Provenance of Cored Sediments from Active Margin off Southwestern Taiwan Deduced from Geochemical Constraints

The cored sediments sampled by R/V Marion Dufrense in the active margin off southwestern Taiwan on the east side of the Manila Trench are mostly muds consisting mainly of quartz, feldspar, illite, chlorite ＋ kaolinite with a trace amount of montmorillonite. The cored sediments have higher Fe, Ti, Cr, Cs, Li, Ni, and V but lower Ca, Na, Mn, Hf, Nb, Sr, Ta, U and Zr contents when compared with the upper continental crust（UCC, Taylor and McLennan, 1985）. The Eu/Eu＊ ratios of the sediments averaging 0.69 indicate that the recycling effect on the Eu anomaly may be minor and the contribution of mafic source rock was unimportant, which can also be depicted in the La-Th-Sc plot（Cullers, 1994） showing a ＂mixed origin＂ close to the felsic end member. The chemistry of the cored sediment can be explained by a mixing model designed by Ho and Chen（1996） with four end members i.e., greywacke, shale, quartzite and limestone, which are widely distributed in the Cenozoic formations of southern Taiwan with greywacke as the dominant contributor followed by shale.

Relation of Tethys Ocean Subduction, Collision and Closure with Regional Metamorphism, Volcanism and Mineralization Exemplified on the Eurasian Active Margin

The geodynamic development of Eurasian active margin is related to subduction,collision and closure of Tethys Ocean.It is divided on pre-collision and post-collision stages.The pre-collision development controlled by subduction,whereas post-collision related by orogenesis,granodiorite magmatism gold base and trace metals(Sb,W,Mo and Hg)metallogeny.The mentioned trace metals association is the geochemical indicator of first stage of post-collision development.The second stage revealed in andesite basalt,shoshonite,olivine basalt and tholeiite volcanic activity.Pre-collision stage is controlled by steady state subduction related with metamorphism and calc-alkaline volcanic activity in subaqual and island conditions of island arc setting.It is lately with steepening of subducting slab and incursion of mantle diapir transferred in interarc-backarc and minor ocean setting with shoshonite-trachyandesite and alkali olivine basalt and tholeiite volcanism and later with ophiolite volcanism,dunite-peridotite magmatism and Cu-pyrite mineralization of minor ocean setting.The pre-collision stage is developed temporally and spacially along dipping of subducted slab in the island arc setting transferring in the backarc-interarc and minor ocean settings.The similar transferring occurs laterally to dipping of slab and ascending succession.The alternation of settings shown the cycling along dipping spatial and temporal alternation of island arc,backarc and minor ocean settings.Laterally to dipping alternation is only spatial,whereas in ascending succession cycling is only temporal,localized spacially.The pre-collision development occurs in subaqual condition,whereas related to orogenesis post-collision development is mainly subaerial.

New Discovery of Holocene Activity along the Weixi-Qiaohou Fault in Southeastern Margin of the Tibetan Plateau and its Neotectonic Significance

Objective The lateral extrusion eastward of the Tibetan Plateau leads to the formation of the Sichuan–Yunnan block, which is the most representative active block in the southeastern margin of the Tibetan Plateau, characterized by strong and frequent seismicity(Li Ping et al., 1975; Zhang Peizhen et al., 2003; Li Yong et al., 2017). Its eastern boundary is composed of sinistral faults including the Xianshuihe, Xiaojiang faults, etc., and the western

Cambrian-early Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France:Continental break-up and rifting of the northern Gondwana margin

The Cambrian-lower Ordovician volcanic units of the South Armorican and Occitan domains are ana- lysed in a tectonostratigraphic survey of the French Variscan Belt. The South Armorican lavas consist of continental tholeiites in middle Camhrian-Furongian sequences related to continental break-up. A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites. The Occitan lavas are distributed into five volcanic phases： （I） basal Cambrian rhyolites, （2） upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, （3） upper lower-middle Cambrian continental tholeiites, （4） Tremadocian rhyolites, and （5） upper lower Ordovician initial rift tholeiites. A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve. It renewed in the Tremadocian with abundant crustal melting due to underplating of mixed asthenospheric and lithospheric magmas. This main tectono-magmatic conti- nental rift is termed the ＂Tremadocian Tectonic Belt＂ underlined by a chain of rhyolitic volcanoes from Occitan and South Armorican domains to Central Iberia. It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that fore- casted the South Armorican-Medio-European Ocean as a part of the Palaeotethys Ocean.

