New constraints on the P-T path of HT/UHT metapelites from the Highland Complex of Sri Lanka

Dharmapriya, PL and Malaviarachchi, Sanjeewa PK and Kriegsman, LM and Galli, Andrea and Sajeev, K and Zhang, Chengli (2017) New constraints on the P-T path of HT/UHT metapelites from the Highland Complex of Sri Lanka. In: GEOSCIENCE FRONTIERS, 8 (6). pp. 1405-1430.

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Abstract

We report here rare evidence for the early prograde P-T evolution of garnet-sillimanite-graphite gneiss (khondalite) from the central Highland Complex, Sri Lanka. Four types of garnet porphyroblasts (Grt(1), Grt(2), Grt(3) and Grt(4)) are observed in the rock with specific types of inclusion features. Only Grt(3) shows evidence for non-coaxial strain. Combining the information shows a sequence of main inclusion phases, from old to young: oriented quartz inclusions at core, staurolite and prismatic sillimanite at mantle, kyanite and kyanite pseudomorph, and biotite at rim in Grt(1); fibrolitic sillimanite pseudomorphing kyanite +/- corundum, kyanite, and spinel + sillimanite after garnet + corundum in Grt(2); biotite, sillimanite, quartz +/- spinel in Grt3; and ilmenite, rulite, quartz and sillimanite in Grt(4). The pre-melting, original rock composition was calculated through stepwise re-integration of melt into the residual, XRF based composition, allowing the early prograde metamorphic evolution to be deduced from petrographical observations and pseudosections. The earliest recognizable stage occurred in the sillimanite field at around 575 degrees C at 4.5 kbar. Subsequent collision associated with Gondwana amalgamation led to crustal thickening along a P-T trajectory with an average dP/dT of similar to 30 bar/degrees C in the kyanite field, up to similar to 660 degrees C at 6.5 kbar, before crossing the wet-solidus at around 675 degrees C at 7.5 kbar. The highest pressure occurred at P > 10 kbar and T around 780 degrees C before prograde decompression associated with further heating. At 825 degrees C and 10.5 kbar, the rock re-entered into the sillimanite field. The temperature peaked at 900 degrees C at ca. 9-9.5 kbar. Subsequent near-isobaric cooling led to the growth of Grt(4) and rutile at T similar to 880 degrees C. Local pyrophyllite rims around sillimanite suggest a late stage of rehydration at T < 400 degrees C, which probably occurred after uplift to upper crustal levels. U-Pb dating of zircons by LA-ICPMS of the khondalite yielded two concordant Pb-206/U-238 age groups with mean values of 542 +/- 2 Ma (MSWD = 0.24, Th/U = 0.01-0.03) and 514 +/- 3 Ma ( MSWD = 0.50, Th/U = 0.01-0.05) interpreted as peak metamorphism of the khondalite and subsequent melt crystallization during cooling. (C) 2017, China University of Geosciences ( Beijing) and Peking University. Production and hosting by Elsevier B.V.

Item Type:	Journal Article
Publication:	GEOSCIENCE FRONTIERS
Additional Information:	Copy right for this article belongs to the CHINA UNIV GEOSCIENCES, BEIJING, 29 XUEYUAN RD, HAIDIAN DISTRICT, 100083, PEOPLES R CHINA
Department/Centre:	Division of Mechanical Sciences > Centre for Earth Sciences Division of Interdisciplinary Sciences > Center for Infrastructure, Sustainable Transportation and Urban Planning (CiSTUP)
Date Deposited:	11 Nov 2017 07:16
Last Modified:	05 Mar 2019 09:18
URI:	http://eprints.iisc.ac.in/id/eprint/58189

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