STRUCTURE AND COMPOSITION OF THE EARLY MESOZOIC VOLCANIC SERIES OF THE TSAGAN-KHURTEI GRABEN, WESTERN TRANSBAIKALIA: GEOLOGICAL, GEOCHEMICAL, AND ISOTOPIC DATA

Abstract

This paper presents geological, isotopic-geochemical, and geochronologic data on bimodal trachybasalt-trachyte and trachybasalt-trachyrhyolite-comendite associations of the Tsagan-Khurtei graben of Western Transbaikalia. This graben extends over more than 150 km and belongs to the sublatitudinal Western Transbaikalia rift zone, which controls the distribution of the Early Mesozoic bimodal volcanic and alkali granitesyenite associations. Rb-Sr isotope dating showed that volcanic sequences of the Tsagan-Khurtei graben formed 207-212 Ma ago. Correlations of lithophile elements in compositionally different rocks of volcanic associations can be explained by crystallization differentiation of primary trachybasalt magma with isotopic and geochemical signatures of OIB-type enriched mantle. The limited isotopic data available on alkaline felsic rocks indicate both mantle and crustal contributions to their formation.