Abstract

Sumatra is the largest volcanic island of the Indonesian archipelago. The oblique subduction of the Indian Ocean lithosphere below the Sundaland margin is responsible for the development of a NW-SE trending volcanic arc, the location of which coincides approximately with the Great Sumatran Fault Zone (GSFZ). We present in this paper ca. 80 new 40K-40Ar ages measured on Cenozoic calc-alkaline to shoshonitic magmatic rocks sampled all along this arc from Aceh to Lampung. The results show that magmatic activity started during the Paleocene (ca. 63 Ma) all along the arc, and was more or less permanent until Present. However, its spatial distribution increased at ca. 20 Ma, a feature possibly connected to the development of the Great Sumatran Fault. The position of Plio-Quaternary magmatic rocks is shifted away from the trench by a few tens of kilometres with respect to that of Paleocene to Miocene ones, a feature consistent with a significant tectonic erosion of the Sundaland margin during the Cenozoic.

The studied samples display typical subduction-related geochemical signatures. However, we have been unable to identify clear geochemical trends, either spatial or temporal. We suggest that the lack of such regular variations reflects a complex igneous petrogenesis during which the contribution of the Sundaland continental crust overprinted those of the mantle wedge and the subducted slab.

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