Characteristics and petrogenesis of the Mesozoic volcanic rocks from the Middle-Lower Yangtze River Belt and the Dabie Orogen

Mesozoic magmatic rocks are widely distributed in the Middle-Lower Yangtze River Belt (MLYRB) and the Dabie Orogen (DB), with the active stages of 135~127 Ma and 133~125 Ma respectively for the volcanic rocks. High potassium calc-alkaline bimodal volcanics and shoshonitic volcanic rocks crop out in the several Mesozoic volcanic basins of the MLYRB. These rocks have consistent characteristics of enriched large ion lithophile elements, depleted high field-strength elements, weak enriched Sr-Nd-Hf isotopies and high radiogenic Pb isotopic components, indicating crustal materials involved into the mantle sources. Among them, the intermediate-basic volcanic rocks derived from enriched lithospheric mantle, which was metasomatized by hydrous melt released from subducted oceanic slab. The late stage super alkaline volcanic rocks originated from the asthenospheric mantle, which suffered matasomatism of the similar metasomatic agent, inferring deepening of the magma source. The Mesozoic volcanic rocks in the DB dominantly include two rock series of high potassium calc-alkaline and super potassic. These rocks show quite different geochemical features from those in the MLYRB, by enriched large ion lithophile elements, depleted high field-strength elements, strongly enriched Sr-Nd-Hf isotopies and low radiogenic Pb isotopic components. The high potassium calc-alkaline volcanic rocks in the DB also derived from enriched lithospheric mantle, but the metasomatic agent was melted form the Indosinian deep subducted continental crust of South China. The late stage super potassic volcanic rocks originated from deeper mantle source, where phengite in the deep subducted continental crust of South China broken down under high pressure. Direct materials contribution from the subducted paleo-Pacific plate imprinted in the mantle-derived volcanic rocks in the MLYRB, but the late Mesozoic magma sources in the DB only recorded information of the Indosinian deep subducted continental crust. The volcanic activities in the two tectonic units indicate a similar process of deepening of magmatic source from early to late, corresponding to low angle subduction and then rollback (ca. 130 Ma) of the paleo-Pacific plate beneath the MLYRB. Due to dynamic triggering of the subduction and rollback of the paleo-Pacific plate, orogenic collapse and lithospheric delamination took place in the DB.