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 2O(2.88-4.36 wt%)but low MgO (0.38-1.98wt%). The Laojungou pluton has relatively high Sr (307-697 ppm) and low Y (12.2-21.0 ppm) with fractionated REE ((La/ Yb) N > 10) 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 (12.2-21.0 ppm) and strikingly low Sr/Y ratios (5-32). The two plutons all exhibit crustal Nd-Sr isotope compositions (s 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 O/Na 2 O 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.