Triassic granitoids in the eastern Songpan Ganzi Fold Belt, SW China: Magmatic response to geodynamics of the deep lithosphere

Abstract

The Songpan Ganzi Fold Belt (SGFB), SW China, was developed from a passive continental margin into an orogenic belt with the consumption of the Paleo-Tethys. During the evolution of the SGFB, numerous Late Triassic granitic plutons formed and exhibited a progressive development from adakite/I-type granite, high Ba–Sr granite, A-type granite and monzonite. Representative Late Triassic plutons were studied to unravel the bewildering evolution of the eastern SGFB. The Menggu Pluton (224 ± 3 Ma) consists of granites with high alkali (K2O+Na2O = 7.85–10.4 wt.%) and adakitic characteristics (Sr/Y = 19–38). The εNdT values (− 2.77 to − 5.03), initial 87Sr/86Sr ratios (0.7050–0.7063) and low Nb/Ta ratios (8–10) are indicative of an origin by partial melting of amphibolitic lower crust. Rocks from the Niuxingou Pluton (215 ± 3 Ma) are richer in K than Na (K2O/Na2O = 1.1–1.5) and contain high Sr (1006–1662 ppm) and Ba (1277–2009 ppm), typical of shoshonite and high Ba–Sr granite. They have less enriched εNdT values (+ 0.08 to − 2.04) and less radiogenic 87Sr/86Sri ratios (0.7047–0.7048), and formed from a mixed melt derived from upwelling asthenosphere and the overlying metasomatised lithospheric mantle. The Taiyanghe Pluton (205 ± 3 Ma) consists of monzonites, with high Al2O3 (> 20 wt.%), but low MgO (0.94–1.39 wt.%). The rocks are richer in Na than K (K2O/Na2O < 0.7), contain high large ion lithophile element (LILE) (681–834 ppm Sr and 2142–9453 ppm Ba) and display strongly fractionated REE patterns ((La/Yb)N = 35–63). These features, together with their enriched Nd–Sr isotopic compositions (εNdT = − 4.78 to − 6.50; 87Sr/86Sri = 0.7074–0.7090), suggest that the monzonite probably formed from low degrees of partial melting of metasomatised lithospheric mantle. Although a continuous compressional regime during the Mid- and Late Triassic has been invoked for the SGFB, the generation of crustally derived adakitic and shoshonitic plutons reflect thickening in response to an arc-continental collision accompanied by fracturing of the lithosphere and an extensional regime in the deep lithosphere in the Late Triassic. The 205 Ma Taiyanghe Pluton was emplaced simultaneously with a rapid uplift of the lithosphere, when surface deposits changed from deep-water turbidite to tidal flat sediments. It was therefore generated during decompression, probably related to the rapid removal of the overthickened lithospheric mantle. The Triassic magmatism in the eastern SGFB is therefore important for probing geodynamic processes in the deep lithosphere.