Metamorphic and chronological constraints on the early Paleozoic tectono-thermal evolution of the Olkhon Terrane, southern Siberia

Abstract

Terranes accreted to the southeastern margin of the Siberian Craton record an important early Paleozoic tectono-thermal event (known as the Baikal orogenic cycle) in the evolution of the Central Asian Orogenic Belt (CAOB). However, the precise metamorphic conditions and relative timing of this event and its linkage to the wider CAOB remain far poorly constrained. The best exposed of these terranes is the Olkhon Terrane on the western bank of Lake Baikal. Here, late Neoproterozoic through early Paleozoic island arc and back-arc assemblages were metamorphosed to form a thin granulite facies belt cropping out adjacent to the Siberian Craton and lower temperature/pressure paragneiss and migmatite towards the southeast. Phase equilibria modelling suggests that the granulite facies belt preserved moderate pressure (c. 0.80 GPa) and high temperature (up to 900°C) conditions while the paragneiss and migmatites in the southeast have peak metamorphic conditions around 700–770°C at 0.60–0.80 GPa. New geochronological data (zircon U–Pb in granulite and monazite U–Pb in paragneiss/migmatite) in combination with phase equilibria modelling and petro-structural analysis suggest that the tectono-metamorphic evolution of the Olkhon Terrane was controlled by a long-lasting (535–450 Ma) and pervasive thermal anomaly. Discrete maxima in the zircon and monazite U–Pb ages at c. 535, 500, and 450 Ma are linked to different stages of a semi-continuous high-temperature metamorphic evolution. Based on existing geological data of the region, a generalized geodynamic model for the Baikal orogenic cycle involving switching between compressional and extensional regimes during the early Paleozoic accretion of ‘exotic’ CAOB-derived material to the southern margin of Siberia is proposed. The tectono-metamorphic evolution of the Olkhon Terrane may represent a world-class example of polyphase shortening of a long-lived hot intra-continental arc–back-arc system during its collision with cratonic blocks.