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The late Mesozoic Era was a time of widespread crustal extension in eastern Asia resulting in both rift basin and metamorphic core complex formation. Two of the more recently documented examples of this extensional phase are the Ereendavaa and Buteel metamorphic core complexes (EMCC, BMCC). Both are located in northern Mongolia proximal to the Mongol Okhotsk Suture Zone (MOSZ). The MOSZ is a profound, yet enigmatic structure that formed due to closure of the Mongol-Okhotsk Ocean, a basin that separated the Siberian and North China cratons and intervening terranes of the Central Asian Orogenic Belt. Based on published work by others, the core complexes record NW-SE extension, cooling and deformation from c. 135 to 120 Ma. We present new data as part of a collaborative research project that aims to constrain the evolution of the MOSZ more broadly and its relationship to intracontinental deformation after suturing. Our methods include analysis of satellite imagery and digital elevation models with synthesis of field, (micro)structural, and geochronologic data with published maps and studies. Based on our findings, the EMCC likely extends several 10's of km to the NE. Satellite imagery and DEMs suggest large-scale corrugations along the N-flank consistent with NW-SE extension. To the SW of the EMCC, Early Cretaceous rift basins are associated with strong NE-SW oriented lineaments. We examined the BMCC along its SW mapped extent, an area for which no data were presented in prior publications; we confirmed the presence of a top-to-the-SE detachment fault. The EMCC and BMCC, like the Yagan-Onch Hayrhan MCC in southern Mongolia, have footwall rocks previously mapped as Precambrian that are, in large part, metamorphosed Paleozoic and Mesozoic igneous and sedimentary rocks. All three MCCs exhibit evidence for structural complexity, such as NE-SW trending lineations orthogonal to the NW-SE extension direction. As in S Mongolia, we hypothesize that the NE-SW lineations in the EMCC and BMCC formed during an earlier phase of shortening. The expression of the Early Cretaceous extension (rift basin vs. MCC) appears to be controlled by the inherited structure. 

The Mongol-Okhotsk suture in NE Mongolia and SE Russia is the last vestige of the Mongol-Okhotsk Ocean, a basin that separated the Siberian from the North China Craton. Although the exact location of the suture is cryptic and the timing and kinematics of closure remains debated, magmatism north and south of the suture zone provide insight into the characteristics of the subduction system during the closure of this ocean. Magmatic provinces recording pre-, syn- and post-collisional processes (and potentially deeper mantle sources) along the Mongol Okhotsk Belt were emplaced from the Permian to Jurassic. Our compilation of geochemical, isotopic, and geochronologic data from 51 published studies shows that the age of peak magmatism and subsequent magmatic lull decreases eastward. We interpret this to record a shift in the locus of collision as the ocean zipped close. Most mafic rocks within the suture zone show the influence of lower crustal components and both spinel and garnet peridotites in the source. However, with one exception, their compositions are generally not consistent with a depleted mantle source (usually associated with MgO-TiO2-rich, isotopically fertile, and OIB components). Additionally, although significant geochemical variability due to crustal petrogenetic processes is observed, the data show a ubiquitous subduction signature characterized by enrichment in large ion lithophile elements, light rare earth elements, and La/Nb ratios, as well as by depletion in high field strength elements and heavy rare earth elements. We interpret the consistent peraluminous nature of the magmatic rocks before, during, and after collision, together with their crust-like Nb/U content, high (Th/Yb)/(Ba/La) and (Th/Nb)/(Ba/Nb) ratios, to reflect the strong influence of sediment melts throughout the collision process. This requires not only unusually high geothermal gradients (ca. 200 °C higher than a normal mantle wedge) that can partially melt sedimentary rocks along the whole subduction system, but also persistence of underplated, assimilated, and/or metasomatically incorporated sediments in the magma source during the continental closure. Together, the compiled data paint a picture of a sediment-rich magmatic engine above a hot, dynamic, subduction system with largely intact slabs. 

The Mongol-Okhotsk belt extends for more than 3000 km, from Mongolia to the Sea of Okhotsk in the northwest Pacific Ocean. It is the relict of the Mongol-Okhotsk ocean, a cryptic basin for which the timing, location and modes of opening and closure are still debated. One of the key components associated with the progressive closure and final collision and suturing of this ocean is the vast magmatic and volcanic record north and south of the presumed suture. Permian to Triassic magmatic and volcanic rocks are particularly abundant in the belt. They are characterized by bimodal volcanism, abundant calc-alkaline activity, secondary alkaline activity, and geochemical signatures that point to both volcanic-arc and within-plate origin as well as crustal and mantle sources. This has led to two main tectono-magmatic interpretations: (1) an Andean-style active margin modified by a mantle plume and/or affected by lithospheric delamination that evolved to a syn-and post-collisional setting, and (2) a mantle plume that recycled geochemical signatures from a long-lived active margin after the ocean was closed either by anatexis or by melting a remnant slab. These two models have first-order implications for the tectonic setting as well as for the location of the proposed mantle plume through time. We reconstruct the distribution of magmatic and volcanic provinces surrounding the Mongol-Okhotsk Suture Zone using G-Plates and the most recent paleomagnetic and mantle tomographic reconstructions to restore the position of the proposed mantle plume and the location of the slab between the Late Permian and Early Jurassic. Our reconstructions enable us to test the kinematic restoration and the viability of both the active subduction and the mantle plume models during the closure of the Mongol-Okhotsk Ocean.