Ga Metamorphic Zircons Research Articles

Assembling the Proto-Yangtze Block in the late Paleoproterozoic: Constraints from a refined Magmatic-Metamorphic framework of the Cuoke Complex, SW China

The Archean-Paleoproterozoic records of the southwestern and northern Yangtze are key components in establishing the early history of the Yangtze Block as a whole. To date, knowledge of the late Paleoproterozoic evolutionary history of the Yangtze Block has been largely derived from its northern margin. This study documents the presence of ca. 1.93 Ga metamorphic zircons and 1.89–1.86 Ga metamorphic-hydrothermal reset zircons from igneous rocks in the Cuoke Complex of southwestern Yangtze, as well as ca. 1.93 Ga and 1.86 Ga xenotime growths in a meta-volcaniclastic rock (tuff). In addition, the origin of 2.0–1.85 Ga zircons from magmatic rocks in both the Cuoke and Phan Si Pan complexes of the southwestern Yangtze Block are reassessed. Based on new and published CL images, U contents and REE patterns, we argue that the 2.0–1.85 Ga ages are reset from primary 2.3 Ga magmatic zircons during a widespread metamorphic-hydrothermal overprinting event. Integrated U-Pb data reveal two metamorphic peaks at 2.0–1.95 Ga and ca. 1.85 Ga in both northern and southwestern Yangtze, indicating the two domains came together at 2.0–1.95 Ga to form the proto-Yangtze Block. The geological records of northern and southwestern Yangtze match well with those of northern Laurentia and Siberia, respectively, suggesting an internal location for the proto-Yangtze Block within Nuna. This contrasts with its inferred external location within the Rodinia supercontinent, suggesting the transition between the two Proterozoic supercontinents involved significant reconfiguration of at least some of the constituent core continental blocks.

Using multiple methods to better constrain the depositional age of a meta-sedimentary succession: An example of the Laoling Group from the northeastern North China Craton

The youngest detrital zircon (DZ) or groups of zircons age is usually interpreted as the maximum depositional age (MDA) and the oldest metamorphic zircon since deposition is used to resolve the minimum depositional age of a meta-sedimentary succession. However, DZ grains with multi-cycle or fingerprinting proximal sources may cause the MDAs older than the true depositional age, whereas post-depositional resetting has the opposite effect. Here, we present DZ U-Pb, Lu-Hf isotopic and trace elemental data, supplemented by whole-rock geochemistry, from the greenschist-facies metamorphosed Laoling Group consisting of clastic and carbonate rocks in the northern margin of the Jiao-Liao-Ji belt (JLJB) in the North China Craton to investigate the use of multiple methods to constrain its true depositional age. The dominant late Neoarchean and middle Paleoproterozoic DZ age peaks from the meta-sandstones, meta-siltstone, meta-mudstone, and meta-carbonate rocks are interpreted to be sourced from the Southern Jilin Complex and the JLJB, respectively. The low Th/U ratios (<0.18) and Ce/Nd ratios (<2.5) of the 1.96–1.77 Ga metamorphic zircons, which is consistent with the monazite ages of 1.95–1.76 Ga, are used to filter the possibly altered detrital magmatic zircons and the source-normalized α-dose metrics bigger than ca. 0.8 imply a predominant first-cycle origin, therefore the true depositional age of the group is constrained at 2.08–2.03 Ga. Additionally, in consideration of the strike of the basin parallel to the boundary between the JLJB and the Longgang Block, two periods of regional extensional tectonism, bimodal provenance, and depositional age coeval with the second magmatic event in the JLJB, we suggest that the Laoling Group most likely deposited in a back-arc basin in the northern margin of the JLJB. Finally, the temporal changes of zircon Hf isotope and Eu/Eu* in zircon crustal thickness proxy, especially the coeval excursion towards more radiogenic Hf signals and crustal thinning at 2.17–2.13 Ga, is consistent with the back-arc basin opening and closure model rather than the intra-continental rift model.

P-T-T TRENDS OF METAMORPHIC TRANSFORMATION OF ECLOGITES IN THE GRIDINO COMPLEX OF THE BELOMORIAN PROVINCE (STOLBIKHA ISLAND AND SAMYLINO AREA)

Eclogites in the Gridino complex of the Belomorian province have complex history of metamorphic transformations. Their polymetamorphism is revealed from relics of early and late high-pressure mineral assemblages (omphacite+garnet), reaction microstructures, changes of the garnet chemical zoning, results of thermobarometric calculations, and isotope-geochronological studies. Geochronological data indicate two stages of eclogitic metamorphism in the Gridino complex at the Archean and Paleoproterozoic. As a result of the study, we obtained two P-T-t trends for the regressive stage of rock evolution with a successive transition from eclogitic facies to granulite (amphibolite). The peak of the first metamorphic event responds to metamorphic zircons with an age of 2,7 Ga. The maximal values of the P-T conditions of the second metamorphic event are reliably correlated with 1,9 Ga metamorphic zircons.

