Mineralogy, Geochemistry and Palaeomagnetism of Mafic Dykes from Kumaun Lesser Himalaya: Implication on Petrogenesis, Tectonic Setting and Timing of Mafic Magmatism in Northern Part of Indian Lithosphere

Abstract

Occurrence of mafic dykes in Himalaya has been intriguing and debated since long because of its difficulty to ascent and emplacement through a thickened crust. Mafic dykes in Kumaun Lesser Himalaya (KLH) of central Indian Himalaya occur abundantly in three major tectonic segments intruding the Krol-Tal Formation of Nainital region (ND) between Main Boundary Thrust (MBT) and South Almora Thrust (SAT), Almora crystalline in Khunt region (AD) between SAT and North Almora Thrust (NAT) and Mandhali-Deoban Formations of Pithoragarh region (PD) between NAT and Main Central Thrust (MCT). Mineralogical, geochemical and palaeomagnetic investigations of dykes (ND, AD, PD) from KLH have been carried out in order to recognize sources, magmatic processes and timing of dyke magmatism, and further to understand extensional regime prevailing in northern part of Indian lithosphere. Modally PD and ND correspond to leucogabbro to gabbro whereas AD represent olivine gabbro to gabbro. Plagioclase in PD (An10-An65), AD (An12-An58) and ND (An17-An61) is normal zoned. Olivine (Fo61 to Fo33) in AD corresponds to crysolite to mostly hortonolite type of olivine. Clinopyroxene varies from Wo36 En36 Fs10 to Wo46 En51 Fs27 in PD, Wo40 En36 Fs15 to Wo46 En42 Fs23 in AD and Wo35 En32 Fs14 to Wo44 En45 Fs25 in ND, which exhibit distinct evolutionary trends along Di-Hd join crystallizing in tholeiitic to mildly alkaline magmas within a range of T=1050o-1200oC at P<3kbar.