Detailed analysis of successive pTRMs carried by pyrrhotite in Himalayan metacarbonates: an example from Hidden Valley, Central Nepal

Abstract

Summary Low‐grade metacarbonates from the Tethyan Himalaya were sampled for palaeomagnetic studies in Hidden Valley (Central Nepal). The remanence is carried by pyrrhotite, evidenced by thermomagnetic runs of susceptibility (Hopkinson peak at ∼300 °C), alternating field demagnetization, isothermal remanent magnetization acquisition and subsequent thermal demagnetization. The palaeomagnetic directions reflect a Tertiary overprint after the main folding event, probably synchronous with the metamorphism. Normal and reverse remanence directions were separated and vary with altitude. It is also possible to retrieve several antiparallel components versus temperature during thermal demagnetization of a single sample. At higher altitudes (4920–5500 m), the first component recorded is reverse (R1). At a lower temperature a normal component can be extracted (N1). For sites sampled at lower altitudes (4700–4900 m), the high‐temperature reverse component disappears but a medium‐temperature reverse component (R2) demagnetized in a narrow temperature range can be identified in between two normal components (N1 at high temperature and N2 at low temperature). At the lowest altitudes (4450–4700 m), only a normal component (N2) appears. The occurrence of successive normal and reverse polarities in one sample is interpreted as the record of successive reversals of the geomagnetic field during the post‐metamorphic Tertiary cooling of the studied area. The polarity versus altitude function is a powerful argument for a thermomagnetic origin of the magnetization. No obvious rotations around a vertical axis with respect to the stable Indian plate are evidenced for the Tertiary. However, the inclination is not consistent with the expected inclination. Main Central Thrust ramping can be invoked to explain our observations. R1, N1 and N2 inclinations are slightly different and their tendency is consistent with tilting towards the north during magnetization acquisition. The minimum total amount of such tilting is around 25°. Accurate geochronological data from the Tethyan Himalaya would be of a great help for better resolution.

Remagnetization of carbonate rocks in southern Tibet: Perspectives from rock magnetic and petrographic investigations

“…Besides the widespread secondary remanence residing in pyrrhotite [e.g., Crouzet et al ., ; Appel et al ., ], we have detected pervasive remagnetization residing in authigenic magnetite in unmetamorphosed carbonate rocks of the TH. Paleogeographic reconstructions of the Tibetan Himalaya and India‐Asia collision could be seriously flawed when remagnetization of carbonate rocks is not identified.…”

Temperature and age constraints on the metamorphism of the Tethyan Himalaya in Central Nepal: A multidisciplinary approach

“…Therefore the 150% thickening associated to D2 could explain the observed temperature. Whatever, palaeomagnetic investigations performed in the Dolpo-Manang area (Appel et al, 1991;Crouzet et al, 2001aCrouzet et al, , 2003Schill et al, 2003) have evidenced a post-deformation remanence acquisition carried by pyrrhotite. This remanence is acquired during the cooling just below 320°C (Rochette, 1987;Dunlop et al, 2000).…”

“…For the other areas, the existence of a major thrust carrying material over the studied areas can be assumed. The existence of such a thrust was proposed by Guillot (1993) and Guillot et al (1995) in order to justify the barometric results in the contact aureole of the Manaslu granite and more recently by Crouzet et al (2001a) in order to explain paleotemperatures higher than 325°C in Triassic metasediments of the Hidden Valley area deduced from palaeomagnetic investigations. Except the Nar Tö thrust, carrying the Ratna nappe over the Triassic of the TH in Himlung Himal area Guillot, 1998, 1999), no other geological surveys have ever evidenced such a thrust in northern Nepal.…”

“…Synthesis of the metamorphic temperature estimations from the sampled areas using different approaches. The temperature estimation with paleomagnetism are deduced from the thermal origin of the natural remanent magnetization demonstrated by Appel et al (1991) in Manang and by Crouzet et al (2001a) in Hidden valley. that the Manang area underwent milder thermal overprint, because complete Miocene reset was not observed.…”

Magnetomineralogical changes along the Kohistan-Karakoram collision zone inNorth Pakistan: implications for variable thermochemical activities

“…Pyrrhotite has been used as an index mineral in metamorphic belts to delineate isograds (Crouzet et al, 2001;Horng et al, 2012). Crouzet et al (2001) reported a pyrrhotite-held postmetamorphic partial thermoremanent magnetization in metamorphic rocks acquired during postorogenic uplift.…”