Decoupling of metamorphic zircon U-Pb ages and P-T paths in the Dunhuang metamorphic complex, northwestern China

U-Pb dating of metamorphic zircon is commonly used to determine the timing of high-grade metamorphism, but linking zircon U-Pb ages to metamorphic P-T paths can be challenging, especially in metamorphic terranes where multiple metamorphic events result in various degrees of re-equilibration and complex metamorphic zircon growth/recrystallization. Here we carried out a detailed P-T path study, as well as zircon/titanite/apatite U-Pb dating and mineral Sm-Nd isochron dating, for garnet amphibolites from the Dunhuang Block, northwestern China, where late Paleoproterozoic metamorphism was extensively overprinted by a Paleozoic tectonothermal event and the P-T conditions of the earlier event remain elusive. Petrographic observation, phase equilibria modeling and geothermobarometry show that the garnet amphibolites share a clockwise P-T path, from 675 – 689 °C/10.0 – 11.6 kbar to 780 – 807 °C/9.9 – 11.8 kbar for the prograde to peak metamorphism, followed by significant cooling and decompression to 612 – 689 °C/4.1 – 6.0 kbar, indicating an orogenic setting. U-Pb dating of metamorphic zircons yields two groups of upper intercept ages of ca. 1.82 – 1.85 and 1.95 Ga. The ca. 1.82 – 1.85 Ga metamorphic zircons have flat HREE patterns and weak Eu* anomalies, suggesting zircon growth when abundant garnet but little plagioclase was present, consistent with high-pressure metamorphism. In contrast, the ca. 1.95 Ga zircons have steep HREE and negative Eu anomalies, suggesting formation at relatively low-pressures. These zircon results provide evidence for 1.8 – 1.9 Ga metamorphism, possibly due to variable crustal thickening during a late Paleoproterozoic orogenic event. However, garnet porphyroblasts and the matrix minerals yield a Sm-Nd isochron age of 333.5 ± 4.0 Ma, whereas metamorphic titanites record a U-Pb age of 345.4 ± 2.7 Ma. Considering the high closure temperatures of the garnet Sm-Nd (650 – 720℃) and titanite U-Pb (710 – 780℃) systems, our data suggest that the late Paleoproterozoic metamorphic mineral assemblages were likely completely re-equilibrated by a Paleozoic tectonothermal event and that the P-T path documented above is a result of Paleozoic orogeny. In addition, apatite U-Pb dating yields 236.6 ± 4.3 and 300 ± 10 Ma for samples from the southern and central Dunhuang area, respectively, indicating different cooling histories. Combining with previous results, we propose that the Paleozoic metamorphism occurred in a long-lived hot continental arc with slow cooling rates (0.3 – 5℃/Myr), rather than an subduction–accretionary complex. Our study highlights the decoupling of metamorphic zircon U-Pb ages and P-T paths, which has significant implications for P-T-t path studies in polyphase metamorphic terranes.

Comment on Volodichev et al. Archean Zircons with Omphacite Inclusions from Eclogites of the Belomorian Province, Fennoscandian Shield: The First Finding. Minerals 2021, 11, 1029

Volodichev et al. (Volodichev et al., 2021) reported on the first finding of omphacite (23%–25% Jd) inclusions in 2.68 Ga metamorphic zircons from Gridino eclogites and presented it as evidence for Archean eclogite-facies metamorphism in the Belomorian Mobile Belt. We believe that the Archean age of the garnets referred to by the above authors was estimated incorrectly. Our interpretation is that omphacite origin is related to Archean high-pressure granulite-facies metamorphism.

Archean Zircons with Omphacite Inclusions from Eclogites of the Belomorian Province, Fennoscandian Shield: The First Finding

Early Precambrian retrogressed eclogites are abundant in the central and northern parts of the Belomorian Province of the Fennoscandian Shield (Gridino + Keret and Salma + Kuru-Vaara study areas, respectively). Older and younger eclogites are recognized and their Archean and Paleoproterozoic ages are argued. Archean eclogites are intensely retrogressed and occur in amphibolite boudins in the tonalite-trondhjemite-granodiorite (TTG) gneiss matrix of the Archean Gridino eclogite-bearing mélange. Less retrogressed Paleoproterozoic eclogites form patches in mafic dikes and some amphibolite boudins; their Paleoproterozoic age is supported by U-Pb/SIMS data on zircons depleted in heavy rare earth elements (REE) with omphacite, garnet, and kyanite inclusions, and Sm-Nd and Lu-Hf mineral isochrons. Archean eclogites contain Archean heavy rare-earth elements (REE)-depleted zircons with garnet and zoisite inclusions and Archean garnets. No omphacite inclusions were found in these zircons, and this fact was considered as evidence against the existence of Archean eclogites. This study reports on the first finding of omphacite (23–25% Jd) inclusions in 2.68 Ga metamorphic zircons from eclogites from the Gridino eclogite-bearing mélange. The zircons are poorly enriched in heavy REE and display a weak negative Eu-anomaly but a poor positive Ce-anomaly typical of eclogitic zircons. Thus, zircons with these decisive features provide evidence for an Archean eclogite-facies metamorphism.

Zircon U-Pb geochronology and Nd-Pb isotope geochemistry of Blue Ridge basement in the eastern Great Smoky Mountains, U.S.A.: Implications for the Proterozoic tectonic evolution of the southeastern Laurentian margin

