GEOCHEMICAL CHARACTERIZATION OF BANGKA ISLAND GRANITOID : TYPOLOGY AND GEOTECTONIC REVIEW

Bangka as an accretion island compiled by variation of granitoids, has special interest due to three microcontinents and the geodynamics involved. This research aims to investigate the characteristics of granitoid P. Bangka to be applied in study of magmatism, tectonic situations and relationships developed between granitoids. The methodology included geochemical spectroscopy via Neutron Activation Analysis (NAA) and X-Ray Fluorescence (XRF) techniques for analyzing trace and rare earth elements. Granitoid samples from Western, Southern, Central and East (Belinyu) with ages from Late Permian to Late Triassic were examined. Petrographic analysis of granitoids classifies them as Alkali Feldspar - Syeno Granite, whereas geochemical analysis classifies them as Alkali-Syeno Granite. SiO2 variation diagrams show declines in the proportion of the major elements with increasing SiO2 influenced by affinity as calc-alkalic magma with high K content. Affinity can be formed on continental margins where subduction and collision are involved. In common, granitoid typology as peraluminous, with I type in Central and Eastern Bangka (Belinyu) characterized by high proportion of magnetite and Y, while S type in the South and West Bangka granitoids are characterized by high K2O and abundance of biotite. Plotting in a spider diagram normalized to chondrite showed LILE against HFSE enrichment, depletion of Nb, and LREE enrichment relative to flat HREE. The condition showed the character of subduction-related in continental arc, whereas Rb abundance indicates the position of Syn Collision. The tectonic conditions affect the formation of granitoid magmatism. Trace elements ratios indicate the source of magmatism in Central and Eastern Bangka (Belinyu) as shosonitik (mixed derived), while the South and West Bangka as crust derived. It also distinguished tectonomagmatism respectively as subduction and collision influenced. Granitoid magmatism formed due to subduction in Central Bangka and Belinyu the Late Permian-Early Triassic along with the closure of Paleo-Tethys due to the collision of East Malaya and Sibumasu Block, while the Middle Triassic - Late collision with the West Sumatra and Sibumasu Block produced granitoid South and West Bangka-influenced crust.