TEXTURAL VARIATION AND ALTERATION OF THE GUNUNG BULIGIR INTRUSIVE COMPLEX, BANTARUJEG, MAJALENGKA REGENCY, WEST JAVA: A PETROGRAPHIC APPROACH

Abstrak

The Gunung Buligir Intrusive Complex in Bantarujeg, Majalengka Regency, West Java, is an andesitic to dioritic intrusive body that penetrates Tertiary sedimentary sequences of the Halang and Kaliwangu Formations, yet its petrographic variability and alteration evolution remain poorly constrained. This study aims to systematically characterize mineral assemblages, textures, and hydrothermal alteration patterns to better understand fluid–rock interaction during arc-related magmatism. This study presents revised and elaborated petrographic characterization of five thin-section samples (BTRJ1–BTRJ5), integrating modal point-counting data (1,000 points per sample), detailed alteration mineralogy, and systematic textural analysis. Four of five samples (BTRJ1, 2, 3, 5) are classified as porphyry microdiorite with a microplagioclase groundmass, while BTRJ4 is classified as a diorite with a more equigranular texture (0.5 mm crystal size). The primary phenocryst assemblage is dominated by plagioclase (averaging ~28% as phenocrysts) and hornblende/amphibole (~13%), with clinopyroxene identified exclusively in BTRJ4 (~6%). Quartz ranges from <1% (BTRJ2) to ~5% (BTRJ3). Secondary alteration is dominated by a propylitic assemblage of chlorite, epidote, calcite, sericite, and secondary quartz. Alteration intensity is ranked BTRJ3 > BTRJ2 > BTRJ4 > BTRJ5 > BTRJ1. The spatial variability in alteration intensity and the occurrence of corona textures indicate a multi-stage hydrothermal system involving evolving fluid composition and temperature conditions. These results provide new insights into the coupling between magmatic crystallization and hydrothermal processes in Neogene arc intrusions and contribute to a better understanding of alteration zoning and fluid evolution in the Bogor Zone, West Java.