The Mesoproterozoic to Paleozoic history of the southeastern Laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events. The Great Smoky Mountains Basement Complex (GSMBC) within the southern Appalachian Blue Ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of Laurentian margin evolution from ca. 1.9 Ga to 450 Ma. The GSMBC consists primarily of: (1) 1.34 to 1.31 Ga (pre-Elzevirian) granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (correlated with pre-Elzevirian crustal components in the Adirondack, Green Mountains, New Jersey Highlands, and French Broad massifs), (2) ca. l.15 to 1.05 Ga augen and granitic orthogneiss produced during Shawinigan and Ottawan phases of Grenville-age magmatism and metamorphism, respectively, and (3) paragneiss derived from protoliths with either Grenville-age (1.1–1.0 Ga) or post-Grenville (Neoproterozoic) depositional ages based on presence/absence of ca. 1.0 Ga metamorphic zircon and 1.9 to 1.1 Ga detrital zircon. All lithologies experienced Taconian metamorphism and variable migmatization. Pre-Ottawan paragneiss exhibits major detrital zircon ages modes at 2.0 to 1.6 and 1.4 to 1.3 Ga that require a component of older Proterozoic crust in the sediment source region. Detrital zircon grains in post-Ottawan paragneiss exhibits the full spectrum of Grenville-age modes that correlate with the five phases of Grenville magmatic/metamorphic events in eastern Laurentia. These paragneiss samples also contain scattered 750 to 600 Ma detrital zircon grains that constrain their maximum depositional age to late Neoproterozoic. The sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all Grenville crustal age components during post-orogenic exhumation and crustal extension leading up to Late Neoproterozoic breakup of Rodinia. Most zircon U-Pb age systematics exhibit variable discordance that can be attributed to disturbance of the U-Pb system and/or new zircon growth during either high-grade Ottawan (ca. 1.04 Ga) or Taconian (ca. 0.46 Ga) regional metamorphism and migmatization. Neodymium TDM model ages for granodioritic orthogneiss and paragneiss range from 1.8 to 1.6 Ga, indicating that the rocks were derived from recycling of Proterozoic crust (that is, they are not juvenile), consistent with the 1.9 to 1.6 Ga detrital zircon grains in pre-Ottawan paragneiss and with 1.8 to 1.7 Ga inherited zircon in some 1.33 Ga orthogneisses and a 1.35 Ga xenolith. Whole rock Pb isotope compositions of GSMBC rocks overlap the field of compositions characteristic of Amazonian crust and of other basement rocks from the south-central Appalachians. The Pb isotopes and geochronology in orthogneiss, mafic xenoliths, and pre-Ottawan paragneiss are consistent with a correlation of the GSMBC with the Mars Hill terrane within the French Broad massif and with the greater Grenvillian south-central Appalachian basement (SCAB) that is considered exotic to Laurentia, and transferred during Rodinian collision prior to ca. 1.2 Ga. Similarities in protolith ages and Pb isotopes point to the Paragua terrane of Amazonia (southwestern Brazil and eastern Bolivia) as a likely match for SCAB. Initial Amazonia-Laurentia collision occurred at ca. 1.35 to 1.32 Ga with final transfer of SCAB to Laurentia occurring after 1.20 Ga within the sinistral oblique collision zone between Laurentia and Amazonia defined by previous workers.

Timing of two separate granulite-facies metamorphic events in the Helanshan complex, North China Craton: Constraints from monazite and zircon U–Pb dating of pelitic granulites

This study presents the results of monazite and zircon U–Pb dating of garnet–biotite gneiss and recently discovered spinel-bearing ultrahigh-temperature (UHT) pelitic granulites from the Paleoproterozoic Helanshan complex of the North China Craton, and considers implications for the formation of the complex and the tectonic history of this region. SHRIMP monazite U–Pb dating of the garnet–biotite gneiss yielded a weighted mean 207Pb/206Pb age of 1944.4 ± 4.2 Ma, which is the same within error as a weighted mean laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) 207Pb/206Pb age of 1959 ± 18 Ma obtained from metamorphic zircons within the same unit. The age of 1959 ± 18 Ma is similar to the published metamorphic ages of ca. 1.96–1.95 Ga from high-temperature (Tmax < 900 °C) pelitic granulites in the Helanshan, which represent the timing of continental collision between the Yinshan and Ordos terranes as suggested by previous published works. Metamorphic monazites from the spinel-bearing UHT pelitic granulite unit in this area yielded weighted mean 207Pb/206Pb ages of 1930.8 ± 2.6 and 1933.6 ± 3.1 Ma, which are consistent with the ages of metamorphic zircons. These ages record the timing of UHT metamorphism within the Helanshan complex and are similar to 1.93–1.92 Ga ages previously reported from typical UHT pelitic granulites within the Jining and Daqingshan complexes of the Khondalite Belt, where ca. 1.92 Ga metamorphic zircons are thought to record the cooling of the UHT rocks to the solidus. The new data presented here indicate that the ca. 1.93 Ga UHT metamorphic event occurred across a wider area than previously thought and extended into the Helanshan complex. Combining these new data with the results of previous research suggests that the entire Khondalite Belt was influenced by a ca. 1.93 Ga UHT metamorphic event that was preceded by metamorphism associated with continental collision between the Yinshan and Ordos terranes at ca. 1.95 Ga. This suggests that the Khondalite Belt underwent a tectonic transition from compression to extension between 1.95 and 1.93 Ga, a process likely controlled by a late-stage shallow slab-breakoff event.

