Post-collisional Extensional Setting Research Articles

Age and Petrogenesis of Paleoproterozoic Diorites from Dikes of the Baikal Uplift of the Siberian Craton

Abstract ––We carried out a detailed geological, geochronological, geochemical, and isotope study of diorites from a dike located in the central part of the Baikal uplift of the Siberian craton. The geochemical and isotope data obtained for diorites of the studied intrusion were compared with coeval mafic and intermediate igneous rocks of the southern part of the Siberian craton. The U–Pb (ID-TIMS) baddeleyite age of 1862 ± 7 Ma has been estimated for diorite from a dike located in the area of the Onguren Village. The obtained data are the first reliable age determination for the Paleoproterozoic mafic and intermediate igneous rocks of the Baikal uplift, which are part of the South Siberian postcollisional magmatic belt. The dike is of NE strike. The dike rocks correspond in chemical composition to diorites and are highly differentiated varieties (mg# = 36.5–37.4). There are no significant variations in the composition of diorites in the marginal and central parts of the dike. The rocks are characterized by low contents of TiO2, P2O5, and Nb, high contents of Th, Zr, Ba, and LREE, and low negative values of εNd(T) (–5.9 to –6.2). We assume that the diorites formed from the enriched subcontinental lithospheric mantle. The obtained geochemical and isotope data show similar negative values of εNd(T) from –4.3 to –11.6 for most of Paleoproterozoic mafic and intermediate igneous rocks of the South Siberian postcollisional magmatic belt within the Aldan Shield, the Baikal uplift, and the Irkut block of the Sharyzhalgai uplift. These rocks correspond in geochemical features to rocks resulted from the melting of subduction-modified lithospheric mantle sources in the postcollisional extension setting at the final Paleoproterozoic stage of formation of the Siberian craton.

The Formation Age and Magma Source of the Xiaonanshan–Tunaobao Cu-Ni-PGE Deposit in the Northern Margin of the North China Craton

The Xiaonanshan–Tunaobao Cu-Ni-PGE deposit is located in the northern margin of the North China Craton (N-NCC) in central Inner Mongolia. However, the age, magma source, petrogenesis, and sulfide mineralization mechanism of the ore-related Xiaonanshan-Tunaobao pluton remain unclear. Zircon U-Pb dating indicates the Tunaobao pluton formed at 275.9 ± 2.8 Ma (Early Permian), similar to the Xiaonanshan pluton (272.7 ± 2.9 Ma). The ore-related gabbro is enriched in LREE and LILE (e.g., Rb) and depleted in HREE and HFSE (e.g., Nb and Ti). It likely originated from enriched mantle metasomatized by subduction fluids, supported by enriched Hf-Nd isotopes (–34.34 to –6.16 for zircon εHf(t) and –7.24 to –5.92 for whole-rock εNd(t) values) and high Ba/La but low Rb/Y ratios. The δ34S values of the Xiaonanshan sulfides range from 4.5‰ to 11.4‰, indicating a mantle origin with contribution from surrounding rocks. Combining previous recognition with this study, we propose that the Xiaonanshan–Tunaobao pluton formed in a post-collision extensional setting.

Geochronology, geochemistry, and isotope signatures of porphyries from the dabate Mo—Cu deposit, Western Tianshan, China: Implications for petrogenesis and geodynamic setting

The Dabate deposit in the West Tianshan Orogenic Belt is a large porphyry Mo-Cu deposit with obvious zoning feature. The Dabate pluton is closely related to mineralization associated with the rhyolite and granite porphyries. Copper (Cu) mineralization occurs on the margin of the rhyolite porphyry, and molybdenum (Mo) mineralization occurs in the interior of the rhyolite porphyry. Petrology, zircon U-Pb isotopic data, whole-rock geochemistry, and Sr-Nb-Hf-Pb isotopic data of the rhyolite and granite porphyries in the Dabate Mo-Cu mining zone and to discuss the properties, sources, evolution, and tectonic environment of the magma. Zircon U-Pb age dating shows that the rhyolite porphyry and granite porphyry were formed in 300.1 ± 1.3 Ma and 290.4 ± 5.2 Ma, respectively, and they are geochemically similar. Both are characterized by negative Eu anomalies (δEu = 0.09–0.15 and 0.10–0.11, respectively) and are slightly rich in light rare earth elements (LREE) but depleted in high-field-strength elements (HFSE), which indicates that they are both A2 type granites that underwent high differentiation. The Sr isotope (ISr) values for the rhyolite and granite porphyries range from 0.70564 to 0.70658 and 0.70833–0.70954, respectively. The Nd isotopic composition [εNd(t)] values for both are negative but close to zero (−0.25 to −0.46 and −0.65 to −1.10, respectively), and the two-stage depleted mantle Nd model ages (TDM2(Nd)) are 1.07–1.10 Ga and 1.11–1.14 Ga, respectively. Zircon εHf(t) values are 7.29–11.97 and 6.26–11.08 for rhyolite porphyry and granite porphyry, respectively, and the two-stage Hf model ages (TDM2(Hf)) are 550–848 Ma and 605–916 Ma, respectively. The initial Pb isotope ratios for the rhyolite and granite porphyry are close at (206Pb/204Pb)i = 18.26–18.65, (207Pb/204Pb)i = 15.54–15.64, and (208Pb/204Pb)i = 38.19–38.57. These results indicate that the two porphyries have a mixed source, likely originating from the partial melting of mantle blended with some recently formed lower crust material. The geochronology, geochemistry, and Sr-Nd-Hf-Pb isotope data of existing intrusions in the West Tianshan imply that the Late Carboniferous to Early Permian Dabate area was underplated in a post-collisional extensional setting, and A2-type granite intrusions with related Mo-Cu mineralization were formed.

