Mesozoic A-type Granites in Mongolia: Geochemistry and Origin

摘要

A-type granites are widespread in the Central Asian Orogenic Belt (CAOB). One of the province with abundant A-type granites is the Khentey area with granite-granodiorite batholith in the core and subalkaline and alkaline granites in margins of batholith, and in surrounding rifts. In margins of the Khentey batholith to A-type are related small one-two stage plutons with subalkaline granites, leucogranites and Li-F granites. In the rifts plutons are characterized by shallow-level emplacement and composed of alkali feldspar granite, leucogranite and quartz syenite (syenite). All granitic rocks are highly evolved with low CaO, MgO, high silica and alkalies, and are related to A (A2) type granite. Alkali granites contain ainphibole, and quartz syenites sometimes pyroxene. Li-F granites are composed of quartz, K-feldspar, albite, biotite and Li-mica, also topaz, and fluorite. Trace element distribution shows low Sr, Ba, and abundance of Li, Rb, Cs, enrichment of LREE with flat HREE and distinct Eu anomaly. Origin of A2 granites suggests the emplacement from relatively low water pressure in high temperature condition that supported by geochemistry. Li-F granites are product of high last stage fractionation characteristic for many evolved granites. Fractional crystallization is a process responsible for granite origin, and syenites are result of assimilation and fractional crystallization.