LA-ICP-MS U-Pb dating of metamorphic monazite and zircon of the Qinling metamorphic complex, Xixia and Neixiang areas, Henan Province

Abstract

西峡-内乡地区位于北秦岭造山带东部， 出露黑云斜长片麻岩、变质砂岩、石英片岩、石墨大理岩、石榴角闪岩等变质岩石。激光拉曼测试表明，变质岩锆石与独居石包裹体中， 未发现高压-超高压变质特征矿物。值得注意的是，显微岩相研究也未见到多期变质作用的证据。对变质独居石和变质锆石进行的LA-ICP-MS U-Pb定年表明， 副变质岩记录的变质高峰期时代为414~405Ma， 石榴角闪岩记录的变质高峰期时代为401~379Ma， 属晚志留世-中泥盆世。区内岩石普遍经历了角闪岩相变质作用， 变质高峰期P-T条件为660~710℃/0.53~1.02GPa， 属于中压变质相系。不同变质岩的变质高峰期压力条件差异明显， 说明这些岩石俯冲深度不同， 在折返阶段才以构造岩片的形式混杂在一起。各种变质岩中退变质矿物组合普遍不发育， 暗示抬升速率较快。;Metapelite, metasandstone, quartzite, marble and garnet amphibolite expose in the Xixia and Neixiang areas, eastern North Qinling Orogenic Belt. No (ultra-) high pressure mineral inclusions were found within the metamorphic monazite or zircon through laser Raman investigation, and no petrographic evidences suggest they experienced polymetamorphism. LA-ICP-MS U-Pb dating of metamorphic monazite and zircon suggest that the metamorphic peak occurred at 414~405Ma recorded by metapelite or 401~379Ma by amphibolite, i.e., Late Silurian to Middle Devonian. The rocks experienced amphibolite facies metamorphism with peak P-T conditions estimated to be 660~710℃/0.53~1.02GPa, which is ascribed to the medium P/T facies series. However, significant diverse pressure conditions of these rocks imply that they reached different depths and later they were amalgamated in the tectonic exhumation. Ubiqutous absence of retrograde mineral assemblages imply that the exhumation was relatively fast.