Petrogenesis of Adang and Talaya volcanic rocks, in the Mamuju area and its surrounding, West Sulawesi

Abstract

The Adang Volcanic and Talaya Volcanic complex located in the Mamuju area are ancient volcanic complexes of the Neogene period. These two complexes consist of pyroclastic, volcaniclastic, and basic-intermediate lava containing the main modal minerals of leucite and clinopyroxene, making these rocks unique and rarely found in Indonesia. This study aims to determine the petrological and geochemical characteristics as well as the tectonic setting of rock formation. The rocks are generally porphyritic and vitrophyric in texture with various rock types as a result of magma differentiation. The Adang Volcanic Lava is composed of trachyandesite, tephriphonolite, trachyte, phonolite, and leucitite. Talaya Volcanic Lava is composed of basalt and trachybasalt rocks. Based on geochemical content, rocks have high alkalinity with wide range concentrations of K2O (0.27 – 10.12%), Na2O (0.54 – 11.06%), and total alkali (4.6 – 12.6%), as well as evidenced by the presence of feldspathoids. Rocks are divided into three groups, sodic, potassic, and ultrapotassic. The ultrapotassic rocks have similarities with Group III (Active Orogenic Zone) and Group I (Lamproite). The intensive enrichment of incompatible elements such as Rb and Th as well as the negative Nb and Ti anomalies reflect that magma derived from mantle metasomatism and is associated with subduction events. The genesis of the rock occurred in the tectonic setting of alkaline arc magmatism or continental extensional zone which was formed after the collision of the Banggai-Sula microcontinent with West Sulawesi. This process is also indirectly influenced by pre-collision subduction of the oceanic lithosphere.