2381/45953 Thomas .C Sheldrick Thomas .C Sheldrick Tiffany L. Barry Tiffany L. Barry Ian Millar Ian Millar Dan N. Barfod Dan N. Barfod Alison M. Halton Alison M. Halton Dan J. Smith Dan J. Smith Evidence for southward subduction of the Mongol-Okhotsk oceanic plate: Implications from Mesozoic adakitic lavas from Mongolia University of Leicester 2019 Adaatsag/Ereendavaa terrane Middle Gobi volcanic belt Central Asian Orogenic Belt Double-sided subduction Sr-Nd-Pb-Hf isotopes 2019-12-02 11:37:38 Journal contribution https://figshare.le.ac.uk/articles/journal_contribution/Evidence_for_southward_subduction_of_the_Mongol-Okhotsk_oceanic_plate_implications_from_Mesozoic_adakitic_lavas_from_Mongolia/10204733 A combination of new 40Ar/39Ar dating results, major- and trace-element data, plus Sr-Nd-Pb-Hf isotope data, are used to investigate the petrogenesis of Triassic high-Si adakite (HSA), Cretaceous low-Si adakite-like (LSA) lavas, and Cretaceous high-K and shoshonitic trachyandesite lavas, from eastern and south-central Mongolia. All samples are light rare-earth element and large-ion lithophile element enriched but depleted in some high-field strength elements (notably Nb, Ta and Ti). Two alternative models are proposed to explain the petrogenesis of the HSA samples. (1) A southward-subducting Mongol-Okhotsk slab underwent partial melting in the Triassic during the closure of the Mongol-Okhotsk Ocean, with the resultant melts assimilating mantle and crustal material. Alternatively (2), a basaltic underplate of thickened (>50 km; >1.5 GPa), eclogitic lower crust foundered into the underlying mantle, and underwent partial melting with minor contamination from mantle material and some shallow-level crustal contamination. The LSA samples are interpreted as melts derived from a lithospheric mantle wedge that was previously metasomatised by slab melts. Similarly, the trachyandesite lavas are interpreted as melts deriving from a subduction-enriched subcontinental lithospheric mantle. The spatial distribution of these samples implies that metasomatism likely occurred due to a southward-subducting Mongol-Okhotsk slab associated with the closure of the Mongol-Okhotsk Ocean. When this interpretation is combined with previous evidence for a northward-subducting Mongol-Okhotsk slab it advocates that the Mongol-Okhotsk Ocean closed with double-sided subduction.