Cobalt remobilization during tectonic–hydrothermal overprinting: A case from the Tuolugou Co(–Au) deposit in East Kunlun Orogenic Belt, China

Stratiform Cu-Co deposits hosted in (meta-) sedimentary and volcaniclastic rocks contribute two-thirds of the global Co supply. However, the mechanisms driving Co remobilization during tectonic-hydrothermal overprinting events remain poorly understood. As a dominant Co repository, pyrite can provide valuable insights into mineralization history due to its refractory nature. Here, we present the first micro-analytical investigation of pyrite from the Tuolugou Co(–Au) deposit in East Kunlun Orogenic Belt. Ore textures indicate that Co mineralization is attributed to two distinct stages: primary enrichment and subsequent overprinting processes. The former is characterized by pyrite (PyI with up to 3.92 wt% Co) remnants exhibiting oscillatory zones, while the latter is marked by deformation of pyrite (PyII) ores. Three overprinting episodes, namely Da, Db, and Dc, are further distinguished according to varying extents of deformation and mineral assemblages. The Da is characterized by synkinematic PyIIa (up to 4.18 wt% Co) augens, which were most likely formed via pressure–solution mechanism. The Db is represented by polygonal PyIIb (up to 4.75 wt% Co) and siegenite with 120° triple–point junctions. The random orientation and rare intragrain misorientation of cobaltiferous sulfides at Db suggest their formation by recrystallization without plastic strains. The Dc resulted in the formation of Co ± As–rich PyIIc (up to 5.36 wt% Co) along with minor occurrence of cobaltite–gersdorffite, pyrrhotite (up to 1.25 wt% Co), and marcasite (up to 1.28 wt% Co). The PyIIc replacing PyI underscores the role of fluid-coupled dissolution and precipitation during fluid–rock interactions in the formation of these cobaltiferous sulfide assemblages at Dc. The presence of PyIIc infilling within the PyI fissures, in conjunction with the transition from siegenite to cobaltite–gersdorffite, suggests a shift towards low fO2 and low temperature conditions under relatively brittle setting. There is a discernible disparity in sulfur isotope compositions between the two mineralization stages. The PyI exhibits a narrow range of δ34SV–CDT values from –2.14 to 1.35 ‰, indicating a magmatic sulfur origin. In contrast, PyII displays a wide δ34SV–CDT range of –5.22 to 6.16 ‰, suggesting an involvement of strata sulfur during the overprinting stage. Monazite U-Pb dating has effectively constrained two mineralization events. The ca. 453 Ma age implies a potential correlation of the primary Co mineralization with the Late Ordovician to Early Silurian exhalative-syngenetic processes in a Proto–Tethyan-related post–collisional extension setting. Conversely, the ca. 190 Ma age indicates that Co remobilization was instigated by the early Mesozoic ductile–brittle deformation associated with the Paleo-Tethyan orogenesis. Finally, geochemical modeling was performed by simulating the interaction of a Co-rich acidic, reducing fluid with typical quartz-albitite rocks. The simulations demonstrate the appearance of independent Co minerals such as Co-pentlandite during the reaction progression, greatly supporting ore upgrading via tectonic-hydrothermal overprinting.

Initial sedimentation after the closure of the Sumdo Paleo-Tethys Ocean: Implications for early Mesozoic tectonics in the East Tethys domain

The closure of the Paleo-Tethys Ocean and subsequent collisional orogenesis in the early Mesozoic played an important role in the formation of the Qinghai–Tibetan Plateau. However, the final closure of the southernmost Paleo-Tethys branching ocean basin in the Qinghai–Tibetan Plateau—known as Sumdo Paleo-Tethys Ocean—and collision between Central and Southern Lhasa terranes remain controversial. In particular, the tectonic significance of early Mesozoic volcanism and sedimentation in the Lhasa terrane is unknown, but crucial for understanding the final stages of ocean closure. Here, we investigate the depositional ages and provenance of detrital zircon in clastic sedimentary rocks, as well as the petrogenesis of the interbedded andesite from the volcano-sedimentary sequence of the Late Triassic to Early Jurassic Xionglai Formation in the Tangjia-Sumdo area. Detrital zircon dates from the Xionglai Formation define five age peaks at 180–200 Ma, 230–340 Ma, 500–620 Ma, 1000–1240 Ma, and 1600–1800 Ma. Detrital zircon trace elements and Hf isotopic characteristics are interpreted to represent derivation from an accretionary wedge and later magmatism related to oceanic slab break-off as well as a minor component from coeval magmatic rocks in the Nyainrong-Nang area in northern Lhasa terrane. Interbedded andesite has a magnesian andesite signature and yields ages of 192–184 Ma with zircon εHf(t) values that range from −3 to +1. We interpret these data as the andesite being sourced from sediment-derived melts that interacted with the overlying mantle wedge in a post-collision extensional setting. Our study suggests that the closure of the Sumdo Paleo-Tethys Ocean and subsequent collision between Central Lhasa and Southern Lhasa terrane may have happened in Late Triassic-Early Jurassic. The final closure of the multi-Paleo-Tethys Ocean and subsequent collision in the Qinghai–Tibetan Plateau may have induced initial subduction of the Meso-/Neo-Tethys Ocean, representing the tectonic transition from collision/post-collision to subduction/accretion in the Qinghai–Tibetan Plateau during the Late Triassic to Early Jurassic.

