass resolution ion microprobe (SHRIMP) single-zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites. Use of CL allowed identification of relict oscillatory igneous zoning, metamorphic overgrowths, and irregular areas of recrystallization. From the SHRIMP data, an age of ca. 2960 Ma is inferred for the gneiss protoliths, which were altered considerably during an important metamorphic event ca. 2490 Ma.
The mid-Paleozoic, greenschist-grade Rockfish Valley Fault Zone (RVFZ) of central Virginia cuts the Grenville-aged Pedlar River Charnockite Suite (PRCS) and contains zircons that underwent brittle failure during ductile deformation. Electron microprobe analyses and scanning electron microscope (SEM) backscattered electron (BSE) imaging show that zircons from the protolith PRCS are concentrically zoned (with alternating U-Hf-rich and U-Hf-poor bands), and contain numerous radial microcracks. Zircons from the RVFZ mylonite are unzoned, fragmented, show no internal microfractures, and have low U and Hf concentrations relative to the PRCS zircons. U-Pb isotopic studies of zircons
from the mylonites and from the charnockitic protolith demonstrate that no preferential Pb loss occurred in the zircons from the mylonite, and that the 207Pb/20Pba ges of the mylonite zircons are identical to those of the protolith zircons. The loss of primary zoning from the zircons of the RVFZ ultramylonites can be explained by the physical removal of microfractured, U-rich, alpha-damaged zircon domains as the result of brittle failure and disaggregation during mylonitization. Mechanically resistant (low-U) portions of zircon grains tended to remain intact in the mylonite. Thus, it may not always be possible to predict whether or not the zircon U-Pb system has been disturbed by mylonitization by using physical criteria (e.g., grain size reduction, obliteration of primary zoning textures) alone. Evidently, fluids present during mylonitization accomplished the hydration of primary mineral assemblages, but did not chemically interact with zircons, and their primary U-Pb and Pb-Pb ages were preserved.
Granulite facies metamorphic events are constrained commonly through application of U-Pb zircon geochronometry. Zircon growth related to high-grade metamorphism is interpreted as reflecting the age of peak pressure-temperature (P-T) conditions. However, these ages obtained from granulites need to be interpreted with considerable care. Under conditions of high-grade metamorphism, it is important that the possible presence of melt is considered. Our modeling of partial melting and its impact on zircon stability implies that zircon crystallization in hot, isothermally uplifted granulites could postdate the pressure peak of the P-T path. In a case study of felsic granulites from the Bohemian massif of Variscan central Europe, it appears likely that most zircons in the rocks would have grown after they were exhumed to medium pressure levels. Thus, zircon growth related to high-grade metamorphism should not be automatically assumed as reflecting the age of peak P-T conditions.
The microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite-granulitic gneisses from the lower crust have been studied. Special interest in the effects of dynamic recrystallization and deformation fabrics has been taken. The plagioclase feldspars (An40-50) have undergone extensive ductile deformation by intracrystalline slip and twinning. They show strong crystallographic preferred orientation which was produced by dominant (010) [001] slip. Dynamic recrystallization is indicated by fine-grained new grains (neoblasts) interspersed with deformed relicts (porphyroclasts). The neoblasts have formed preferentially along grain boundaries, kink band boundaries (KBBs), intracrystalline shear zones, and around solid inclusions, such as garnet and pyroxene. Optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low strain. The orientation of neoblasts is closely related to the kinematic framework of flow (X-Y-Z), not to the original orientation of their host grains. The stable preferred orientation developed by dynamic recrystallization favours the easy glide of dislocations on the (010) [001] system. We suggest that recrystallization-accommodated dislocation creep is an important process in the development of preferred orientations in plagioclase feldspars, which will result in geometric softening.
Oscillatory zoning is a common feature in zircons from acid igneous rocks and is believed to form during crystallisation of zircons from a magma by a mechanism which is not yet understood. Many zircons with oscillatory zoning also show a patchwork replacement of zoned by unzoned zircon. The unzoned zircon occurs as rounded, transgressive patches distributed throughout the zoned zircon and as areas of transitional replacement where zoned zircon is progressively replaced by unzoned zircon such that only faint traces of original zones remain. This structure is interpreted as a progressive recrystallisation of the oscillatory zoned zircon made unstable by the incorporation of high concentrations of contaminant elements during magmatic crystallisation. Recrystallisation overprints oscillatory zones and appears to have occurred after completion of primary crystallisation. It is accompanied by loss of U, Th and Pb and the removal of oscillatory zones. The recrystallised unzoned zircon is extremely stable with respect to later Pb loss and tends to retain a concordant or slightly discordant U–Pb age. Recrystallisation provides a mechanism for resetting zircon U–Pb ages which is independent of the degree of radiation damage of the zircon lattice. This differs from other models of discordance which involve a leaching of radiogenic Pb as a consequence of a progressive breakdown of the zircon structure through time-integraded radiation damage further enhanced by high concentrations of trace-element contaminants. The U–Pb age of the unzoned zircon may date the recrystallisation event, which may be close to the age of primary crystallisation or reflect a later metamorphism.
