Early Yanshanian NE Trending Sn/W-Bearing A-Type Granites in the Western-Middle Part of the Nanling Mts Region
In the western-middle part of the Nanling Mountains region there develops an Early Yanshanian (Jurassic) Sn/W bearing A-type granite belt. It is composed of Huashan, Guposhan, Jiuyishan and Qitianling granite batholiths and nearby granite stocks. This belt extends in a NE direction for more than 250 km. The total exposure area is over 3000 km2. These granitic bodies are usually multiple phased intrusions. The emplacement ages of their major phases are in the range of 165 ~153 Ma. The contemporaneous igneous rocks of intermediate composition (diorite, granodiorite, quartz monzonite), acidic extrusive rocks and mafic microgranular enclaves with mingling character are very common. The major phase granites are mostly porphyritic biotite granite, sometimes contain hornblende. They are acidic to ultra-acidic, weakly metaluminous to weakly peraluminous, rich in K2O and total alkalis, rich in LILE and HFSE such as Rb, Cs, U, Th, LREE, Y, Nb, Ta, Zr, Hf, Ga, as well as Sn, W etc. oreforming elements and F, Cl etc. volatile elements. On the discrimination diagrams of Whalen et al (1987) they are plotted in the A-type granite field. Their ISr values vary in the range of 0.7063～0.7182, εNd(t) values in the range of -1.7 ~ -8.0, t2DM values in the range of 1.1 ~ 1.6 Ga, which indicates that different proportions of mantle component were involved in their source materials, especially for the Huashan and Guposhan granites. These granite bodies are usually strongly fractionated. The emplacement ages of the later phase (or the additional phase) fine-grained granites are in the range of 146 ~151 Ma. Compared with the major phase, they are more ultra-acidic, per-aluminous, more enriched in Rb, Cs, U, Y, Sn, W etc. trace elements, but more depleted in LREE and Zr etc. HFSE. Petrochemically, they are close to the S-type granites. The Sn/W etc. mineralizations were found throughout the whole process of granite emplacement and evolution, beginning from the major intrusive phase, through the additional intrusive phase, up to the post-magmatic hydrothermal stage. The mineralization types are various, including skarn type, greisen type, quartz vein type, Li-F granite type, cassiterite-sulfide type, chloritized and structurally hydrothermal alteration type etc. The large even giant tonnages are not rare. It was generally considered, that the Sn/W mineralizations are usually related with the S-type granites. Establishment of Sn/W bearing A-type granite in Nanling Mts region provides important evidence for close relationship of Sn/W deposits with A-type granites. It is also significant in exploring the new Sn/W deposits in South China and worldwide. The Early Yanshanian post-orogenic extensional and thinning tectonic environment, upwelling of asthenospheric mantle and underplating of mantle mafic magma, crust-mantle interaction, high temperature melting of the lower crust, fractional crystallization and differentiation of granitic magmas, hydrothermal filling and alteration, these are key factors for rock-forming and ore-forming processes in the Nanling Mts. region.
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