Abstract
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 ~200km 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 99.2Ma. 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 (NbLa and TaLa≤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 ~125Ma.By contrast, geochemical and isotope data from the youngest magmatic rocks in the area — 107-99Ma 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 ~107Ma) with patchy areas thinning sufficiently to enable the generation of relatively small-scale asthenospheric-derived magmatism to predominate in the late Cretaceous.
Highlights
Mesozoic mafic-intermediate magmatic rocks crop out over an estimated 9800 km2 across southern and eastern Mongolia (Fig. 1A)
We investigate the petrogenesis of Mesozoic magmatic rocks in southern Mongolia to constrain the nature of the source of the magmatism, the timing of magmatic activity, and the implications of the timing and changes in source chemistry for understanding underlying lithospheric controls
There are a number of ways this source could be generated for the Gobi lavas: (1) interaction between mantle peridotite and a melt derived from a rutile-bearing, recycled eclogite (e.g., Liu et al, 2008); (2) the involvement of previous subducted continental crust trapped in the lithospheric mantle (e.g., Fan et al, 2004) and (3) interaction between mantle peridotite and fluids with high concentrations of light rare earth elements (LREE) but depleted in Nb and Ta
Summary
Mesozoic mafic-intermediate magmatic rocks crop out over an estimated 9800 km across southern and eastern Mongolia (Fig. 1A). The only plate boundary that was active in the vicinity of the Mongolian magmatic fields during the earlier part of their formation was the subduction zone associated with the closure of the Mongol-Okhotsk Ocean, which was active until latest Jurassic - earliest Cretaceous time (Cogné et al, 2005; Van der Voo et al, 2015) This boundary was distant from the penecontemporaneous magmatism in the North China Craton where volcanic rocks and xenolith data have led to suggestions for widespread lithosphere removal during the Mesozoic (e.g., Gao et al, 2002; Menzies et al, 1993; Windley et al, 2010). We focus on an area in the Gobi Altai, located south of the western end of the Mongol-Okhotsk Suture (Fig. 1B), where well-dated, well-described basaltic successions span the long duration of Triassic to the Late Cretaceous, straddling the latest Jurassic-earliest Cretaceous closure, ~200 km to the north of the study area, of the Mongol-Okhotsk Ocean that once extended between the Siberian and the Mongol-China terranes/crustal blocks (Cogné et al, 2005; Van der Voo et al, 2015)
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