Petrogenesis of Late Triassic adakitic plutons in the East Kunlun Orogen, Northern Tibet: Geodynamic implications for the Paleo-Tethyan orogeny and crustal evolution

Adakitic rocks found within continental arcs are pivotal in unraveling the geodynamic processes and tectonic evolution of East Kunlun Paleo-Tethyan orogen. We present an integrated study on the petrography, zircon U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-Hf isotopes of newly identified Late Triassic adakitic plutons in the East Kunlun Orogenic Belt (EKOB), Northern Tibet, aiming to constrain their petrogenesis and geodynamic implications. These plutons comprise by mylonitized granite, monzogranite, granodiorite and dioritic porphyrite, exhibiting consistent zircon U-Pb ages of approximately 204–212 Ma. They have high SiO2, K2O and Sr (average of 479 ppm) contents, but low contents of Y (5.6–18 ppm) and Yb (0.5–1.0 ppm) with high Sr/Y ratios (17–63), resembling adakitic rocks. These adakitic rocks can be classified into high-Mg (Mg#= ∼51) and low-Mg groups (Mg#= ∼35), although they all display enriched large ion lithophile elements and light rare earth elements but depleted high field strength elements. The studied low-Mg adakitic rocks have low MgO (∼0.68 wt%) and Cr (∼35 ppm) with enriched zircon Hf isotopes (εHf(t) = − 1.16 t 2.44), suggesting their derivation by partial melting of thickened juvenile crust. Conversely, the high-Mg adakitic rocks exhibit high MgO (∼1.94 wt%) and Cr (∼61 ppm) with relatively high zircon εHf(t) values (−0.68 to 4.95), indicating their derivation from foundered crustal root with subsequent interaction with enriched mantle-derived magma. Tectonic reconstructions suggest that the formation of these Late Triassic adakitic rocks occurred in a post-collisional extension setting and the East Kunlun Paleo-Tethyan Ocean closed no later than Norian.

Neoarchean and Paleoproterozoic magmatism in the Longjiang area, NE China: Implications for the recycling history of supracrustal components in the eastern Central Asian Orogenic Belt

Substantial Phanerozoic crustal growth and reworking in the eastern Central Asian Orogenic Belt has partly obscured the Precambrian geological record. Accurate identification of such Precambrian magmatism and its petrogenesis is paramount to unveiling the early crustal evolution of the eastern Central Asian Orogenic Belt. Two outcrops of Neoarchean and Paleoproterozoic magmatism in the Longjiang area of NE China, eastern Central Asian Orogenic Belt, were recently discovered. The Kaoquantun outcrop consists of monzogranites that yield a zircon 207Pb/206Pb age of 2568 ± 8 Ma and are similar to peraluminous A1-type granites. They were derived from the partial melting of a hybrid crustal source comprising newly accreted Mesoarchean crust and abundant supracrustal metapelites in a post-collisional extensional setting. Such metapelites were initially part of the upper crust but were buried in the middle−lower crust due to crustal thickening prior to ca. 2.6 Ga. In the Zhanbeitun outcrop, basaltic andesites overlie monzogranites, and both have been intruded by syenogranites. The monzogranites and syenogranites yield zircon 207Pb/206Pb ages of 1881 ± 10 Ma and 1843 ± 3 Ma, respectively, whereas the basaltic andesites are inferred to have formed at 1.88−1.84 Ga. The 1.88 Ga monzogranites have relatively enriched zircon Hf-O isotopic compositions and were derived from a hybrid melt of mafic rocks in the lower crust and ancient metabasalts that had undergone low-temperature alteration at supracrustal depths. The 1.88−1.84 Ga basaltic andesites are Nb-rich and were generated by the partial melting of a mantle wedge that was metasomatized by subduction-derived fluids, recycled sediments, and slab-derived melts. The 1.84 Ga syenogranites are peraluminous and K-rich, and record a profound mixing and assimilation process involving melts derived from supracrustal sediments and the lower crust. Three stages of Paleoproterozoic magmatism emerged from subduction-related crustal thickening, slab breakoff, and collision, respectively. The zircon Hf isotopic data further indicate that the eastern Central Asian Orogenic Belt underwent at least one stage of proto-crustal growth at ca. 3.0 Ga, followed by multiple phases of crustal reworking during the Neoarchean and Paleoproterozoic, with dominant involvement of supracrustal components.

New precise age constraints of the Hatu gold belt, west Junggar, NW China: Implications for a 300 Ma magmatic hydrothermal event in post-collisional setting

A 300 Ma magmatic hydrothermal ore-forming process is identified in the Hatu gold belt in west Junggar, northwest China, based on precise age constraint by secondary ion mass spectroscopy U-Pb dating of hydrothermal zircon and Ar-Ar dating of muscovite. The gold deposits in the Hatu belt (Hatu, Qi-V, Huilvshan, etc.) are similar in geology, with NE- and NW-trending orebodies composed of gold-bearing quartz veins and auriferous altered tuffaceous wall-rocks. Hydrothermal zircon grains separated from gold-bearing quartz veins in the Qi-V gold deposit provide a concordia age of 299.6 Ma and a nearly identical weighted mean 206Pb/238U age of 300 ± 2 Ma. Muscovite samples also from the gold-bearing quartz veins yield 40Ar/39Ar plateau ages of 299.6 ± 1.7 Ma, 299.9 ± 1.8 Ma, and 300.6 ± 1.9 Ma. By comprehensive compilation of geological and geochemical characteristics, tectonic evolution, and geochronology of the gold deposits in the Hatu gold belt, our new precise age data confirm that the gold deposits in the Hatu belt formed simultaneously at ca. 300 Ma, and likely formed during a post-collisional extensional setting by magmatic hydrothermal fluid from cooling magmatic sources.

