Abstract Recent research on Paleo-Tethys tectonics has identified a huge late Paleozoic to Mesozoic igneous belt that extends more than 2500 km in the northeast Tibetan Plateau. However, the magma genesis and evolution in this belt remains a subject of considerable debate. This paper presents a combination of zircon U-Pb ages, mineral compositions, major and trace element concentrations, and Sr-Nd-Hf isotopic data for the plutons across the Zhiduo arc belt that marks the site connecting different tectonic-magmatic units. The studied rocks from one quartz diorite, two granodiorite plutons, and their mafic enclaves define a continuous compositional evolution varying from high- to medium-K calc-alkaline gabbroic diorite to granodiorite. Laser ablation–inductively coupled plasma–mass spectroscopy U-Pb analyses of zircons from these three plutonic suites and one mafic enclave yield Late Triassic ages of 222–217 Ma, establishing that the mafic and felsic magmas were nearly coeval. All these rocks are featured by zoned hornblende and plagioclase with Mg- and Ca-rich mantles or oscillatory change in compositions. They exhibit high and variable MgO (up to 4.88–5.66 wt%), Cr, and Ni contents except that one granitoid pluton (Dangjiangrong) possesses high Co (up to 145.0 ppm). They are characterized by subduction-type trace element patterns, with prominent positive Rb, Th, Pb, and K anomalies and negative Ba, Nb, P, and Ti. Together with continuous and heterogeneous Sr-, Nd-, and zircon Hf-isotopic compositions, it suggests that these Late Triassic high-Mg diorites and associated granitoids were generated through magma mixing and fractional crystallization accompanied by chemical exchange. Taking into account the magmatic record from nearby regions, we suggest that double-sided subduction and rollback of the subducting Paleo-Tethys oceanic slab is the main mechanism to generate geochemically-varied magmatism in the northeast Tibetan Plateau, and eventually close the Paleo-Tethys Ocean during much of the Late Triassic.
The Liaodong gold province, along the eastern margin of the North China craton, is a large productive base of gold in China. The Wulong gold deposit hosted in granitoid is a famous Au-bearing quartz vein gold deposit in this region, and located at the northern side of Sanguliu granite. The ages of Sanguliu granite determined by the conventional Rb-Sr isochron and U-Pb zircon methods are 131 ± 4.5 Ma and 129 ± 2.9 Ma respectively, thus the comprehensive isotopic age of Sanguliu granite is 130 Ma. Lode gold deposits commonly consist of pyrite and lesser amounts of galena and sphalerit sulfides accompanied by quartz and calcite. Here we use direct Rb-Sr dating of pyrite from Wulong gold deposit, and determined the age of Wulong gold deposit. A positive correlation between present-day 87Sr/86Sr and 87Rr/86Sr ratios of pyrite from the Wulong gold deposit corresponds to an age of 120 ± 3 Ma, which dates the age of gold mineralization. The Sr initial ratios (0.714816 to 0.7148927 of Sanguliu granite and 0.715280 to 0.715504 of pyrites in Au-bearing quartz veins) and Pb isotopic compositions in pyrites and Sanguliu granite indicates that lead was derived partially from the similar magmatic source, and the liner array of Pb isotope composition in pyrite show a incorporation of two end members (may be incorporation of basic and acid wall rocks). The studies of H and O isotopes of fluid inclusions in auriferous quartz veins demonstrate that the magmatic water predominates in ore fluids, and also reflect a little formation waters.
Xiasai is the largest magmatic-hydrothermal Ag–Pb–Zn–Sn deposit in the central Yidun Terrane (SW China). Two generations of pyrite are associated with Ag–Pb–Zn mineralization. The coarse-grained pyrite (PyI) occurs together with arsenopyrite and pyrrhotite, and the fine-grained pyrite (PyII) is associated with sphalerite and chalcopyrite. This study documents LA-ICPMS mapping, trace element and LA-MC-ICPMS sulfur isotopic analyses of the two types of pyrite and pyrrhotite, to further constrain the formation of Ag–Pb–Zn veins. Micro-textures reveal that pyrite and pyrrhotite have a homogeneous distribution of trace elements at the individual grain scale, however, some micro-inclusions containing trace elements, including Co, Ni, Cu, Pb, Zn, Ag, Bi, and As, are identified. Pyrite and pyrrhotite are enriched in Co, Ni, Cu, Pb, Zn, Ag, Bi, As, Se, and Sb. However, PyII has higher Cu, Pb, Zn, Ag, and Bi, but lower Co, Ni, As, Sb, and Te concentrations than PyI. The variations of Co and Ni concentrations, as well as the Co/Ni ratios in pyrite and pyrrhotite demonstrate a link with the distance to ore-related granite. PyI has in-situ δ34S values of –6.8 to –3.4 ‰ (average of –5.9 ‰), and PyII exhibits values ranging from –6.7 to –5.4 ‰ (average of –6.0 ‰). Pyrrhotite has δ34S values ranging from –6.9 to –5.5 ‰. The in-situ sulfur isotopic compositions demonstrate a magmatic source for ore-forming materials, which were derived from the Early Cretaceous monzogranite. Trace elements and sulfur isotopic compositions of pyrite and pyrrhotite indicate that the formation of the Ag–Pb–Zn veins at Xiasai is due to a decrease of temperature, salinities, fS2 and fO2 of the hydrothermal fluids.
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.
Abstract The Huaixi copper-gold polymetallic deposit of SE Zhejiang Province, China, is a typical hydrothermal-vein ore body. The Caomen K-feldspar granite porphyry, the dominant intrusion in the mining district, has been dated by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses of zircon, which yielded a weighted mean 206Pb/238U age of 101.6 ± 0.9 Ma (MSDW = 1.3). Rb-Sr dating of fluid inclusions in auriferous quartz from ore bodies yielded an isochron age of 101 ± 2 Ma. These results indicate that intrusion of the Caomen pluton and Cu-Au mineralization was contemporaneous and corresponds to the third episode of Mesozoic magmatism and metallogenesis in South China. Calculated δ18O values of fluid inclusions from ore-bearing quartz range from −0.89 to −1.98‰ and δD from −42.60 to −60.20‰, suggesting that the ore-forming fluids are derived from a mixed source of magmatic and meteoric waters. δ18S values of 8 pyrites range from −2.14 to +4.14‰ with a mean of +1.67‰, similar to magmatic sulphur. These isotope data support a genetic relationship between the Huaixi copper-gold deposit and the Caomen alkaline granite and probably indicate a common deep source. Petrography and chemical compositions show that the Caomen alkaline granite crystallized from shoshonitic magmas characterized by high SiO2 (75.64–78.00%) and alkali (K2O + Na2O = 7.96–8.82%) but low FeOT (1.34–3.31%), P2O5 (0.05–0.13%) and TiO2 (0.12–0.18%). The granitic rocks are enriched in Ga, Rb, Th, U, and Pb but depleted in Ba, Nb, Sr, P, and Ti. REEs are characterized by marked negative Eu anomalies (Eu/Eu* = 0.06–0.13) and exhibit right-dipping ‘V’ patterns with LREE enrichment. These are similar to the Late Cretaceous alkaline granites in the coastal areas of Zhejiang and Fujian provinces, implying that the Caomen granite formed in a post-collisional extensional tectonic setting. Combined with previous studies, we interpret the Huaixi copper-gold deposit and the associated Caomen alkaline granite as related to back-arc extension due to high-angle subduction of the palaeo-Pacific plate, caused by northward movement of the Indian plate. Keywords: LA–ICP–MS zircon U-Pb datingRb-Sr datingalkaline graniteback-arc extensionHuaixi copper-gold vein depositSE China Acknowledgements This study was financially supported by the Programme for Innovative Research Team in University (IRT0755), the Programmes of Superseding Resources Prospecting in Crisis Mines in China (No. 2006020035 and 2007020047) and the Key Laboratory of Geological Progress and Mineral Resources (GPMR), China University of Geosciences (CUG). We acknowledge the kind help of Mr Roger Mason and Mr Jianwei Li, CUG, for reading through the paper and providing numerous corrections in the usage of English that led to great improvement in the presentation of the paper. Ms Yafei Ge from CUG and Dr Jiajia Zheng from the University of Queensland are acknowledged for their constructive suggestions. We also thank Ms Haihong Chen and Mr Shu Zheng, GPMR, for their analytical support and No. 11 Geological Team of Zhejiang Bureau of Geology and Mineral Resource for their help in the field work.
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