Petrogenesis of Granitoids from Silicic Large Igneous Provinces (Central and Northeast Asia)

Abstract

Large granitoid provinces of Central and North-East Asia (Angara–Vitim, Khangai, Kalba-Narym, and Kolyma) can be divided into areal and linear types, which differ significantly in the area and volume of granitoids in their composition. It is assumed that these differences are caused by the structure of pregranitic basement and the degree of thermal impact on the lower and middle continental crust. An important factor in the formation of granitoid provinces is a mantle mafic magmatism, the estimated scale of which correlates with the volumetric and areal characteristics of the granitoid provinces. The role of mafic magmatism is an additional input of heat from the fluids into the melting region of crustal protoliths, as well as a material contribution through various mechanisms of magma mixing. Mixing at a deep level is the most efficient, resulting in the formation of significant volumes of increased basicity silicic magmas. The petrogenetic role of contrasting magmas mixing at the mesoabyssal crustal level, as well as at hypabyssal conditions is not great, but mingling dikes formed in this process serve as a key argument in justifying the simultaneous formation of mafic and granitoid magmatism. Granitoids of Silicic Large Igneous Provinces (SLIPs) are characterized by a heterogeneous isotopic composition generally corresponding to the parameters of the continental crust. The extremely high heterogeneity of spatially conjugate granitoids is caused by mixing of silicic magmas formed through the melting of a few isotopically contrasting sources, including mixing with magmas of mantle origin. The mafic rocks ascribed to the granitoid provinces correspond to the isotopic composition of the enriched mantle (Angara–Vitim batholith) or indicate a significant crustal contribution (Khangai area). The metallogeny of SLIPs is determined by the degree of erosional section and the crustal protolith type, the metamorphic grade of which largely determines the initial fluid content of silicic magmas. The melting of high-grade ancient crustal protoliths produces relatively “dry” silicic melts, the melting of low-grade crustal sources leads to the formation of “aqueous” melts, the differentiation of which ends with pegmatite formation with rare metal mineralization. The formation of non-subduction SLIPs is associated with the mantle plume impact (in the form of simultaneous basaltic magmatism) on the heated crust of young orogenic regions, where tectonic processes were completed no more than a few tens of Ma.