Geochemistry and petrogenesis of Laojungou and Mengtonggou granites in western Sichuan, China: Constraints on the nature of Songpan-Ganzi basement

Abstract

The Mesozoic Laojungou and Mengtonggou granitic plutons in the eastern part of the Songpan-Ganzi terrane represent products of a major orogenic granitic magmatism and contain important information about the nature of the basement.These granites are intermediate-to high-K calc-alkaline,with intermediate to high SiO_2(63.9～75.0 wt%)and metaluminous to mildly peraluminous characteristics(ACNK=0.86～1.08).Rocks of the Laojungou pluton contain high Al_2O_3(14.2～16.0wt%)and K_2 O(2.88～4.36 wt%)but low MgO(0.38～1.98 wt%).The Laojungou pluton has relatively high Sr(307～697 ppm)and low Y(12.2～21.0 ppm)with fractionated REE((La/Yb)_N10)and relatively high Sr/Y ratios(22～55),and resembles adakitic crustal melts.The Mengtonggou pluton is quite similar to the Laojungou pluton in most major and trace element characteristics,except for its relatively low Sr(110～397 ppm),high Y(12.2～21.0ppm)and strikingly low Sr/Y ratios(5～32).The two plutons all exhibit crustal Nd-Sr isotope compositions(e_(Nd)(T)=-6.02～3.11;I_(Sr)=0.7053～0.7075),indicating that these granites were not mantle-derived, but most likely resulted from partial melting of lower crustal materials in the garnet stability field.The high Rb(106～202 ppm), Cs(3.79～25.6 ppm)contents and intermediate to high K_2O/Na_2O ratios(0.56～1.12)of both plutons are consistent with their generation from dehydration of biotite.The relatively high temperature for biotite dehydration precludes shear-heating as the cause for the genesis of these plutons,and requires an underplating or intraplating scheme of mantle-derived magma to fuse the crustal materials. The two plutons possess similar,Mid-proterozoic T_(DM)ages(T_(DM(Ⅱ))=1.23～1.44 Ga),which together with their geochemical compositions,indicate that the Songpan-Ganzi terrane is underlain with the Yangzi-type basement.