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Deformation Structures of the Madao Gneiss in South Qinling: Structural Analysis, Geochronological Constraints, and Tectonic Implications

The tectonic evolution of South Qinling, which is a main part of the Qinling orogenic belt, is still in dispute and deformation history of South Qinling is poorly studied. In this paper, detailed structural, microstructural, quartz c‐axis fabric analysis, and geochronology results for the Madao gneiss in South Qinling are presented to characterize the deformation history. Results show that rocks in the northern part (Tiefodian‐Laozhanggou) experience general shearing and deform at relative low temperature. The shear sense generally is south to north. In contrast, rocks in the southern part (Laozhanggou‐Panjiahe) are weakly sheared with pure shear features and evidence of high‐temperature deformation. Based on the analyses, we conclude that there exist two distinct deformation geometries in the Madao gneiss and accordingly we can divide the deformation into two stages. The early stage is represented by regional shortening, while the late stage features northward thrust shearing and evidence shows that it was a progressive process between them. LA‐ICP MS U‐Pb dating of zircons from pre‐deformational migmatite veins yields age of 198.5 ± 2.0 Ma. This result, in combination with the age of post‐deformational granite, indicates that the northward thrust shearing of the Madao gneiss occurred in the Late Triassic. In view of these results and other reported data in South Qinling, we propose that deformation in Madao gneiss may result from the initial collision and subsequent northward accretion in Late Triassic.