Biodiversity across the Guadalupian-Lopingian Boundary: first results on the ostracod (Crustacea) fauna, Chaotian section (Sichuan Province, South China)

International audienceThe Middle Permian-Late Permian boundary (Guadalupian-Lopingian boundary, GLB) interval is characterised by important faunal assemblage changes. This extinction-turnover episode is considered by some authors to be the first step of the end-Permian biodiversity drop. The forty-five meters thickness of sediment encompassing the GLB in Chaotian section (Sichuan Province, South China) was sampled and processed for ostracod study. This study presents the first analysis of ostracod faunas in the GLB interval. A total of 154 species belonging to 29 genera are identified. Three species are described as new: Bairdia chaotianensis Zazzali, n. sp., Microcheilinella wujiapingensis Zazzali, n. sp., Microcheilinella pagodaensisZazzali, n. sp. All the ostracods discovered in the section belong to shallow marine taxa. So these results are not consistent with previous interpretations (lagoonal environment or deep water setting) based on other evidences. Abundance and diversity present a rapid and noticeable decline in the Early Capitanian. Recovery is then recorded about three meters above the GLB. At specific level, a 93% extinction rate and a 96% turnover rate are recorded at the GLB. Moreover, Palaeocopida, straight dorsal border ostracods known to progressively disappear from the Late Permian to the basal Middle-Triassic, are here less abundant and diversified after the GLB. This could reflect the first step of their disappearance at the end of the Palaeozoic

Early Paleoproterozoic magmatism in the Yangtze Block : evidence from zircon U-Pb ages, Sr-Nd-Hf isotopes and geochemistry of ca. 2.3 Ga and 2.1 Ga granitic rocks in the Phan Si Pan Complex, north Vietnam

This study was financially supported by projects from the China Natural Science Foundation (41672222) and State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Wuhan (MSFGPMR201802). PAC acknowledges support from Australian Research Council grant FL160100168. This study was also supported by the Ministry of Natural Resources and Environment of Viet Nam, Project BĐKH.29/16-20 to Dung My Tran.Our understanding of the early evolution of the Yangtze Block is limited by the sparsely dispersed nature of pre-Neoproterozoic exposures. New, integrated petrographic, zircon U-Pb age and Hf-Nd isotope analyses, and whole-rock geochemical data for early Paleoproterozoic granites in the Phan Si Pan Complex provides new insights into the evolution of the Yangtze Block as well as its role in the Pre-Nuna supercontinent. LA-ICP-MS zircon U-Pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207Pb/206Pb ages of 2306 ± 12 Ma and 2096 ± 15 Ma, respectively. Zircons from the quartz monzonite have εHf(t) values ranging from -4.1 to -2.1, corresponding to TDM2 model ages of 3002–2890 Ma, whereas zircons in the gneissic granite have εHf(t) values between -0.95 and +1.72 and corresponding TDM2 model ages of 2660–2516 Ma, which are consistent with their whole-rock Nd isotope values. Geochemically, the quartz monzonites are I-type granites. Combined with their relatively high Sr/Y ratios and low Y concentrations, as well as fractionated REE patterns with relatively high LREE but low HREE concentrations, they were probably generated by partial melting of the thickened middle-lower crust under elevated temperature. Geochemical and isotopic signatures suggest that the ca. 2.1 Ga gneissic granites are high-K calc-alkaline, ferroan A-type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material. The Phan Si Pan complex has a distinct early Paleoproterozoic crustal evolution history compared with the other crustal provinces of the Yangtze Block, suggesting independent histories that were not unified until the late Paleoproterozoic during the assembly of Nuna. Moreover, the magmatism and tectonic evolution of the north Vietnam region is broadly similar to that of the Arrowsmith Orogen of the Rae craton in Laurentia suggesting a potential spatial linkage. The geologic record of the Yangtze Block does not support an early Paleoproterozoic shutdown of plate tectonics.PostprintPeer reviewe

Neoarchean and Paleoproterozoic K-rich granites in the Phan Si Pan Complex, north Vietnam : constraints on the early crustal evolution of the Yangtze Block

