Magnetic fabric studies of Wenquan granite pluton in western Qinling Mountains and implications for emplacement mechanism

Wenquan pluton is situated in western Qinling Mountains and is adjacent to the south of 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 (k(m)) 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, 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 (P-j) of less than 1. 2 can be regarded as flow magnetic fabrics based on microscopic observations. The shape parameters (T) of most 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 the interior magnetic lineations and foliations appear relatively disordered. Hence it can be inferred 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 compression as a whole, the compression caused by the closure of Shangdan suture zone in Indosinian became much weaker than the one in main orogeny 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 extension environment can be probable and understandable, which was consistent with the regional tectonic setting in the extension stage of the emergence of delamination when the Qin ling orogenic belt evolved to post-collision stage.