Pyrrhotite remagnetizations in the Himalaya: a review

Abstract

AbstractThis paper reviews results on the nature and thermo-tectonic interpretation of widespread pyrrhotite remagnetizations in the Himalaya. Throughout the last two decades a large dataset has been acquired, in particular from low-grade metamorphic rocks of the Tethyan Sedimentary Series (TSS). The nature of this magnetization is a thermoremanence when the peak metamorphic temperature Tmax exceeded the Curie temperature (Tcc. 325 °C) of pyrrhotite; in this case the remanence age can be related to last metamorphic cooling. For Tmax<Tc, the remanence can be of chemical, thermochemical or thermoremanent origin. Cooling ages show a systematic trend of c. 50–20 Ma from the western to the eastern Himalaya. The pyrrhotite remagnetizations post-date main Himalayan folding and record late orogenic long-wavelength rotations and tiltings around vertical and horizontal axes. Remanence directions in the western Himalaya are well matching with large-scale deformations of rotational shortening and oroclinal bending, while in the central and eastern Himalaya they are predominantly controlled by meso-scale effects due to crustal doming. Stable pyrrhotite remanences are especially typical for low-grade marly limestones in the TSS, but were also found in medium-grade rocks of the Lesser Himalaya, highly metamorphic rocks of the Higher Himalayan Crystalline and diorite dykes intruding into the TSS.