Composition and origin of peculiar pyropes from lherzolites: evidence for the evolution of the lithospheric mantle of the Siberian Platform

Abstract

More than 1000 pyropes from the Muza (J3) and Ivushka (D–C) (northeastern Siberian Platform, and Khorkich (Mz) (southwestern part of the platform) kimberlite pipes, alluvial deposits of the Muna-Markha area, and granular peridotites of the Udachnaya pipe have been analyzed for major and some minor elements. As a result, a group of pyropes was distinguished whose composition is not typical of the lherzolite paragenesis (LAC pyropes). They are predominant in the Muza pipe and are widespread over the world. This group is described as a separate paragenetic type. In all known cases, LAC pyropes belong to granular clinopyroxene-bearing harzburgites, and in situ conditions for this suite are typically below 50 kbar and 1000 °C. Our own and literature data suggest that LAC pyropes may appear when the magmas corresponding to the high-degree melting of the primary magma affect the depleted peridotites of the lithosphere mantle. The character of paleogeotherm and distribution of LAC pyropes in kimberlites and secondary collectors of varying age that occur on the Siberian Platform indicate that the lithosphere mantle was considerably thinner in the northeast and the rocks characterized by LAC pyropes played an increasingly important role in this region in the period from Paleozoic to Mesozoic and that these rocks were abundant in the lithosphere mantle of the platform’s interior. These facts as well as a considerable change in the rock composition in the lithospheric mantle and in the southwestern part of the platform in the same range of time suggest that the effect of the Permian-Triassic Siberian plume on the lithospheric mantle of the platform considerably changed its composition and structure in its separate parts.