This study was financially supported by projects from the China Natural Science Foundation (41672222) and State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Wuhan (MSFGPMR201802). PAC acknowledges support from Australian Research Council grant FL160100168. This study was also supported by the Ministry of Natural Resources and Environment of Vietnam, Project BĐKH.29/16-20 to Dung My Tran.Precambrian igneous and metamorphic rocks of the Phan Si Pan Complex, North Vietnam, constitute the southern extension of the Yangtze Block, and provide a valuable record of the early evolution of the continental crust. We present results of U-Pb zircon geochronology and geochemistry for Precambrian granites in this complex to constrain their emplacement age and genesis. Granites from three plutonic bodies yielded ages of 2848 ± 15 Ma, 2768 ± 19 Ma and 1869 ± 30 Ma, which represent newly-recognized late Archean to Paleoproterozoic potassic granite plutonism in the southern Yangtze Block. The average εHf(t) values range from −6.2 to 0.1 for the 2.85–2.77 Ga granitic rocks and -13.1 to -9.2 for the ca. 1.86 Ga granitic rocks, with two-stage model ages of 3.64 to 3.20 Ga and 3.31 to 3.07 Ga, respectively, suggesting derivation from partial melting of Paleoarchean and Mesoarchean crust. The late Archean potassic granites exhibit high K2O, and high Sr/Y and (La/Yb)N ratios with negligible Eu anomalies, indicating derivation from melting of the thickened lower crust, which is inferred to have occurred in an active margin setting. The late Paleoproterozoic alkali feldspar granites are characterized by high FeOT/(FeOT + MgO)(0.96–0.99) and 10000∗Ga/Al (2.75–2.94) ratios, showing an affinity of A-type granite. These A-type granites exhibit flat chondrite-normalized HREE patterns and strong negative Eu anomalies, and low Sr/Y and (La/Yb)N ratios, corresponding to melting at a shallow depth, probably in a post-collisional extension setting. Comparison of the rock units and events recorded by the Phan Si Pan complex with other Archean to Paleoproterozoic complexes (Houhe, Dongchuan, Yudongzi, Douling, Zhongxiang and Kongling complexes) in the Yangtze Block indicate spatially distinct histories of crustal growth, and thus may reflect independent terranes. The ca. 1.86 Ga post-collisional magmatism, which succeeds a 2.0–1.9 Ga metamorphic event, is distributed throughout the Yangtze Block, including the Phan Si Pan Complex, suggesting assembly of the disparate terranes and final cratonization of the Yangtze Block overlaps with, and may be related to, assembly of the Nuna supercontinent.PostprintPeer reviewe

Late cretaceous granitoids along the northern Kuching zone: implications for the Paleo-pacific subduction in Borneo

The EW-trending Kuching zone in Borneo is a target region for exploring the southern continuation of Paleo-Pacific subduction from South China, Vietnam to SE Asia. Previous studies mainly focused on mafic igneous rocks, and poor attention has been paid to the contemporaneous granitoids in this zone. This study presented detailed zircon U-Pb geochronology and Lu-Hf isotope and whole-rock geochemistry analyses for late Cretaceous granitoids (granodiorites and granites) in the northern Kuching zone. These granitoids are dated at ~77.5-83.6 Ma with younger ages than the igneous rocks in the southern Kuching zone (~130-144 Ma). The granitoids have variable SiO2 (64.86-77.37 wt.%) and A/CNK (0.7-1.5) and are strongly enriched in LILE and depleted in HFSE with significant Ba, Nb, Sr, and Ti negative anomalies. They have variable (87Sr/86Sr)i (from 0.70656 to 0.71208), ε Nd(t) (from -4.4 to +0.9), and zircon ε Hf(t) (from -1.2 to +12.4) with high (206Pb/204Pb)i ratio of 18.78-19.74, suggesting derivation from a hybrid source involving meta-sedimentary and meta-igneous rocks. Combined with previously-published data, two episodes of Cretaceous (~77-98 Ma and ~130-144 Ma) magmatic activities are identified in the Kuching zone, showing a younging age trend from south to north. These episodes of Cretaceous igneous rocks and their spatial distribution in the Kuching zone can be totally comparable to those in South China and Vietnam. Thus, the Kuching zone was likely a part of the Paleo-Pacific subduction system during the Cretaceous, northerly linking to Vietnam and South China

