Distinct metamorphic evolution of alternating silica-saturated and silica-deficient microdomains within garnet in ultrahigh-temperature granulites: An example from Sri Lanka

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 metape...

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Bibliographic Details
Published in:Di xue qian yuan. 2017-09, Vol.8 (5), p.1115-1133
Main Authors: Dharmapriya, P.L., Malaviarachchi, Sanjeewa P.K., Kriegsman, Leo M., Sajeev, K., Galli, Andrea, Osanai, Y., Subasinghe, N.D., Dissanayake, C.B.
Format: Article
Language:English
Subjects:
Complex
Sri
Crystallization
Deformation
Dehydration
Evolution
Garnet
Garnets
High temperature
Highland Complex
Laminates
Lanka
Mantle
Metamorphism (geology)
Metapelites
Microdomains
Mineral inclusions
Minerals
Sediments
Self diffusion
Silica
Silicon dioxide
Sri Lanka
UHT
UHT
Metapelites
Garnet
Microdomains
Highland
Ultrahigh temperature
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Summary: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 silicasaturated 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.
ISSN:1674-9871
2588-9192
DOI:10.1016/j.gsf.2016.11.008