Petrochemistry of corundum-bearing rock in Wellawaya, Sri Lanka

Abstract

This study is aimed to characterize corundum-bearing rocks in Wellawaya. Petrochemistry and mineral chemistry of rock samples were then investigated prior to reconstruction of metamorphism of the area. Rock collection can be divided, based on petrographic description, into 2 types including corundum-bearing rock and mica schist. Moreover, corundum-bearing samples, the main focus of this study, can also be divided into 2 types which are obvious foliated rock and non-foliated rocks. All corundum-bearing rock samples have similar mineral composition and microscopic texture characterized by alkali feldspar and plagioclase which usually form granoblatic grains with well-developed triple junctions. Essential minerals contain about 50-55% plagioclase, 25-30% alkali feldspar, 10- 15% biotite and 1-5% corundum. Corundums have been found as granoblastic and porphyroblastic grains that usually formed as very large crystals in both sample groups. These petrographic features indicate high-grad metamorphism. However, non-foliated samples appear to have occurred as a part of felsic layer in foliated rocks. Whole-rock geochemistry shows somewhat difference within these rocks. All corundum-bearing rocks are classified as peraluminous rocks on the basis of alumina saturation. Mineral chemistry shows similarity of assemblages observed in both corundum-bearing groups. They have similar mineral chemistry of corundum, alkali feldspar, plagioclase zircon, and biotite. Corundum may have crystallized during the peak metamorphism equilibrated with biotite, plagioclase and alkali feldspar. Corundum forms at high temperature conditions with a wide range of pressure conditions during regional metamorphism. Their protoliths would be alumina rich provenance prior to high grade metamorphism belonging to granulite facies.