Distinct metamorphic evolution of alternating silica-saturated and silica-deficient microdomains within garnet in ultrahigh-temperature granulites: An example from Sri Lanka |
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Authors: | P.L. Dharmapriya Sanjeewa P.K. Malaviarachchi Leo M. Kriegsman K. Sajeev Andrea Galli Y. Osanai N.D. Subasinghe C.B. Dissanayake |
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Affiliation: | 1. Postgraduate Institute of Science, University of Peradeniya, 20400, Sri Lanka;2. Department of Geology, Faculty of Science, University of Peradeniya, 20400, Sri Lanka;3. Department of Geology, Naturalis Biodiversity Center, Darwinweg 2, 2333 CR, Leiden, The Netherlands;4. Department of Earth Sciences, University of Utrecht, NL-3584 CD, Utrecht, The Netherlands;5. Center for Earth Sciences, Indian Institute of Science, Bangalore, 560012, India;6. Department of Earth Sciences, ETH Zurich, Sonnegstrasse 5, CH-8092, Zurich, Switzerland;7. Division of Earth Sciences, Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, 744 Motooka, Fukuoka, 819-0395, Japan;8. National Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka |
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Abstract: | Here we report the occurrence of garnet porphyroblasts that have overgrown alternating silica-saturated and silica deficient microdomains via different mineral reactions. The samples were collected from ultrahigh-temperature (UHT) metapelites in the Highland Complex, Sri Lanka. In some of the metapelites, garnet crystals have cores formed via a dehydration reaction, which had taken place at silica-saturated microdomains and mantle to rim areas formed via a dehydration reaction at silica-deficient microdomains. In contrast, some other garnets in the same rock cores had formed via a dehydration reaction which occurred at silica-deficient microdomains while mantle to rim areas formed via a dehydration reaction at silica-saturated microdomains. Based on the textural observations, we conclude that the studied garnets have grown across different effective bulk compositional microdomains during the prograde evolution. These microdomains could represent heterogeneous compositional layers (paleobedding/laminations) in the precursor sediments or differentiated crenulation cleavages that existed during prograde metamorphism. UHT metamorphism associated with strong ductile deformation, metamorphic differentiation and crystallization of locally produced melt may have obliterated the evidence for such microdomains in the matrix. The lack of significant compositional zoning in garnet probably due to self-diffusion during UHT metamorphism had left mineral inclusions as the sole evidence for earlier microdomains with contrasting chemistry. |
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Keywords: | UHT Metapelites Garnet Microdomains Highland Complex Sri Lanka |
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