What Drove Mongolian Mesozoic Volcanism: Was It Crustal Or Mantle Processes?
thesisposted on 2019-07-16, 15:00 authored by Thomas C. Sheldrick
Mesozoic intraplate volcanism occurred over a wide region of East Asia. Although there are a variety of hypotheses to explain the volcanism, the lack of studies on the Mesozoic volcanism from Mongolia has hindered the ability to test these hypotheses. Furthermore, the existence of the Palaeozoic and Mesozoic Mongol-Okhotsk Ocean is evident from the Mongol-Okhotsk suture, which stretches from central Mongolia to the Sea of Okhotsk. Insufficient palaeomagnetic data has led to difficulties in reconstructing the history, geometry and closure of this ocean. However, this ocean closure may have implications for Mesozoic magmatic/metasomatism processes. Samples were collected from traverses parallel and perpendicular to, the Mongol-Okhotsk suture. Newly discovered low-silica adakite-like (LSA) and high-silica adakite (HSA) samples from eastern and south-central Mongolia, on the southern side of the Mongol-Okhotsk suture, are used to investigate the possibility of a southward-subducting Mongol-Okhotsk oceanic slab. The LSA are interpreted as being derived from a lithospheric mantle wedge that has undergone metasomatism by slab melts. This mantle was likely metasomatised by a southward-subducting Mongol-Okhotsk slab associated with the closure of the Mongol-Okhotsk Ocean. Two alternative models are proposed to explain the petrogenesis of the HSA samples: (1) a southward-subducting Mongol-Okhotsk slab underwent partial melting during the closure of the Mongol-Okhotsk Ocean and assimilated minor mantle and crustal material; or (2) basaltic underplating of a thickened (>50 km; >1.5 GPa), eclogite lower crust, foundered into the underlying mantle, assimilated minor mantle material and finally underwent crustal contamination. Mongolian basaltic samples >107 Ma have geochemical signatures consistent with a metasomatised subcontinental lithospheric mantle source. However, the geochemistry also indicates increasing asthenospheric input throughout the Mesozoic (~220-107 Ma), with enough lithosphere being stripped away by 107 Ma for asthenospheric mantle derived magmatism to predominate. An extensive comparative study, using geochemical data from the literature, on volcanic samples from Russia, Mongolia, the North China Craton and Korea is combined with this new data. The North China Craton magmatism also has a geochemical signature consistent with increasing asthenospheric input throughout the Mesozoic. Both Mongolia and the North China Craton switch to asthenospheric magmatism at ~107 Ma; this might reflect a largescale process for Late Mesozoic magmatism in East Asia.