Late Permian -- early Cretaceous igneous events in the Erguna Massif, NE China : constraints on the timing of closure of the Mongol-Okhotsk Ocean

Abstract

The Mongol-Okhotsk suture zone, a 3000-km-long suture extending from central Mongolia to the Sea of Okhotsk, resulted from the closure of the MongolOkhotsk Ocean, which had separated the Mongolian terranes from the Siberian Craton in the Late Paleozoic and most of the Mesozoic. Controversy has long surrounded this ancient ocean over (1) whether it was consumed by northward single-sided or divergent double-sided subduction, and (2) its timing of closure, especially in the central part of the suture zone where considerable discrepancies exist between paleomagnetic reconstructions and other regional geological records, with the former favoring a wide ocean in the Middle Jurassic and the latter advocating a post-collisional setting during that period. The Erguna Massif in northeastern (NE) China forms part of the southern boundary of the Mongol-Okhotsk suture zone, making it an ideal area to study and reconstruct the tectonic history of the southern margin of the ocean. This MPhil project aims to provide constraints on the ocean’s pre-collision subduction polarity and its timing of closure through an integrated field-based petrological, geochronological and geochemical study on less-studied Late Permian – Early Cretaceous granitoids and volcanic rocks in the Erguna Massif. In this study, new zircon U-Pb ages, zircon Hf isotopic data, and whole-rock major and trace element data are reported. Key findings of this study are summarized as follows: (1) a suite of high-K calc-alkaline, metaluminous to weakly peraluminous monzogranite-granodioritequartz diorite assemblage were emplaced in the Erguna area during 263 – 243 Ma. They resemble I-type granitoids formed in active continental margins; (2) a silicarich, peraluminous S-type monzogranite intrusion in the Manzhouli area was emplaced during 212 – 203 Ma, likely in a continental arc setting; (3) rhyolites and trachytes erupted in the Manzhouli-Erguna area at 158 – 124 Ma are alkalic, metaluminous to weakly peraluminous, predominantly ferroan and possess high REE content, akin to post-collisional A-type rocks; and (4) volcanic rocks from the Guanghua and Tamulangou Formations in the Mohe area were erupted at 125 – 124 Ma. Weakly negative to moderately positive εHf (t) values in magmatic rock samples in this study suggest that the Late Permian – Early Cretaceous rocks were mostly derived from reworking of ancient crust, or materials extracted from the lithospheric mantle. New data, alongside other data from literature, suggest that intermittent southward subduction of the Mongol-Okhotsk plate triggered two stages of calcalkaline, dominantly I-type magmatism in the Erguna Massif in Late Permian – Early Jurassic. Collison between the Erguna Massif and the Siberian Craton led to a magmatic gap in the massif at ca. 175 – 165 Ma, followed by a prolonged stage of A-type-dominated, subalkaline-to-alkaline transitional volcanism in the Late Jurassic – Early Cretaceous, in association with post-collision extension. This study concludes that the Mongol-Okhotsk Ocean was consumed by divergent double-sided subduction, at least from the Late Permian to the Early Jurassic, and that the central part of the ocean was closed in the Middle Jurassic, which are both supported by other lines of geological evidence.