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Volcanic activity and associated plutonism, ranging in age from the Carboniferous to the Late Cretaceous, has made an important contribution to the Pre-Tertiary geological evolution of Sumatra. This chapter summarizes the known occurrences of Pre-Tertiary volcanic rocks and their geological settings (Fig. 6.1 & Table 6.1). There has been no systematic isotopic dating programme directed at determining the ages of the volcanic rocks, but dating of volcanic episodes in Sumatra has benefited greatly from stratigraphic palaeontological studies on the associated sedimentary units, summarized by Fontaine & Gafoer (1989). Unfortunately, little progress has been made in determining the chemistry of the volcanic rocks of Sumatra, subsequent to the compilation of analses reported by Rocks et al., but the initial results of a programme of detailed mapping studies by geological Research and Development Centre, Bandung promises an improved understanding of the geochemistry of both volcanic and plutonic rocks in the island (Suwarna et al. 2000).
According to the tectonic synthesis which has been presented in this volume (Chapter 14), in the late Palaeozoic (Fig. 602a) the eastern half of sumatra formed a segment of the margin of the southern Gondwana Supercontinent facing the Palaco-Tethys ocean, off NW Austrialia, while Austrialia was undergoing glaciation. On the other hand, the western half of Sumatra lay in tropical latitudes, beyond the Greater Sula Spur of Eastern Indonesia, at the junction zone between Gondwana and the Indochina Block of the northern Cathaysian Continent (Fig. 14.11). Palaeo-Tethys was subducted beneath the Indochina Block in the Late Palaeozoic
