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6.7 Discussion

The Gumburanjun leucogranite is one of the pure crustal melt granites cropping out along the top of the High Himalayan Crystalline Sequence, in close association with the Zanskar Shear Zone, a segment of the South Tibetan Detachment System. This granite has a very homogenous mineral and major-element composition. Two slightly different types of leucogranites were however observed, essentially on the basis of the presence or absence of biotite. This intrusions is very similar to the other leucogranites of Zanskar which in turn show almost no mineralogical or chemical difference with other Himalayan leucogranites. Marked differences are observed in the trace-element contents of the leucogranites. These differences mainly result from the melting of an inhomogeneous source region composed of metapelites and Cambro-Ordovician orthogneisses.

Mineralogical and textural observation coupled with analytical results concur to indicate that the migmatitic zone forms the source region of the leucogranitic melts and that melting occurred through vapour-absent breakdown of muscovite, without external fluids flushed into the system. A chronological relation can be established between the biotite leucogranites and the tourmaline leucogranites. The intrusion of the former ones seems to precede the intrusion of the latter ones, which are often coarser grained and form pegmatites. It is our strong belief that the biotite leucogranites represent early melts produced in the migmatitic zone and injected in the overlying units through fracture propagation. The crystallisation of these biotite leucogranites liberated volatiles that favoured the production of additional melts of the second type (tourmaline leucogranite). This «in-situ» production of melt is testified by the presence of country-rock xenoliths whose strong depletion in muscovite argues for a restitic nature.

Vapour-absent melting of muscovite in the migmatitic zone was triggered by isothermal decompression. Isothermal decompression resulted from the rapid exhumation of the HHCS along the Zanskar Shear Zone. The production of leucogranitic melts is thus the direct consequence of extensional movements along the ZSZ.

The leucogranites cooled below the closure temperature (725°±25°C) of monazite at 22±0.2 Ma. This age is a close estimate of the «real» age of these granites as they underwent rapid cooling (> 165°C/Ma) and the melting temperature of the protolith was ~770°C.

The reason why leucogranites are abundant in south-east Zanskar but diminish towards the north-west is not clear. The same metamorphic conditions and extensional tectonics do indeed prevail for more than 150 km along the ZSZ. Therefore, we suggest that the main factor influencing the formation of leucogranites lies in the composition of the protolith. To produce leucogranites through decompression melting, a muscovite rich source is needed. Hence metapelites represent a more fertile source than orthogneisses or metapsammites. We could indeed observe that the studied area was rich in metapelitic horizons, but that to the north-west, (Reru-Mune area), the HHCS shows a more psammitic nature, with increasing bodies of orthogneisses, precluding the formation of anatectic melts leaving their source.


Origin of the leucogranites and melt migration Chapter7: The ZSZ: age and amount of shear next

©Pierre Dèzes