Petrogenesis Of The Triassic Granitoids And Skarn Mineralization In The Qimantag Area, Qinghai Province, And Their Geodynamic Setting

Based on geological, mineralogical, petrological, geochemistrical data and previous studies, this paper discusses the relationship among the Trassic geodynamic setting, petrogenesis and mineralization, aiming to find out factors controlling Fe, Cu and Pb-Zn skarn mineralization in the Qimantag area of Qinghai Province.According to petrographical and geochemistrical data, two suites of the Triassic granitic rocks can be classified: 250~237Ma diorites, granodiorites, and 235~204Ma monzonites, granodiorites, monzogranites, grantites and porphyritic granites. Sr-Nd-Hf isotopic data show that the two discrete suites of intrusions were both derived from an enriched lithospheric mantle. The rocks of suite 1 have relatively lower degree of differentiation and magma temperatures, but formed at relatively high pressures. These rocks were generated by mixing of mafic magmas derived from an enriched lithospheric mantle, and felsic magmas produced by partial melting of mantle-derived mafic crust materials in a subduction setting. The intrusions of suite 2 have formed at relatively lower pressures, but higher magma temperatures and degree of differentiation. These granitic rocks are associated with partial melting of mantle-derived mafic crust materials under an extensional setting.Two discrete suites of the Triassic skarn mineralization and related intrusions are recognized in the study area:(1) 243~237Ma granodiorites, and together with associated 241~238Ma porphyry-skarn Cu, Mo polymetallic deposits;(2) 235~213Ma monzonites, granodiorites, monzogranites and granites, together with related skarn Fe, Pb-Zn polymetallic deposits. These two suites of intrusion and mineralization are spatially, temporally, and genetically associated with different geodynamic setting, and two suites of the Triassic magmatism in the study area. Geological, mineralogical, petrological and geochemical data indicate that, in the Qimantag area of Qinghai Province, the subduction setting, mixing of mantle- and crust-derived magmas, lower degree of differentiation, and magmatic fluids with relatively higher Cl, H2 O and lower F concentrations, control skarn Cu polymetallic mineralization; the extensive setting, crust thinning, mantle-derived mafic magma underplating, high proportion of mantle components in the magma source, multi-stage evolution of magma, and magmatic fluids with relatively higher Cl and lower H2 O, F concentrations play a key role in skarn Fe polymetallic mineralization; skarn Pb-Zn polymetallic mineralization are associated with the extensive setting, crust thinning, mantle-derived mafic magma underplating, high proportion of mantle components in the magma source, high degree of differentiation during the evolution of the magma, and magmatic fluids with relatively higher F and lower H2 O, Cl concentrations. In summary, geodynamic setting, characteristic of magma source, fractional crystallization, and features of magma fluid may be important factors of controlling skarn polymetallic mineralization in the study area.