Fragment of the Early Paleozoic (∼500 Ma) island arc in the structure of the Olkhon Terrane, Central Asian fold belt

Abstract

The volcanic (basaltic, basalt andesitic, andesitic, and rhyolitic) porphyric rocks of the Tsagan-Zaba complex are studied in the Olkhon composite terrane of the Central Asian foldbelt. The concordant U-Pb (SHRIMP-II) age of single zircon grains from rhyolites (492 ± 5 Ma) may be interpreted as the period of formation of the Tsagan-Zaba complex. The volcanic rocks of this complex are characterized by clear suprasubduction geochemical features and positive ɛNd(t) values. The similar ages, compositions, and ɛNd(t) values of the studied volcanic rocks and gabbroic rocks of the Birkhin pluton allow us to combine them into a common Birkhin volcano-plutonic association, which may be considered as a fragment of a section of the mature island arc of ∼500 Ma in age. The gabbroic rocks may be interpreted as the middle part of this section, whereas the volcanic and volcanosedimentary rocks belong to its upper part. The section was disintegrated 470–460 Ma ago, when the Early Paleozoic island arc was accreted to the southern flank of the Siberian craton in the course of the oblique collision and became a part of the Olkhon composite terrane.