Finding of overprinted granulites from the Da Nang-P Rao shear zone in the Truong Son Belt, central Vietnam

Several Indosinian NW to EW-trending shear zones develop in the Truong Son Belt, central Vietnam. High-grade rocks in amphibolite facies are commonly found along these shear zones, although few petrological works have been conducted. Here we firstly report overprinted granulites along the P Rao- Da Nang shear zone (DPSZ) which is an EW shear zone located in southern Truong Son Belt consisting of Early Paleozoic and Triassic rocks. Along this shear zone, high-grade pelitic granulites crop out as mega blocks enclosed in Devonian granitoids of the Dai Loc complex. We provide petrography, geochemistry and P-T evolution of the granulites in order to understand the formation process of these rocks. Through petrographic observations from these metapelitic rocks, three distinct metamorphic stages (M1a, M1b, and M2) are recognized. A peak granulite stage (M1a) was characterized by mineral assemblage of garnet (Grt1 core) + orthopyroxene (Opx core) + biotite (Bt1) + plagioclase (Pl1) + K-feldspar (Kfs) + cordierite (Crd) + rutile + quartz (Qz) ± spinel ± corundum. The M1b retrograde stage is recognized by the rim of porphyroblastic garnet and orthopyroxene and also matrix minerals (Crd, Pl1, Qz, Kfs). The M2 overprinting stage are characterized by garnet (Grt2) + orthoamphibole + kyanite/sillimanite + biotite (Bt2) + plagioclase (Pl2) + rutile ± staurolite ± ilmenite produced by several breakdown reactions including Crd + H₂O → orthoamphibole + kyanite + Qz, Kfs + H₂O → Bt2 + kyanite + Qz, Pl1 → kyanite + Pl2, and orthopyroxene + H₂O → Bt2 + Qz. The P-T condition of each metamorphic stage was estimated by using conventional thermobarometers. P-T conditions of 700-770 °C and 5-6 kbar is defined for peak granulite facies, the cooling stage under P-T conditions of ~ 600 °C and 4 kbar. The overprinting occurred in a higher-pressure condition (7.5-8.2 kbar at 670-700 °C) in amphibolite facies. The overall P-T path shows an early clockwise and later anticlockwise trajectories, this result could be related to two distinct tectono-thermal events. The granulite facies stage may be related to a collisional event between Kon Tum terrane and Truong Son terrane during Early Paleozoic. The overprint metamorphism is a consequence of crustal thickening with east-west shearing when Indochina block assembled with South China block in Early Triassic.