The Petrogenesis Of The Mesozoic-Cenozoic Volcanic Rocks In Central Inner Mongolia

It has been reached a consensus that the lithosphere of North China Craton (NCC) had been thinned (～100 km) during the Mesozoic time. However, the mechanism of the lithosphere thinning remains the problem, and there is also spatial limitation on the lithosphere thinning. In recent years, based on the geophysical observation that the paleo-Pacific plate lying under the continental China, more and more people realized that destruction of the NCC and the large scale of the Mesozoic-Cenozoic volcanisms are closely related to the subduction of the paleo-Pacific plate, which is most probably the immediate cause or indirect cause to the Mesozoic-Cenozoic volcanism occurred in East China. However, the causal relationship between destruction of NCC together with Mesozoic-Cenozoic volcanism and the subduction of the paleo-Pacific plate need more evidences, and it is remains the problem that how the subducted paleo-Pacific plate affect and what is the geodynamic processes?There are scattered earlier Cretaceous volcanic rocks in Siziwang Qi (Central Inner Mongolia) and amounts of Cenozoic basalts outcropped in Central Inner Mongolia (CIM) area. The geochemistry of these volcanic rocks can inverse the lithosphere thinning process in the regional area, and constrain the lithosphere thinning spatially and temporally. In this context, we studied the petrology and geochemistry of the Siziwang Qi earlier Cretaceous volcanic rocks and the Cenozoic basalts from Jining, Abaga and Chifeng volcanic fields, and obtained following results:1. The Cenozoic basalts from CIM are the special products of the plate tectonicsAll the three series of the Jining, Abaga and Chifeng Cenozoic basalts show enrichment in incompatible elements with [Sm/Yb]N= 1.66-4.01. These basalts are more enriched in more incompatible elements except for the obviously positive anomalies of Nb-Ta and Ba-Sr-Pb, indicating the pre-enrichment before the magmatism. Despite the high [Nb/Th]N and [Ta/U]N ratios of the CIM basalts and striking high 87Sr/86Sr, high 143Nd/144Nd for several samples suggesting the contribution of the ocean crust components, the low MgO together with the depletion in incompatible elements of the oceanic crust suggest it is impossible for oceanic crust to be the major material of the magma source regions. Therefore, the hydrous low degree partial melts metasomatism is considered to be the best mechanism to explain the incompatible elements enriched source region to CIM basalts. Based on the observation that the paleo-Pacific plate lying under the continental China, this study propose that the origin of the metasomatic agents should be the low degree partial melting of the enriched lower portion of the subducted oceanic lithosphere mantle, with some contribution of the ocean crust and the carbonate subducted together with the ocean crust. Such a model not only well accounts for the incompatible elements enriched character of the Cenozoic basalts from CIM area, but also explains the positive anomalies of Nb-Ta and Ba-Sr-Pb in the CIM basalts. Thus, we consider the Cenozoic basalts from CIM area as the special products of the plate tectonics.Furthermore, there are no correlations between parent/daughter ratios (Rb/Sr, Sm/Nd, Lu/Hf) and Sr-Nd-Hf isotopes. This is also the case for [La/Sm]N, [Sm/Yb]N with Sr-Nd-Hf isotopes. There is also significant decoupling of incompatible elements with Sr-Nd-Hf isotopes. That is, the radiogenic isotopes are unsupported by parent/daughter ratios of the basalt sources. Such decoupling of incompatible elements with radiogenic isotopes manifests that the rather recent (or current) low-F melt refertilization in the CIM mantle source regions with inadequate time for radiogenic ingrowth.2. The lithosphere thinning of the west NCCIn west NCC, there are scattered earlier Cretaceous volcanic rocks in Siziwang Qi area. These volcanic rocks are mainly basaltic andesite and trachybasalts, which are characterized by enrichment in incompatible elements and high 87Sr/86Sr, low εNd (t) and low εHf (t). We propose that these volcanic rocks originated from partial melting of ancient metasomatized lithospheric mantle by comparing study of the Wulanhada high-Mg andesite from Liaoxi area. Considering the geophysical observation that the lithosphere thickness of some area in the west NCC is～80-100 km, we propose that the lithosphere had been thinned and the Siziwang Qi earlier Cretaceous volcanic rocks were the direct products of the lithosphere thinning in west NCC.Both the Miocene basalts from Siziwang Qi and the Quaternary basalts from Baiyinchagan, a place near the Siziwang Qi, show low SiO2, high MgO, incompatible element enriched characters with low 87Sr/86Sr, high εNd(t) and εHf(t), implying their enriched asthenospheic mantle origin. However, Miocene basalts have basically higher moderately/weakly incompatible element ratios and more garnet signatures than the Quaternary basalts, indicating the higher partial melting of the source region for the Quaternary basalts, and the Miocene basalts were produced in the context of thicker lithosphere than that of Quaternary basalts produced. We, thus, infer that the lithosphere has been thinning since Miocene in the west NCC.3. The petrogenetic model of the Mesozoic-Cenozoic basalts in CIMBased on the petrological and elemental-isotopic study of the Mesozoic-Cenozoic volcanic rocks from the CIM area, we propose that the "basal hydration of lithosphere" is the most suitable mechanism to explain the destruction of NCC or lithosphere thinning occurred in East China and the large scale of the volcanism during Mesozoic time. We infer that the paleo-Pacific plate had been under the East China during the Mesozoic time, and the westernmost part was beneath the Siziwang Qi area. The "water" released from the transition-zone slabs with the heating by the abyssal mantle and hydrous melt can hydrate and weaken the basal portion of the existing lithosphere, whose melting produced the Siziwang Qi earlier Cretaceous volcanism. The lithosphere under such area evolved, from the early Cretaceous to the Miocene, to become more depleted as the enriched components have been progressively melted and extracted; this process eventually transformed the lower portion of the ancient lithospheric mantle into the asthenospheric mantle in terms of physical properties.The West NCC lithosphere had been thinned when it comes the Cenozoic time. Due to the western Pacific wedge suction, asthenospheric materials flow from the west to east beneath continental China. Such asthenospheric flow from beneath the plateaus to beneath eastern China must experience decompression, and this decompression can cause the flowing asthenosphere to melt and give rise to the Cenozoic volcanism in eastern China. Because Siziwang Qi area located the westernmost of the destructed NCC, it should be the first place to experience the decompression of eastward asthenospheric flow with intensive volcanism occurred during Cenozoic time. Note that despite we recognized the recycled ocean crust components in the Cenozoic basalts from CIM area, the ocean crust still cannot be the major source materials for these basalts because the subduced ocean crust is depleted in incompatible element, and also because ocean crust is characterized with low MgO. Therefore, the mantle metasomatism, which occurred recently or currently, is the best mechanism to produce the incompatible element enriched sources to the CIM Cenozoic intra-plate basalts.