Corundums from basalt fields, particularly in Australia and Asia, include a dominant blue-green-yellow zoned “magmatic” suite
(BGY suite) and subsidiary vari-coloured “metamorphic” suites. The BGY corundums have distinctive trace element contents (up
to 0.04 wt% Ga2O3 and low Cr/Ga and Ti/Ga ratios <1). Different melt origins for BGY corundums are considered here from their inclusion and
intergrowth mineralogy, petrologic associations and tectonic setting. Analysed primary inclusion minerals (over 100 inclusions)
cover typical feldspars, zircon and Nb-Ta oxides and also include hercynite-magnetite, gahnospinel, rutile-ilmenite solid
solution, calcic plagioclase, Ni-rich pyrrhotite, thorite and low-Si and Fe-rich glassy inclusions. This widens a previous
inclusion survey; New England, East Australia corundums contain the most diverse inclusion suite known from basalt fields
(20 phases). Zircon inclusion, intergrowth and megacryst rare earth element data show similar patterns, except for Eu which
shows variable depletion. Temperature estimates from magnetite exsolution, feldspar compositions and fluid inclusion homogenization
suggest that some corundums crystallized between 685–900 °C. Overlap of inclusion Nb, Ta oxide compositions with new comparative
data from niobium-yttrium-fluorine enriched granitic pegmatites favour a silicate melt origin for the corundums. The feasibility
of crystallizing corundum from low-volume initial melting of amphibole-bearing mantle assemblages was tested using the MELTS
program on amphibole-pyroxenite xenolith chemistry from basalts. Corundum appears in the calculations at 720–880 °C and 0.7–1.1 GPa
with residual feldspathic assemblages that match mineral compositions found in corundums and their related xenoliths. A model
that generates melts from amphibole-bearing lithospheric mantle during magmatic plume activity is proposed for BGY corundum
formation.
An Fe–1.06%V alloy was used to study the control of recrystallization through fine VN precipitates in cold-worked and nitrided ferritic matrix. Nitriding was carried out at 475C in ammonia atmosphere. Subsequent high temperature annealing process was performed in hydrogen gas for 795, 820, 860 and 880C, respectively. The data on recrystallization indicates that kinetics of recrystallization obeys an Avrami type equation with a temperature independent time exponent. Transmission electron microscopy techniques were used to measure the precipitate sizes and to study the changes in the microstructure. The activation energy evaluated for recrystallization was found to be consistent with that governing the VN particle coarsening. From these findings it was concluded that the initiation of recrystallization is dominated by the interface reaction controlled growth of VN stringers at subgrain boundaries. After the disappearance of stringers, the interaction of fine precipitates with subgrain boundaries is minimized and hence recrystallization starts. 1998 Chapman & Hall
The Proterozoic sequence in the central part of the Georgetown Inlier has been divided into the Etheridge, Langlovale, Croydon and Inorunie Groups. The Early to Middle Proterozoic Etheridge Group consists of shallow-water, fine-grained, clastic metasedimentary rocks, and minor basaltic lavas and related dolerite intrusions. It was deposited in a uniformly subsiding basin in a continental setting sometime between 2400 and 1570 Ma. It was first deformed at ∼ 1570 Ma (D1) and again at ∼ 1470 Ma (D2) by major folding events accompanied by low- to intermediate-pressure metamorphism, mainly in the greenschist and amphibolite facies, but ranging into the granulite facies in places during D2. The grade of metamorphism increases from west to east. Extensive S-type plutonic magmatism accompanied D2. In the west the Langlovale Group, a fluviatile to marine, pro-deltaic facies, post-dates D1, and was deposited before D2. It is moderately folded but not regionally metamorphosed. Post-orogenic magmatism (also S-type) is represented in the western part of the inlier by the subaerial felsic ignimbrite of the Croydon Volcanic Group and related subvolcanic granitoids. The volcanic rocks are overlaim by the fluviatile Inorunie Group which is of uncertain age. Neither the Croydon Volcanic Group nor the Inorunie Group is appreciably deformed or metamorphosed.
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.
Zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium (Zr). We have measured the concentration of Zr in the major minerals in granulite-facies rocks of differing bulk composition. Both garnet and hornblende contain tens of parts per million Zr. and no other major phase contains significant Zr. Simple calculations show that reaction of either garnet or hornblende to form non-Zr-bearing phases will release sufficient Zr to account for at least some new zircon growth. L-Pb ages from new zircon, grown as a result of either hornblende or garnet breakdown, are not expected to record the time of peak metamorphism, but rather will record the time of particular metamorphic reactions, allowing direct correlation of zircon ages with petrologically derived pressure-temperature-time paths. This approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible.
