Volcanic Rock Characteristics of Arjuno–Anjasmoro Volcanoes in Tulungrejo Geothermal Prospect Area, East Java

Abstract

Tulungrejo is situated between the Arjuno and Anjasmoro volcanoes, a region with geothermal potential. The area exhibits hot-water manifestations and contains volcanic and pyroclastic rocks within its geological framework. This study aims to identify the petrographic characteristics of the region's volcanic rocks to provide insights into their crystallization and magmatic evolution and to support assessment of the geothermal system. The research involved geological mapping, the study of hot water manifestations, representative rock sampling based on lithological variations, and petrographic analysis. The results indicate that the volcanic rocks consist of basaltic andesite, andesite, pyroxene andesite, and hornblende andesite. These rocks exhibit porphyritic, pilotaxitic, and trachytic textures, with primary constituent minerals comprising plagioclase, pyroxene, hornblende, and opaque minerals. Plagioclase is the dominant mineral, with compositions ranging from andesine to labradorite. The variety of volcanic rocks present indicates a non-linear differentiation trend. The abundance of phenocrysts is characteristic of an island-arc setting. Evidence of magmatic activity and hot springs suggests that the region holds promising geothermal energy potential. Therefore, this study is expected to be followed by further exploration aimed at developing the area into a new geothermal field to increase East Java's electricity supply potential.