Discussion On The Genesis Of Foping Dome In Southern Qinling: Evidence From Structural Analysis

Foping dome, which located in the junction of Eastern and Western Qinling, is belongs to South Qinling tectonic belts (Qinling micro-plate). This dome sites between Shangxian-Danfeng suture zone and Mianxian-Lueyang suture zone, and it is also one of the strongest compression areas by North China Block and Yangtze Block. Foping area is characterized by outcropping of Precambrian metamorphic basement and Indosinian granite. The rock assemblage in this area can be classified into crystalline basement, translation basement, caprock rocks, and intermediate-acid intrusive rocks. The crystalline basement mainly outcrops at Foping town and Longcaoping. It suffered multi-period and multilevel deformation in Foping area, and the structural characteristics here show a series of fold structure, which spreads nearly east-west and reflects a strong north-south compression. The Foping town dome and Longcaoping dome locate at the nuclear of two anticlines.The metamorphism in Foping area shows:the granulite facie-High-amphibolite facies-Low-amphibolite facie-High-greenschist facie-Low-greenschist facie spread in the shape of concentric rings and to the core of Foping dome in the plane, which can also proves the "Dome" structure features. The granulite facie rock shows lenticulars, mainly outcrops at about 500 meters to the town, east of river. The major minerals of these rocks are diopside, plagioclase, hornblende, biotite and garnet. The migmatization developed well in this region, which maybe caused by deep magmatism.The study of paleostress fields by systematic measurement and statistics on the tectonic foliation and lineation in the Foping dome and its adjacent areas. This can reveal the distribution of regional paleostress fields, by which we can discuss the genetic mechanism of the Foping dome. The results show that the foliation in Foping and Longcaoping uplifting and their adjacent stratum proves the circle features with outward trend, and longer in E-W, shorter in S-N in the plane. It proves that the view Foping uplifting is a thermal dome, which is transformed by latter tectonic, that is the nearly N-S compression tectonic in Indosian.The recovery of the paleostress field is based on systematic measurement of the occurrences and the judge of the kinematic meaning in the field. The characteristics of the paleostress fields in Foping County and it northern Longcaoping revealed that the similar paleostress fields. The first phase structure shows:the compression stress (al) generally proves outward trend with high angles, with the core of the core of the domes, and the tensile stress (σ3) generally proves nearly horizontal radial spreading. The second phase structure shows:the compression stress (σ1) generally spread nearly horizontal, N-S or NE-SW, and the tensile stress (σ3) generally mainly proves extension with E-W or NW-NE. It reveals that the town uplifting and Longcaoping uplifting shared the same of similar genetic mechanism and tectonic evolution. The uplifting area suffers two stages'deformations, which are vertical uplifting in the early stage and N-S compression in the latter stage.The first stage structure was caused by the northward subduction of Yangtze Plate. The uplifting of the basement of Foping dome was closely related with the deep magmatism and granite intrusion, which was generated by the subduction. The second stage structure was caused by the collision orogeny between Yangtze Block and Qinling Micro-Plate, which leaded the N-S compression in the island-arc zone, and it superposited on the early structures and formed a series E-W structures. The combination of regional tectonic interpretation and the analysis of principal stress field demonstrate that Foping dome underwent a sub-vertical uplifting period, which was superimposed by the latter tectonic of the N-S compressive.The result of U-Pb zircon dating shows that the main magmatic crystallization is 212±1.5Ma, which is later on the time of main orogenic period. The magma intrusion may be a result of delamination. We also get the dating 182.3±1.1Ma from the migmatites in this region, it reveals a migmatization in this period. The result proves that the structural transformation had occurred in Qinling belt and had been in the extensional tectonic stage. In Yanshania, because of the NE direction compression of Bashan arc thrust structure, Foping area, which sited the most narrow zone of Qinling orogeny, suffered the most intense compression. The Foping dome continued uplifting and was further exhumated to the surface.