Late Triassic Granites From the Quxu Batholith Shedding a New Light on the Evolution of the Gangdese Belt in Southern Tibet

The Gangdese magmatic belt formed during Late Triassic to Neogene in the southernmost Lhasa terrane of the Tibetan plateau. It is interpreted as a major component of a continental margin related to the northward subduction of the Neo-Tethys oceanic slab beneath Eurasia and it is the key in understanding the tectonic framework of southern Tibet prior to the India-Eurasia collision. It is widely accepted that northward subduction of the Neo-Tethys oceanic crust formed the Gangdese magmatic belt, but the occurrence of Late Triassic magmatism and the detailed tectonic evolution of southern Tibet are still debated. This work presents new zircon U-Pb-Hf isotope data and whole-rock geochemical compositions of a mylonitic granite pluton in the central Gangdese belt, southern Tibet. Zircon U-Pb dating from two representative samples yields consistent ages of 225.3~=1.8 Ma and 229.9~1.5 Ma, respectively, indicating that the granite pluton was formed during the early phase of Late Triassic instead of Early Eocene （47-52 Ma） as previously suggested. Geochemically, the mylonitic granite pluton has a sub-alkaline composition and low-medium K calc-alkaline affinities and it can be defined as an I-type granite with metaluminous features （A/CNK〈I.1）. The analyzed samples are characterized by strong enrichments of LREE and pronounced depletions of Nb, Ta and Ti, suggesting that the granite was generated in an island-arc setting. However, the use of tectonic discrimination diagrams indicates a continental arc setting. Zircon Lu-Hf isotopes indicate that the granite has highly positive till（t） values ranging from ＋13.91 to ＋15.54 （mean value ＋14.79）, reflecting the input of depleted mantle material during its magmatic evolution, consistent with Mg# numbers. Additionally, the studied samples also reveal relatively young Hf two-stage model ages ranging from 238 Ma to 342 Ma （mean value 292 Ma）, suggesting that the pluton was derived from partial melting of juvenile crust. Geochemical discrimination diagrams also suggest that the granite was derived from partial melting of the mafic lower crust. Taking into account both the spatial and temporal distribution of the mylonitic granite, its geochemical fingerprints as well as previous studies, we propose that the northward subduction of the Neo-Tethys oceanic slab beneath the Lhasa terrane had already commenced in Late Triassic （-230 Ma）, and that the Late Triassic magmatic events were formed in an active continental margin that subsequently evolved into the numerous sub- terranes, paleo-island-arcs and multiple collision phases that form the present southern Tibet.

Zircon U-Pb Dating on Granitoids from the Northern South China Sea and its Geotectonic Relevance

Zircon U-Pb ages of 163.8-100.4 Ma and 146.6-134.5 Ma are obtained for the granitoids from the Pearl River mouth basin, and from southern Guangdong Province, respectively. These new dating data accord well with the crystallization ages of Yanshanian granitoids broadly in the Nanling. The active continental margin of South China, as revealed by a combination of zircon U-Pb data, underwent a key granitoid-dominated magmatism in 165-100 Ma. Its evolution varied temporally, and spatially, registering under control of the paleo-Pacific slab subduction. The granitoids that occurred in 165-150 Ma broadly from the South China Sea to the Nanling are preferably related to two settings from volcanic-arc to back-arc extension, respectively. The activities of Cretaceous granitoids migrated from the southeastern Guangdong （148-130 Ma） to the Pearl River Mouth basin （127-112 Ma）, corresponding to the model of a retreating subduction. The subduction-related granitoid magmatism in South China continued until 108-97 Ma. A tectonic transformation from slab-subduction to extension should occur at -100 Ma.

Zircon U-Pb geochronology,geochemistry and Sr-Nd-Pb isotopes from the metamorphic basement in the Wuhe Complex:Implications for Neoarchean active continental margin along the southeastern North China Craton and constraints on the petrogenesis of Mesozoic granitoids

We report zircon U-Pb geochronology,geochemistry and Sr-Nd-Pb isotope data from mafic granulites and garnet amphibolites of the Wuhe Complex in the southeastern margin of the North China Craton (NCC).In combination with previous data,our results demonstrate that these rocks represent fragments of the ancient lower crust,and have features similar to those of the granulite basement in the northern margin of the NCC.A detailed evaluation of the Pb isotope data shows that Pb isotopes cannot effectively distinguish the role of the Yangtze Craton basement from that of the NCC basement with regard to the source and generation of magmas,at least for southeastern NCC.The age data suggest that the protoliths of the granulites or amphibolites in the Wuhe Complex were most likely generated in Neoarchean and that these rocks were subjected to Paleoproterozoic(1.8-1.9 Ga) high-pressure granulite facies metamorphism. This study also shows that the Precambrian metamorphic basement in the southeastern margin of the NCC might have formed in a tectonic setting characterized by a late Neoarchean active continental margin.