Geochronological evidence for Archaean and Palaeoproterozoic polymetamorphism in the Central Zone of the Limpopo Belt, South Africa

The Central Zone of Limpopo Belt is a polymetamorphic and complexly deformed Precambrian terrane whose tectonic evolution is still uncertain because details on the timing of its evolution are not well established. We report zircon ages for newly discovered rocks in the Central Zone which document three distinct high-grade tectono-metamorphic events at 3.22, 2.62 and 2.02 Ga. The youngest of these has been well established, but the older two events were so far largely based only on rare zircon overgrowth rims. Most of our samples were collected from a large enclave in the 2.61 Ga Bulai pluton and constitute a strongly deformed, brecciated high-grade assemblage of supracrustal rocks, meta-anorthosite and granitoid gneisses, some of which are most likely equivalents of the pre-3.33 Ga Beit Bridge Complex. Magmatic zircon ages of 3.34–3.33 Ga for anorthositic and tonalitic gneisses in the enclave breccia support this correlation. Ca. 3.22 Ga metamorphic zircons, possibly reflecting the D1/M1 event, were only found in one felsic granulite sample, and their regional significance remains uncertain. Abundant metamorphic zircons grew at ca. 2.62 Ga under granulite-facies conditions (D2/M2), and some of these experienced variable degrees of recrystallization and/or Pb-loss during the third granulite-facies event at ca. 2.0 Ga (D3/M3). Rocks outside the Bulai enclave contain abundant newly-formed ca. 2.02 Ga metamorphic zircons but only rarely preserve evidence for the ca. 2.62 Ga event since these late Archaean zircons show strong recrystallization and/or Pb-loss at ca. 2.0 Ga. We suggest that the Bulai pluton “protected” the rocks inside the enclave from severe overprinting at ca. 2.0 Ga because these rocks remained dry, whereas the assemblages outside the Bulai pluton experienced fluid-assisted metamorphic overprinting at ca. 2.0 Ga.Our new data sugggest that the tectonic evolution of the Central Zone was more complex than previously thought. We favour a scenario where collision involving the Limpopo Belt occurred in the Neoarchaean, whereas the high-grade Palaeoproterozoic event was most likely intracontinental and records far-field effects of plate movements elsewhere. The tectonic setting controlling the early Archaean D1/M1 event remains unknown, largely because of insufficient data and extensive overprinting during later times.

The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: An example from the late Archean TTG terrane of western Shandong Province, China

There are different viewpoints on metamorphic and anatectic zircons recording ages of 2.45–2.48Ga or even younger in some areas of the North China Craton where both late Neoarchean and late Paleoproterozoic tectono-thermal events are well developed. These are: 1) partial resetting of the U-Pb isotopic system in the late Neoarchean zircons, 2) metamorphism lasting from the late Neoarchean to the earliest Paleoproterozoic, and 3) earliest Paleoproterozoic metamorphism as separate different event. Western Shandong Province is an area where the late Neoarchean tectono-thermal event is widely developed but the late Paleoproterozoic event has not been identified. This provides an opportunity to understand the geological processes around the Archean-Proterozoic boundary. Based on a field study, we carried out SHRIMP U-Pb zircon dating on seventeen samples of ∼2.5Ga old metamorphic and anatectic rocks, including tonalite, trondhjemite, granodiorite, gabbro, quartz diorite, granite and paragneiss with primary emplacement or depositional ages of 2.52–2.68Ga. Anatectic zircons show some textural and compositional features: a) Homogenous or blurred oscillatory zoning, b) high U contents and low in Th/U ratios (commonly <0.3), c) rare idiomorphic morphologies, and d) commonly containing inherited (xenocrystic) cores. We conclude that the strong late Neoarchean event is widespread in western Shandong and is limited between ∼2.50 and 2.54Ga. In contrast, apparent 2.45–2.48Ga metamorphic zircon ages in some other areas of the North China Craton may be the result of overprinting and partial recrystallization of Neoarchean metamorphic zircons during the late Paleoproterozoic event. The Archaean rocks of western Shandong can be divided into three belts, namely Belts A, B and C from the northeast to the southwest. The difference between Belts A and B in ∼2.5Ga metamorphic and anatectic intensity may indicate that the former was elevated to a higher crustal level compared with the latter at the end of the Neoarchean.

Comparison of metamorphic zircons from granulite xenoliths and granulite terrain in northern North China Craton

We conduct a study of petrography, geochronology, trace element and Hf isotope compositions on zircons from granulite terrain and granulite xenoliths in the Zhangjiakou region, northern North China Craton. Most zircons from both types of granulite have core-rim structure in their CL images. All the magmatic zircon cores yield essentially identical ∼2.5Ga ages, whereas metamorphic zircons yield ages of ∼1.85Ga and 120–140Ma, respectively. Most the ∼1.85Ga metamorphic zircons have Th/U ratios lower than 0.4 with a range of 0.01 to 0.63. However, the range of Th/U ratios of Mesozoic (120–140Ma) metamorphic zircons (0.12–3.48) overlaps that of the magmatic zircons (0.12–2.44). The Mesozoic and ∼1.85Ga metamorphic zircons have REE patterns consistent with magmatic zircons, except that the ∼1.85Ga ones show less enrichments and much less negative Eu anomalies than those of magmatic zircons. Therefore, Th/U ratios and the REE patterns cannot be used to distinguish metamorphic zircons from magmatic ones. The ∼1.85Ga metamorphic zircons, however, have lower formation temperatures, oxygen fugacity and 176Lu/177Hf ratios than the Mesozoic ones. We speculate that the ∼1.85Ga metamorphic zircons represent newly grown zircons coexisting with garnets and melt/fluid while the Mesozoic metamorphic zircons were formed quickly without the presence of garnet. The Mesozoic metamorphic zircons vary in their origins; some of these zircons were metamorphic newly grown while the others experienced recrystallization from a U-depleted melt/fluid. Our results suggest that P-T conditions, the melt/fluid and oxygen fugacity play important roles in controlling the trace element distribution and Hf isotope compositions of metamorphic zircons in the granulites.

