Microstructural Constraints on Syn- to Post-Magmatic Deformation of North Bangka Granitoids, Indonesian Segment of the Southeast Asian Tin Belt

Granitoids in North Bangka, Bangka Belitung Province, Indonesia, are an integral part of the Southeast Asian Tin Belt magmatism, yet their deformation history from crystallization through tectonic cooling remains incompletely documented. This study characterizes the petrography and reconstructs the microstructural deformation history of granitoids. Nine representative granitoid samples were collected from outcrops at Bukit Bias, Tempilang, Siangau, Mangkol, Kelapa, Jebus, Pemali, Plangas, and Menumbing (Nikon Eclipse LV100NPOL (Nikon Corporation, Tokyo, Japan) with NIS-Elements BR software, Basic Research Version) and modal point counting (3608 points per section) were applied to ensure a more detailed description and accurate quantification of mineral compositions. The granitoids are classified as syenogranite, monzogranite, and quartz syenite based on the QAP modal diagram. The dominant minerals are K-feldspar (24.89–54.27 vol.%), quartz (16.47–37.13 vol.%), and plagioclase (10.89–24.45 vol.%), with biotite as the femic phase. Microstructural analysis reveals three successive deformation stages: (1) a suprasolidus stage (T > 650 °C) marked by submagmatic fractures, grain boundary migration (GBM) in feldspar and chessboard pattern quartz; (2) a high-temperature subsolidus stage (650–450 °C) characterized by GBM and subgrain rotation recrystallization in quartz, myrmekite, and flame perthite; and (3) a low-temperature subsolidus stage (T < 450 °C) evidenced by undulose extinction, bulging recrystallization (BLG), deformation twins in feldspar, and kink bands in feldspar and mica. The progressive microstructural sequence demonstrates that deformation was initiated during the late magmatic stage and persisted under decreasing temperature in the solid state, indicating that the North Bangka granitoids record a prolonged history of syn- to post-magmatic deformation linked to the regional tectono-magmatic evolution of the Southeast Asian Tin Belt. These granitoids belong to the Late Triassic–Early Jurassic (228–198 Ma) Klabat Granite Complex, emplaced within the Indonesian segment of the belt during the Sibumasu–Indochina collision, and the microstructural evolution documented here is consistent with this regional tectono-magmatic history.