Geochronology and Petrogeochemistry of Metallogenic Intrusions in the Shanmen Large-scale Silver Deposit in the Siping Area of Jilin Province, NE China

The Shanmen deposit, located in the Siping area of Jilin Province, is one of large-scale silver deposits in Northeast(NE) China. Due to its high Ag grade, associated gold resources and special tectonic location, this deposit has important theoretical and ore-prospecting significance. To present new data on the ore genesis, mineralization time and tectonic settings, the relationship between silver mineralization and intrusions in this deposit has been studied and some analyses have been carried out, including LA-ICP-MS zircon U-Pb dating, whole-rock major and trace element analysis and Sr-Nd-Pb isotope analysis of the granitoids associated with silver mineralization. Studies on deposit geology indicate that the ore-hosting granodiorite and monzogranite have intimate genetic relationships with silver mineralization. LA-ICP-MS zircon U-Pb dating results of the both intrusions are 167.6 ± 1.9 Ma and 167.0 ± 1.5 Ma, respectively, implying that the two intrusions and associated silver mineralization in the Shanmen deposit formed during the Middle Jurassic. Major element analytical data suggest that the two intrusions are high-K calc–alkaline series I-type granites and belong to metaluminous and peraluminous rocks. Both intrusions have similar REE characteristics with regard to the relative enrichment of LREEs, depletion of HREEs, obvious fractionation of LREEs and HREEs(ΣLREE/ΣHREE = 8.68–14.09,(La/Yb)_N = 12.51–21.96), moderately negative Eu anomalies as well as weakly negative Ce anomalies(δEu = 0.56–0.71, δCe = 0.93–1.09). Moreover, the samples are generally enriched in LILEs and depleted in HFSEs. The rock assemblages and geochemical characteristics of granitoids in the Shanmen deposit suggest that they formed in an active continental margin associated with the Paleo-Pacific plate subduction. The Sr-Nd-Pb isotope compositions show that both the granodiorite and monzogranite have low Sr initial ratios and high Sm/Nd initial ratios. The ε_(Nd)(t) values and young Nd-model ages are-3.2 to-1.7 and 990 Ma to 1110 Ma, respectively. Considering the Sr-Nd isotope compositions, in conjunction with the relatively high ~(206)Pb, ~(207)Pb and ~(208)Pb, alongside the relatively low ~(204)Pb, it can be concluded that the Middle Jurassic intermediate-acidic magma represented by the studied monzogranite and granodiorite in the Shanmen deposit, was derived from the partial melting of juvenile lower crust enriched in mantle-derived materials and affected by the mixing of ancient crustal materials during the ascension and intrusion processes.

Devonian continental arc magmatism in the southern Central Asian Orogenic Belt:Evidence from the Dunhuang Block,NW China

Reconstruction of periods and sources of continental arc magmatism provides critical insights for deciphering the thermal evolution and geodynamics of the continental lithosphere and unveiling subduction-related episodes of accretion and orogeny.The Dunhuang Block,located in the southern Central Asian Orogenic Belt,is built upon a Precambrian metamorphic crystalline basement that is intruded by numerous Paleozoic granitoids.In this paper,we present new zircon U-Pb ages,wholerock geochemistry and Sr-Nd-Pb-Hf isotopes from the Hanxiakouzi and Shuangta granodiorite plutons outcropped in the central and northern Dunhuang Block,respectively.The U-Pb zircon dating indicate that the Hanxiakouzi and Shuangta granodiorites were emplaced at ca.366–363 Ma,i.e.,in the late Devonian.Geochemically,the Hanxiakouzi granodiorites are medium-to high-K,I-type granites possessing high Sr and Sr/Y,i.e.,similar to adakites.The Hanxiakouzi granodiorites show negativeε_(Nd)(t)values(-8.2 to-6.6),significant positive eHf(t)values(+0.7 to+8.1),and evolved Pb isotope features suggesting their derivation by melting of subducted terrigenous sediments,interaction of melts with mantle peridotite,and assimilation of ancient crust during magma ascent.The Shuangta granodiorites belong to high-K calc-alkaline I-type granites with medium Sr and low Sr/Y.The Shuangta granodiorites yield negativeε_(Nd)(t)values(-2.9 to-3.1)and negative to positive zircon eHf(t)values(-1.8 to+3.6),suggesting their formation by partial melting of ancient mafic crust with addition of minor mantle-derived melt and subducted terrigenous sediments.In general,Hf-in-zircon isotopes characteristics of the late Paleozoic granitic rocks in the Dunhuang Block indicate that the southern Dunhuang area carries more ancient crust materials than the central and northern Dunhuang areas,indicating a lateral heterogeneity of the middle Paleozoic crust.Conclusively,the middle Paleozoic granitoids of the Dunhuang Block were emplaced at an active continental margin of the southern Paleo-Asian Ocean.