Geochronology And Metamorphism Of The Qinling Complex In The Weiziping Area Of The Qinling Orogen,Central China

The Qinling Complex is the major part of North Qinling belt in Qinling Orogen belt,and is thought to be the oldest and highest metamorphic grade unit.Therefore,it is of great significance to understand the geological evolution of the Qinling Orogenic belt.The Qinling Complex has experienced multiple metamorphism during the Paleozoic,including HP eclogite?510–480 Ma?and MP granulite?455–400 Ma?.There is no consensus on the genesis of ca.455–400 Ma granulite-facies metamorphic rocks from the Qinling Complex.Therefore,more detailed metamrohic petrology and geochronology are needed to clarify the P-T evolution and tectonic setting of ca.455–400 Ma granulite-facies metamorphism.In this study,we conducted detailedzircondating and metamorphic petrology for the migmatite from the Weiziping area of the Qinling Complex,North Qinling belt,to constrain the Early to Middle Devonian granulite-facies metamorphic evolution of the Qinling Complex and its tectonic significance.The mesosomes of the migmatites are composed of garnet-bearing biotite–hornblende–plagioclase and biotite–plagioclase gneisses,and the leucosomes can be divided into two types:wide leucosomes that were deformed,and narrow leucosomes.Laser ablation–inductively coupled plasma–mass spectrometry?LA–ICP–MS?zircon U–Pb analyses yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 398.8±4.7,394.6±2.6,399.5±7.3,372.5±4.2,370.8±4.3,and 368.5±4.2 Ma for metamorphic zircons within the sampled mesosomes,and a weighted mean age of 408.7±5.7 Ma for igneous zircons within a leucosome.In addition,LA–ICP–MS zircon U–Pb dating on metamorphic zircons from amphibolite lens or interlayer in the migmatite yielded a weighted mean ²⁰⁶Pb/²³⁸U ages of 375±9 Ma,391±5 and 386±5 Ma.Combined with the published data and the results in this study,we constrained the timing of granulite-facies metamorphism of the Qinling Complex in the Weiziping area to be ca.410–390 Ma.Based on the field observation and petrography,the mesosome sample has a peak mineral assemblage of garnet,biotite,plagioclase,hornblende,quartz,ilmenite and melt.Phase equilibrium modeling in the NCKFMASHTO system were carried out on mesosome sample,and the peak metamorphic condition was constrained to be 8.5–9.7 kbar at 745–820? by isopleths of X_An and X_Py in a P–T pseudosection,which indicate that these mesosome samplex has underwent middle-pressuregranulite-faciesmetamorphism.Conventional garnet–biotite–plagioclase–quartz?GBPQ?geothermobarometry yielded P–T conditions of703? at 10 kbar and 685–748? at 7.1–8.9 kbar,which corresponding to prograde and retrograde metamorphic stages,respectively.On the basis of these P–T results,the studied samples document a clockwise P–T path characterized by temperature increase and pressure decrease during the prograde stage,and temperature decrease and pressure decrease during the retrograde stage.Ti-in-zircon thermometry yielded a temperature range of 781–793? for metamorphic zircons from the mesosomes,further demonstrating that they underwent granulite-facies metamorphism at ca.410–390 Ma.The prograde metamorphic stage characterized by temperature increase and pressure decrease suggests that thermal relaxation after crustal thickening has induced the partial melting and migmatization.Combined with regional geological data,crustal thickening possibly associated with Paleozoic continental collision between the NCB and the SCB should occur at least before ca.410 Ma.The main results are summarized as follow.?1?The timing of granulite-facies metamorphism of the Qinling Complex in the Weiziping area was constrained to be ca.409–390 Ma.?2?Phase equilibrium modeling combined with conventional geothermobarometry reveal that the migmatites of Weiziping area experienced MP granulite-facies metamophism,and prograde metamorphic stage,peak metamorphic stage and retrograde metamorphic stage are constrained to be 703°at10 kbar,745–820? at 8.5–9.7 kbar,685–748? at 7.1–8.9 kbar,respectively.The studied samples document a clockwise P–T path characterized by temperature increase and pressure decrease during the prograde stage,and temperature decrease and pressure decrease during the retrograde stage.?3?The prograde metamorphic stage characterized by temperature increase and pressure decrease suggests that thermal relaxation after crustal thickening induced granulite-facies metamorphism and migmatization.Crustal thickening possibly related with continental collision between the NCB and the SCB should occur at least before ca.410 Ma.