Constraining lithospheric removal and asthenospheric input to melts in Central Asia: A geochemical study of Triassic to Cretaceous magmatic rocks in the Gobi Altai (Mongolia)
journal contributionposted on 06.04.2018, 13:29 by Thomas. C Sheldrick, Tiffany. L Barry, D. J. J Van Hinsbergen, P. D Kempton
Throughout northeast China, eastern and southern Mongolia, and eastern Russia there is widespread Mesozoic intracontinental magmatism. Extensive studies on the Chinese magmatic rocks have suggested lithospheric mantle removal was a driver of the magmatism. The timing, distribution and potential diachroneity of such lithospheric mantle removal remains poorly constrained. Here, we examine successions of Mesozoic lavas and shallow intrusive volcanic plugs from the Gobi Altai in southern Mongolia that appear to be unrelated to regional, relatively small-scale deformation; at the time of magmatism, the area was ~ 200 km from any active margin, or, after its Late Jurassic-Early Cretaceous closure, from the suture of the Mongol-Okhotsk Ocean. 40Ar/39Ar radiometric age data place magmatic events in the Gobi Altai between ~ 220 to 99.2 Ma. This succession overlaps Chinese successions and therefore provides an opportunity to constrain whether Mesozoic lithosphere removal may provide an explanation for the magmatism here too, and if so, when. We show that Triassic to Lower Cretaceous lavas in the Gobi Altai (from Dulaan Bogd, Noyon Uul, Bulgantiin Uul, Jaran Bogd and Tsagaan Tsav) are all light rare-earth element (LREE) and large-ion lithophile element (LILE)-enriched, with negative Nb and Ta anomalies ( and ≤ 1). Geochemical data suggest that these lavas formed by low degrees of partial melting of a metasomatised lithospheric mantle that may have been modified by melts derived from recycled rutile-bearing eclogite. A gradual reduction in the involvement of garnet in the source of these lavas points towards a shallowing of the depth of melting after ~ 125 Ma. By contrast, geochemical and isotope data from the youngest magmatic rocks in the area — 107-99 Ma old volcanic plugs from Tsost Magmatic Field — have OIB-like trace element patterns and are interpreted to have formed by low degrees of partial melting of a garnet-bearing lherzolite mantle source. These rocks did not undergo significant crustal contamination, and were derived from asthenospheric mantle. The evidence of a gradual shallowing of melting in the Gobi lava provinces, culminating in an asthenospheric source signature in the youngest magmatic rocks is similar to examples from neighboring China, emphasising the wide-scale effect of a regional Mesozoic magmatic event during similar time periods. We suggest that Mongolia underwent lithospheric thinning/delamination during the Mesozoic (between ~ 125 and ~ 107 Ma) with patchy areas thinning sufficiently to enable the generation of relatively small-scale asthenospheric-derived magmatism to predominate in the late Cretaceous.