GEOCHEMICAL CHARACTERIZATION OF BANGKA ISLAND GRANITOID : TYPOLOGY AND GEOTECTONIC REVIEW

Bangka as an accretion island compiled by variation of granitoids, have special interest due to of three microcontinent and geodynamic 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 granitoid. The methodology used mainly by geochemical spectroscopic. Spectroscopic by Neutron Activation Analysis (NAA) and X-Ray Fluorescence (XRF) were trace and rare earth elements analyzed. Irradiated samples conducted at the Reactor "Siwabessy", BATAN-Indonesia. Granitoid sampel from Western, Southern, Central and East (Belinyu) with ages from Late Permian to Late Triassic. Petrographic analysis of granitoid dominant as Alkali Feldspar - Syeno Granite, whereas geochemical analysis as Alkali-Syeno Granite. SiO₂ variation diagram showed declining 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 involve. 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 abundant of biotite. Plotting in a spider diagram normalized to chondrite showed LILE against HFSE enrichment, depletion consistent 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.