A ca. 2.60 Ga tectono-thermal event in Western Shandong Province, North China Craton from zircon U–Pb–O isotopic evidence: Plume or convergent plate boundary process

Voluminous Neoarchean plutonic and supracrustal rocks are well developed in the Western Shandong Province of the North China Craton, which has been divided into a late Neoarchean crustally-derived granite belt, a middle to early Neoarchean ancient rock belt and a late Neoarchean juvenile rock belt. Earlier studies provided reconnaissance evidence for ∼2.60Ga metamorphism in some areas. This study presents SHRIMP zircon U–Pb dating and O isotope analysis on nine metamorphosed igneous rock samples (gneissic tonalite, gneissic trondhjemite, gneissic quartz diorite and meta-gabbro) from the ancient rock belt. Magmatic zircons vary in age from 2.74Ga to 2.60Ga, whereas metamorphic and anatectic zircons vary in age from 2.63Ga to 2.59Ga, with an age peak of 2.61Ga. In this study, we identified 2.69–2.67Ga metamorphic zircons for the first time. Most of zircons, whether magmatic or metamorphic, have δ18O values of +4.5‰ to +6.5‰ (V-SMOW). However, the oldest tonalite sample with a protolith age of 2735Ma has zircons showing low δ18O (V-SMOW) values, particularly in recrystallized domains and overgrowths. This suggests at least in local crustal extension and influx of meteoric water during the evolution of the rocks. Combined with data from earlier studies, we draw the conclusion that the Western Shandong Province experienced a regional tectono-thermal event at ∼2.60Ga, as a result of high heat flow and crustal extension, perhaps caused by a mantle plume or mantle overturn activity. Therefore we conclude that 2.60Ga can be regarded the break between early and late Neoarchean crustal accretion. The O isotope study indicates that in some cases, trondhjemitic leucosome was not derived from nearby gneissic tonalite, although they show a close relationship in outcrop.

Zircon U–Pb ages of Paleoproterozoic mafic granulites from the Huai’an terrane, North China Craton (NCC): Implications for timing of cratonization and crustal evolution history

The North China Craton (NCC) is considered to have been incorporated into the Columbia supercontinent during late Paleoproterozoic. The Huai’an terrane located in north-central part of the NCC is an important tectonic window to investigate the Paleoproterozoic crustal evolution history of this craton, particularly due to the wide occurrence of high-grade metamorphic rocks including retrograded eclogites and high-pressure granulites. Available data show two controversial peak metamorphic ages of these rocks as 1.95–1.90 or ∼1.85–1.80Ga. Thus, determining the precise timing of metamorphism of the high-pressure granulites is important in reconstructing the collisional history. In this study, we present zircon U–Pb ages, bulk chemistry, and mineral inclusions in zircons and matrix mineral assemblages from the high-pressure granulites to evaluate the timing of metamorphism. Our results reveal ca. 2.05–2.0Ga ages from magmatic zircons in these rocks representing the timing of emplacement of mafic magmas. The metamorphic zircons yield two groups of ages at 1.95–1.90 and ∼1.85∼1.80Ga. Clinopyroxene, plagioclase, titanite, and apatite occur as common mineral inclusions within the ∼1.85–1.80Ga metamorphic zircons. The chemical compositions of clinopyroxene and plagioclase occurring as inclusions are very similar to those of the rim compositions of those minerals in the matrix, suggesting their formation during retrograde granulite metamorphism, and are distinctly different from those of the core domains of the minerals in the matrix which represent high-pressure granulite metamorphism. These results indicate that the metamorphic zircons formed during retrograde granulite metamorphism dated as ∼1.85–1.80Ga and the age range of ∼1.95–1.90Ga corresponds to high-pressure granulite metamorphism. The dual nature of the geological and geochronological features suggest that the Archean gray gneisses in the Huai’an terrane represent the basement rocks detached from the Khondalite Belt of the NCC after collision. Integrating with published data, a double-side collision with scissor-like suturing model is proposed for the Paleoproterozoic cratonization of the NCC.

Stages and conditions of metamorphism of mafic granulites in the Early Precambrian complex of the Angara–Kan terrane (southwestern Siberian Craton)

We present results of study of mineral assemblages and PT-conditions of metamorphism of mafic garnet–two-pyroxene and two-pyroxene granulites in the Early Precambrian metamorphic complex of the Angara–Kan terrane as well as the U–Pb age and trace-element and Lu–Hf isotope compositions of zircon from these rocks and the zircon/garnet REE distribution coefficients. The temperatures of metamorphism of two-pyroxene granulites are estimated as 800–870 to ~900 °C. Mafic garnet–two-pyroxene granulites contain garnet coronas formed at 750–860 °C and 8–9.5 kbar. The formation of the garnet coronas proceeded probably at the retrograde stage during cooling and was controlled by the rock composition. The age (1.92–1.94 Ga) of zircon cores, which retain the REE pattern typical of magmatic zircon, can be taken as the minimum age of protolith for the mafic granulites. The metamorphic zircon generation in the mafic granulites is represented by multifaceted or soccerball crystals and rims depleted in Y, MREE, and HREE compared to the cores. The age of metamorphic zircon in the garnet–two-pyroxene (~1.77 Ga) and two-pyroxene granulites (~1.85 and 1.78 Ga) indicates two episodes of high-temperature metamorphism. The presence of one generation (1.77 Ga) of metamorphic zircon in the garnet–two-pyroxene granulites and, on the contrary, the predominance of 1.85 Ga zircon in the two-pyroxene granulites with single garnet grains suggest that the formation of the garnet coronas proceeded at the second stage of metamorphism. The agreement between the zircon/garnet HREE distribution coefficients and the experimentally determined values at 800 °C suggests the simultaneous formation of ~1.77 Ga metamorphic zircon and garnet. Zircon formation by dissolution/reprecipitation or recrystallization in a closed system without exchange with the rock matrix is confirmed by the close ranges of 176Hf/177Hf values for the core and rims. The positive εHf values (up to +6.2) for the zircon cores suggest that the protolith of mafic granulites are derived from depleted-mantle source. The first stage of metamorphism of the mafic granulites and paragneisses of the Kan complex (1.85–1.89 Ga) ended with the formation of collisional granitoids (1.84 Ga). The second stage (~1.77 Ga) corresponds to the intrusion of the second phase of subalkalic leucogranites of the Taraka pluton and charnockites (1.73–1.75 Ga).

