Geochemical evolution of Phanerozoic magmatism in Transbaikalia, East Asia: a key constraint on the origin of K-rich silicic magmas and the process of cratonization

During the Phanerozoic, granitoid magmatism occurred on a vast scale in Transbaikalia and Mongolia in east central Asia. Within this huge and geologically complex region, many hundreds of individual plutons of syenite, quartz syenite, quartz monzonite, and K-rich granite have been intruded within NE-SW oriented belts 200-400 km wide and 2000-3000 km long. Five compositionally distinct, major stages of magmatism are distinguished: 1) Ordovician-Silurian, 2) Devonian, 3) Early Permian I, 4) Early Permian II, and 5) Permo-Triassic. The systematic evolution of geochemical and isotopic composition with time allows the development of a model explaining the progressive increase in alkalinity (especially K₂O) and decrease in δ ¹⁸O over 250 m.y. of plutonism. These constraints may be satisfied by progressive hybridization of K-rich (alkali) basalt magmas with crustally derived silicic melts, accompanied by fractional crystallization of K-poor phases such as plagioclase and pyroxene. -from Authors