The origin of Cenozoic basalts from central Inner Mongolia, East China: The consequence of recent mantle metasomatism genetically associated with seismically observed paleo-Pacific slab in the mantle transition zone

Abstract

We present new major element, trace element and Sr–Nd–Hf isotope data on Cenozoic basalts from central Inner Mongolia (CIM) in eastern China to study the origin of the incompatible-element enriched component in these basalts by testing whether or not the paleo-Pacific plate lying in the mantle transition zone beneath eastern China is the immediate cause. The Cenozoic CIM basalts have a large variation in major element, trace element and isotope compositions. Fractional crystallization of olivine and clinopyroxene can readily explain much of the major element compositional variation, while trace element and isotope ratio variation largely reflect source heterogeneities and source histories. The variably low 87Sr/86Sr, high εNd, high εHf and elevated ratios of high field strength element over large ion lithophile element (HFSE/LILE, e.g., Nb/U, Nb/La) indicate that the CIM basalts are of asthenospheric origin, which is characterized by mixing between DMM and EM1. However, the CIM basalts are enriched in incompatible elements and enriched in the progressively more incompatible elements (e.g., variably high [La/Sm]N=1.66–3.38), suggesting that the magma source(s) must have been enriched prior to the major episode of the magmatism. Participation of subducted ocean crust in the mantle source region of these basalts is recognized, but cannot be the major source material because the subducted ocean crust is expectedly too depleted in incompatible elements (e.g., [La/Sm]N≪1) to produce magmas highly enriched in incompatible elements with [La/Sm]N≫1. With the new data, we consider that low mass fraction (low-F) melt metasomatism in the seismic low velocity zone (LVZ) beneath eastern China as the most likely process to generate incompatible-element enriched source(s) for mantle melts parental to the Cenozoic CIM basalts. The low-F metasomatic agent most likely resulted from dehydration melting of the transition-zone paleo-Pacific slab, which has been taking place in the Cenozoic. This recent/current metasomatic process also explains the strong decoupling of the abundances and ratios of incompatible elements (e.g., Rb/Sr, Sm/Nd, Lu/Hf, La/Sm, Sm/Yb) from Sr–Nd–Hf isotopes because of the inadequate time for radiogenic ingrowth. In this case, we emphasize that the Cenozoic volcanism in eastern China (including the CIM basalts) is a special consequence of plate tectonics.Because of the known presence of subducted oceanic lithosphere beneath eastern China and because of the close resemblance of the intra-plate CIM alkaline basalts with the present-day ocean island basalts (OIBs) in terms of incompatible element systematics, we emphasize the potential role of subducted ocean crust in mantle source regions of oceanic basalts, including E-MORBs (enriched type mid-ocean ridge basalts) and OIBs, but low-F melt metasomatism in the source regions is required as the principal enrichment mechanism because otherwise it is unlikely to produce incompatible-element enriched signature in E-MORB and OIB.