Origin of the Triassic Lincang granites in the southeastern Tibetan Plateau: Crystallization from crystal mush

The crustal generation of the Paleo-Tethys orogenic belt in the southeastern Tibetan Plateau is a major unresolved geological problem. The Changning-Menglian suture in the southeastern Tibetan Plateau represents a remnant of the Paleo-Tethys Ocean, The Lincang batholith and associated volcanic rocks (the Manghuai rhyolites and Xiaodingxi basalts) are located to the east of the Changning-Menglian suture, and represent important consftainis for the abovementioned geological problem. New zircon U-Pb ages, together with previously published results, indicate that the Manghuai rhyolites were emplaced at 229-239 Ma and the Lincang bail-1014h yielded longer age span (211-239 Ma). Broad age span of zircons (to 28 million years) from these rocks record a long-lived crustal magmatic system in the Changning-Menglian Paleo-TeLhys orogenic bell.. These intrusive and associated volcanic rocks have significant differences between them in terms of geochemistry. In this study, these rocks are geochemically classified into three groups: (1) Group 1 comprises biotite granite and granodiorite rocks; (2) Group 2 includes alkali feldspar granite, two mica granite, and rhyolite, which are high-silica (generally >70 wt%) rocks that have low TiO2, Al2O5, FeOtot, MgO CaO, P2O5, Eu, and Si contents, compared to those of Group 1. Based on theft geochemical relationships and mineral textures, Groups 1 and 2 include cumulative- and fractionated-type granitoids, respectively; (3) while Group 3 is comprised of dioritic enclaves and the Xiaodingxi basalts. Based on more robust evidence, dioritic enclaves are derived from a mantle source magma. The Lincang granites have high and variable initial Sr-87/Sr-86 ratios (0.703122-0.728476), significantly negative epsilon(Nd) (t = 220 Ma) values (-134 to -7.5), negative epsilon(Hf)(t) values (-17.7 to -6.8), and old T-DM2 ages. The Manghuai rhyolites and Xiaodingxi basalts have lower initial Sr-87/Sr-86 ratios (0.695970-0.709398), and higher epsilon(Nd) (t) values (-34 to -0.6) and epsilon(Hf)(t) values (-12 to +3.7) than those of the Lincang granites. Geochemical data suggest that the Manghuai rhyolites could be seen as fractionated and erupted melts from crystal mush reservoirs, which later crystallized to form the Lincang granites. The Manghuai rhyolites provide an instantaneous snapshot of the stale of the crystal mush at the time of eruption. The Lincang granites are interpreted to record prolonged histories of open-system magmatic evolution, involving magma mixing, wall-rock assimilation, and multiple cycles of crystallization from the crystal mush reservoirs. Magmatic evolution was likely related to mantle upwelliing in a post-collisional setting, forming large-volume granites and volcanics in the Changning-Menglian Paleo-Tethys orogenic belt. Thus, this crystal mush model provides an integrated picture of the Triassic silicic magmatism, linking the evolution of the Lincang batholith and Manghuai rhyolites before storage in the Changning-Menglian Paleo-Tethys orogenic belt. (C) 2020 Elsevier B.V. All rights reserved.