P-T HISTORY OF GARNET-WEBSTERITES IN THE SHARYZHALGAI COMPLEX, SOUTHWESTERN MARGIN OF SIBERIAN CRATON: EVIDENCE FOR PALEOPROTEROZOIC HIGH-PRESSURE METAMORPHISM

Abstract

The Saramta massif in the Paleoproterozoic Sharyzhalgai complex, the southwestern margin of the Siberian craton, is mainly composed of spinel-peridotites with garnet-websterites; it is enclosed within granitic gneisses and migmatites with mafic intercalations of granulite-facies grade. The garnet-websterites occur as lenses or layers intercalated within spinel-harzburgite and spinel-lherzolite. They consist mainly of clinopyroxene (Cpx), garnet (Grt), and orthopyroxene (Opx): Grt often includes Cpx, Opx, and pargasite (Prg). Opx also occurs as kelyphite with plagioclase (Pl), spinel, olivine, Prg, and biotite. Relationships between textures and chemical compositions of these minerals suggest the following P–T stages: stage 1 (pre-peak), 0.9–1.5 GPa at 640–780 °C; stage 2 (peak), 2.3–3.0 GPa at 920–1030 °C as the minimum estimate; and stage 3 (post-peak), 750–830 °C at 0.5–0.9 GPa. Finally, the garnet-websterites are veined with lower amphibolite- to greenschist-facies minerals (stage 4).These results suggests that the Saramta massif was carried to depths of c. 100 km by subduction, and metamorphosed under eclogite-facies conditions in the Paleoproterozoic, despite the commonly held view that high geothermal gradients in those times would have prevented such deep subduction. Paleoproterozoic plate subduction at the southwestern margin of the Siberian craton might have caused subduction-zone magmatism and mantle metasomatism similar to those in the Phanerozoic.