Carboniferous–Early Permian volcano-sedimentary units from the northern margin of the North China Craton recorded subduction initiation in the Paleo-Asian Ocean

The precise timing of subduction initiation of the Paleo-Asian Ocean (PAO) in the northern margin of the North China Craton (NCC) remains controversial. In this study, we have conducted zircon UPb dating, whole-rock major and trace elements, and zircon Hf isotope analyses on metadacites and metamorphic greywackes from the Changde volcano-sedimentary rocks, feldspar quartz sandstones and argillaceous siltstones from the Luquantun Formation and the Sidao sedimentary rocks in central-eastern Jilin Province (NE China), to constrain the initial closure of the PAO. We obtained Early Carboniferous zircon UPb ages (345 ± 2 Ma and 351 ± 7 Ma) from the Changde volcano-sedimentary rocks. The metadacites are characterized by the enrichment in light rare earth elements and large ion lithophile elements, depletion in heavy rare earth elements and high field strength elements, and rare earth element ratios similar to those of the continental crust, indicating that these rocks originated from crustal melting. The metadacites exhibit affinity with the peraluminous A-type granite, with high zircon εHf(t) values (+3.46 − +12.21) and TDM2 of 1132–571 Ma, suggesting that they formed in a post-collisional extensional setting. Detrital zircon UPb ages from the Luquantun Formation and Sidao sedimentary rocks constrain their maximum depositional ages to the Late Carboniferous (319 ± 5 Ma) and Early Permian (284 ± 4 Ma), respectively. Most detrital zircons have age peaks at 1800 Ma with negative zircon εHf(t) values, and 2500 Ma with positive zircon εHf(t) values, suggesting that the Carboniferous–Early Permian sediments from these two formations were mainly sourced from the NCC. Moreover, the chemical index of alteration, index of compositional variability, and plagioclase index of alteration values suggest that the sediments of the Changde volcano-sedimentary rocks originated from a mafic igneous source with a low degree of weathering and a sedimentary recycling history, while the samples from the Luquantun Formation and Sidao sedimentary rocks were sourced from an intermediate-felsic igneous rock with a moderate to intense degree of weathering and mainly influenced by source composition. The crystallization age curves and summary plots of detrital zircons indicate that the Early Carboniferous metamorphic greywackes from the Changde Formation were mainly deposited in a passive margin, while the Carboniferous–Early Permian sediments of the Luquantun Formation and Sidao sedimentary rocks were mainly deposited in an active margin. We suggest that the tectonic transition from passive to active margin occurred at ∼333 Ma, marking subduction initiation in the PAO close to the northern margin of the NCC.

The genesis of the Baguamiao orogenic gold deposit linked to multistage ore-forming processes in Triassic tectonic transition of South Qinling, China

The Baguamiao gold deposit represents one of the largest Triassic orogenic gold deposits in the South Qinling metallogenic belt of central China. Here we investigate Ar-Ar dating of sericite and muscovite, fluid inclusion of quartz, the trace element and sulfur isotopic composition of pyrite from different mineralization stage in the Baguamiao gold deposit to gain insights into the ore genesis. The Ar-Ar dating results yield 40Ar/39Ar plateau ages of 217.5 ± 2.1 Ma, 209.2 ± 0.2 Ma and 208.8 ± 0.2 Ma, indicating a two-stage gold mineralization during the tectonic transition from the early metamorphic deformation (at ca. 218 Ma) to the late extension (ca. 209 Ma) in the post-collision environment. Fluid inclusion, trace element and sulfur isotopic analyses reveal that the two stages gold mineralization show different sources. The early mineralization period developed CO2-rich inclusions with medium temperature (317–413 °C) and medium–low salinity (4.62–18.81 wt% NaCleqv.), and is characterized by formation of Py1 and Py2 with depleted Au, Cu, Pb, Zn, Ag, Te and Bi. The sulfur isotope data of Py1 and Py2 are between 3.6 ∼ 10.5 ‰, suggesting that the sulfur source was mainly derived from the surrounding rock (Xinghongpu Group). The late mineralization period developed medium–high temperature (321–453 °C), medium–high salinity (3.3–19.62 wt% NaCleqv.) CO2-rich inclusions and high-temperature and high-salinity daughter crystal inclusions, and is characterized by formation of Py3 and Py4 with obviously higher arsenic and metal contents, such as Au, As, Cu, Pb, Zn, Ag, Cr, Ti, Mn, Sb. The δ34S values of Py3 and Py4 are 8.3 ∼ 18.8 ‰, suggesting a mixed fluid of the Xinghongpu Group and Gudaoling Group with deep magmatic-hydrothermal fluids. We propose that the two periods of gold mineralization in the Baguamiao deposit, in response to the collision assembly of Qinling Orogenic Belt, preserve evidence for diverse metal sources with the early mineralization period sourced from the metamorphism of the upper crust (Xinghongpu Group) in the compressional stage (ca. 218 Ma), and the late mineralization period characterized by the contribution of the Xinghongpu Group, Gudaoling Group and deep magmatic-hydrothermal fluids in a post-collisional extensional setting (ca. 209 Ma). The role of metamorphism and magmatic activity during the Triassic Qinling orogenic processes is considered to be important in the remobilization, transport and enrichment of sulfur, arsenic and gold (and potentially other trace elements) from the earlier formed seafloor exhalation sequence.

