Formation of ekanite in “antiskarn” by carbonatitic melt infiltration into charnockitic gneiss, Ampegama, southwestern Highland Complex, Sri Lanka

Abstract

Carbonatitic melt–rock interactions in the crust provide key insights into element mobility, metasomatic processes, and the formation of rare mineral assemblages in high-grade metamorphic terrains. Ekanite-containing calc-silicate dykes at Ampegama, southwestern Highland Complex, Sri Lanka, provide compelling evidence for the interaction of crustal-derived carbonatitic melt with charnockitic gneiss wall rocks, forming an “antiskarn”-type assemblage with ekanite. The dykes, reaching up to 2 m in width, contain variously oriented, irregularly shaped fragments of charnockitic gneiss and form gradational boundaries and reaction zones with wall rocks / enclosed fragments. Field occurrence, textures and variations in mineralogical and chemical composition from charnockitic gneiss over contact zone toward calc-silicate indicate formation of silicate minerals – such as wollastonite, scapolite, clinopyroxene, titanite and ekanite (Ca 2 Th 0.9 U 0.1 Si 8 O 20 ) – via metasomatic reactions between carbonatitic melt and wall rocks. Other minerals (e.g. K-feldspar) reflect assimilation from wall rocks. In addition, there are reaction textures that indicate late-stage overprint by CO 2 and F-rich fluids. Unaltered ekanite is bottle-green and transparent and may have gem quality. It is metamict (glassy) but still contains crystallograpically oriented inclusions. The Th-U-Pb age of ekanite, determined by electron probe micro-analyser (EPMA) chemical dating, is 524.4 ± 6.4 Ma (2σ), assigning primary ekanite growth to late-stage regional metamorphism in the Highland Complex. Phase equilibria modeling of the host charnockitic gneiss gives the peak pressure-temperature conditions of ca. 850 ± 50 °C and 6 ± 1 kbar and the carbonatitic melt infiltration has occurred at or close to the peak conditions. Thorium necessary for ekanite formation is proposed to have been taken up from monazite- and thorite-bearing wall rocks by the ascending carbonatitic melt. This melt was likely derived from the anatexis of crustal carbonate rocks during regional metamorphism.

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