Origin of Permian OIB-like basalts in NW Thailand and implication on the Paleotethyan Ocean

Financial support from National Science Foundation of China (41190073, 41372198 and 40490613), National Basic Research Program of China (2014CB440901 and 2016YFC0600303) and “the Fundamental Research Funds for the Central Universities to SYSU” are gratefully acknowledged.The basaltic rocks in NW Thailand belong to part of giant Southeast Asian igneous zone that delineates the extension of the Paleotethyan Ocean from SW China into NW Thailand. The Chiang Mai basaltic samples from the Chiang Dao, Fang, Lamphun and Ban Sahakorn sections are divisible into two groups of high-iron basalt. Group 1 has SiO2 of 38.30–49.18 wt.%, FeOt of 13.09–25.37 wt.%, MgO of 8.38–1.60 wt.%, TiO2 of 3.92–6.30 wt.%, which is rarely observed in nature. Group 2 shows SiO2 = 44.71–49.21 wt.%, FeOt = 10.88–14.34 wt.%, MgO = 5.24–16.11 wt.%, TiO2 = 2.22–3.07 wt.% and mg# = 44–70. Olivine and pyroxene are responsible for the fractionation of the Group 2 magma whereas low oxygen fugacity during the late-stage differentiation of the Group 1 magma prolonged fractionation of ilmenite and magnetite. The onset of ilmenite and magnetite fractionations controls the distinct differentiation commencing at MgO = ~ 7 wt.%. Both groups show similar REE and primitive mantle-normalized patterns with insignificant Eu, Nb-Ta and Zr-Hf anomalies. They have similar Nd isotopic compositions with εNd(t) values ranging from + 2.8 to + 3.7 and similar Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y and Zr/Y, resembling those of OIB-like rocks. The representative basaltic sample yields the argon plateau age of 282.3 ± 1.4 Ma, suggestive of early Permian origin. Our data argue for Group 1 and Group 2 are coeval in the intra-oceanic seamount setting within the Paleotethyan Ocean, which at least continued till 283 Ma. These data, along with other observations, suggest that the Inthanon zone defines the main Paleotethyan suture zone, which northerly links with the Changning-Menglian zone in SW China.PostprintPeer reviewe

Earliest Triassic microbialites in the South China Block and other areas; controls on their growth and distribution

Earliest Triassic microbialites (ETMs) and inorganic carbonate crystal fans formed after the end-Permian mass extinction (ca. 251.4 Ma) within the basal Triassic Hindeodus parvus conodont zone. ETMs are distinguished from rarer, and more regional, subsequent Triassic microbialites. Large differences in ETMs between northern and southern areas of the South China block suggest geographic provinces, and ETMs are most abundant throughout the equatorial Tethys Ocean with further geographic variation. ETMs occur in shallow-marine shelves in a superanoxic stratified ocean and form the only widespread Phanerozoic microbialites with structures similar to those of the Cambro-Ordovician, and briefly after the latest Ordovician, Late Silurian and Late Devonian extinctions. ETMs disappeared long before the mid-Triassic biotic recovery, but it is not clear why, if they are interpreted as disaster taxa. In general, ETM occurrence suggests that microbially mediated calcification occurred where upwelled carbonate-rich anoxic waters mixed with warm aerated surface waters, forming regional dysoxia, so that extreme carbonate supersaturation and dysoxic conditions were both required for their growth. Long-term oceanic and atmospheric changes may have contributed to a trigger for ETM formation. In equatorial western Pangea, the earliest microbialites are late Early Triassic, but it is possible that ETMs could exist in western Pangea, if well-preserved earliest Triassic facies are discovered in future work

Flores et dynamique évolutive au passage de la limite Permien - Trias dans la zone de contact entre le Guizhou et le Yunnan, Chine du Sud

PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceChinaFRC