Compositional change from high-Mg to low-Mg magmatism at ca. 150 Ma in the central Lhasa terrane, Tibet: Switching from advancing to retreating subduction of the Bangong Tethyan slab

Abstract

Differentiating between advancing and retreating subduction zones and delineating the timing and processes for switching between these end-member types are critical factors in understanding orogenic evolution. In this study, we used temporal composition variations and spatiotemporal distribution of igneous rocks to constrain the types and directions of the subduction zone beneath the Lhasa terrane in southern Tibet during the mid-Mesozoic. Comprehensive geochronological and geochemical data from mid-Mesozoic magmatic rocks in five areas of the central Lhasa terrane document two compositionally distinct magmatic suites: older (ca. 172−150 Ma) high-Mg diorite-granodiorite and younger (ca. 150−130 Ma) low-Mg granodiorite-syenogranite. The high-Mg rocks with enriched isotopes were likely generated by the mixing of Lhasa terrane basement-derived magmas with mantle-derived magmas that incorporated extensive contributions from subducted sediments. The low-Mg rocks with relatively depleted isotopes are interpreted as derived from a mixed lower-crust source (ancient basement and juvenile crust) with minor mantle contributions followed by crystal fractionation. Both high-Mg and low-Mg rocks show arc signatures and formed the 1200 km Shiquan-Zhegu-Menba arc. A combination of the Mesozoic geological and geochemical data suggests that both the Neo-Tethyan and Bangong Tethyan slabs were likely subducting beneath the Lhasa terrane, creating two opposing subduction zones. The change from high-Mg to low-Mg magmatism within the Shiquan-Zhegu-Menba arc likely records a tectonic switch from advancing to retreating subduction of the south-dipping Bangong Tethyan slab. Our results provide a good example for identification of subduction polarities and types of ancient orogenic belts through spatiotemporal distributions and changes in geochemical compositions of igneous rocks.