U-Pb zircon SHRIMP ion-microprobe ages are presented for five Proterozoic igneous rocks from the Georgetown Province of northeastern Queensland. Four of these are interpreted as the igneous crystallisation ages of the Croydon Volcanic Group (1548 ± 18 Ma), Lighthouse Granite (1561 ± 10 Ma), Digger Creek Granite (1554 ± 10 Ma), and Mount Hogan Granite (1549 ± 25 Ma. The U-Pb zircon emplacement ages are internally consistent in constraining the age of a prograde, intense tectonothermal event (D2) in the province to 1554 ± 10 Ma. -from Authors
We attempted to obtain the magmatic crystallization age of a metamorphosed (granulite facies, T = 800 ± 90°C; P = 8 ± 1 kbar) massif-type anorthosite from the Ankafotia body of southwest Madagascar. The sample studied is a coarse-grained leuconorite with good preservation of igneous texture and mineralogy, although plagioclase, which contains abundant rutile and zircon inclusions, has been slightly recrystallized. Thirty three isotope dilution U-Pb analyses of zircons representing single-grain fragments (29 analyses) and multi-grain fractions (4 analyses) yield a spectrum of concordant ages from 631 to 549 Ma, a time span of more than 80 myr. Back-scattered electron and cathodoluminescence images show that most grains are either homogeneous, structureless fragments (35%), or are permeated to a variable degree by anastamosing cracks occupied by relatively U- and/or Th-enriched zircon (45%); a smaller percentage of grains show relict magmatic zoning (20%). Thin, U- and/or Th-rich overgrowths occur on about 25% of grains. Raman spot analyses demonstrate that all fragments are highly crystalline and non-metamict. There are marked correlations between zircon grain size and internal features, such that the oldest grains are larger, and show relict magmatic zoning; the youngest grains are small fragments containing high-U crack networks. Ion microprobe spot analyses show that each zircon grain preserves a distinct trace element signature; rare earth element patterns show heavy REE-enrichment, with negative Eu anomalies and positive Ce anomalies. We suggest that the ca. 80 myr spread in concordant U-Pb ages in this sample is indicative of high-temperature Pb-loss during one or more protracted periods of granulite facies metamorphism, with only minor episodic or continuous metamorphic zircon growth. Volume diffusion and/or fracture-assisted diffusion seems to be the dominant mechanism of Pb-loss. Cooling curves, calculated using recently-measured Pb diffusion parameters, conform to the age-size relationship, and imply very slow cooling rates (1–2°C/myr or less), as might be expected for a terrane in which granulite conditions were maintained for an extended period of time. Our results, therefore, suggest a note of caution for interpretation of concordant zircon ages in meta-igneous rocks affected by high-grade metamorphism of long duration.
Diffusion rates for three rare-earth elements (REEs: Sm, Dy, Yb) have been measured in synthetic and natural zircon. REE-phosphate powders were used as the source of diffusant, with Rutherford backscattering spectrometry (RBS) used to measure REE depth profiles.Over the temperature range 1150–1400°C, the following Arrhenius relations were obtained (diffusion coefficients in m2 s−1 ): Results for synthetic: and natural zircons were quite similar, and no evidence of significant anisotropy was observed when comparing transport normal and parallel to the c-axis.The data show a systematic increase in diffusivity with decreasing ionic radius (i.e. faster diffusion rates for the heavier REEs). Given these trends the diffusion rates of Lu and La should differ by over two orders of magnitude. Diffusive fractionation is unlikely in the Sm-Nd system because differences in diffusivities are relatively small, but may be a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations.The very slow diffusion rates measured for the REEs suggest that they are essentially immobile under most geologic conditions, thus permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited cores.
We report UPb and ages for multigrain fractions, single grains, and grain domains in zircons from highgrade igneous and sedimentary rocks of the Vavuniya Charnockite Province (VCP) in northern Sri Lanka, using the ionmicroprobe SHRIMP, a new vapour digestion technique (VDT) and evaporation. Charnockitic, enderbitic and mangeritic gneisses are derived from I-type tonalitic, syenitic, granodioritic and granitic precursors for which the UPb systematics of zircons as determined by SHRIMP suggest crystallization ages between ∼ 1000 and ∼ 1100 Ma. Many translucent, round and multifacetted zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrystallization. The large majority of zircons exhibit severe and variable Pb-loss some 540–580 Ma ago which we ascribe to granulite-grade metamorphism, almost immediately followed by local retrogression. Most of the zircons lost Pb from the entire grain with no domains remaining that reflect the original Pb isotopic composition. It was therefore impossible to obtain meaningful evaporation ages for such grains, and this cautions against use of this method in zircons that experienced severe Pb-loss. However, zircons formed during high-grade metamorphism apparently retain their primary Pb isotopic composition and were dated successfully by the evaporation technique, yielding ages between 553 ± 26 and 560 ± 30 Ma. Conventional multigrain analysis and small fractions following VDT for two samples, although much more precise than the SHRIMP data, yielded only strongly discordant data points with patterns reflecting variable Pb-loss. These data alone would not have made it possible to obtain reliable primary zircon crystallization ages.
Zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium (Zr). We have measured the concentration of Zr in the major minerals in granulite-facies rocks of differing bulk composition. Both garnet and hornblende contain tens of parts per million Zr, and no other major phase contains significant Zr. Simple calculations show that reaction of either garnet or hornblende to form non Zr-bearing phases will release sufficient Zr to account for at least some new zircon growth. U-Pb ages from new zircon, grown as a result of either hornblende or garnet breakdown, are not expected to record the time of peak metamorphism, but rather will record the time of particular metamorphic reactions, allowing direct correlation of zircon ages with petrologically derived pressure-temperature-time paths. This approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible.
The Einasleigh Metamorphics are the lowest exposed component of the Etheridge Group, part of the Etheridge Province, in the Georgetown region of north Queensland. Previous dating of granites has imposed a younger age limit of about 1550 Ma for the depositional age of the Etheridge Group. Based on SHRIMP U‐Pb analyses of zircon from mafic (1674.9 ±3.3 Ma and 1655.9 ±2.2 Ma) and felsic (1695.8 ± 1.5 Ma and 1684.2 ± 2.1 Ma) intrusive rocks in the Einasleigh Metamorphics, it can now be demonstrated that deposition commenced at about 1700 Ma, and continued for an estimated 100 million years. The stratabound, base‐metal deposits in the Mt Isa Inlier, Broken Hill Block and McArthur Basin formed in the early part of this time span, thereby suggesting potential for these styles of mineralisation in the Georgetown region.
A series of natural zircon samples representing various degrees of metamictization were investigated by Raman microprobe (RMP) analysis. Systematic changes in wavenumbers and half-widths of the Raman bands were found, caused by increasing irregularities of bond-lengths and bond-angles and a general breaking-up of the structure as a result of metamictization. Therefore, Raman spectroscopy can be used to measure the crystallinity of zircons. The Raman parameters seem to be almost uninfluenced by chemical variations. -from Authors
Primary and secondary internal structures observed in zircons from granites from the Late Archaean Darling Range Batholith record stages in the evolutionary history of the granites and provide a basis for a SHRIMP U-Pb study on the timing of granite evolution. Many granite zircons contain cores. A few low-U cores retain concordant to slightly discordant U-Pb ages of ∼2·8 Ga, some show discordant intermediate ages, but most have nearly concordant ages within the range 2690-2650 Ma. These ages are interpreted as dating the source rocks of the Darling Range granites, or as representing different degrees of isoiopic resetting owing to reciystallization of protolith zircon during prograde metamorphism and subsequent melting. The zircon cores are enclosed by inner rims of oscillatory zoned zircon, which are interpreted as zircon growth during the main crystallization phase of the granite magma. SHRIMP ages of zoned zircon, of between 2648 and 2626 Ma, suggest an extended period of granite crystallization. The oscillatory zoned inner rims are surrounded by weakly zoned to unzoned outer rims which transgress primary zoning of the inner rim, suggesting corrosion followed by new zircon deposition. However, the preservation of weak zoning in the outer rim and the euhedral nature of external zircon faces, which are identical to those developed in the inner rim, suggests that the outer nms are in fact recrystallized outer parts of inner rims. This conclusion is supported by the younger ages (2628-2616 Ma) determined for outer rims. These results indicate that formation of outer rims and accompanying loss of radiogenic Pb occurred dunng or soon after granite crystallization and before zircons had time to accumulate significant radiation damage, suggesting that the recrystallization process is independent of the degree of metamictization. The history of formation of the Darling Range granites contained within the zircon crystals suggests initial magma formation between 2690 and 2650 Ma, crystallization and emplacement of the granite magma at 2648-2626 Ma, and slow cooling, indicated by marginal recrystallization and continued Pb loss from the zircons, until 2628-2616 Ma.
Compositional and textural characteristics of the accessory minerals monazite, xenotime and zircon in the Sweetwater Wash granites and related aplites indicate that these phases not only controlled much of the trace-element geochemistry of the suite, but that they also record the melt compositional changes that occurred during magmatic differentiation. Fractionation of monazite due to decreasing saturation levels of its essential structural constituents with falling temperature was likely responsible for an ongoing trend of light rare-earth element (REE) depletion. This was accompanied by increasing heavy-REE concentrations until xenotime joined the crystallizing assemblage; subsequently, combined monazite-xenotime fractionation resulted in lowering of the entire REE budget. Zircon, which contains inherited cores that were apparently resorbed and rounded during initial anatexis, was saturated throughout the differentiation history of the Sweetwater Wash suite.
