Geochemistry and Geochronology of Diorite in Pengshan Area of Jiangxi Province:Implications for Magmatic Source and Tectonic Evolution of Jiangnan Orogenic Belt

Magmatic activities associated with tectonic events play a significant role in understanding the evolution of an orogenic belt.The Jiangnan orogenic belt has been regarded as the collisional suture zone between the Yangtze Block and the Cathaysia Block.Although the magmatic activities during the period of intra-plate extension after the collision have been well studied in recent years,some remaining issues,including source nature and geodynamic mechanism,need to be further addressed.In this paper,based on a detailed field geological,petrological,geochemical and geochronological study,we focus our work on diorites in the Pengshan area located at the northwestern margin of the Jiangnan orogenic belt.The mineral assemblages are mainly composed of plagioclase(55 vol.%-65 vol.%)and hornblende(35 vol.%-45 vol.%).One diorite sample yields zircon^(206)Pb/^(238)U mean age of 768±8 Ma(MSWD=0.29).The diorites have enriched large ion lithophile elements(Ba,K and Rb)and incompatible elements(Th and U),and are depleted in high field-strength elements including Ta,Ti and Nb.Diorites in this study have relatively high MgO content(6.56 wt.%-7.58 wt.%,7.07 wt.%on average)and Mg number values(65-67,65.8 on average).The diorites are metaluminous,high K calc-alkaline series rocks with high contents of K_2O(1.59 wt.%-1.97 wt.%)and total alkali(Na_2O+K_2O=5.56 wt.%-6.05 wt.%).The Nd/Th ratio(4.34-5.27)is higher than that of crust-derived rocks and lower than mantle-derived rocks.The Rb/Sr ratio(0.19-0.22)is slightly lower than crust,but significantly higher than upper mantle.Based on the above geochemical and geochronological analyses,we suggest that the diorites in the Pengshan area were mainly derived from crustal materials with a small amount of mantleoriginated materials involved,and possibly produced from an extensional tectonic setting after the collision between the Yangtze Block and Cathaysia Block.

Timing of Metamorphism and Provenance of the Metamorphic Basement of the Xiangshan Uranium Orefield, Jiangxi Province, China

The origin, age and evolution of the Precambrian metamorphic basement of southern China provide useful insights into early crustal development. Here, we present new laser ablation-inductively coupled plasma-mass spectrometry （LA-ICP-MS） U-Pb age data for detrital zircons from five samples of the Precambrian metamorphic basement of the Xiangshan uranium orefield. Two of these samples, from the northern Xiangshan volcanic basin, yielded a total of 140 U-Pb ages that cluster within the Neoproterozoic （773-963 Ma; 79.3% of data points）, with the rest being scattered through the Paleoproterozoic and Mesoproterozoic, along with a single Archean age. These ages indicate that this basement material is associated with the Cathaysia Block. In comparison, the 172 concordant ages from the other three samples from the southern part of the Xiangshan volcanic basin cluster within the Neoproterozoic （767-944 Ma; 59.8%） as well as the Proterozoic （37.8%） and the Archean （2502-2712 Ma; 14.5%）. These samples are also free of zircons with Grenvillian ages, indicating that these units are associated with the southeastern Yangtze Block. Combining these data with the geochemistry of these units, which suggests that the metamorphosed sedimentary rocks within the northern and southern parts of the Xiangshan basin have a common component from a magmatic island arc that formed during the early Neoproterozoic, we infer that the basin was located along the boundary between the Cathaysian and Yangtze blocks. In addition, the zircons within the samples from the southern and northern parts of the Xiangshan basin show different pre-Neoproterozoic （963 Ma） age populations but similar post- Neoproterozoic zircon populations, indicating that the amalgamation of the Cathaysian and Yangtze blocks occurred after the Neoproterozoic （960 Ma）, with magmatism peaking at 830 Ma and rifting starting at -770 Ma, leading to the subsequent deposition （from bottom to top） of the Shenshan, Kuli, and Shangshi formations.

Hydrothermal alteration processes in the giant Dahutang tungsten deposit, South China: Implications from litho-geochemistry and mass balance calculation

The giant Dahutang tungsten(W)deposit has a total reserve of more than 1.31 Mt WO3.Veinlet-disseminated scheelite and vein type wolframite mineralization are developed in this deposit,which are related to Late Mesozoic biotite granite.Four major types of alterations,which include albitization,potassic-alteration,and greisenization,and overprinted silicification developed in contact zone.The mass balance calculate of the four alteration types were used to further understanding of the mineralization process.The fresh porphyritic biotite granite has high Nb,Ta,and W,but low Ca and Sr while the Jiuling granodiorite has high Ca and Sr,but low Nb,Ta,and W concentrations.The altered porphyritic biotite granite indicated that the Nb,Ta,and W were leached out from the fresh porphyritic biotite granite,especially by sodic alteration.The low Ca and Sr contents of the altered Neoproterozoic Jiuling granodiorite indicate that Ca and Sr had been leached out from the fresh granodiorite by the fluid from Mesozoic porphyritic biotite granites.The metal W of the Dahutang deposit was mainly derived from the fluid exsolution from the melt and alteration of W-bearing granites.This study of alteration presents a new hydrothermal circulation model to understand tungsten mineralization in the Dahutang deposit.