The multi-stage metamorphism of the Dongzaoyuan garnet amphibolite in the Qinling Complex and its geological significance
The Qinling Complex in the North Qinling Belt is a high-pressure-ultra-high-pressure metamorphic zone which had experienced subduction-deep subduction and exhumation processes of massive continental crust materials in the early Paleozoic.The garnet amphibolite exposed in the Qinling Complex had recorded important information of multi-stage metamorphic ages and P-T conditions,which can provide important constraints for the reconstruction of the early Paleozoic metamorphic evolution history of the North Qinling Belt.In this paper,detailed zircon U-Pb geochronology,geochemistry,and mineral chemistry of garnet amphibolite samples collected from the migmatized felsic gneiss in the Qinling Complex in the Dongzaoyuan area,Danfeng County,Shaanxi Province have been studied.The results show that there are two kinds of amphibole(Amp1 and Amp2)and plagioclase(Pl1 and Pl2)in the garnet amphibolite.There is no obvious compositional zoning in garnet.The garnet amphibolite is characterized with two sets of metamorphic mineral assemblages including(1)amphibole(Amp1)+plagioclase(Pl1)+quartz+ilmenite/sphene and(2)garnet+amphibole(Amp2)+plagioclase(Pl2)+quartz+ilmenite.The LA-ICP-MS zircon U-Pb dating yielded four metamorphic ages of 497±3 Ma,450±3 Ma,419±5 Ma,and 399±5 Ma,respectively.According to the REE compositions and mineral inclusion assemblages of zircons,it is considered that the four metamorphic ages correspond to those of eclogite facies,granulite facies,and two amphibolite facies,respectively.In addition,the garnet amphibolite has similar geochemical characteristics of the eclogite in the Qinling Complex.It is suggested that the garnet amphibolite was formed by the multi-stgae retrograde metamorphism of original eclogite which underwent eclogite facies metamorphism at～500 Ma,and retrograde metamorphism of two stages at～450 Ma,and～420 Ma,respectively.The garnet amphibolite finally experienced a prograde metamorphism of high amphibolite facies at～400 Ma,which may be related to the rotation of slab detached from the subducted continental crust in the slab exhumation process.
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