Early Triassic Tectonic Evolution of the Northeastern Kontum Massif: New Constraints from the S-type Granite in Ba To Area, Quang Ngai Province, Central Vietnam

The granitoids exposed in Ba To area, northeastern portion of the Kontum Massif, central Vietnam were previously assigned as part of the Hai Van complex. Field investigation and petrographic study reveal that the magmatic rocks in Ba To area comprise dominantly medium- to coarse-grained, per-aluminous two-mica granite of S-type origin. These rocks are strongly overprinted by several phases of tectonic activities including the emplacement of felsic and mafic dikes and veins, formation of weak ductile foliation, brittle fracturing, and widespread alteration under the influence of several post-magmatic thermotectonic processes. Results of U–Pb dating of zircons collected from granite in Ba To area show a crystallization age of 245.8 ± 1.5 Ma for the magmatic rock. The petrographic study and absolute age dating results conducted by this work are comparable to other recent works on similar granitoid occurred in the northern margin of the Kontum Massif. This indicates that the widespread Ca 245 Ma Hai Van-type S-type granitoid along the north and northeastern portions of the Kontum Massif were derived from a collisional orogenic event, which extended from Late Paleozoic onward during the assembly of Sibumasu Block to Indochina Block to form the Proto-Southeast Asia, which is correspondent to the Indosinian Orogeny. Several post-intrusive thermotectonic events due to the tectonic activities along the newly formed Proto-Indochina Block had resulted in subsequent deformation, injection of late magmas and alterations of the Hai Van—Type magmatic rocks.