Episodic Paleoarchean-Paleoproterozoic (3.3–2.0 Ga) granitoid magmatism in Yangtze Craton, South China: Implications for late Archean tectonics

The Archean Kongling Terrane preserves voluminous Paleoarchean-Paleoproterozoic granitoids, recording the early formation and evolution history of the Yangtze Craton, South China. We present petrography, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of twelve gneissic granitoids from this terrane, including trondhjemites, biotite-granites, and two-mica granites. Magmatic zircons in these granitoids yield emplacement ages of 3.31–3.29Ga for trondhjemites, 2.81–2.78 and 2.66Ga for biotite-granites, as well as 2.70–2.64, 2.42, and 2.00Ga for two-mica granites. The 2.8–2.6Ga granites contain minor 3.4–2.9Ga inherited zircons and 2.0Ga metamorphic zircons that formed mainly by recrystallization. Major and trace element compositions of the trondhjemites suggest a garnet amphibolite-facies low-K mafic source, similar to the typical Archean medium-pressure trondhjemite-tonalite-granodiorites (TTGs), whereas those of the biotite- and two-mica granites indicate tonalitic and tonalitic-sedimentary sources, respectively. The Lu–Hf data (chondritic ɛHf(t) values −0.8 to 0 and Eoarchean two-stage Hf model ages 3.75–3.70Ga) for the trondhjemites support partial melting of a long-lived ancient depleted-mantle-derived or a juvenile lower-mantle-derived mafic crust. The Lu–Hf data and inherited zircon ages also suggest that the 2.8Ga granites were derived from the local 3.4–2.9Ga TTGs. Together with previously addressed 3.2 and 3.0–2.9Ga TTG magmatism, a remarkable change is observed at 2.8Ga in the petrology, geochemistry, and petrogenesis of the episodic 3.3–2.0Ga granitoid magmatism from TTG- to granite-dominated in the Kongling Terrane, which may reflect a transition from subduction- to collision-related tectonics. In addition, we note that the 2.70–2.64Ga biotite- and two-mica granites (this study) as well as 2.67–2.62Ga A-type granites (published data) were emplaced shortly after a 2.75–2.72Ga high-grade metamorphic event (published data) in the Kongling Terrane. These granites could be generated as a result of orogenic root collapse and subsequent mantle upwelling, implying a 2.7Ga orogenic event in the Yangtze Craton.

Ca. 2.9 Ga granitoid magmatism in eastern Shandong, North China Craton: Zircon dating, Hf-in-zircon isotopic analysis and whole-rock geochemistry

Eastern Shandong is one of the important early Precambrian areas in the North China Craton, where extensive 2.5, 2.7 and 2.9Ga magmatic activities have been identified, followed by very strong late Neoarchean and Paleoproterozoic tectono-thermal events. We carried out SHRIMP zircon dating, Hf-in-zircon isotope analysis and a whole-rock geochemical study on ca. 2.9Ga Mesoarchean magmatic rocks. These include gneissic diorite, gneissic quartz diorite, gneissic tonalite and gneissic high-Si trondhjemite and were identified in five new areas. All these rocks contain ca. 2.5Ga metamorphic zircons. The dioritic and tonalitic rocks are enriched in large ion lithosphile (LIL) elements (K, Rb, Ba) and depleted in Nb and P with moderate to highly fractionated REE patterns, similar in chemical compositions to Archean TTG rocks worldwide. The high-Si trondhjemites are characterized by very high SiO2 and very low FeOt+MgO. The magmatic zircon grains from all these rocks are similar in their Hf isotopic compositions and have ɛHf(t) values ranging from +1.3 to +9.9. Combined with previous work, our main conclusions are: (1) eastern Shandong underwent a major magmatic event at ca. 2.9Ga; (2) diorites and tonalites formed by melting of thickened lower crust or the root of an oceanic plateau, whereas the high-Si trondhjemites were derived from melting of intermediate to felsic rocks; (3) the Mesoarchean (ca. 2.9Ga) was a major period of juvenile crustal addition from the depleted mantle, earlier than the major crustal growth period at 2.7–2.8Ga in the North China Craton; (4) the 2.7–2.9Ga rocks underwent strong metamorphism at ca. 2.5Ga, probably as a result of magmatic underplating.

