长江中下游—大别造山带中生代火山岩特征及成因

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

中国中东部长江中下游地区和大别造山带广泛发生了中生代岩浆作用,火山岩均集中在中间阶段,时代分别为135~127 Ma和133~125 Ma。长江中下游地区多个中生代火山盆地发育高钾钙碱性系列双峰式火山岩和中基性橄榄安粗岩系列火山岩。这些岩石富集大离子亲石元素,亏损高场强元素,弱富集Sr-Nd-Hf同位素,具有高放射成因Pb同位素组成,指示地幔源区地壳组分的加入。其中,中基性火山岩起源于富集的岩石圈地幔,受到俯冲大洋板片析出的含水熔体交代,晚阶段的超碱质火山岩起源于类似交代介质交代的软流圈地幔,指示岩浆源区的加深。大别造山带中生代火山岩包括高钾钙碱性系列和超钾质系列,均富集大离子亲石元素,亏损高场强元素,高度富集Sr-Nd-Hf同位素,具有低放射成因Pb同位素组成,与长江中下游地区差异显著。其中,高钾钙碱性系列火山岩起源于交代富集的岩石圈地幔,交代介质为印支期深俯冲的华南陆壳析出熔体,而晚阶段的超钾质火山岩起源更深,是深俯冲的华南陆壳在高压下,多硅白云母分解产生熔体交代的地幔源区。长江中下游地区幔源火山岩记录了俯冲的古太平洋板块的直接物质贡献,而大别造山带地幔源区记录了印支期俯冲陆壳的信息。两个构造单元火山岩早、晚阶段均表现出岩浆源区的加深,长江中下游地区对应了古太平洋板块的低角度俯冲及俯冲板片的回卷(约130 Ma),而大别造山带在古太平洋板块俯冲回卷的动力学机制下,发生造山带的垮塌和岩石圈的拆沉。

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.