The Tibetan Slab gneisses are currently considered as the source for the High-Himalayan leucogranites. The rare-earth element distributions in zircons from the Tibetan Slab migmatites and the Manaslu leucogranite (Nepal Himalaya), were investigated by in situ ion probe analysis. These data combined with textural information have been used to elucidate the zircon growth conditions and, indirectly, the processes involved in incipient anatexis and evolution of granitic magmas.In the migmatites, the zircons from gneisses and melanosomes have rounded shapes and variable REE patterns with low Yb contents (145–700 ppm) and chondrite-normalized (Yb/Sm)N ratios (≤81). Zircons from low-Zr tonalitic leucosomes are morphologically and chemically indistinguishable from those of the gneisses and melanosomes. Zircons from the high-Zr tonalitic leucosomes and granitic leucosomes are euhedral and show higher Yb contents (409–2820 prim) and (Yb/SM)N ratios (≥145) than those of the gneisses and melanosomes. The euhedral shapes and distinctive REE patterns of zircons from the high-Zr leucosomes and granitic leucosomes are consistent with crystallization from a melt, whereas the morphological and chemical similarities of zircons from the low-Zr leucosomes with those from the gneisses and melanosomes suggest inheritance without significant chemical change.In the Manaslu granite, zircons have rounded cores with REE patterns distinct from those of the rims (e.g., 250 ppm ≤Yb≤710 ppm in the core, 965 ppm ≤Yb≤ 2775 ppm in the rim) but comparable to those from the Tibetan Slab gneisses suggesting inheritance. The rim compositions, however, are distinct from those of either zircon types of the Tibetan Slab leucosomes, indicating that the leucosomes cannot be the unsegregated equivalent to the Manaslu granite parental magma. Comparison of the rim compositions with fractional crystallization models suggests that the range in zircon Sm and Yb contents are consistent with zircon crystallization from a monazite-saturated, xenotime-undersaturated melt. The Yb contents in the different zircons studied, and their variation within a single zircon, further suggest boundary-layer effects and magma compositional heterogeneity, in agreement with previous models which considered that the Manaslu granite resulted from the aggregation of magma batches.
The occurrence of sector zoning in minerals of regional metamorphic or low-temperature intrusive origin implies that rapid growth is not required for the development of this particular form of homogeneous disequilibrium. It is shown here that sector zoning can be a natural consequence simply of slow lattice diffusion. Known diffusivities of rare earth elements in zircon and titanite lead to the expectation of sector zoning in these minerals with respect to REEs for reasonable geologic growth rates. Even in the case of clinopyroxene, diffusion of REEs and high field-strength elements may be slow enough to contribute to the development of sector zoning in laboratory-grown crystals. -from Authors
The distribution of U, Y, P, and Ca was studied with an electron microprobe in detrital zircons from paragneisses of amphibolite to granulite facies as well as in zircons formed in their present-day host rocks, such as a granite, anatectic and aplitic gneisses and a migmatite from the Southern Alps. Detrital paragneiss zircons have low trace element contents, whereas zircons formed in their present-day host rocks have elevated trace element contents. According to the isotopic U-Pb age pattern one may distinguish two types of zircon populations formed in their present-day host rocks: one which lost lead without any known exogene cause, and one the U-Pb system of which was only opened in response to a geological event. Different modes of incorporation of the trace elements in the zircon crystals are thought to be responsible for this different behaviour and are discussed.
The relative growth rates of zircon crystal faces are recorded by growth zoning, which is recognizable in cathodoluminescence photographs of oriented crystal sections. The kinematics of zircon growth is graphically presented by the slopes of Grabahnen between crystal sectors. The relative velocities of advance of crystal faces correlate to the pattern of growth zoning. Widely spaced zoning, interrupted by surfaces of dissolution, on the one hand, and narrowly spaced uninterrupted oscillatory zoning, on the other hand, are interpreted as markers of low and high zircon-supersaturation of the melt, respectively. The following model is deduced from this correlation. The prism {110} is that crystal form, the growth rate of which reacts most sensitively to the zircon-supersaturation of the melt. The growthrate of the steep pyramid {211} is not primarily controlled by zircon-supersaturation. Instead, the growth of {211} is delayed by the adsorption of foreign atoms on its faces. The kinematics of zircon growth in anatectic, plutonic and meta-rhyolitic host rocks in the pre-Mesozoic basement of the Tauern Window (Eastern Alps) is explained by this model. Results show that the kinematics of zircon growth carry a much greater petrogenetic significance than has been previously suggested for the final shape of crystals alone.
