PETROLOGY, GEOCHEMISTRY, AND MECHANISMS OF BASALT EMERGENCE BEHIND VOLCANIC ARCS: A CASE STUDY OF BASALT SUKADANA, LAMPUNG

• Main Author: Pardamean Siringoringo, Luhut
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/86630
• Institution: Institut Teknologi Bandung

The emergence of Basalt on the earth's surface, especially behind volcanic arcs, is still unresolved, including Sukadana Basalt in Lampung Province. Petrogenesis and the mechanism of its emergence on continental plates, especially behind volcanic arcs, are still debated. Therefore, this study will reveal the petrogenesis of Sukadana Basalt and its emergence mechanism behind the Sumatran volcanic arc through field observation methods, petrography, gravity geophysical methods, general geochemical analysis using the XRF ICP-OES method (Inductively Coupled Plasma Optical Emission Spectroscopy; and the ICP-MS method (Inductively Coupled Plasma Mass Spectrometry), Sr-Nd-Pb-Hf Isotopes, and age determination using the 40Ar/39Ar method. Sukadana Basalt has petrological characteristics, namely aphanitic texture, common phenocryst minerals, namely plagioclase, pyroxene, and olivine minerals, with a massively developed structure, namely vesicular, and has iddingsite rim alteration. General geochemical data show that Sukadana Basalt has two petrogenesis groups, namely group A (<Nb 22 ppm) and group B (Nb>22 ppm), each group has Calc-Alkaline and Ocean Island Basalt like characteristics. Basalt, has minimal contamination level, and minor crystallization fractionation level. Group A is estimated to erupt at 299.6 ± 11.2 Ka and group B is estimated to erupt at 266.7 ± 3.7 Ka. Integration of whole rock geochemical data with Sr- Nd-Pb-Hf isotope data shows that group A is a product of metasomatism by the decay of sedimentary material or crust of the Indo-Australian plate subducted beneath the Eurasian plate, while group B is a product of mixing magma from the deep asthenosphere with magma from metasomatism by the melting of sedimentary material or crust of the Indo- Australian plate subducted beneath the Eurasian plate. Magma from the asthenosphere rises upward through slab tearing on the oceanic plate. Both groups of magma rise to the earth's surface through normal faults through the mechanism of decompression decay because of tensional deformation in the continental crust. Decompression melting is indirectly caused by the activity of the Sumatra Fault. This study contributes to the model of magma emergence in orthogonal and oblique subduction transition zones, the change in contrast from ARC-type magma to OIB like-type magma because of slab tearing and a new contribution that high Nb values can originate from OIB-type source mantle that has undergone partial decay (characterized by MORB-type isotope values and OIB like-type trace elements).