北秦岭高压-超高压岩石的时空分布、P-T-t演化及其形成机制

The Distribution, P-T-t Evolution and Formation Mechanism of HP-UHP Metamorphic Rocks in the North Qinling Orogenic Belt

北秦岭构造带早古生代的构造格局和演化过程一直是地学界比较关注也是存在较大争议的问题之一.在已有研究基础上,系统总结了本课题组近年来在北秦岭早古生代高压-超高压变质作用研究方面的进展,从变质作用角度对北秦岭早古生代的构造演化提供重要限定.丹凤斜长角闪岩中柯石英的发现为区内超高压变质作用的存在提供了最直接的矿物学证据;东秦岭秦岭杂岩中的斜长角闪岩普遍经历了高压-超高压榴辉岩相变质,具面状分布的特征,是陆壳俯冲/深俯冲作用的产物;高压-超高压榴辉岩和围岩片麻岩都记录了顺时针的P-T-t轨迹,峰期变质时代为500~490 Ma,之后主体又经历约470~450 Ma和约420~400 Ma两期抬升退变质叠加和部分熔融作用;高压-超高压岩石两期退变质和部分熔融发生的时代与北秦岭460~440Ma和~420Ma的两期岩浆事件的时代一致,说明北秦岭早古生代岩浆作用是深俯冲陆壳板片断离和碰撞造山结束后地壳伸展作用的岩浆响应;高压-超高压榴辉岩原岩形成时代约800 Ma,具有与南秦岭新元古代中晚期岩浆岩一致的地球化学特征,北秦岭超高压岩石的形成可能是商丹洋关闭后洋壳拖曳着南秦岭陆壳物质向北发生大陆深俯冲的结果,商丹洋在500 Ma主体应该已经关闭;秦岭岩群是部分而不是整体经历了大陆的深俯冲,现今的秦岭岩群是一个俯冲碰撞杂岩带而不是一个岩石地层单元或微陆块;北秦岭早古生代造山作用在中泥盆世已经结束,整体处于构造隆升后的剥蚀阶段,是南秦岭刘岭群碎屑岩的主要蚀源区,刘岭群沉积盆地形成于碰撞造山后的伸展构造背景而非弧前环境.

The Early Paleozoic tectonic framework and evolution history of the North Qinling orogenic belt(NQOB)is in great dispute and has drawn wide concern.In this contribution,we systematically summarized our recent progresses on Early Paleozoic HP-UHP metamorphism in the NQOB and constrained the Early Paleozoic tectonic evolution of the NQOB from the perspectives of metamorphism.The discovery of coesite in the Danfeng amphibolite provides a conclusive evidence for the UHP metamorphism in the NQOB.The amphibolites,which are widely distributed in Qinling Complex in East Qinling,commonly experienced HPUHP metamorphism and are products of continental(deep)subduction.HP-UHP eclogites and host gneisses have clockwise P-T-t paths and similar peak metamorphic ages of 500-490 Ma,and experienced two stages retrogression and anataxis at 470-450 Ma and 420-400 Ma,respectively.The times of the two stage retrogression and anataxis are coincident with the magmatism in the NQOB,which occurred at 460-450 Ma and^420 Ma,suggesting that the Early Paleozoic magmatism in the NQOB were formed under the tectonic setting of breaking-off of deeply subducted continental slab and post-orogenic extension-thinning,respectively.The HP-UHP rocks have protolith ages of ca.800 Ma,and geochemical characteristics similar to Mid-Late Neoproterozoic magmatic rocks from South Qinling,indicating that continental materials from South Qinling were dragged down to mantle depth and experienced UHP metamorphism by the north dipping Shangdan oceanic lithosphere,and the Shangdan Ocean was already closed at 500 Ma.The Qinling Group is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent and only part of it underwent continental deep subduction.The Early Paleozoic orogenisis in North Qinling was ended and the HPUHP rocks were exhumated up to the surface in Mid-Devonian,and became the main provenance of sedimentary rocks in Liuling group in South Qinling.The Liuling basin is a post-orogenic extensional basin,rather than a fore-arc basin.