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. Granites and syenites occur within each suite, but the proportion of syenite increases with time, as does the alkalinity of all magmas. Synplutonic mafic rocks are associated with plutons of all five suites and mafic/felsic magma-mixing textures are common. Laser fluorination oxygen isotope analysis (Sharp, 1990) of titanite from granitoids indicates that within and among plutons of a given suite O-18/O-16 is fairly constant. This indicates that these magmas probably formed with a high degree of isotopic (and chemical) homogeneity. However, there is a progressive decrease in delta(18)O of titanite with time from about +6.5 in the earliest suite to about +1.5 in the youngest suite (corresponding to a decrease in magmatic delta(18)O from similar to +11 to similar to +6). The systematic evolution of geochemical and isotopic composition with time allows us to develop a model explaining the progressive increase in alkalinity (especially K2O) and decrease in delta(18)O 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 (cf. Barker et al., 1975). Such a mechanism could form large volumes of syenitic residual liquids, having the appropriate isotopic composition. Younger suites were in part derived through remelting and hybridization of material formed or intruded during earlier magmatic episodes, leading to a progressive increase in overall alkalinity and decrease in delta(18)O. The repeated cycles of magmatism involved significant anorogenic crustal growth and a gradual cratonization of the continental crust of East Asia, caused by multiple melting and remelting events and associated large influxes of mantle-derived alkalic mafic magma.