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Ding X.,CAS Guangzhou Institute of Geochemistry | Hu Y.,Zhejiang Institute of Geological Survey | Zhang H.,CAS Guangzhou Institute of Geochemistry | Zhang H.,Northwest University, China | And 3 more authors.
Journal of Geology | Year: 2013

Mobilities and fractionations of high-field-strength elements, especially Nb and Ta within a subducting slab, are important for deciphering the formation of the continental crust (CC). Here we report geochemical results on an epidote garnet amphibolite facies metagabbro body in the Tongbai-Dabie orogenic belt, central China. Our samples were hydrated during prograde metamorphism of the Triassic plate subduction. Major minerals such as amphibole, garnet, rutile, and ilmenite and garnet amphibolite bulk rocks show varied and overall lower Nb/Ta and/or Zr/Hf ratios than the continental crust. Magma differentiation might have contributed to variations of Zr/Hf but not those of Nb/Ta, suggesting major Nb/Ta fractionations during plate subduction. LA-ICPMS in situ trace element analyses of amphibole and especially rutile grains exhibit obvious chemical zonations. Typically, the rutile cores are usually small with higher Nb and Ta concentrations and lower Nb/Ta ratios compared to the thick rims. Chemical and fabric characteristics of the zonations may be explained by diverse external fluid activities: the gabbro first absorbed low Nb/Ta fluids that were released during blueschist to amphibolite transformation in deeper portions of the subducting slab, followed by acquiring external fluids with elevated Nb/Ta released during amphibolite to eclogite transformation. Our results imply that fluids with low Nb/Ta released during blueschist to amphibolite transformation can be transferred to cold regions within a subducted plate and also to the mantle wedge through fluid-rock reaction. Such regions are more easily melted during further subduction, especially in the early history of the earth, providing a plausible explanation for the low Nb/Ta in the CC. © 2013 by The University of Chicago. Source

Zeng J.-N.,Wuhan University | Li J.-W.,Wuhan University | Chen J.-H.,Wuhan University | Chen J.-H.,Zhejiang Institute of Geological Survey | Lu J.-P.,Wuhan University
Diqiu Kexue - Zhongguo Dizhi Daxue Xuebao/Earth Science - Journal of China University of Geosciences | Year: 2013

The SHRIMP zircon U-Pb dating of granodiorite-porphyry, the main rock type of Anjishan rock mass yields the age of 106.9±0.9 Ma, as the formation age of the Anjishan rock mass, which shows that the Anjishan rock mass formed in the late Early Cretaceous. Anjishan rock mass is younger than the high-K calc-alkaline intermediate-acidic intrusive rocks in Tongling and other districts and it illustrates that the intermediate-acidic intrusive rocks in Ningzhen District belongs to another diagenetic event in the middle to lower Yangtze metallogenic belt's Mesozoic large magmatism. The Ningzhen District's Cu-polymetallic mineralization is closely related with the 107 Ma calc-alkaline intermediate-acidic magmatism in time, space and origin. The petrogenesis- mineralization has been a continuous process in this district. The geodynamics background of the Ningzhen District's calc-alkaline intermediate-acidic magmatism corresponds to the Mesozoic lithospheric delamination thinning event in Eastern China. It demonstrates that the middle to lower Yangtze lithosphere delamination began at about 130 Ma, and lasted to 107 Ma or later. Source

Ling M.-X.,CAS Guangzhou Institute of Geochemistry | Zhang H.,Northwest University, China | Li H.,CAS Guangzhou Institute of Geochemistry | Liu Y.-L.,CAS Guangzhou Institute of Geochemistry | And 5 more authors.
Lithos | Year: 2014

Granitoids near the Bayan Obo giant rare earth element (REE) deposit at the north margin of the North China Craton (NCC), the world's largest light REE (LREE) deposit, have been taken by some authors as the key factors that controlled the mineralization. In contrast, others proposed that the REE deposit has been partially destructed by these granitoids. Here we report systematic studies on geochronology and geochemical characteristics of granitoids of different distances from the orebodies, to investigate the genesis and their relationship to the giant Bayan Obo deposit. Granitoids studied here, including granites and quartz monzonites, are peraluminous with A/CNK=0.99-1.11, LREE enriched and heavy REE (HREE) depleted, with variable REE concentrations (total REE=54-330ppm) and large negative Eu anomaly (δEu=0.19-0.70). The REE patterns are distinct from those of ore-bearing dolomites. Some samples have slightly higher LREE concentrations, which may have been contaminated by the orebodies during intrusion. Trace elements of the granitoids are characterized by positive Pb anomaly, strong negative Ti anomaly and Nb, Ta and Sr anomalies. The granites exhibit negative Ba anomaly. The granitoids plot within the post-collision granite field in the Pearce diagram, which is consistent with the tectonic regime. The quartz monzonites and one granite have A-type granite characteristics and belong to the A2 subgroup. Zircons in these granitoids have high Th/U values, which are typical for magmatic zircons. High precision U-Pb dating for these zircons by secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields Permian-Triassic 206Pb/238U ages ranging from 243.2 to 293.8Ma. The formation of the granitoids is >55Ma later than the latest ore forming age. The zircons have low La concentrations (0.02-12ppm), high (Sm/La)N (0.8-685) and Ce/Ce* (1.4-80). The Ti-in-zircon temperature of the granitoids ranges from 590 to 770°C. All these evidences suggest that the granitoids have no contribution to the formation of the Bayan Obo deposit. Granitoids that are close to the orebodies had limited interaction with it and gained some LREE-enriched characteristics during magmatisms. Nevertheless, their effects to the orebodies are subtle. All the granitoids formed in a post-collisional tectonic regime at convergent margins, which is consistent with plate subduction during the closure of the Palaeo-Asian Ocean, which started in the Neoproterozoic and lasted until the Carboniferous/Permian. © 2014 Elsevier B.V. Source

