Petrogenesis Of Granitic Plutons Associated With Rare And Polymetallic Mineralization In The Eastern Segment Of The Qin-Hang Metallogenic Belt, And Their Constraints On Metallogenesis

The Qin-Hang (Qinzhou-Hangzhou) metallogenic belt is the most important rare and polymetallic mineralization belt of South China. In this paper, the author selected two granitic plutons (i. e., the Yashan pluton and the Xiongjiashan pluton) associated with rare and polymetallic mineralization in the eastern segment of the Qin-Han metallogenic belt as examples, and conducted a detailed studies including petrology, whole-rock elemental and Sr-Nd isotopic geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes. Based on these studies, this paper has revealed the geological and geochemical characteristics, and genetic types of the two plutons, and comprehensively discussed their source components, rock-forming process and petrogenesis. In combination with a synthesis of the magma evolution and tectonic setting, this paper has also probed the metallogenic mechanism of the rare and polymetallic deposits. Therefore, this study has further enriched the understanding of granitic magmatism and its relation to rare and polymetallic mineralization in South China.The Yashan pluton is located in the west end of Pingxiang-Shaoxing suture zone which is referred as the eastern section of Qin-Hang metallogenic belt. It is one of the most important rare-metal granitic plutons with Ta-Nb mineralization in South China. This pluton was formed via multi-stage magmatic evolution, which can be roughly distinguished into protolithionite-muscovite granite, lepidolite granite and topaz-lepidolite granite from early to late. Zircon LA-ICP-MS U-Pb dating yields an age of about 150Ma, indicating it was generated in Late Jurassic. Geochemically, the Yashan granites are enriched in silica, depleted in magnesium and iron, thus are highly-fractionated with differentiation index (D.I) values mostly larger than 93. They also show subalkaline, and strongly peraluminous signatures with agpaitic index (AKI) values less than 0.80 and aluminum saturation index (A/NKC) values over 1.20. The granites are high in Cs, Rb, Th, U, Nb, Ta, low in Ba, Sr, REE, and exhibit distinct negative europium anomalies (8Eu=0.04-0.25). With increasing degrees of fractionation, the Na2O/K2O and Ta/Nb ratios, the fluorine contents, the europium depletions and the REE tetrad effect of the pluton increase, while the total REE abundances and the crystallization temperatures decrease. The Yashan granites have low d(t) values (=-10.4～-10.7) and CaO/Na2O ratios(<0.3), indicating that they were likely originated from partial melting of metamorphic pelitic rocks in crustal basement. However, the granites show highly variable zircon Hf isotopic compositions with εHf(t) values range from-14.8 to 0.7, implying that little mantle or juvenile materials might also have contributed to magma genesis. Based on a synthesis of petrology, geochronology, elemental and isotopic geochemistry, it is suggested that the Yashan pluton should be ascribed to highly-fractionated S-type granites. The Ta-Nb mineralization of the pluton was controlled jointly both by highly fractionation of the magma and by fluid-melt interaction during the late stage, while the former is likely the main factor controlling the enrichment of tantalum and niobium. In addition, the different importance of magma fractionation and fluid metasomatism in mineralization seems to be the reason that result in distinguishing metallogenic specialisation of A-and S-type granites in south China.Both the Caledonian and the Yanshanian granites are spatially coexisted within the Xiongjiashan molybdenum deposit in Jinxi, Jiangxi Province. Zircon LA-ICP-MS U-Pb dating results show that the ages of the Caledonian and the Yanshanian granites are-440Ma and-155Ma, respectively. Chemically, the both stage granites show subalkaline and Fe-poor signatures, and are enriched in Cs, Rb, Th, Pb, and LREE, depleted in Ba, Sr, P, Ti, Nb, Ta. However, the Caledonian granites are more enriched in aluminum, while the Yanshanian granites are more enriched in alkaline, and have higher concentrations of Nb, Ta, Th. The Yanshanian granites also show higher Rb/Sr and Rb/Ba, lower K/Rb and LREE/HREE ratios relative to the Caledonian granites, and exhibit more distinct negative europium anomalies, implying they have suffered a higher degree of differentiation. Integrated geochemical features indicate that the Caledonian and the Yanshanian granites should be grouped into different genetic types, i. e., the Caledonian granites belong to S-type, while the Yanshanian granites belong to I-type. Sr-Nd-Hf isotopic compositions indicate that the Caledonian granites were originated mainly from partial melting of Paleo-to Mesoproterozoic basement crustal rocks, and mantle-derived or juvenile crustal materials were less involved in petrogenesis. The Yanshanian granites have similar Sr-Nd-Hf isotopic compositions to that of the Caledonian granites, and contain more inherited magmatic zircons with Caledonian ages. Integrated geological and geochemical data, and partial melting simulation suggest that the Yanshanian granites were generated from jointly remelting of the Caledonian granites and the further evolved basement crustal rocks, i. e., the Caledonian granites were involved in the formation of the Yanshanian granites as parts of the source components. The molybdenum mineralization in this district was controlled mainly by the Yanshanian magmatic event, and the cryptoexplosion of granitic magma is a critical factor for metallogenesis. The role of the Caledonian granites in the ore-forming process are reflected mainly in two aspects, the first is that they were directly involved in the generation of Yanshanian granites as components of source rocks, thus might supply some ore-forming materials; the second is that they promoted the maturity of crust and enrichments of ore-forming elements, thus indirectly affect the metallogenic process.