New insights into the emplacement mechanism of the Late Triassic granite plutons in the Qinling orogen: A structural study of the Mishuling pluton

摘要

Numerous geochronological and geochemical studies of the Late Triassic granite plutons in the Qinling orogen have been conducted over the past few decades. These studies have extensively discussed the genesis and correlations of granite plutons with the collisional processes between the North and South China blocks. However, several contradictory conclusions on the tectonic settings of these plutons (subduction, syncollision, postcollision, or intraplate) have been reached. Moreover, in all these studies, compressional or extensional structures have been always considered to control the magma emplacement, but no direct evidence has been presented so far. In order to clarify the emplacement mechanism of these Late Triassic plutons and avoid the ambiguities from purely geochemical studies, we conducted a multidisciplinary structural study on the Mishuling pluton in West Qinling, one of the biggest Late Triassic plutons in the orogen. New single-grain zircon dating confirmed that the Mishuling pluton was emplaced at 212 +/- 2 Ma. Gravity investigations show an overall wedge shape to the pluton: A deep root is located in its western part, the thickness gradually decreases to the east, and it finally pinches out at its eastern extremity. We studied internal structures of the Mishuling pluton through field, microscopic, and AMS (anisotropy of magnetic susceptibility) observations, which show that the pluton did not undergo penetrative secondary deformation, and preserved cryptic internal fabrics were mainly acquired during the final stage of emplacement. In the western part of the pluton, internal fabrics display a concentric pattern. In the eastern part, the lineations mainly plunge west, while the foliations form an analogous W-plunging synform, mainly dipping SW in the northern part and dipping NW in the southern part. The three-dimensional shape and internal structures of the Mishuling-pluton are concordant with the wrench tectonics of country rocks that developed during the late stage of the Late Triassic collision between the North and South China blocks. We propose an integrated emplacement model involving both the three-dimensional shape and internal fabrics of the Mishuling pluton, and regional structures of the country rocks. In this model, the magma rose in the western part and then spread eastward parallel to the regional strike-slip movements.