Ancient crustal rocks provide the only direct evidence for the processes and products of early Earth differentiation. SHRIMP
zircon U-Th-Pb dating has identified, amongst the Acasta gneisses of the western Slave Province, Canada, two metatonalites
and a metagranodiorite that have igneous ages of 4002 ± 4, 4012 ± 6 and 4031 ± 3 Ga respectively. These are the first identified
Priscoan terrestrial rocks. A record of metamorphic events at ∼3.75, ∼3.6 and ∼1.7 Ga also is preserved. These discoveries
approximately double, to ∼40 km2, the area over which ∼4.0 Ga gneisses are known to occur. A single older zircon core in one sample suggests that rocks as
old as 4.06 Ga might yet be found in the region. As early as 4.03 Ga, terrestrial differentiation was already producing tonalitic
magmas, probably by partial melting of pre-existing, less differentiated crust.
Ion microprobe U-Th-Pb analyses of residual cores and metamorphic mantles of zircons from three high grade paragneisses from the Seve Nappe Complex, north-western Sweden, show that: 1) The sediments comprising the protolith of the Seve Nappes gneisses over a distance of 250 km were probably derived from similarly-aged source terranes. 2) Those source terranes were dominated by rocks with ages in the range 1400 to 1730 Ma, with minor components at least as young as 1000 Ma. The oldest component identified is 173012 Ma old. 3) Those rocks had U and Th contents normal for felsic igneous rocks. 4) The gneiss protolith was metamorphosed to granulite grade during the Caledonian. There is evidence to suggest that the peak of metamorphism may not have been synchronous throughout the Nappes. 5) The metamorphism probably included a reduction of about a factor of ten in the gneisses' Th/U.
Zircons from a metasedimentary and a meta-igneous quartz-feldspar granulite from the Val Sesia and Val Mastallone area of
the Ivrea Zone (Southern Alps) differ in their response to granulite facies metamorphism with respect to crystal morphology
and U/Pb ages. Detrital zircons in the metasediment developed an isometric overgrowth by the addition of Zr derived mainly
from co-existing minerals, most probably biotite, decomposing during anatectic melting. The overgrowth started in the pelitic
layer of the metasediment in the Late Carboniferous at approximately 296 Ma, significantly earlier than in the adjacent psammitic
layer where it started only at 261 ± 4 Ma (95% confidence level). These ages are ascribed to the differential initiation of
anatexis in the two layers. The delay of melting in the psammitic layer was probably due to the different position and less
steep slope of its solidus in P-T-space, as compared to the solidus in the pelitic layer. Accordingly, the melting in the psammitic layer at 261 Ma was initiated
by a thermal pulse and/or by a decompression event. Decompression melting is supported by a characteristic shell morphology
of the zircon overgrowth in the psammitic layer, which might have grown under shear movements during high-temperature extensional
faulting. The typically magmatic zircon population of the meta-igneous granulite crystallized at 355 ± 6 Ma (95% confidence
level). The morphology of the zircons and the chemistry of the rock suggest that the magma was calcalkaline. A minor subpopulation
of crystals is morphologically similar to the zircons in the pelitic layer of the metasediment. This points to the admixture
of a minor sediment component and, thus, to a largely volcaniclastic origin of the protolith. In contrast to the detrital
zircons in the metasediment, the magmatic zircons show rare and little overgrowth and, instead, have been strongly resorbed
by anatectic melt. In addition, they became partially recrystallized and the rejuvenated ages from the most thoroughly recrystallized
domains indicate that the rock was subject to prograde metamorphism after 279 Ma. This may correspond to the regional temperature
increase prior to the climax of metamorphism or to a local thermal pulse due to nearby mafic intrusions. An Upper Triassic
event at 226 ± 5 Ma is reflected by distinct peripheral zones in the overgrowths of some zircons in the metapelite. These
are interpreted as a second metamorphic pulse, possibly induced by the infiltration of fluids.
Internal structures in zircons from granitoids from the late Archaean Darling Range Batholith show secondary features revealed
by HF etching, which record reconstitution of the zircons and modification of the distribution of trace elements during post
crystallisation cooling of the granitoid. Zircons from the granites commonly contain unzoned to weakly zoned cores surrounded
by rims showing oscillatory zoning which has been modified by recrystallisation. The most striking feature is the development
of high trace element concentration areas found in zircons from a number of granites. These structures range from enhanced
trace element concentrations in primary zones to a single accumulation of most trace elements in one band, about half way
between the outer edge and the centre of the zircon. In any zircon the extent of the concentration of trace elements towards
the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory
zones in the outer rim. This suggests that the trace element bands formed by migration of trace elements from the outer primary
zones to new concentration sites on an inner set of primary zones. This explanation is supported by the formation of multiple
curved trace element bands that transgress primary zoning and the determination of younger SHRIMP ages on depleted zircon
outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres. Also observed in some granite
zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs
throughout the zircon. This structure is explained in terms of secondary migration and reconcentration of trace elements in
curved bands. All structures can be transgressed by generally rounded lobes and patches of low U, weakly nebulously zoned
zircon. This is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation
of the granitoid, resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements
from the zircon.
