Mineralogical and geochemical constraints on the provenance and depositional setting of Sri Lankan marbles

Abstract

Proterozoic high-grade marbles cover a significant portion of the Highland Complex (HC) of Sri Lanka. Although many studies on metamorphic silicate rocks of the country have been carried out, marbles, which can be used as a tool to interpret the evolution history of the crust, has been paid little attention. The present study aims to understand the genesis and characteristics of protolith of these marbles by investigating their mineralogy and geochemistry. The studied marbles occur as bands intercalated with pelitic gneisses and quartzites of the HC, indicating the pre-existing sandstone-shale-carbonate sedimentary sequence. Except along the lithological contacts, the studied marbles contain low amounts of silicate minerals (<5%) and are dominated by dolomites (>90%) with minor calcites. Calcites occur as inclusions within dolomites or as fine grains along the grain boundaries of dolomites. Silicate minerals always occur as lensoid or layered aggregations. Dolomites are related to the protolith composition, while most of the calcites are metamorphic products formed due to the reaction of silicate minerals with dolomite. Silicate mineralogical zones suggest the inherited compositional variations of the protolith. Although the marbles have undergone granulite-grade metamorphism, their trace elemental contents have been well preserved. Concentrations of Fe, Mn, Sr, Na, and K as well as rare earth element (REE) contents of the marbles are comparable to those of marine origin carbonates. Hence, marine dolostone can be proposed as the protolith for Sri Lankan marbles. The distinct mineralogy as well as elevated trace element and REE concentrations of marble in the lithological contact zones are evidenced for prevailed syn- to post-metamorphic fluid activities in the HC. Mineralogical and trace elemental composition of Sri Lankan marbles are correlated well with the marbles from other East-Gondwanaland crustal units.