Structural Characteristics And Exhumation Process Of The North Qinling Ultrahigh-pressure Metamorphic Rock Belt

The Qinling Group(CG)outcrops in the North Qinling Orogenic Belt(NQB)and preserves crucial evolutionary information of NQB during the early Paleozoic.The QG is composed of tectonic slices,which are bounded by ductile shear zones.Based on the published researches,high and ultra-high pressure(HP-UHP)metamorphic rocks are widely distributed in Guanpo-Shuanghuaishu,Songshugou,Qingyouhe,Zhaigen and Xixia in different tectonic slices of QG,while no relevant reports have been found in Qihe tectonic slice of central QG.Whether the Qihe tectonic slice underwent the similar UHP–HP metamorphism,and what is the relationship between the different tectonic slice of QG.Using phase equilibrium modeling,metamorphic pressure-temperature conditions of amphibolite in Qihe tectonic slab are limited in 2.95–4.6 Kbar and 481–568 ?,which are consistent with the approximately uppergreenschist–low-grade amphibolite facies.The U-Pb dating of the amphibolite in Qihe tectonic slice provides a protolith age of 714 ± 46 Ma and the ages of three-stage metamorphism of 514±4 Ma,462±3 Ma,and 418±5 Ma.The first group(514±4 Ma)is the metamorphic age of eclogite facies,which is related to the subduction-deep subduction of the QG.The second group of ages(462±3 Ma)is associated with the retrograde metamorphism during an initial rapid exhumation to a middle–lower crust level.The third age(418±5)is the metamorphic age of the amphibolite in Qihe tectonic slice and represents age of the subsequent exhumation to the middle–upper crust.Based on the structural analysis of different tectonic slices in the QG,four stages of ductile–brittle-ductile deformation are determined.The D1 is a regionally penetrative schistosity and gneissosity,with some intrafolial tight isoclinal folds.The eclogite and retrograded eclogite occur as lenses or layers within gneissic rocks,indicating the paleo-stress characteristics of bedding stretching caused by vertical foliation extrusion.The age of D1 should be approximately ca.517–485 Ma based on the age of the migmatitc orthogneiss and the granitic dikes in the research area.The D2 is mainly formed in the north-south compression structure,including the ductile shear zone and fold superposed on D1.The ductile shear zone is characterized by the strong orientation of amphibolite,stretched quartz and plagioclase along mylonitic foliation,indicating that plastic deformation developed in amphibolite-facies conditions.The D2 age is defined as ca.437–445 Ma according to the age of granitic veins and granites.The D3 is low-angle detached ductile shear superimposed on the D1/D2 deformation,which is related to the metamorphism of high-greenschist facies–low-amphibolite facies.Combined with metamorphism age and granite age,D3 deformation occurred at ca.418 Ma.The D4 is defined by the ductile shear zone developed in the QG near the Zhuxia and Shangdan Fault.The metamorphic mineral,such as albite,fine sericite,epidote,and chlorite,replaces the early assemblage,particularly along C-surface.This mineral assemblage indicates greenschistfacies metamorphism,and the D4 should occur at the ca.368–386 Ma.In addition,the study on the detrital zircon ages of the parametamorphic rock,which is the surrounding rock of eclogite and retrograde eclogite,show similar age concentrations(ca.1000–1850 Ma)and age peaks in different structural zones of the QG,indicating the QG is a unified structural unit and the structural affinity of HP–UHP metamorphic rocks are more closely related to the QG.Based on the above research results,the QG is a uniform stratigraphic unit and partially evolved into subduction erosion and eclogite-facies metamorphism at ca.500 Ma.Then,the QG was superimposed by the deformation structures and retrograde metamorphism during the exhumation.