A Neogene back-arc origin for the Banda Sea basins: geochemical and geochronological constraints from the Banda ridges (East Indonesia)

Dredgings conducted during the French-Indonesian cruises Banda Sea II and I II collected volcanic rocks from several ridges of the Banda Sea area (Tuka ng Besi ridge, site 218; Lucipara ridge, sites 214 and 305; Nieuwerkerk-Emp eror of China, sites 219 and 220), With the exception of one 36-Ma-old N-MO RB type basalt, thought to belong to an ophiolitic complex, K-Ar and Ar-Ar datings indicate that all the dredged volcanics are Neogene. They range in age from ca. 10 Ma (Tukang Besi back-are basalts) to 8-7 Ma (Nieuwerkerk-Em peror of China calc-alkaline andesites) and to 7-3 Ma (Lucipara GIB-type tr ansitional basalts and cordierite-bearing andesites). Radiogenic isotopic s ignatures of andesites are consistent with an AFC (Assimilation coupled wit h Fractional Crystallization) process involving assimilation of continental crust. 8-3-Ma-old calc-alkaline volcanic activity is also recorded on the Wetar segment, an inactive part of the East Sunda are, and corresponding is otopic compositions are also consistent with an AFC process involving conti nental crust. These features suggest that Lucipara-Nieuwerkerk-Emperor of C hina ridges and the Wetar segment were representing a single volcanic are 8 -7 Ma ago. The corresponding calc-alkaline activity was related to the subd uction of the Indian oceanic lithosphere beneath continental blocks of Aust ralian origin. Back-are opening processes occurred from 6 to 3 Ma as a mult i-rift opening for the Wetar basin and as a single-rift opening for the Dam ar basin while subduction-related magmatism was still active in the Wetar s egment. Volcanic activity stopped at 7 Ma in the Nieuwerkerk-Emperor of Chi na ridge. On Lucipara ridge, 6-3 Ma volcanic activity emplaced concomitantl y transitional basalts and cordierite-bearing andesites, The mineralogical and chemical features of the latter are consistent with an AFC process invo lving assimilation of continental crust by mantle-derived basaltic magmas, The end of magmatic activity on both volcanic segments at 3 Ma is thought t o result from the collision of Timer with the Wetar segment of the Sunda ar e, (C) 1998 Elsevier Science B.V. ALL rights reserved.