VOLCANOSTRATIGRAPHY AND PETROGENESIS BAWEAN ISLAND: SPECIAL STUDY ON THE KASTOBA LAKE AND ITS SURROUNDINGS AREA, BAWEAN ISLAND, GRESIK REGENCY, EAST JAVA

• Main Author: Hafizh, Muhammad
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/69534
• Institution: Institut Teknologi Bandung

Bawean Island is an island with volcanoes located in the north of Java Island, Java Sea, and it’s included Quaternary Volcano. Administratively, Bawean Island is in Gresik Regency, East Java. If viewed from the position of Bawean Island and the chemical composition of rocks that have high potassium content, it’s estimated that Bawean Island is associated with subduction that occurred in the south of Java Island so that it becomes a back-arc volcano. This research aims to map the volcanostratigraphic unit, determine the magma series and tectonic in the area, and determine the evolution of magma, based on petrogenesis. The data used are microtextures in pyroxene minerals, and the chemical content of rocks obtained from rock samples. Based on vulcanostratigraphic analysis, it’s known that Bawean Island has 4 Khuluks and 2 Gumuks. Bawean Island has magma evolution characteristics like magma mixing due to the injection of new basalt magma and crystal fractionation. This is reflected in fluctuations in oxides such as SiO2, K2O, MgO, and CaO. These changes are also reflected in the microtexture including fractionation, magma mixing, cooling due to eruption, and decompression due to eruption. The results of geochemical data analysis using discriminant diagrams in the form of bivariate diagrams and triangle diagrams, spider diagrams, and comparisons using the mayor and trace elements, obtained that Bawean Island is a back-arc island arc caused by subduction between the Sunda Plate and the Indo-Australia Plate. These results are also supported by the content of TiO2 < 1.3% and the high content of K2O in the rock samples. However, on Bawean Island there is the mixing of subduction-related magma with nonsubduction-related, characterized by a high ratio of Rb/K. The Bawean Island magma series is undersaturated silica and high potassium - ultrapotassic