Geochronologies of the Huaixi copper-gold deposit and the Caomen alkaline granite, SE China: implications for tectonic evolution
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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.Keywords:
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Ore genesis
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Radiogenic nuclide
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Abstract Thirteen samples of Värmland granites from south-eastern Värmland, Sweden, have been subjected to a Rb—Sr dating study. Most of the studied samples subdivide into two groups defining two separate isochrons of closely similar age but different initial 87Sr/86Sr ratios. Five samples from a southern sub-area of Filipstad granite around Filipstad and Karlskoga have yielded a Rb-Sr age of 1700 ± 37 Ma at an initial 87Sr/86Sr ratio of 0.7068 ± 0.0021. Five samples from the northern sub-area of Hagfors—Ekshärad—Uvanå, together with two samples from the vicinity of Kristinehamn, form another isochron of 1689 ± 30 Ma with an initial Sr-isotopic ratio of 0.7021 ± 0.0006. One sample could not be assigned to either isochron and is intermediate between them. The differentiation of Värmland-group granites into separate complexes and types is also brought out by field data and geochemical characteristics.
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Abstract Based on the Rb‐Sr isotopic study of the granitoids in the Hengduan Mountains, the classification and geologic significance of whole‐ rock Rb‐ Sr isochrons as well as the factors influencing homogenization of the isotopic systems are discussed. Usually, there is no good linear correlation of isochrons for diorites and alkali‐rich intrusive rocks (including alkaline rock and alkalic granite). But by means of the external isochron of monominerals from the rocks, isochrons with good linear correlation as well as ages and Sr initial ratios with a high confidence can be obtained. In order to obtain a satisfactory isochron, the discriminant parameters should be calculated from the Rb / Sr ratios and estimated ages of samples. Only those that meet the requirements can be used as the Rb‐Sr isochrons. The quality of an isochron should be judged from three factors, i.e. correlation coefficient, MSWD and homogenization degree.
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Rb-Sr isochrons were obtained on the cretaceous acid volcanic rocks of Himeji district, Hyogo prefecture, Japan. The isochron of the Tenkadaiyama forInation, younger rocks in this area, yielded T=67.5±5. 9m. y. and (87Sr/86Sr) . =0.7105±0.0006by both whole rock specimens and separated fractions. On the contrary, it was determined two ages on the Aioi formation, older extrusive rocks. One age was obtained by the whole rock isochron which gave T=105±16m. y. and (87Sr/86 Sr) 1= 0. 7057±0.0008. The other was obtained by internal isochrons. Two rock specimens of the Aioi formation were separated for internal isochrons. One gave T=70±7m.y. and (87Sr/86Sr) =0.7079±0.0007, and the other gave the same age and higher initial strontium ratio, respectively. This indicates that the Aioi formation erupted at about100m. y. BP and was influenced at about70m. y. BP by the eruption of the Tenkadaiyama formation which caused re-equilibration of the Rb-Sr system in the mineral parts of the Aioi formation to give a younger internal isochron age.
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