Meng H.-H.,CAS Xishuangbanna Tropical Botanical Garden | Meng H.-H.,University of Chinese Academy of Sciences | Jacques F.M.B.,CAS Xishuangbanna Tropical Botanical Garden | Su T.,CAS Xishuangbanna Tropical Botanical Garden | And 5 more authors.
BMC Evolutionary Biology | Year: 2014

Background: Given that most species that have ever existed on earth are extinct, it stands to reason that the evolutionary history can be better understood with fossil taxa. Bauhinia is a typical genus of pantropical intercontinental disjunction among the Asian, African, and American continents. Geographic distribution patterns are better recognized when fossil records and molecular sequences are combined in the analyses. Here, we describe a new macrofossil species of Bauhinia from the Upper Miocene Xiaolongtan Formation in Wenshan County, Southeast Yunnan, China, and elucidate the biogeographic significance through the analyses of molecules and fossils. Results: Morphometric analysis demonstrates that the leaf shapes of B. acuminata, B. championii, B. chalcophylla, B. purpurea, and B. podopetala closely resemble the leaf shapes of the new finding fossil. Phylogenetic relationships among the Bauhinia species were reconstructed using maximum parsimony and Bayesian inference, which inferred that species in Bauhinia species are well-resolved into three main groups. Divergence times were estimated by the Bayesian Markov chain Monte Carlo (MCMC) method under a relaxed clock, and inferred that the stem diversification time of Bauhinia was ca. 62.7 Ma. The Asian lineage first diverged at ca. 59.8 Ma, followed by divergence of the Africa lineage starting during the late Eocene, whereas that of the neotropical lineage starting during the middle Miocene. Conclusions: Hypotheses relying on vicariance or continental history to explain pantropical disjunct distributions are dismissed because they require mostly Palaeogene and older tectonic events. We suggest that Bauhinia originated in the middle Paleocene in Laurasia, probably in Asia, implying a possible Tethys Seaway origin or an "Out of Tropical Asia", and dispersal of legumes. Its present pantropical disjunction resulted from disruption of the boreotropical flora by climatic cooling after the Paleocene-Eocene Thermal Maximum (PETM). North Atlantic land bridges (NALB) seem the most plausible route for migration of Bauhinia from Asia to America; and additional aspects of the Bauhinia species distribution are explained by migration and long distance dispersal (LDD) from Eurasia to the African and American continents. © 2014 Meng et al.; licensee BioMed Central Ltd. Source

Rong J.Y.,CAS Nanjing Institute of Geology and Palaeontology | Zhan R.B.,CAS Nanjing Institute of Geology and Palaeontology | Xu H.G.,Zhejiang Institute of Geological Survey | Huang B.,CAS Nanjing Institute of Geology and Palaeontology | Yu G.H.,Zhejiang Institute of Geological Survey
Science China Earth Sciences | Year: 2010

Located northwest of the Jiangshan-Shaoxing Fault Zone, western and northwestern Zhejiang and northeastern Jiangxi provinces experienced major changes in bio- and litho-facies and paleogeography through the Ordovician-Silurian transition (late Katian, Hirnantian, and early Rhuddanian), as manifested by stratigraphic, paleontologic and synecologic records. Three geographic units under consideration are South, Central, and North areas. The western margin of the South area was occupied by the Huaiyu Mountains, whereas the other two parts were covered by the Zhe-Gan Sea during late Katian (Late Ordovician) time. In the early stage of the Changwu Formation deposition (late Katian), the sea was deepening northeastward, but with shallower conditions in the southwest and deeper conditions in the northeast. During mid to late stages of Changwu Formation deposition (latest Katian), the sea became much shallower and the sea bottom was uplifted substantially, which occurred somewhat prior to the global trend. During the Hirnantian (latest Ordovician) and early Rhuddanian (earliest Silurian), the study region became a shallow bay under expansion of the Cathaysian Oldland. There occurred a major drop of sea level and great changes in benthic biota with the occurrence of many new immigrants through the Ordovician-Silurian transition, which are closely related to a unique regional orogeny. The Yangtze and Cathaysian blocks may have amalgamated within the South China Paleoplate during this interval to cause the continuous uplifting and northwestward expansion of the Cathaysian Oldland. © Science China Press and Springer-Verlag Berlin Heidelberg 2010. Source

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