Petrogenesis of early Paleozoic peraluminous A-type granites from southern China

ABSTRACT Early Paleozoic A-type granites in South China are rare, and their petrogenesis is still highly debated. This study focuses on the controversial Epo pluton in the Wuyi-Yunkai Orogen and conducts petrological and petrogeochemical research to discuss its origin. The Epo granites were emplaced at approximately 414 Ma and exhibit petrological and geochemical features of A-type granites. They contain only one ferromagnesian mineral, ferrum biotite. The granites mostly have high alkalis contents (Na2O + K2O = 6.48–8.85 wt.%), high field strength element (HFSE) contents (Zr + Nb + Ce + Y = 353–461 ppm), and high Ga/Al (104×Ga/Al = 2.50–3.72) and FeOT/(FeOT + MgO) (0.74–0.85) ratios. Besides, they formed under high-temperature (>850°C) and low-pressure (≤5 kbar) conditions. Their high A/CNK (average 1.20) and Eu/Eu* (average 0.46) values as well as Sr (average 128 ppm) and Ba (average 628 ppm) contents but relatively low Zr contents (approximately 200 ppm) further suggest a strongly peraluminous A-type affinity, a distinctive subtype of A-type granites. The Epo granites display enriched Sr-Nd-Hf isotope compositions with a whole-rock87Sr/86Sri average ratio of 0.712, whole-rock εNd(t) values ranging from − 8.9 to − 7.6, and zircon εHf(t) values clustering at − 8. Their two-stage Nd and Hf isotope model ages are 1.7 Ga to 2.0 Ga, suggesting ancient crustal sources. Furthermore, Nb/U (average 7.46), Ce/Pb (average 4.06) and other trace element ratios indicate negligible contributions from juvenile mantle materials. Our data on the Epo pluton, coupled with other contemporary A-type granites in South China, suggest that the high-temperature melting of metasedimentary basements generated strongly peraluminous A-type granites, while the involvement of metaigneous rocks or mantle-derived magmas resulted in metaluminous to weakly peraluminous A-type granites. Source diversity leads to different aluminium saturations. The heat required to produce high-temperature A-type magmas was supplied by upwelling mantle-derived magma in a postcollisional extensional setting.

Early Permian Post-Collision Extensional Setting in the Southern Beishan Orogenic Belt: Evidence from the Zhangfangshan Granodiorite and the Baishantang Bimodal Volcanic Rocks

Outcrops of late Paleozoic magmatic rocks are common in the Southern Beishan Orogenic Belt (SBOB), Southern Central Asian Orogenic Belt (CAOB), which is a key object for the understanding of regional tectonism and defining the final closure time of the Paleo-Asian Ocean (PAO). We present zircon U-Pb chronology and whole-rock geochemistry data for late Paleozoic granodiorites and bimodal volcanic rocks from the Shuangyingshan-Huaniushan unit in the north Huitongshan-Zhangfangshan ophiolitic belt in the SBOB. The Zhangfangshan granodiorites (LA-ICP-MS, Ca. 288 Ma) are A2-type granite enriched in Rb, Th, Pb and LREEs and depleted in Nb, Ta, Ti, Sr, Ba and HREEs. They have varying MgO and TFe2O3 contents with high Mg# (38.56~48.97) values; the Lu/Yb ratios (0.14~0.15) of these granodiorites are similar to mantle-derived magma. A clear plagioclase zoning structure and acicular apatite occur in mineral assemblages derived from magma mixing between mafic and felsic magmas. The Baishantang bimodal volcanic rocks (272 Ma) consist of rhyolite and basaltic andesite. Baishantang rhyolites are A2-type felsic rock enriched in Rb, Th, Pb and LREEs and depleted in Nb, Ta, Ti, Sr, Ba and HREEs, with negative εNd(t) and εHf(t) (−5.2~−4.8 and −2.2~−1.9, respectively). Rhyolites originated from the partial melting of the crust, influenced by mantle material. Basaltic andesites belong to calc-alkaline series and have an enrichment of Rb, Ba, Th, U, Pb and LREEs, are weakly enriched in Zr-Hf, and are depleted in Nb, Ta, Ti and HREEs. The Nd-Hf isotopes of these basaltic andesites are not coupled with negative εNd(t) (−2.8~−0.4) and positive εHf(t) (1.8~5.5) values. These characteristics indicate that they originated from the partial melting of the mantle mixed with sediment-derived melts. In combination with previous studies, our findings show that the early Permian Zhangfangshan granodiorites and Baishantang bimodal volcanic rocks formed in a post-collision extensional setting, and the Huitongshan-Zhangfangshan ocean had been closed before early Permian.

Eclogite in the East Kunlun Orogen, northwestern China: A record of the Neoproterozoic breakup of Rodinia and early Paleozoic continental subduction

An early Paleozoic eclogite belt extends discontinuously for ∼500 km in the East Kunlun Orogen, northwestern China, which provides insight into the evolution of the Proto-Tethys Ocean. However, the nature and age of the eclogite protoliths and subsequent metamorphism are unclear. Here, we report mineral chemical, zircon, and titanite geochronological; whole-rock major and trace elemental; and Sr-Nd isotopic data for eclogites from the Xiarihamu area in the western East Kunlun Orogen. At a minimum, these eclogites underwent peak eclogite-facies metamorphism at 27−28 kbar and 630−650 °C, post-peak decompression, and retrograde amphibolite-facies overprinting. Geochemically, eclogites have characteristics typical of enriched mid-oceanic-ridge basalts and oceanic-island basalts. Inherited magmatic zircon cores yielded a protolith age of 828 Ma, and metamorphic zircons yielded peak and post-peak decompression ages of 443 Ma and 422 Ma, respectively. A titanite U-Pb age of 413 Ma constrains the timing of later amphibolite-facies overprinting. The protoliths of the Xiarihamu eclogites were the products of continental rift-related basaltic magmatism, which was generated during the initial breakup of Rodinia. The protoliths experienced eclogite-facies metamorphism in a continental subduction/collisional setting during the early Paleozoic. The formation of the Xiarihamu eclogites suggests that the Proto-Tethys Ocean had closed by 443 Ma. Given the spatio-temporal relationship between the Xiarihamu magmatic Cu-Ni sulfide deposit and the eclogites studied, the former may have formed in a post-collisional extensional setting. The eclogites and magmatic Cu-Ni sulfide deposit from the Xiarihamu area jointly reveal the evolution of the East Kunlun Orogen from Neoproterozoic supercontinent breakup to early Paleozoic tectonic transitions from collisional convergence to post-collisional extension.

Contrasting Nb-Ta mineralization between the Mufushan and Lianyunshan granites, South China: Evidence from whole-rock and zircon geochemistry and geochronology

The Mufushan (MFS) and Lianyunshan (LYS) granitic complexes are located in northeastern Hunan (South China) and host important Nb-Ta mineralization. Fine-grained muscovite granites (FGMGs) occur as intrusions within the Lengjiaxi Group and show a spatial relationship with the ore-bearing pegmatites. Here, geochemical, geochronological, and isotopic investigations were carried out on the FGMGs of the MFS and LYS complexes to compare their magma sources and metallogenic potential. The MFS and LYS FGMGs are strongly peraluminous (A/CNK > 1.5) and highly fractionated S-type granites. Comparatively, the MFS FGMGs possess more negative Eu (Eu/Eu* = 0.19–0.60), Ba (3.3–360 ppm), Sr (6.2–156 ppm), and Zr (4–26 ppm) anomalies, and more positive Ta (1.1–15.8 ppm) anomalies than the LYS FGMGs, suggesting a stronger magma differentiation. In addition, the zircons from the MFS FGMGs exhibit higher contents of Hf (MFS = 34336 to 60010 ppm, LYS = 6795 to 28937 ppm), Nb (MFS = 16.49 to 401.56 ppm, LYS = 1.12 to 20.22 ppm), Ta (MFS = 33.45 to 138.65 ppm, LYS = 1.11 to 7.19 ppm), Th (MFS = 458 to 3630 ppm, LYS = 11 to 710 ppm), and U (MFS = 8400 to 30100 ppm, LYS = 1285 to 5220 ppm) than those from the LYS FGMGs, suggesting that the MFS FGMGs are more fractionated and their zircons have undergone hydrothermal alteration. Zircon U-Pb dating yielded concordant ages of 140.6 ± 0.5 Ma and 139.9 ± 0.7 Ma for the MFS and LYS FGMGs, respectively, implying an Early Cretaceous magmatic-hydrothermal event. Moreover, the geochemical and isotopic (whole-rock Sm-Nd and zircon Lu-Hf) compositions show younger sources for the MFS FGMGs compared to the LYS FGMGs, which were derived from the partial melting of Paleoproterozoic basement rocks with clay-rich material in a post-collision extensional setting. Our results, combined with previous studies, indicate that the studied FGMGs are the parental rocks of extensive Nb-Ta mineralization, and the highly fractionated MFS FGMGs are more conducive to hosting Nb-Ta mineralization than the LYS FGMGs. The strongly peraluminous and highly-fractionated S-type FGMGs can be used as good targets for further exploration of Nb-Ta deposits in South China and elsewhere.

Timing of Transition from Proto- to Paleo-Tethys: Evidence from the Early Devonian Bimodal Volcanics in the North Qaidam Tectonic Belt, Northern Tibetan Plateau

The transition from the Proto- to the Paleo-Tethys is still a controversial issue. This study reports a new petrology, zircon U–Pb geochronology, and whole-rock geochemistry of volcanic rocks from the Maoniushan Formation in the Nankeke area, northern Qaidam (NQ) of the Tibetan Plateau, to provide new evidence for the transition from the Proto- to the Paleo-Tethys oceans. The volcanic suite consists mainly of rhyolitic crystal lithic tuff lavas and minor basalts. Zircon U–Pb data indicate that the bimodal volcanic rocks were formed during the Early Devonian (ca. 410–409 Ma). Geochemically, the basalts have low contents of SiO2 (48.92 wt.%–51.19 wt.%) and relatively high contents of MgO (8.94 wt.%–9.99 wt.%), TiO2 (1.05 wt.%–1.29 wt.%), K2O (2.35 wt.%–4.17 wt.%), and K2O/Na2O ratios (1.04–2.56), showing the characteristics of calc-alkaline basalts. Their rare earth element (REE) patterns and trace element spider diagrams are characterized by enrichments in LREEs (LREE/HREE = 18.31–21.34) and large ion lithophile elements (LILEs; Rb, Th, and K) and depletion in high-field-strength elements (HFSEs; Nb, Ta, P, and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.82–0.86), which are similar to Etendeka continental flood basalts (CFB). These features suggest that the basalts were most likely derived from low degree (1%–5%) partial melting of the asthenospheric mantle, contaminated by small volumes of continental crust. In contrast, the felsic volcanics have high SiO2 (68.41 wt.%–77.12 wt.%), variable Al2O3 (9.56 wt.%–12.62 wt.%), low MgO, and A/CNK ratios mostly between 1.08 and 1.15, defining their peraluminous and medium-K calc-alkaline signatures. Their trace element signatures show enrichments of LREEs and LILEs (e.g., Rb, Th, U, K, and Pb), depletion of HFSEs (e.g., Nb, Ti, Ta, and P), and negative Eu anomalies (Eu/Eu* = 0.22–0.66). These features suggest that the felsic volcanics were derived from partial melting of the middle crust, without interaction with mantle melts. Considering all the previous data and geochemical features, the Maoniushan Formation volcanic rocks in NQ formed in a post-collisional extensional setting associated with asthenospheric mantle upwelling and delamination in the Early Devonian. Together with the regional data, this study proposed that the Proto-Tethys Ocean had closed and evolved to the continental subduction/collision orogeny stage during the Middle to Late Ordovician, evolved to the post-collisional extensional stage in the Early Devonian, and finally formed the Zongwulong Ocean (branches of the Paleo-Tethys Ocean) in the Late Carboniferous, forming the tectonic framework of the Paleo-Tethys Archipelagic Ocean in the northern margin of the Tibetan Plateau.

Crust–mantle interaction beneath the Pamir orogenic belt: Insights from the Miocene Dunkeldik carbonatite–syenite complex in Tajikistan

Only some carbonatite–syenite complexes in the world could form carbonatite-related rare earth element (REE) deposits, which is regarded as the main REE origin. The Miocene Dunkeldik carbonatite–syenite complex is located in the Dunkeldik magmatic field in Tajikistan between the Pamir and Tibetan plateaus. So far, the origin, tectonic setting, and REE mineralization potential of the Dunkeldik carbonatite–syenite complex remain unclear due to the limited isotopic values, which inhibit the further understanding the genesis of such carbonatite complex. This study presents zircon U–Pb age, whole-rock geochemical, and Sr–Nd–Pb isotope, and zircon Hf–O isotope data for the Dunkeldik carbonatite–syenite complex on the Pamir Plateau. Zircon U–Pb dating of syenite yielded two concordant ages of 10.27 ± 0.16 and 10.48 ± 0.21 Ma, indicative of formation in a post-collisional extensional setting related to India–Asia collision. Both the carbonatite and syenite exhibit similar enrichments in large-ion lithophile elements (e.g., Sr and Ba) and light REEs, depletions in high-field-strength elements (e.g., Nb and Ta) and heavy REEs, and small Eu anomalies. Whole-rock Sr–Nd–Pb isotopic compositions are highly enriched, with high (87Sr/86Sr)i (0.70971 to 0.70979), low εNd(t) (–8.9 to –9.2), and radiogenic Pb isotope composition (206Pb/204Pb (18.322–18.358), 207Pb/204Pb (15.689–15.695), and 208Pb/204Pb (39.091–39.149)). Zircons Hf-O isotope analyses yield low εHf(t) values (–9.5 to –19.0) and high δ18O value (7.4 ‰–9.5 ‰), respectively. These geochemical and isotopic features indicate that the complex was formed by partial melting of enriched lithospheric mantle during the Miocene India–Asia collision and was assimilated by upper crustal material during magma ascent. The carbonatite and syenite in the complex formed by melt immiscibility. The involvement of crustal material in the magma and less development of fenitization may responsible for the limited REE mineralization potential as compared with other large REE deposits that formed during coeval India–Asia collision.

Compositions and ages of Early Cretaceous volcanic and plutonic rocks in central Tibet: Insights into the magmatic and uplift response to slab breakoff

AbstractWe present new zircon U-Pb ages and Hf isotope compositions as well as whole-rock major- and trace-element geochemical and Sr-Nd isotopic data for silicic plutonic and volcanic rocks from the Duolong area of central Tibet. Combined with existing data, our new data indicate that these plutonic and volcanic rocks were formed in two stages ca. 120 Ma and ca. 110 Ma, respectively, in a postcollisional extensional setting that was triggered by slab breakoff. The similar geochemical compositions of granitoids and rhyolites, combined with their close spatial and temporal relationships, suggest that they were both derived from juvenile crustal material within a single magmatic system. We propose that the two inferred crustal melting events in the Duolong area were caused by two episodes of deep mantle activity triggered by the transition of the plate subduction angle from steep to shallow in response to the ascent of buoyant continental lithosphere during slab breakoff. Furthermore, rapid surface uplift during the late Early Cretaceous caused by slab breakoff made an important contribution to the formation of the proto–Tibetan Plateau. This study provides new insights into postcollisional tectonomagmatism and plateau uplift in central Tibet triggered by slab breakoff. We propose more generally that tectonic uplift during postcollisional processes (i.e., slab breakoff and lithospheric delamination) is a major contributor to plateau uplift in collision zones.

Contribution of Triassic Tectonomagmatic Activity to the Mineralization of Liziyuan Orogenic Gold Deposits, West Qinling Orogenic Belt, China

The Western Qinling orogenic belt (WQOB) is one of the most important prospective gold districts in China, with widely distributed Indosinian intermediate–acidic intrusions. The Liziyuan Au deposit is a representative orogenic deposit in the northern WQOB, hosting several sections spatially associated with igneous rocks. The Au deposit is hosted by meta-sedimentary volcanic rocks of the Cambrian–Ordovician Liziyuan Group and the Tianzishan monzogranite. Two periods, including five stages of mineralization, are recognized in this area: an early metamorphic mineralization period (PI), including quartz–pyrite (Stage I) and banded quartz–polymetallic sulfide (Stage II) veins, and a later magmatic mineralization period (PII) including quartz–K-feldspar–pyrite–molybdenite veins (Stage III), quartz–polymetallic sulfide–chlorite ± calcite veinlets and stockwork (Stage IV), and late calcite–quartz veinlets (Stage V). Geochronological studies indicate a SHRIMP zircon U-Pb age of 236.1 Ma for the Tianzishan monzogranite, and our published ages of ore-bearing diorite porphyrite of the Suishizi section and granite porphyry of the Jiancaowan section being 213 and 212 Ma, respectively. Pyrites formed in association with PI and PII mineralization have well-defined Rb–Sr ages of 220 ± 7.5, 205.8 ± 8.7, and 199 ± 15 Ma, with close temporospatial coupling between mineralization and magmatism. The δ18O and δD values of fluid inclusions in Stage IV auriferous quartz veins range from −0.03‰ to +5.24‰ and −93‰ to −75‰, respectively, suggesting that mineralizing fluid was likely of magmatic origin. Three distinct ranges of δ34S values are identified in the studied sections (i.e., 7.04‰–9.12‰, −4.95‰ to −2.44‰, and 0.10‰–3.08‰), indicating a source containing multiple sulfur isotopes derived from magmatic and metamorphic fluids. The Liziyuan Au deposit is thus likely an orogenic deposit closely related to magmatism. Geochemical characteristics indicate that Tianzishan monzogranite is adakitic and was derived from thickened lower crust during Triassic orogenesis. The ore-bearing diorite porphyrite and granite porphyry formed in a post-collision extensional setting. Together with previous geological and geochemical data, our results indicate that the Liziyuan orogenic Au deposit was formed by early collisional–compressional metamorphism and late post-collision extensional magmatic fluids related to the evolution of the WQOB.

Late Paleozoic tectonics of Southern Central Asian orogenic belt: Evidence from magmatic rocks in the northern Alxa, Northwest China

Late Paleozoic magmatic rock outcrops are common in the Northern Alxa, Southern Central Asian Orogenic Belt (CAOB), which is a key area for understanding tectonic processes and defining the final closure time of the Paleo-Asian Ocean (PAO). We present zircon U‒Pb chronology and whole-rock geochemistry data for late Paleozoic magmatic rocks from the Yagan area of northern Alxa. This paper reveals two periods of magmatism: syenogranite (374.8 Ma) and bimodal intrusive rocks, which consist of gabbro (261.4 Ma), diabase (262.9 Ma) and biotite monzogranite (263.4 Ma). The syenogranite is high-K calc-alkaline and shows enrichments in Th, Zr, Hf and LREEs; depletions in Sr, Nb, Ta, and Ti; and low Mg# values (6.9–13.2); the syenogranite was derived from partial melting of the crust and has volcanic arc characteristics. The gabbro and diabase have similar geochemical characteristics, such as enrichments in Pb, Rb, Sr, Zr, and Hf and depletions in Nb, Ta, and Ti, with positive εHf(t) values (+0.9–+2.7 and +2.6–+3.6, respectively), indicating that they originated from partial melting of depleted mantle and experienced crustal contamination during magma emplacement. The biotite monzogranite shows depletions in Nb, Ta, and Ti and εNd(t) values of -2.6 to −2.4 and resulted from partial melting of the lower crust caused by asthenospheric underplating. The bimodal intrusive rocks formed in a postcollision extensional setting. Combined with previous data, we conclude that northern Alxa was an active continental margin during the late Devonian and that the final closure of the Yagan branch ocean of the PAO occurred prior to the middle Permian.

Growth of early Paleozoic continental crust linked to the Proto-Tethys subduction and continental collision in the East Kunlun Orogen, northern Tibetan Plateau

The East Kunlun Orogen (EKO) in the northern Tibetan Plateau records two continental collisional orogenic events and magmatism in early Paleozoic and early Mesozoic. However, possible magmatic additions to the continental crust growth of the EKO in different tectonic stages of early Paleozoic collisional orogeny have been overlooked. Three phases of early Paleozoic plutons from the Xiangride-Kuhai area in the east of the EKO have been chosen for detailed investigation and the results are reported here. The oldest magmatic suite (Stage 1) includes the ca. 471 Ma Qurelong Monzodiorite and ca. 454 Ma granodiorite in the Zhiyu Intrusive Complex. The monzodiorite has a sanukitoid-like composition with high TiO2 and Y contents and is interpreted as being derived from partial melting of metasomatized mantle wedge lherzolite. The granodiorite is typified by its high SiO2 content, high Sr/Y ratio, and depleted Hf isotope, and is interpreted as an adakite-like melt derived from the melting of a subducted Proto-Tethys oceanic crust. The magmatism can be linked to northward subduction of the Proto-Tethys Ocean between 520 and 450 Ma. Stage 2 magmatism is represented by a plutonic suite emplaced during ca. 450−431 Ma with an I-type granitic composition. Of these, the ca. 447 Ma Kengdenongshe Intrusion composed of peraluminous granite with enriched Nd-Hf isotopes is indicative of a Mesoproterozoic igneous source in the orogen. The ca. 450−434 Ma monzogranite and granodiorite in the Walega and Zhiyu intrusive complexes exhibit variable element and isotope compositions. They would have been generated by magma mixing of felsic melts from the old crust and mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion of >30%. The ca. 431 Ma quartz diorite in the Walega Intrusive Complex is formed through crustal assimilation and fractional crystallization of mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion >60%. Stage 2 suite was emplaced during the closure of Proto-Tethys oceanic branches and subsequent continental collision during 450−426 Ma. Magmatism diminished between ca. 426 and 410 Ma during exhumation of the continental lithosphere as indicated by the presence of retrograde eclogites in the EKO. Stage 3 magmatic suite includes the ca. 408 Ma Langmuri Intrusion and ca. 403 Ma Niantang Syenogranite. These plutons are adakite-like or have an A-type granitic composition and are enriched in Nd-Hf isotopes. They might have been derived from the remelting of old and juvenile continental crust in a post-collisional extensional setting during 410−390 Ma. Identification of partial melts, derived from the subducted Proto-Tethys oceanic crust and metasomatized lithospheric mantle in stage 1 and 2 plutons, show that the subcrustal materials have been significantly transferred to the overlying continental crust. Hence the magmatism in oceanic subduction (Stage 1) and continental collision (Stage 2) settings contributes to the early Paleozoic juvenile continental crust growth of the EKO. The post-collisional extensional setting (Stage 3) is dominated by the reworking of a pre-existing continental crust. The early Paleozoic continental crust growth processes in the EKO are different from the previous view in which the continental collision orogens have no crust growth, and inconsistent with the proposal that crust growth is significant only in a continental collision setting.