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 worldclass example of polyphase shortening of a long-lived hot intra-continental arc-back-arc system during its collision with cratonic blocks.

Peri‐Siberian Ordovician to Devonian Tectonic Switching in the Olkhon Terrane (Southern Siberia): Structural and Geochronological Constraints

“…However, in some places the hinges of Fu 3 folds show steep plunges to the NE or SW which may imply moderate to steep dip of Sh 1 MIG prior to Fu 3 folding (Figure 5a). Our previous study (Z. Y. Li et al., 2023) revealed that the metamorphic cycle associated with the development of the Sh 1 MIG reached the P max at 8 kbar at 770°C and was followed by a second phase of HT ‐ low pressure (LP) metamorphism marked by M2 re‐equilibration at around 4 kbar and 700°C. The structures related to this event are widely discussed and will be assigned as D 2 MIG .…”

“…The Chernorud Zone is characterized by mafic garnet‐ clinopyroxene‐amphibole bearing granulite and intermediate to felsic granulite mainly consisting of garnet, clinopyroxene, orthopyroxene, plagioclase, K‐feldspar, and quartz (Z. Y. Li et al., 2023). Apart from the mafic and felsic granulite, the Chernorud Zone contains numerous highly elongated layers of marble and quartzite that are together with felsic granulite wrapped around mafic and ultra‐mafic lenses and boudins (Fedorovsky & Sklyarov, 2015).…”

“…In a recent zircon U‐Pb geochronological study, Z. Y. Li et al. (2023) inferred two discrete age groups from the granulite at c. 530–500 and c. 450 Ma to represent the timing of granulite and amphioblite facies metamorphism, respectively.…”

“…(2020a, 2020b) reported significantly lower metamorphic peak P–T conditions of approximately 4.4–4.8 kbar at 620°–650°C for similar rocks of the region. The timing of this metamorphic evolution was deduced from monazite U‐Pb ages, which yielded c. 500 Ma for the P max conditions and c. 450 Ma for the later re‐equilibration phase (Z. Y. Li et al., 2023).…”

“…The fine‐grained plagioclase and K‐feldspar indicate HT deformation (Figure 4b). The P–T conditions of relict granulite facies fabric were estimated to c. 8 kbar and 850°C and retrogression to ∼6 kbar and 700°C in association with M 2 amp overprint (Z. Y. Li et al., 2023). The northern part of the Chernorud Zone is formed by an up to kilometer wide zone of amphibolite facies deformation reworking the granulite facies rocks, which is called the “collisional suture zone” in the Russian literature (Figure 2a) (Fedorovsky et al., 2020).…”