Evidence for prograde metamorphic evolution of Sri Lankan pelitic granulites, and implications for the development of continental crust

Abstract

Petrological evidence for prograde metamorphic evolution is obtained for pelitic granulites of the newly defined Highland Complex in Sri Lanka. It includes the occurrences of relict kyanite±quartz, staurolite, corundum±kyanite, hercynite+kyanite and sapphirine+kyanite±spinel within garnet in the sillimanite-quartz-rich rocks. Of special interest is the mineralogy of khondalites (graphitic garnet-sillimanite-K-feldspar-quartz gneisses), the matrix of which is free of hydrous minerals indicating that dehydration reactions have been completed in this kind of rocks. However, relict hydrous minerals such as AlTi-rich (more than 58 wt% Al2O3 and more than 1 wt% TiO2) staurolite and extremely Ti-rich (up to 9.8 wt% TiO2) biotine occur as inclusions within garnet. The local relict mineral assemblages within garnet in khondalites indicate the following continuous staurolite-breakdown reactions which may have progressed as temperature increased and pressure decreased: staurolite+quartz=kyanite+garnet+H2O; staurolite=corundum+kyanite+garnet+H2O; staurolite=kyanite+hercynite+garnet+H2O; staurolite=sillimanite+hercynite+garnet+H2O. Some of these dehydration reactions are inferred to have also taken place in other widespread pelitic granulites of the Highland Complex. Moreover, garnet in the Highland Complex pelitic granulites occasionally preserves growth zoning, though modified slightly to extensively by post-growth volume diffusion. Based on the evidence, a clockwise prograde P-T path of evolution is inferredfor meta-sedimentary granulites belonging to the Highland Complex. On the other hand, no evidence for prograde metamorphism has so far been obtained in “lower grade” rocks of the Wanni Complex to the northwest of the Highland Complex. Such a difference between these complexes may be the first petrological feature which supports the new geotectonic division of the Sri Lankan basement rocks based virtually on the distinctly different neodymium model ages. It suggests that these two complexes were amalgamated during the very last part of prograde metamorphism of the Highland Complex rocks, that is during decompression and heating, probably by the lower crustal lateral flow after tectonic overthickening and delamination. The clockwise prograde P-T-t path followed by the meta-sedimentary granulites of the Highland Complex is in marked contrast with the near isobaric cooling path for meta-igneous granulites suggested by some authors. It is proposed that both types of P-T-t paths albeit for different rock types, are possible and resulted from diverse deep crustal processes, such as underplating of continental crust by sediments and magmas or near convergent plate boundaries.