Metamorphism of the ancient ophiolites of the Sharyzhalgai massif

The metamorphic features of mafic-ultramafic and associated rocks in the we st of the Sharyzhalgai massif are discussed. By the association type and ge ochemical composition, metamorphosed peridotite, pyroxenite, gabbro, and vo lcanics here correspond to ophiolites. A detailed study of mineral paragene ses in metabasic rocks and metapelites revealed two stages of metamorphism and synmetamorphic deformations: early high-pressure metamorphism (P = 12-1 4 kbar, T = 520-540 degrees C) with a regressive stage (up to P = 6 kbar, T = 600 degrees C) and late low-pressure metamorphism (P from 6 to 3 kbar, T from 720 to 470 degrees C). Omphacite relies and Cpx-Pl symplectites in me tabasic rocks and Grt-Bt-Ky-St paragenesis in metapelites are indicators of high-pressure metamorphism. Parageneses of hornblende with cummingtonite i n metabasic rocks and (andalusite)-garnet-biotite-cordierite sillimanite in metapelites are the result of low-pressure metamorphism. The first stage o f metamorphism was probably related to the subduction of oceanic plate and subsequent exhumation of subductional complexes. The second stage of metamo rphism, accompanied by granite intrusion, migmatization of protolith, and i ntense deformations, was related to collisional processes. The age of these tectonic events is presumably 2000-1800 Ma.