Field relations, petrogenesis and emplacement of the Bhagirathi leucogranite, Garhwal Himalaya

Abstract

The Bhagirathi leucogranite forms a series of low-angle en echelon, lensoidal intrusions at the top of the High Himalayan slab in the central Himalaya of Garhwal, northern India. The leucogranite comprises the assemblage: K-feldspar + quartz + plagioclase + tourmaline + muscovite ± biotite ± garnet. Compared to other High Himalayan leucogranites it is particularly rich in tourmaline. The granite is generally compositionally homogeneous although it is magmatically banded in both the upper and lower portions. The Bhagirathi leucogranite is situated structurally above the kyanite and sillimanite gneisses of the Vaikrita Group which, in turn, overlie the north-dipping Main Central Thrust zone of inverted metamorphic isograds. A pegmatite — aplite leucogranite sill and dyke swarm is present around the margins of the leucogranite. Vaikrita Group gneisses below the leucogranite contain a pronounced northeasterly component to generally randomly orientated mineral stretching lineations. This reflects localized reorientation of early, coaxially constrained, mineral growth by later, non-coaxial deformation. Various shear criteria in the gneisses immediately below the granite document the existence of a major zone of ductile NNE-SSW-directed extension across a northeastward dipping, low-angle normal fault zone. The top of the 1500–1800 m thick leucogranite sheet exposed on the peaks of Bhagirathi, Shivling and Thalay Sagar are intrusive into Martoli Formation metasediments of the Tethyan sequence which locally contain andalusite and staurolite. The roof complex shows numerous stoped blocks, xenoliths and elongate rafts of host rock in the top 100 m together with an extensive zone of layer-parallel leucogranite veining. The northward-dipping Bhagirathi leucogranite was intruded syn-tectonically during the ductile to brittle transition and has been deformed into linked, en echelon bodies within the extensional shear zone at the interface between the Vaikrita Group gneisses and the Tethyan sedimentary cover. The long axes of the leucogranite lenses lie parallel to the y-z plane of the finite strain ellipsoid for this extensional duplex. During extension, sub-orthogonal dilatational forces exceeded sub-horizontal shear stresses thus facilitating the repeated emplacement of sheeted granite melt, a process analogous to low-angle tension gash development. The final emplacement level in the crust must have been ultimately controlled by the density contrast between melt and country rocks, the thermal blanketing of the Tethyan sedimentary cover, and the extensional stress field along the top of the High Himalayan slab.

Channel flow and ductile extrusion of the high Himalayan slab-the Kangchenjunga–Darjeeling profile, Sikkim Himalaya☆

“…4), shows several reflectors, which have been successfully matched to the major faults of the Himalaya to the south, notably the STD and the Main Himalayan Thrust (MHT; Hauck et al, 1998;Alsdorf et al, 1998). It has been suggested that the 'bright spots' imaged on the INDEPTH profile beneath south Tibet are leucogranites forming today, at a similar structural horizon and depth to the High Himalayan leucogranites which crystallised during the Miocene and were extruded southward to their present position along the High Himalaya (Searle et al, 1993(Searle et al, , 1997Searle, 1999a,b). It has further been suggested that ages of crustal melt leucogranites in the footwall of the STD decrease to the north (Wu et al, 1998) and that the Greater Himalayan crystalline rocks bounded by the STD above and the MCT below have been effectively extruded southwards from beneath the middle crust of south Tibet (Grujic et al, 1996(Grujic et al, , 2002Searle, 1999a).…”

“…leucogranite sills (Gansser, 1983;personal observations 2001). These leucogranites all occur in the footwall of the STD (Burg et al, 1984;Searle et al, 1993Searle et al, , 1997, so the ages of the leucogranites constrain the maximum age of brittle faulting along the STD.…”

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

“…These results support the view that the magma migrated upward with limited, if any, chemical or thermal modifications. There is field evidence that the HHL magmas moved upward through narrow dykes (Le Fort, 1981;Scaillet et al, 1996;Walker et al, 1999), some of which connect to the base of the pluton, such as in the Garwhal Himalaya (Searle et al, 1993;Scaillet et al, 1995a). Harris (1998), together with the P-T conditions inferred for the top and bottom contact aureoles of Manaslu (Guillot et al, 1995a).…”

Crustal structure, restoration and evolution of the Greater Himalaya in Nepal-South Tibet: implications for channel flow and ductile extrusion of the middle crust

“…The 'bright spots' imaged on the INDEPTH profile beneath south Tibet have been interpreted by some as leucogranitic melts forming today, at a similar structural horizon and depth to the High Himalayan leucogranites which crystallized during the Miocene and were extruded southward to their present position along the High Himalaya (Searle et al 1993(Searle et al , 1997Searle 1999a, b;Gaillard et al 2004). It has further been suggested that the ages of crustal melt leucogranites in the footwall of the STD decrease to the north (Wu et al 1998) and that the Greater Himalayan crystalline rocks, bounded by the STD above and the MCT below, have been effectively extruded southwards from beneath the middle crust of south Tibet (Grujic et al 1996(Grujic et al , 2002Searle 1999a;Searle & Szulc 2005).…”