Dismembered Ophiolite of the Olkhon Composite Terrane (Baikal, Russia): Petrology and Emplacement

Abstract

Dismembered ophiolites in the Early Paleozoic Olkhon terrane, a part of the Baikal collisional belt in the southern periphery of the Siberian craton, occur as fault-bounded blocks of ultramafic and mafic rocks from a few meters to hundreds of meters in size. The ultramafic rocks are mainly dunite–harzburgite peridotites with gradual transitions between the lithologies, as well as moderate amounts of enstatitite, wehrlite, and clinopyroxenite, but no lherzolite. Most peridotites have strongly depleted chemistry and a mineralogy corresponding to the harzburgite type usual for ophiolites of suprasubduction zones (SSZ). The mafic rocks are leuco- to melanocratic gabbros with different relative percentages of clinopyroxene, olivine, and plagioclase, which enclose thin layers and lenses of clinopyroxenite and anorthosite. They bear back-arc basin geochemical signatures, a setting inferred for the Neoproterozoic southern Siberian craton. The gabbroic rocks are of two geochemical groups; most of their trace-element patterns show Ta-Nb minimums and Sr maximums common to suprasubduction zone ophiolites. Judging by the Ol + Opx + Chl + Chr mineral assemblages, the Olkhon peridotites underwent low amphibolite and amphibolite regional metamorphism at 500–650 °C. The occurrence of the ultramafic and mafic bodies is consistent with formation in an accretionary wedge metamorphosed during a collisional orogeny. The mantle and crustal parts of the Olkhon ophiolite suite apparently were incorporated into the terrane during the frontal collision of perio-oceanic structures with the Siberian craton. Then, in a later oblique collision event, they became dismembered by strike-slip faulting into relatively small bodies and fault blocks exposed in the present erosional surface.

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

“…Kelly F I G U R E 1 5 Relative probability plot of new and published geochronological data from the Olkhon Terrane. Published geochronological data are from: (Donskaya et al, 2017;Donskaya, Gladkochub, Fedorovsky, et al, 2013;Li et al, 2009;Makrygina et al, 2014;Sklyarov et al, 2009;Sklyarov, Lavrenchuk, Fedorovsky, Pushkarev, et al, 2020;Volkova et al, 2010Volkova et al, ) et al, 2012 or mixing of different age domains rather than a true spread of ages. Alternatively, the spread of ages is not an artefact but may reflect the semicontinuous crystallization of monazite over a prolonged (c. 50 myr) time period associated with a persistent and long-term thermal anomaly.…”

“…The thrusting of Neoproterozoic ophiolites and deposition of arc-related sediments have been observed in many places along the southern margin of the Siberian Craton (Gladkochub et al, 2022(Gladkochub et al, , 2013Glorie et al, 2014;Khain et al, 2002;Powerman et al, 2015;Sklyarov, Lavrenchuk, Fedorovsky, Pushkarev, et al, 2020); these geological records were traditionally interpreted to reflect an early Paleozoic collision between the Siberian Craton and its peripheral continental fragments to the south (Zorin et al, 2009). Such a collision has been taken as a hallmark of an early event (namely the Baikal orogenic event), which shaped the initial evolution of the CAOB (Khain et al, 2003).…”

“…Available U–Pb zircon ages for different rocks in the region are also indicated. The data sources are 1‐Donskaya et al (2017), 2‐Mikheev et al (2017), 3‐Volkova et al (2010), 4‐Li et al (2009), 5‐Mekhonoshin et al (2013), 6‐Vladimirov et al (2011), 7‐Gladkochub, Donskaya, Wingate, et al (2008), 8‐Makrygina et al (2014), 9‐Sklyarov, Lavrenchuk, Fedorovsky, Pushkarev, et al (2020), and 10‐Sklyarov et al (2009).…”

Marble Dikes in the Olkhon Composite Terrane (West Baikal Area)

“…In another scenario, amalgamation of the Olkhon collisional terrane and its accretion to the Siberian craton involved oceanic crust of a back-arc basin composed of ophiolites, sediments [Donskaya et al, 2017;Sklyarov et al, 2020], island arc complexes, and a cratonic block with an Early Precambrian basement and Neoproterozoic passive margin sediments, mostly of carbonate lithologies. The Precambrian cratonic block may be a fragment detached from Siberia (judging by the presence of Neoproterozoic carbonates in its neighbor parts) or from some other craton.…”

METATERRIGENIOUS ROCKS OF THE OLKHON TERRANE OF THE CENTRAL ASIAN OROGENIC BELT: U-Pb ZIRCON AGE, GEOCHEMICAL CHARACTERISTICS, AND FORMATION MODELS OF SEDIMENTARY PROTOLITHS

“…Комплекс Ольхон включает в себя мраморы и мраморные меланжи, кварциты, ортосланцы, амфиболиты. С породами этого комплекса пространственно ассоциируют метагабброиды и метагипербазиты, которые интерпретируются как фрагменты офиолитов, формирующихся в надсубдукционной обстановке [Sklyarov et al, 2020b].…”