Field and microanalytical isotopic investigation of ultradepleted in 18O Paleoproterozoic “Slushball Earth” rocks from Karelia, Russia

The 1.85 Ga Belomorian Belt, Karelia, Russia, hosts ultralow δ^(18)O and δD (as low as −27.3‰ and −235‰ standard mean ocean water [SMOW], respectively), high-Al gneisses and amphibolites that we attribute to the Paleoproterozoic “Slushball Earth” glaciation. They now occur in at least 11 localities spanning 450 km. To constrain distribution of ^(18)O-depleted rocks, we performed detailed field mapping in Khitostrov, where δ^(18)O values are the lowest. Using 430 new and previously published laser fluorination isotope analyses, we show that the elongated, concentrically zoned area of δ^(18)O depletion is greater than 6 × 2 km in areal extent, ∼10 times larger than previously thought. Relationships between δ^(17)O versus δ^(18)O strictly adhere to the equilibrium terrestrial mass-dependent fractionation with a slope of 0.527. We also report the results of ion microprobe U-Pb geochronology of zircons coupled with co-registered oxygen isotope spot analyses for mafic intrusions and host gneisses in six localities. The 2.9–2.7 Ga gneiss zircon cores are normal in δ^(18)O (5‰–7‰). They show truncated oscillatory cathodoluminescence (CL) patterns and rounded shape indicative of original igneous crystallization with subsequent detrital overprinting. A younger 2.6–2.55 Ga metamorphic zircon domain with normal δ^(18)O, low Th/U, dark cathodoluminescence, and also with rounded crystal morphology is commonly preserved. Cores are surrounded by ubiquitous rims highly depleted in δ^(18)O (re-)crystallized with Svecofennian (1.85–1.89 Ga) ages. Rims are interpreted as metamorphic due to bright and uniform CL and Th/U <0.05. Mafic intrusions preserve few igneous zircon crystals between ca. 2.23 and 2.4 Ga in age, but neoblastic zircon in these intrusions originated mostly during 1.85 Ga Svecofennian metamorphism. The δ^(18)O-age relationship for metamorphic rims in zircon and corundum grains suggests that δ^(18)O values of fluids were subtly increasing with time during metamorphism. Large metamorphic corundum grains have ∼3‰ intracrystalline δ^(18)O isotope zonation from –24 to –21‰, which likely developed during interaction with metamorphic fluids. The Zr-in-rutile geothermometer temperatures are in the range of 760 to 720 °C, in accordance with mineral assemblages and amphibolite metamorphic grade. High and irregular rare-earth element (REE) abundance in cores and rims of many zircons correlates with high phosphorus content and is explained by nanometer-scale xenotime and monazite inclusions, likely in metamict zones during 1.85 Ga Svecofennian metamorphism. A survey of oxygen isotopes in ultrahigh-pressure diamond and coesite-bearing metamorphic terrains around the world reveals normal to high-δ^(18)O values, suggesting that the low δ^(18)O in metamorphic rocks of Dabie Shan, Kokchetav, and in Karelia, are genetically unrelated to metamorphism. We discuss alternative ways to achieve extreme δ^(18)O depletion by kinetic, Rayleigh, and thermal diffusion processes, and by metamorphism. We prefer an interpretation where the low-δ^(18)O and high-Al signature of the rocks predates metamorphism, and is caused by shallow hydrothermal alteration and partial dissolution of the protolith surrounding shallow mafic intrusions by glacial meltwaters during pan-global Paleoproterozoic “Slushball Earth” glaciations between ca. 2.4 and ca. 2.23 Ga.

1.8 billion years of fluid–crust interaction: A zircon oxygen isotope record for the lower crust, western Churchill Province, Canadian Shield

The western Churchill Province of the Canadian Shield experienced a prolonged and complex formation history (ca. 4.04 to 1.70Ga), with evidence for multiple episodes of orogenesis and regional magmatic activity. Here we report on the oxygen isotopic compositions of garnet and zircon recovered from lower crustal xenoliths, which have U–Pb ages between ca. 3.5 and 1.7Ga. Overall, zircon from four metabasite xenoliths from the Rankin Inlet sample suite have δ18O values ranging from +5.5 to +8.6‰. Zircon from three metatonalite/anorthosite xenoliths and five metabasite xenoliths from the Repulse Bay sample suite have δ18O values of +5.6 to +8.3‰. High δ18O values (>+6.0‰) for the oldest igneous zircon cores (ca. 3.5Ga and 3.0–2.6Ga) indicate that their metatonalite/anorthosite protolith magmas were generated from, or had assimilated, supracrustal rocks that interacted previously with surface-derived fluids. Igneous zircon cores (ca. 2.9–2.6Ga) from one metabasite xenolith have δ18O values of +5.6 to +6.4‰, which suggests a formation from a mantle-derived basaltic/gabbroic magma. Metamorphic zircon cores (ca. 2.0–1.9Ga) from one metabasite xenolith commonly have δ18O values between +6.0 and +6.3‰, which is indicative of a basalt/gabbro protolith and localized reworking of the lower crust caused by regional-scale plate convergence. The wide range of δ18O values (+5.5 to +8.3‰) for ca. 1.75–1.70Ga metamorphic zircon rims (identified in all xenoliths) indicates regional transient heating and reworking of mantle- and supracrustal-derived crust, induced by magmatic underplating along the crust–mantle boundary.

