THE ORIGIN OF THE POTASSIC ROCK SUITE FROM BATU TARA VOLCANO (EAST SUNDA ARC, INDONESIA)

Batu Tara is an active potassic volcano in the eastern Sunda arc. Its leucite-bearing rock suite can be subdivided into two groups, one less evolved with Th < 20 ppm, the other more evolved with Th > 20 ppm. Sr-87/Sr-86, deltaO-18 and trace-element systematics in the less evolved group suggests the existence of parental magmas with different mantle origins. The mantle below Batu Tara is most likely heterogeneous and several source components are involved in magma genesis. Trace element and isotopic compositions of Batu Tara and adjacent volcanoes are consistent with the involvement of a subducted sedimentary/crustal component as well as MORB and OIB mantle, the latter with geochemical characteristics comparable to the mantle underlying Muriah (Java). Melt extraction from this complex mixture is envisioned as a two-stage process: partial melts of the crust-contaminated MORB mantle mix in the mantle wedge with partial melts of OIB domains. Different mixtures of these two melts provide the parental magmas that enter the volcanic plumbing system, where crystallization, hybridization and refilling processes occur. The calcalkaline volcanoes in the arc segment show stronger signatures for a subducted crustal component than Batu Tara, which displays a greater influence from the OIB mantle source. The potassium enrichment can therefore be attributed to contributions both from the enriched mantle and from subducted crustal material. Mantle-type deltaO-18 values of the Batu Tara magmas indicate that the mantle wedge below potassic orogenic volcanoes is not necessarily strongly enriched in O-18.