Ion microprobe U-Th-Pb analyses of zircons from a granulite-grade orthogneiss from Mount Sones, Enderby Land, Antarctica, record the ages of four principal events in the history of the gneiss, three of which already have been recognized through previous isotopic dating of other samples. The structure of the zircons indicates at least four different stages of growth. The several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate U-Pb discordance patterns for each stage. The stratigraphically oldest zircon (rare discrete cores) is indistinguishable in age from the most common, euhedrally zoned zircon. Both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 392710 Ma ago, making the orthogneiss currently the oldest known terrestrial rock. The outer parts of most grains and some whole grains recrystallized at 294831/–17 Ma, during or immediately after possibly 100 Ma of high granulite grade metamorphism. The recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the Napier Complex at 1000 Ma. In the intervening time, at 247923 Ma, the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic Pb which left most of the crystals with radiogenic Pb deficiencies, but produced local radiogenic Pb excesses in others. A new generation of zircon, characterized by very high Th/U and low U, grew at that time. That event — deformation and possibly a minor rise in temperature — produced widespread perturbations of other isotopic systems throughout the Napier Complex.
Zircon and monazite from granulite- to amphibolite-facies rocks of the Vosges mountains (central Variscan Belt, eastern France)
were dated by ion-microprobe and conventional U-Pb techniques. Different granulites of igneous (so-called leptynites) and
sedimentary origin (kinzigites) and their leucosomes were dated at 334.9 ± 3.6, 335.4 ± 3.6 and 336.7 ± 3.5 Ma (conventional
age 335.4 ± 0.6 Ma). Subsequent growth stages of zircon were distinguished by secondary electron (SEM) and cathodoluminescence
(CL) imaging: (1) subsolidus growth producing round anhedral morphologies and sector zoning; (2) appearance of an intergranular
fluid or melt phase at incipient dehydration melting that first resulted in resorption of pre-existing zircons, followed by
growth of acicular zircons or overgrowths on round zircons consisting of planar growth zoning; (3) advanced melting producing
euhedral prismatic zircons with oscillatory zoning overgrowing the sector zones. Two further lithologies, the Kaysersberg
granite and the Trois-Epis units, were both formerly considered as migmatites. The intrusion of the Kaysersberg granite was
dated at 325.8 ± 4.8 Ma. The Trois-Epis unit was found to be the product of volume recrystallization of a former granulite,
which occurred under amphibolite-facies conditions 327.9 ± 4.4 Ma ago. The amphibolite-facies overprint of the Trois-Epis
zircons led to the complete rejuvenation of most of the zircon domains by annealing and replacement/recrystallization processes.
Annealing is assumed to occur in strained lattice domains, which are possibly disturbed by high trace element contents and/or
large differences in decay damage between adjacent growth zones. Investigation of cathodoluminescence structures reveals that
the replacement occurs along curved chemical reaction fronts that proceed from the surface towards the interior of the zircon.
The monazite U-Pb system still records the age of high-grade metamorphism at around 335 Ma. The chemical reagent responsible
for the rejuvenation of zircon obviously left the monazite unaffected.
Diffusion rates for the three tetravalent cations U, Th and Hf have been measured in synthetic zircon. Diffusant sources
included oxide powders and ground pre-synthesized silicates. Rutherford backscattering spectrometry (RBS) was used to measure
depth profiles. Over the temperature range 1400–1650 °C, the following Arrhenius relations were obtained (diffusion coefficients
in m2sec−1):
log D
Th = (1.936 ± 0.9820) + (− 792 ± 34 kJ mol−1 /2.303 RT)
log D
U = (0.212 ± 2.440) + (− 726 ± 83 kJ mol−1 /2.303 RT)
log D
Hf = (3.206 ± 1.592) + (− 812 ± 54 kJ mol−1 /2.303 RT)
The data show a systematic increase in diffusivity with decreasing ionic radius (i.e., faster diffusion rates for Hf than
for U or Th), a trend also observed in our earlier study of rare earth diffusion in zircon. Diffusive fractionation may be
a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations when compared with the trivalent
rare earths. The very slow diffusion rates measured for these tetravalent cations suggest that they are essentially immobile
under most geologic conditions, permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited
cores.
