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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.
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Magmatism during the closure of the Mongol-Okhotsk Ocean: a tale of a sediment-rich collision
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.
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- Award ID(s):
- 1917640
- PAR ID:
- 10345287
- Date Published:
- Journal Name:
- AGU Fall Meeting Abstracts
- Page Range / eLocation ID:
- T42C-08
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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