3.45 Ga granitic gneisses from the Yangtze Craton, South China: Implications for Early Archean crustal growth

Zircon U–Pb–Lu–Hf–O isotopic compositions of two granitic gneisses from the Kongling Terrain in the Yangtze Craton, South China were determined by SIMS, LA-ICP-MS and LA-MC-ICP-MS. Whole rocks of the two samples were analyzed for major and trace element compositions. The SIMS and LA-ICP-MS data reveal similar five zircon age groups of 3.4, 3.3, 2.9, 2.7, and 2.0Ga for both gneisses. Three groups (magmatic Group A, metamorphic Group B, and overgrowth Group C) of the 3.4Ga zircons were identified based on their CL images. These three groups have indistinguishable ages and Th/U ratios. Groups A and B show identical 176Hf/177Hf (t), although Group C was too thin to be analyzed by LA-ICP-MS. Taken together, zircons from the two samples with 98–102% age concordance give weighted average SIMS ages of 3434.3±9.6Ma (2σ, MSWD=13, n=8) for Group A, 3446.0±8.8Ma (2σ, MSWD=10.7, n=15) for Group B, and 3479±26Ma (2σ, MSWD=0.49, n=2) for Group C. Groups A and B together yield an upper intercept age of 3457±14Ma (2σ, MSWD=0.85, n=23). The LA-ICP-MS data yield weighted average ages of 3442±19Ma (2σ, MSWD=0.17, n=7) for Group A and 3435±11Ma (2σ, MSWD=0.44, n=16) for Group B. They yield an upper intercept age of 3443±13Ma (2σ, MSWD=0.63, n=23). These SIMS and LA-ICP-MS ages are consistent. We propose that the above SIMS and LA-ICP-MS ages of Groups A and B are the best estimates of the granitic magmatism and the subsequent metamorphism. The metamorphism must have occurred after the granitic magmatism within a few tens of million years, as constrained by their age errors. Accordingly, these two granitic gneisses represent the oldest rocks currently known in South China. They predate the previously reported 3300-Ma-old trondhjemitic gneiss from the Kongling Terrain by 150Ma.The 3.4Ga zircons show near chondritic ɛHf (t) (−0.7±1.0, 2σ, MSWD=1.14, n=8), which is below the coeval value of the depleted mantle. This suggests that the granitic magma contained materials of pre-existing continental crust. Their higher-than-mantle δ18O values (6.1–6.4‰) imply that such materials must have been interacted with surface water. Crust formation ages (TDM2) of the 3.4Ga zircons vary from 3.9 to 3.6Ga with a weighted average of 3703±27Ma (2σ, MSWD=1.05, n=7). Our results support previous studies that the Yangtze Craton may have contained the continental crust as old as 3.8Ga.Among the younger age groups, the 3.3Ga zircons exhibit 176Hf/177Hf (t) and δ18O values similar to the 3.4Ga zircons, suggesting that they were altered from the 3.4Ga zircons. The 2.9 and 2.7Ga zircons in both samples are rare and magmatic. Their 176Hf/177Hf (t) ratios are distinct from the 3.4Ga zircons, indicating different sources. These two age groups are consistent with the 2.9Ga trondhjemitic–tonalitic–granodioritic and the 2.7Ga A-type granitic magmatism in the Kongling Terrain. The 2.0Ga metamorphic zircons, regardless of being concordant or discordant, have 176Hf/177Hf (t) ratios overlapping those of the 2.7Ga zircons, suggesting a common source. In contrast, δ18O of the 2.0Ga zircons is strongly variable and positively correlated with age concordance. The low δ18O (down to 3.1‰) requires interaction with hydrothermal fluid. These results suggest that at least some of the 2.0Ga zircons were likely to have been altered from the 2.7Ga zircons by hydrothermal fluid.

Decoding multiple tectonothermal events in zircons from single rock samples: SHRIMP zircon U–Pb data from the late Neoarchean rocks of Daqingshan, North China Craton

A complex sequence of tectonothermal events shaped the Neoarchean–Paleoproterozoic crustal evolution in the North China Craton (NCC). Here we present a systematic SHRIMP zircon U–Pb investigation on a suite of metamorphic rocks from the Hademengou area of Daqingshan within the Khondalite Belt of the Inner Mongolia Suture Zone. We analyzed zircons from two meta-volcanic rocks of the Sanggan “Group” and four meta-igneous rocks of late Neoarchean to early Paleoproterozoic. Zircons from these rocks display typical core–rim or core–mantle–rim structures. The relict magmatic zircons record crystallization ages of ca. 2.5Ga followed by variable Pb loss, showing a range of ages in the concordia up to 2.1Ga. The recrystallized domains of zircons show sector zoning or are composed of inner dark domains, and outer lighter domains. The overgrowth mantles are homogeneous. The recrystallized zircons and overgrowth mantles show similar age data distribution along a concordia from ~2.5 to ~2.1Ga. The overgrowth rims are poorly developed but record ages (1.96–1.84Ga) similar to those displayed by metamorphic zircons in the late Paleoproterozoic “Upper Wulashan Subgroup” as well as the charnockites and UHT granulites reported in previous studies from this region. A meta-syenogranite intruding the Sanggan “Group” and Hypersthene dioritic gneiss contains 1.95Ga magmatic and 1.85Ga metamorphic zircons. Magmatic zircons of the late Neoarchean–early Paleoproterozoic metavolcanic rocks have εHf(t) of 1.2 to 7.0, tDM1(Hf) model ages of 2687 to 2532Ma and tDM2(CC)(Hf) model ages of 2837 to 2559Ma. The recrystallized zircons and mantle domains display Hf isotope composition similar to that of the magmatic domains. Our study is the first report from this region that identifies multiple tectonothermal events ranging from late Neoarchean through early to late Paleoproterozoic from zircons within single rock samples in the NCC. A strong overprint of late Paleoproterozoic metamorphism traced commonly in all the rock units indicates that these rocks were involved in the tectonothermal event associated with the final amalgamation of the NCC.