Magnetic mineralogy and fabric reliability constraints of Late Triassic Yanzhiba pluton in South Qinling Mountain

The Yanzhiba pluton is one of the typical Late Triassic granite plutons in Qinling Mountain, and lots of geochronological and geochemical work has been done for the Yanzhiba granite pluton in South Qinling Mountain recently. However, opinions of its tectonic settings are contradictory. Some recent studies have placed some new constraints on this problem based on the magnetic fabrics of these granite plutons. These studies have successfully elucidated the correlation between regional tectonics and granite emplacement. However, the reliability of the magnetic fabric method has not been specifically studied. In this paper, the authors present an elaborate magnetic mineralogy analysis, combined with previous petrographical data, to shed light on this problem. Mean susceptibility K_m and corrected anisotropy degree P_J of Yanzhiba pluton are low, which are consistent with data of paramagnetic granite. Hysteresis loops show that susceptibility of most samples originates from paramagnetic minerals, with a small portion of ferrimagnetic contribution, whose concentration, however, is very low. Isothermal remanent magnetization acquisition (IRM) and DC-Field demagnetization curves show that the ferrimagnetic component in pluton is mainly soft magnetic minerals, and χ-T curves confirm that it is magnetite. The hysteresis parameter ratios on Day plot further indicate multi-domain magnetite. The results obtained by the authors prove that the magnetic fabric result from Yanzhiba pluton is reliable and has recorded actual mica fabrics.