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Thermal and baric evolution of garnet granulites from the Kharta region of S Tibet, E Himalaya

¹ Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso 35, I-10125, Torino, Italy
² C.N.R. - Istituto di Geoscienze e Georisorse, Sez. di Torino, c/o DSMP, Via Valperga Caluso 35,I-10125, Torino, Italy
³ Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, I-35137 Padova, Italy
⁴ C.N.R. - Istituto di Geoscienze e Georisorse, Sez. di Padova, Corso Garibaldi 37, I-35137 Padova, Italy

^* E-mail: bruno.lombardo{at}unito.it

Granulite-facies garnet-bearing metapelites, metabasics and calc-silicate rocks from the lower metamorphic complex (Kharta Gneiss) of the Greater Himalayan Crystallines in the Kharta region of S Tibet, E Himalaya, preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–720°C and pressures around 8 kbar during the main event of the Himalayan metamorphism. Post-deformational reaction textures include clinopyroxene (± orthopyroxene) - plagioclase symplectites after garnet in calc-silicate rocks, and cordierite ± spinel coronas on sillimanite and garnet in metapelite granulites. These assemblages indicate a decompressional pressure-temperature path that is confirmed by the geothermobarometry of zoned and symplectite minerals as well as by calculated phase equilibria. Isothermal decompression through ca. 3 kbar occurred at temperatures of about 700°C, and was followed by further decompression to P 3 kbar, and T 710°C. At this point, decompression was replaced by quasi-isobaric cooling ending in the andalusite stability field at P ca. 2.5 kbar.

The P-T path of the Kharta Gneiss appears to be similar to those inferred for the lower Greater Himalayan Crystallines exposed in the nearby Dudh Kosi and middle Arun valleys of eastern Nepal. This type of clockwise P-T path, with most of the exhumation occurring at relatively constant metamorphic temperatures, requires a high exhumation rate and suggests that extrusion tectonics of crustal-scale wedges may have been operative during post-collisional exhumation of the Greater Himalayan Crystallines.

Key-words: granulite, geothermobarometry, petrogenetic grid, P-T path, Himalayas..

This article has been cited by other articles:

				M. J. Jessup, D. L. Newell, J. M. Cottle, A. L. Berger, and J. A. Spotila Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal Geology, July 1, 2008; 36(7): 587 - 590. [Abstract] [Full Text] [PDF]

				S. Ferrando, F. Rolfo, and B. Lombardo Fluid evolution from metamorphic peak to exhumation in Himalayan granulitised eclogites, Ama Drime range, southern Tibet European Journal of Mineralogy, July 1, 2007; 19(4): 439 - 461. [Abstract] [Full Text] [PDF]