| Nancun granite plutons, located in southwest of Dawangshan metallogenic belt, the molybdenum tungsten ploymetallic mineralization zone of Nancun- Gugang- Majizhang, can be found in the area of Nancun, Shanxin and Xiaodu village in Le’an county, Jiangxi province. Several small scale tungsten deposites(related to the granite) such as Pangling, Qingluokeng and Xiaodu deposites were found in Nancun granite pluton. Therefore, this paper choses the three granites in Nancun as the main research object, studies their formation age, genesis, tectonic setting and relationship with molybdenum tungsten ploymetallic deposit through research its geological features, petrology, geochemistry, zircon geochronology.The cover area of the Nancun Yanshanian granitic plutons is about 10.1 km2.The lithologies of the granitic plutons with similar characters are fine-medium giant biotite granite, medium-fine griant biotite granite, micro-fine griant biotite granite, fine griant little phyenocryst two-mica granite, fine granite less phyenocrysts two-mica granite, fine griant porphyraceous two-mica or biotite granite, fine griant two-mica alkali feldspar granite and little granite porphyry. Through precise LA-ICP-MS U-Pb dating of zircon from Shanxin fine two-mica alkali-feldspar granite, the crystallization age of the intrusion was determined to be 152.5±1.3Ma, it indicates that the pluton is Yanshanian granites formed in the Late Jurassic era.The results of mineral chemistry are that micas are muscovite and ferrimuscovite, feldspars are albite and orthoclase. The all feladspars are alkali feldspar. The granties are characterized by high contents of silicon, Na2O+K2O, aluminum, low in calcium and magnesium, metaluminous- peraluminous, belonging to high K calc-alkaline granitoids. The granites also show strong depletion of Sr, Ba, Ti, P and Eu, enrichment of Rb, Th, Ta and U, the characteristics of high Ga/Al, medium to strong negative Eu anomaly, the unabvious differentiation between LREE and HREE, the similar trace elements spider diagram. The chondrite-normalized patterns of rare earth elements are in seagull forms. The characteristics, which are similar to the aluminous A-type granite of the Nanling area, indicate that the Nancun Yanshanian granites may be the separation and crystallization of high differentiation aluminum A- type granite. The characteristics of high Rb/Sr and Rb/Ba ratios, low Nb/Ta, Zr/Hf and K/Rb ratios indicate that the magma experienced a strong fractional crystallization process and fluid process. By the study of the Sr-Nd-Pb isotope in the rocks, we find that the value of εNd(t) is-8.9 and the T2 DM is 1662 Ma. On the Pb isotopic diagram,all the samples locate between the crust and mantle evolution curves, and this phenomenon indicates that rock-forming substances mainly came from the Proterozoic crust and little from the mantle.Based on the rock association, structure, mineralogical, lithogeochemical characteristics, reached the conclusion that Nancun granitoid plutons are high differentiation aluminum A- type granites. The genesis of Nancun plutons are that lower crust partial melting as temperature rise and pressure decrease caused by lithospheric delamination of the Late Jurassic in South China triggered after orogeny, and crustal stretching, thinning of lithosphere, hot asthenosphere surge basaltic magma underplating mantle source heating fluid injected into the Earth’s crust and mantle. Many fracture structures, pegmative veins, quartz veins and greisen veins developed in the study area. These granites are with higher concentration of W, Sn, U and more metallic elements. The characteristics that had introduced and the characteristics of W bearing, W-Sn bearing, high fractionated, high heat and the participation of hydrothermal fluid in Nancun granites indicate that Nancun plutons have a close connection to mineralization. Moreover, there may be a large concealed rock that it has good potential for tungsten-tin mineralization. |
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