Emplacement Mechanism And Its Tectonic Implications Of The Dongjiangkou Pluton,Qinling Orogen

The Indosinian Orogeny played a special role in the whole evolutionary history of the Qinling orogen.It not only laid a foundation for the formation of the Qinling but also represented an important transition stage between the Paleozoic plate tectonics and the mesozoic-cenozoic intra-continental tectonic regime.However,the end time of this orogenic episode is uncertain due to the debate on the tectonic setting for those Late Triassic plutons with proposals of syn-subduction,syn-collision,and post-or late-collision.In spite of those plausible models,it should be noted that most previous works have focused on petrological and geochemical studies and neglected to detect the structural information in these granites.However,it remains to be demonstrated if the geochemistry of granites can discriminate the tectonic setting in which they emplace.In this study,multidisciplinary studies of the Late Triassic Dongjiangkou granite pluton and its country rocks have been conducted to explore the emplacement mechanism of the pluton and further to constrain the tectonic setting of the Late Triassic magmatism in the Qinling Orogen.The following salient insights can be drawn from this study:(1)The Qinling orogen was still under the syn-collisional convergent setting during the Late Triassic orogeny,according to evidences of extensive syn-plutonic deformation that were recognized both in the Dongjiangkou pluton and its country rocks.There are extensive syn-plutonic deformations can be recognized within the Late Triassic Dongjiangkou pluton and its country rocks.In the pluton,syn-plutonic shear deformations that can be inferred from the mafic microgranular enclaves and granitic dikes attest to the existing of regional deformation during the growth pcocess of the pluton.At the immediately north of the pluton,the sinistral transpressional kinematics recorded in the Shagoujie ductile shear zone is deformation partitioning of the oblique convergence along the Shangdan belt,and the timing of deformation can be limited at the interval of ca.219-210 Ma.At the immediately south of the pluton,the Shanyang-Fengzhen fault is characterized by thrust and nappe tectonics combining with sinistral movement that were indicated by sheath folds,imbricate thrust-nappe structure,asymmetry rotated porphyroclast and structural lenticle.Meanwhile,the high-temperature deformation in the contact between the pluton and its country rocks also suggests the coexistence of the regional deformation and the magmatism.Furthermore,formation ages(210-201 Ma)of the syntectonic granitic dikes emplaced within the Liuling group can provide the upper limit on the sinistral transpressional deformation.Therefore,we argue that the Qinling orogen was still under the syn-collisional convergent setting during the construction of the Late Triassic Dongjiangkou pluton,and the coeval"rapakivi-textured" granites in Qinling could not be used to mark the end of the Late Triassic orogeny.(2)Laterally,the Dongjiangkou pluton is assembled by amalgamation from two separated plutons of different ages.Vertically,it is characterized by incremental growth mechanism.The concentric petrologic zoning of the pluton cannot be interpreted as having resulted from in situ magma differentiation but rather have been inherited from the source region before emplacement.Internal fabrics of the Dongjiangkou pluton show two different concentric zonings.Granites collected from the smaller zoning close to the Shagoujie ductile shear zone yielded a crystallization age of 210 Ma,while formation age of granites from the bigger zoning close to the Shanyang-Fengzhen fault is ca.200.6～199.2 Ma.Taken together,our structural and geochronological studies indicate a composite origin of the Dongjiangkou pluton that it is laterally assembled by amalgamation from two separated plutons of different ages(～210 Ma and～200 Ma).Meanwhile,crystallization age of granites from the bigger zoning show a trend of getting younger from margin to core.Moreover,results of Al-in-hornblende geobarometer show each petrological facies emplaced within different level of the crust,i.e.intrusion level of the granites are getting deeper from core to margin.Therefore,the emplacement of the Dongjiangkou pluton can be characterized by incremental growth mechanism,and the concentric petrologic zoning of the pluton is probably resulted from multiple magma batches rather than in situ magma differentiation.(3)Regional tectonics probably controlled the emplacement of Late Triassic granite magmatism.The bilateral extrusion(escape)tectonics model is proposed for better understanding the coeval opposite kinematics in the West and East Qinling which were developed in response to the Late Triassic collision.Structural studies of the Dongjiangkou pluton demonstrate that regional tectonics controlled the emplacement of the pluton as well as the formation of the internal fabrics during late stage of the crystallization process.Moreover,accroding to previous studies,emplacement of many other Late Triassic plutons also show close relationship with regional tectonics,such as the Baliping pluton,the Laocheng pluton,the Yanzhiba pluton and the Mishuling pluton that were developed within the Shangdan belt,the South Qinling and the West Qinling,respectively.It is also noteworthy that almost all these Late Triassic plutons expose in/near the narrower segment,and mainly bounded by the crustal-scale fault zones.Therefore,we argue that regional tectonics probably controlled the emplacement of granite magmatism during the Late Triassic orogeny.Furthermore,the coeval opposite kinematics in the West and East Qinling is probably resulted from the bilateral extrusion(escape)tectonics in response to the Late Triassic oblique convergence.