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.
