PETROLOGY, GEOCHEMISTRY AND TECTONIC EVOLUTION OF THE SOUTH SULAWESI BASEMENT ROCKS, INDONESIA

摘要

This study is primarily concerned with the petrological, geochemical and tectonic evolution of the South Sulawesi basement rocks, particularly the high-pressure metamorphic and the ultramafic rocks. New petrographic, mineral chemistry and geochemical analyses are used to determine their tectonic evolution.The South Sulawesi basement rock complex consists of two separated blocks, Bantimala and Barru block. The metamorphic and ultramafic rocks assemblages from these two blocks show quite different characteristic and different metamorphic history.Whole rock geochemistry indicates that both the Bantimala and Barru blocks were accreted slices from a wide range of tectonic environments. Five different tectonic settings for protolith have been recognised in the Bantimala block, mid oceanic ridge basalt, oceanic island basalt, island arc volcanics, cumulates and continental granodiorities or sediments, and is dominated by the oceanic basalt types. Conversely, the quartzofeldspathic gneisses that make up much of Barru are more felsic and show a consistent arc affinity. The Barru block also shows intrusion of late dacites cutting through the ultramafics, which is not known from Bantimala.Both blocks are heterogeneous and have complex accretion histories. Although roughly the same age (Cretaceous) and probably both situated at the southeast margin of Sundaland, they may not have been geographically as close as they are now. The Bantimala block records deep subduction of cold ocean floor including MORB, arc-related lavas and seamounts, and exhumation of deeply subducted material, prior to collision with microcontinents to the East and obduction of the ultramafics. Conversely, the Barru block is interpreted to preserve the roots of an old island arc, subduction of some ocean floor with seamounts, and obduction of quite different ocean floor material from the North, and was evidently too warm to preserve blueschist or eclogites. Therefore, these two blocks must have derived from different sources and tectonic setting.