THE GANGOTRI GRANITE (GARHWAL HIMALAYA) - LACCOLITHIC EMPLACEMENT IN AN EXTENDING COLLISIONAL BELT

The Gangotri Miocene leucogranite is composed of several laccoliths (6 -7 km long, 1.5-2 lan thick), which can be divided into two sets of le nses: the southern lenses, intruded in the lower part of the High Hima laya sedimentary cover, and the northern lenses, intruded in an older porphyritic biotite-bearing granite, In both cases, the magmatic fabri c is commonly weak with no dominant stretching direction, although a r ough E-W trend is present in the northern lenses. The magnetic fabric is characterized by the fact that drastic changes in the direction, in the magnitude of the magnetic parameters, or in the fabric type (plan ar or linear) may take place over very short distances. Measurements o f preferred orientations in thin sections indicate that the strain reg ime was largely dominated by a coaxial component for both sets of lens es, in agreement with the large dispersion observed in both the field and magnetic lineations. The comparison of the granite and host rock s tructures shows that the leucogranite emplacement dates the onset of t he extensional tectonism in the High Himalaya range and is not related to a southward directed thrust event associated with the Main Central Thrust, This is exemplified by the vertical attitude of the feeder di kes that intrude the metasedimentary rocks beneath the southern lenses . The presence of these dikes indicates in turn that magma ascent occu rred by fracture propagation. The spatial disposition of the southern lenses could have resulted either from the disruption of a single lacc olithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the Tibetan Slab or from the intrusion of ind ependent laccoliths. The use of the elastic bending theory of Pollard and Johnson (1973) shows that in both hypotheses, the current laccolit h sizes are compatible with a laccolithic mode of magma emplacement. H owever, neither the density contrast between the magma and its enclosi ng rocks nor the lithological boundary between the Tibetan Slab and th e overlying Tibetan metasedimentary series controlled the level of mag ma emplacement. Rather, flat-lying collapse structures, which intersec ted the upward propagating magma dikes, are the most likely causes of magma arrest. Such a mechanism was favored by the schist-rich litholog y of the metasedimentary host rocks. In addition, field relationships indicate that the melt supply through the dike system was a continuous , rather than pulsed, process. Existing numerical treatments on the ra tes of magma transport through fractures show that in such a case, the laccoliths could have been built in less than 100 years. This short t ime of emplacement, the small size of the laccoliths, and their periph eral disposition relative to the Badrinath granite suggest that the Ga ngotri lenses may represent the initial stage of pluton accretion in t he High Himalaya which ultimately gave rise to a much larger massif su ch as the Manaslu granite.