Zircon exhibits an extraordinary memory. Its stability, durability, low solubility and low elemental diffusivities combine to preserve in it a record of most of the important events that have affected it, its host rocks, and the crust of which it is a part. Zonation in zircon grains delineates the boundaries of discrete geochemical packages formed at different times, each effectively a closed system. The elemental and isotopic compositions of these packages reflect the timing and conditions of growth events, and the morphology of the zonation indicates qualitatively the nature of both growth and intervening degradation events.Cathodoluminescence (CL) and backscattered electron (BSE) imaging reveals detailed zonation patterns that are commonly invisible or barely visible with conventional transmitted and reflected light microscopy. Characteristic patterns are visible in almost all zircons that serve to distinguish igneous from metamorphic growth, to distinguish truncation surfaces of different types (e.g., sedimentary fracturing vs. resorption), and possibly to identify ancient metamictization. Zircons from many rocks record multistage histories that reflect two or more events; those from rocks such as peraluminous granites and high-grade paragneisses are especially likely to reveal long and complex histories.Studies of zonation patterns in zircons provide a clear, though qualitative, history of a rock and its heritage. Furthermore, they provide the basis for a quantification of that history. Elemental and isotopic compositions can reveal the environment in which a zone grew. U-Pb analysis of a zone provides an age for its growth. shrimp analyses that are not guided by detailed knowledge of zonation can straddle two (or more) zones and a discordant U-Pb result from such an analysis may falsely suggest Pb loss, and important growth zones may be missed entirely. Thus, the combined use of CL, BSE, electron microprobe and ion probe methods can elucidate complex crustal histories.
The incorporation of Pb into zircons grown from Pb-rich solutions was evaluated using three different approaches: (1) high-temperature growth of large crystals from Pb-silicate melts; (2) hydrothermal overplating of thin epitaxial layers on substrates of natural zircon; and (3) growth of small, homogeneously nucleated crystals from aqueous fluids. The melt-grown zircons (50–400 μm) were crystallized from PbOSiO2ZrO2 (±P2O5) liquid at atmospheric pressure by cooling from 1430° to 1350°C. In the P2O5 free system, despite 66 wt% PbO in the melt, these zircons contain < 1 ppm Pb, yielding an apparent crystal/melt partition coefficient (DPb) for Pb2+ of 7 × 10−7. Addition of ∼ 5 wt% P2O5 to the melt results in uptake not only of P ( ∼ 3400 ppm) in the zircons but also Pb (∼ 1500 ppm), increasing the apparent DPb to about 10−3. Hydrothermal overplating of ZrSiO4 was carried out at 1.5 GPa in a piston-cylinder apparatus by slow cooling from 500°C or 550°C to 140°C of polished slabs of natural zircon immersed in zircon-saturated aqueous solutions containing either PbO2 or PbO + P2O5. In both cases, the resulting epitaxial layers of ZrSiO4 (∼ 60 nm thick) contain > 3 atom% Pb, with apparent zircon/fluid partition coefficients of 4.2 and 2.6, respectively, for Pb4+ and Pb2+. In contrast to the case of melt-grown zircons, available P is excluded from the aqueous epitaxial zircon, suggesting that charge balance is accomplished by H+ instead. Small (2–5 μm) zircons grown by cooling aqueous solutions (PbO + SiO2 + ZrO2 ± P2O5) from 800°C or 900°C contain ∼ 0.25–0.5 atom% Pb (∼ 2–4 wt% PbO), yielding apparent DPb values of ∼ 0.2–0.3. Available P5+ is incorporated in a 2:1 ratio with Pb2+, suggesting a specific charge-balance mechanism: [2P5+ + Pb2+] = [2Si4+ + Zr4+]. However, Pb enters the zircon even when P is unavailable, so H+ may again play a charge-balancing role.
Detrital zircons with 207Pb/206Pb ages between 3908 and 4270 Ma from the Narryer Gneiss Complex, Western Australia, are the oldest terrestrial minerals found to date. They occur in small proportions (2–3%) together with 3.0–3.75 Ga zircons in quartzites and metaconglomerates of the 3.0 Ga Mt. Narryer (MN) and Jack Hills (JH) metasedimentary belts. We report the results of a geochronological and geochemical study of these zircons which, together with data on grain morphology and inclusion mineralogy, are used in an attempt to place constraints on their source rocks.
New SHRIMP and TIMS zircon ages, 40Ar/39Ar ages, and eclogite locations contribute significantly to our understanding of the ultrahigh-pressure Dabie Shan. (1) The geographic extent of the Yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the Dabie Shan. (2) The northern half of the Dabie Shan is a magmatic complex, intruded over a 10-Myr interval between 137 and 126 Ma, that accommodated ∼100% N–S stretching of the pre-existing collisional architecture. (3) Granitic orthogneisses and enclosing ultrahigh-pressure paragneisses have indistinguishable zircon populations. The population of Triassic zircon ages ranges from ∼219 to ∼245 Ma, leading us to question the prevailing assumption that 219 Ma zircons formed at ultrahigh pressure, and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation.
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438 P. W. O. HOSKIN & L. P. BLACK Hoskin, P. W. O., Kinny, P. D., Wyborn, D. & Chappell, B. W., D. Robinson is acknowledged for efficient editorial 2000. Identifying accessory mineral saturation during differen-advice and assistance. L.P.B. publishes with the tiation in granitoid magmas: an integrated approach. Journal permission of the Executive Director, Australian of Petrology, in press. Geological Survey.