THE TECTONIC EMPLACEMENT OF SUMBA IN THE SUNDA-BANDA ARC - PALEOMAGNETIC AND GEOCHEMICAL EVIDENCE FROM THE EARLY MIOCENE JAWILA VOLCANICS

The island of Sumba is a continental fragment in the fore-are region n ear the transition between the Sunda Are and Banda Are in southeastern Indonesia. Paleomagnetic and geochemical evidence from the early Mioc ene volcanics of the Jawila Formation in western Sumba constrain the f inal drift stage and tectonic emplacement of the island. The lavas ran ge from predominantly andesites to dacites, and display textural evide nce for a weak metamorphism. Rock magnetic and mineral chemical data p oint to pseudo-single- to multi-domain (titano)magnetite (Fe2.5-3Ti0.5 -0O3), with grain sizes up to 10 mu m, as the main carrier of remanenc e. The Jawila Formation reveals a ChRM direction with declination = 4. 6 degrees, inclination = - 19.2 degrees, alpha(95) = 9.9 degrees and a paleolatitude of 9.9 degrees S, which corroborates earlier results(Ch amalaun and Sunata, 1982). Taking paleomagnetic evidence from other fo rmations on the island into account, we conclude that the Sumba fragme nt has occupied approximately its present position since the Miocene. The calc-alkaline affinity and trace-element signatures of the lavas p oint to an origin in an are environment. This occurrence of subduction -related volcanic activity in the early Miocene on Sumba implies that a volcanic are existed south of the present-day East Sunda Are, or tha t the island was located within the latter are between Sumbawa and eas tern Flores, and still had a minor southward drift to cover.