Characteristics of Magnetic Fabric of Wenquan Granite Pluton in the Western Qinling Mountains and Implications for Emplacement Mechanism

Abstract

AbstractWenquan pluton is situated in the western Qinling mountains and is adjacent to the south of the Shangdan suture zone. Through analyses of regional tectonics and the anisotropy of low‐field magnetic susceptibility (AMS) together with rock magnetism of 125 oriented samples, the emplacement mechanism of the pluton is studied. The values of most mean susceptibilities (km) show a wide variation range. Thermomagnetic curves demonstrate that the paramagnetic minerals such as biotite, acting as the main magnetic carriers, made dominant contributions to the magnetic susceptibility of the samples with low values, also including a small contribution to remanence from small amount of magnetite. The magnetite, however, acted as the dominant magnetic carrier of the samples with high magnetic susceptibility values. The corrected anisotropy degree (PJ ) less than 1.2 can be regarded as flow magnetic fabrics based on microscopic observations. The shape parameters (T) of most of magnetic susceptibility ellipsoid display that the Wenquan pluton is dominated by oblate compressional fabric, and the magnetic foliations are dominant over magnetic lineations. In addition, it is well marked that most magnetic foliations of the samples on the border seem parallel to the edge of the granite pluton, which have relatively steep inclinations, and interior ones appear relatively disordered besides magnetic lineations. Hence it can be revealed that the magnetic fabrics of the pluton are mainly formed by the lateral compression during the emplacement. Though the magnetic fabrics show N‐NEE and SW directed compression as a whole, the compression caused by the closure of the Shangdan suture zone during the Indo‐China period became much weaker than that in the main orogenic stage. Consequently it can be concluded that the weak compression setting can be reflected by the characteristics of magnetic fabrics when the pluton emplaced, and furthermore the relatively sustained and even extensional environment was probable and understandable, which was consistent with the regional tectonic setting in the extension stage of the emergence of delamination when the Qinling orogenic belt evolved to post‐collision stage.