Thermal state and nature of the lower crust in the Baikal Rift Zone: Insight from xenoliths of Cenozoic and Paleozoic magmatic rocks

Abstract

Preliminary studied lower crust and Moho of Baikal Rift (BR) was investigated using pyroxenite xenoliths and xenocrysts from Cenozoic volcanoes to determine the structure, thermal conditions and composition of the transitional zone from the crust to mantle and influence of plumes on it. Samples from Vitim Plateau (Miocene picrite basalts) SW part of BR, Dzhida River Basin (Bartoy volcanoes) located at SE of BR and Tunka axial Valley (Karierny volcanoes) to the West from Baikal were studied for major components using electron microprobe and electron microscope and for trace elements with the inductively connected mass-spectrometry with laser ablation. For the comparison, the lower crust xenocrysts from the Angara Vitim batholite were also included in this study. The PT estimates for minerals are mostly refer to the Moho boundary or are locating just beneath it giving the vast range of temperatures. Beneath the Moho, they trace 90 mw/m² geotherm. Within the crust, temperature regime varies from the conductive to advective. Pyroxene xenocrysts and pyroxenite xenoliths mainly trace 90 mw/m² SEA plume geotherm. The levels of the melt intrusions are overheated to 1350°C. The granulites are typically represent the colder conditions than SEA geotherm. Xenocrysts from Angara Vitim batholith magmas reveal more depleted material of lower crust than those found in Cenozoic lavas and possibly are skialites. The xenocrysts and granulite xenoliths in Cenozoic lavas are mainly basic cumulates. The rocks of the lower crust became more acid to the upper part.

The lateral variations in the lower crust sampled material show enrichment in K₂O at the boundary with the Siberian Craton in Tunka, more metasomatic and hydrous nature in Dzhida zone and more basic and CaO rich characteristic in Vitim area. These data give the evidence for the conditions of origin of the magmas of Angara-Vitim Batholith (AVP) (275–310 Ma), which was created due to interaction of hot spot with the crust in Baikal and Transbaikalia. Initially this hot generated kimberlites and basalts in northern (420–390 Ma) and central part of Yakutia (370–440 Ma) in time, migrated to – and eeastern Sayan Mountain formed Ingashi lamproites -kimberlites (310 Ma), than plume was spreading to south beneath the crust in Transbaikalia and created AVP. After it returned to central and northern Siberia, it generated Permo-Triassic Siberian trap province.