PERMO-TRIASSIC TECTONIC EVOLUTION OF THE KIM SON FORMATION OF THE KAN NACK COMPLEX IN THE KONTUM MASSIF, CENTRAL VIETNAM

The Kontum Massif in central Vietnam features high-grade amphibolite facies metamorphosed mélange, which was considered as a Precambrian continental core within the Indochina Block. However, recent geochronology and petrology data showed most Archean ages come from detrital zircons revealed these metasedimentary rocks had complex and exotic origins. The tectonic evolution of the magmatism and metamorphism is more likely to be related to the evolution of the closure of Paleo-Tethys followed by Indosinian orogenic events, along with subsequent magmatic intrusions and structural deformation. In this study, Biotite gneiss, amphibolite and granodiorite from the same outcrop within the Kim Son Formation were analysed by ⁴⁰Ar-³⁹Ar step heating. Mineral separates of Hornblende and Biotite are analysed aiming to provide a well constrained set of thermo-chronological evolution. The highly deformed biotite gneiss and amphibolite slivers are truncated by undeformed granodiorite. Amphibolite appears as inclusions within the granodiorite and as phenocrysts within the primary foliation of the biotite gneiss. The biotite in the biotite gneiss shows age spectra ranging from 181.54±52.60 to 202.16±13.61 Ma, with a plateau age of 212.87±2.28 Ma (>65% 39Ar cumulative release; MSWD = 1.3). The amphibole in the amphibolite shows age spectra ranging from 266.06±36.37 to 281.23±16.77 Ma, with a plateau age of 270.57±3.02 Ma (>90% 39Ar cumulative release; MSWD = 4.5). The amphibole in the granodiorite shows age spectra ranging from 252.04±69.01 to 279.84±15.12 Ma, with a plateau age of 280.76±1.48 Ma (>75% 39Ar cumulative release; MSWD = 4.3). The biotite in the granodiorite shows age spectra ranging from 176.34±64.28 to 227.18±27.85 Ma, with a plateau age of 221.73±3.05 Ma (>65% 39Ar cumulative release; MSWD = 2.4). Combining the contact relationships and our ⁴⁰Ar-³⁹Ar dating results, we conclude that the metamorphism and deformation of amphibolite and biotite gneiss occurred between 266 Ma and 281 Ma, while the intrusion and cooling of the undeformed granodiorite occurred around 280 Ma. The metamorphic processes were influenced by the granodiorite intrusion, which subsequently uplifted to shallow crustal levels around 190 Ma. This study provides critical geochronological data that enhances our understanding of the Permo-Triassic Tectonic evolution of the Kontum Massif.