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Tso Morari eclogitic rocks
Photomicrographs from thin sections of EC series and TS samples (HK Sachan)

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Metamorphic history of Tso Morari eclogites
17 August 2007

The metabasic eclogites have complex microstructures that record a good deal of information about their prograde and retrograde evolution. They show some similarities to the Tauern eclogites of the Eastern Alps, and to certain lower-grade eclogites in the Western Gneiss complex of Norway. However, I have had trouble reconciling my own observations with those in published papers on Tso Morari, particularly in that (i) glaucophane is rare in these samples, the optics of the coarse bluish amphibole indicating that it is a calcic clinoamphibole, and (ii) the published papers do not distinguish an amphibole-phengite overprint and make no mention of the distinctive atoll garnets.

I recognise the following stages of metamorphic evolution: (1) Pre-eclogite stage with blue-green amphibole (barroisite?), clinozoisite, etc., now enclosed in garnet core zones. (2) Eclogite stage, represented mainly by rim zones of garnet, omphacite, and more rarely prismatic amphibole. An important deformation occurred at eclogite facies conditions in which omphacite was dynamically recrystallised. (3) Hydrous high-P overprint, prominent in most samples, with coarse, largely static growth of pale greenish-blue Ca-amphibole and phengite. Cores of garnet were preferentially replaced leading to striking atoll textures. (4) Narrow symplectite rims formed around omphacite and locally also phengite. (5) Low-pressure overprint, in some samples obliterating all high-P minerals, at lower amphibolite facies, with Hbl/Act, sodic Pl, Chl, Bt, Czo, etc..