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Formation of ekanite in “antiskarn” by carbonatitic melt infiltration into charnockitic gneiss, Ampegama, southwestern Highland Complex, Sri Lanka
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DOI：10.1007/s00410-026-02330-z.png)
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Abstract
En 中文
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 (Ca2Th0.9U0.1Si8O20) – 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 CO2 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.
Keywords:
Ekanite
Carbonatitic melt
Calc-silicate rock
Antiskarn
Sri Lanka
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