Mechanism of crustal thickening and exhumation of southern Lhasa terrane during the Late Cretaceous: Evidence from high-pressure metamorphic rocks of the Eastern Himalayan Syntaxis

Abstract

The mechanism of Late Cretaceous crustal thickening and exhumation of the southern Lhasa terrane is critical for understanding the tectonic evolution of the Tibetan Plateau. High-pressure metamorphic rocks from the lower crust are good candidates for addressing this issue. In this study, we focus on Late Cretaceous, high-pressure, garnet-bearing amphibolites from the Nyingchi Complex of the Eastern Himalayan Syntaxis and present an integrated study of geochronology, petrography, mineral chemistry, and thermodynamic modeling. Petrographic data determine three metamorphic stages (M1–M3). The M1 stage is characterized by a peak mineral assemblage of garnet + hornblende + albite + rutile + muscovite + quartz, which is followed by a post-peak (M2) assemblage of garnet + hornblende + plagioclase + epidote + biotite + rutile + quartz. The late retrograde stage (M3) is defined by hornblende + plagioclase symplectites surrounding garnet porphyroblasts. Mineral chemistry, with thermodynamic modeling, constrains the P-T conditions of the M1– M3 stages to 14–19 kbar/660–720 °C, 8–10 kbar/650–660 °C, and <7 kbar/<600 °C, respectively. Metamorphic zircons yield a concordant age at 90 Ma, which indicates the formation of garnet-bearing amphibolites. These results indicate a P-T-t path involving near-isothermal decompression for garnetbearing amphibolites, which suggests that the Nyingchi Complex underwent peak-pressure metamorphism (M1) at 90 Ma, followed by rapid exhumation to the depth of 32–26 km along the subduction channel. Moreover, the garnet-bearing amphibolites are considered to be the product of high-pressure metamorphism of mafic crust at the base of the Gangdese belt. Hence, the crust of the Gangdese belt experienced significant crustal thickening of up to 60 km at 90 Ma.