The distinctive tectonic and metamorphic history of the Barru Block, South Sulawesi, Indonesia: Petrological, geochemical and geochronological evidence

Abstract

This study describes the field relationships, petrography and geochemistry of various metamorphic and magmatic rocks from the Barru Block, South Sulawesi, Indonesia, and reports Rb-Sr isochron ages for clastic metasedimentary rocks. The Barru Block is composed of a tectonic stack of various lithological units, including cherts, radiolarian mudstone, breccias, shales, phyllites, mica schist, gneisses, amphibolite and ultramafic rocks. Dacitic rocks intrude the metamorphic and ultramafic sequences. Greenschist- to amphibolite-facies metasedimentary rocks represent detritus from nearby continental blocks that have been redistributed by turbidity currents. Quartzofeldspathic gneisses are derived from andesitic and dacitic protoliths, whereas amphibolites have basaltic precursors. The clastic metasedimentary rocks show enrichment in LILE and HFSE, particularly Nb, but depletion in Sr. The amphibolites display trace element patterns typical of N-MORB rocks, whereas compositional characteristics of gneisses and dacites suggest that they originated in an arc-related environment.

Geothermobarometric calculations and mineral assemblages of greenschist facies-rocks suggest metamorphic equilibration at 250–300 °C and a maximum pressure of 5 kbar. Temperature estimates for amphibolites are 640 ± 20 °C at a pressure of 1.3–3 kbar, whereas the temperature estimates for quartzofeldspathic gneisses range from ca. 630–660 °C for peak conditions to 430–520 °C for the retrograde stage. Internal Rb-Sr mineral isochrons for three medium-pressure mica schists indicate metamorphic ages of ca. 70.7 ± 2.4 Ma, 107.3 ± 0.7 Ma and 110 ± 2.4 Ma, respectively. This age range confirms the importance of Early Cretaceous (Albian) metamorphism within the region, but also indicates a previously unknown Late Cretaceous (Maastrichtian) metamorphic or deformational event at ca. 70 Ma.

The Barru rocks show a range of tectonic environments as diverse as that of the adjacent Bantimala Block, but overall, the Barru rock association is more felsic in composition. Although the two blocks accreted approximately at the same time (Early Cretaceous), differences in protoliths, geochemistry and metamorphic P–T conditions imply that they were not geographically close together at that time, but were subsequently juxtaposed tectonically.