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6.3 Petrography

Leucogranites in Zanskar make no exception to the rule and their mineralogical composition is remarkably constant. As for the other Himalayan regions, the leucogranites in the studied area can be grouped in two categories, the tourmaline facies and the biotite facies, depending essentially on the presence or absence of biotite or garnet and the amount of tourmaline and muscovite. The texture of both leucogranitic facies is equigranular.

The tourmaline facies is formed of quartz (30 - 35 %), plagioclase (35 - 40 %), K-feldspar (15 - 25 %), muscovite (~ 5 %) and tourmaline (~ 5 %).

The biotite facies is formed of quartz (30 - 35 %), plagioclase (35 - 40 %), K-feldspar (15 - 25 %), muscovite (10 - 15 %), biotite (5 - 10%) and tourmaline (~1 %).

In general both type of leucogranites are exceptionally fresh in thin sections and sericitisation of the F-feldspar or chloritisation of the biotite is very rarely observed.

  • Garnet was only observed as an accessory phase in the tourmaline facies. This mineral is generally anhedral and sometimes grows as an interstitial phase.

  • Biotite is very rarely observed as an accessory phase in the tourmaline facies, when the two granite types are in close contact.

  • Plagioclase is the most abundant mineral in both facies. It is slightly zoned and optical determinations show a composition range from oligoclase (~ An20) in the core to albite (~ An2) at the rim. This mineral sometimes forms symplectites with K-feldspar.

  • K-feldspar forms among the largest crystals in the leucogranites and contains all the other phases as inclusions. They are thus the last phase to crystallize.

  • Muscovite is always present partly as large flakes, but its abundance decreases from the biotite facies to the tourmaline facies leucogranites. This mineral is evenly distributed throughout the granite and can be found as inclusions within plagioclase, quartz or K-feldspar. In the biotite facies, both phyllosilicates are intimately intergrown without enclosing relationships.

  • Tourmaline shows colour zoning from lavender blue to yellow-brown. The blue colour is generally, but not necessarily, restricted to the core of the crystals. This phase is usually euhedral although it sometimes shows a skeletal habit.

  • Accessory phases in both leucogranitic types are apatite, monazite, ilmenite, zircons beryl and copper mineralisations. In one aplitic sample of the Reru valley, fuchsite was observed as the dominant phyllosilicate.

Pegmatites and aplites belong to the tourmaline facies because they have all the mineralogical characteristics of this category of leucogranites. As the pegmatites and aplites are often undeformed by extensional movements, they are among the last intrusives.

Consequently we interpret the tourmaline facies as intruding chronologically later than the biotite facies and representing a later stage of the magmatic evolution.



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©Pierre Dèzes