18.104.22.168 The Kyanite Zone
Kyanite often grows as elongated tabular crystals at the contact between quartz veins and pelitic horizons of the Phe formation. Such association between kyanite and quartz veins is frequently observed also in other metamorphic terranes like the Alps and is of great help to detect the kyanite isograd. Locally kyanite forms decimetric lenses in association with staurolite, quartz and plagioclase. These lenses are wrapped by biotite-rich layers. Kyanite also occurs in pegmatitic rocks in association with quartz and plagioclase where both kyanite and plagioclase sometimes form 10 centimetre long individual crystals. Kyanite however also occurs as millimetric grains disseminated within the metapelitic horizons of the Phe Formation. The metapelitic rocks of the structurally upper part of the kyanite zone are characterized by the mineral assemblage:
kyanite ± staurolite + biotite + muscovite + plagioclase + quartz
In these rocks garnet is only preserved as inclusions inside staurolite or kyanite grains and was otherwise completely consumed by the reaction: Grt + Chl = St + Bt. Chlorite is also absent from these rocks, most likely because it was consumed to form kyanite by the reaction Chl + St = Bt + Ky. This reaction stopped once all the chlorite was consumed, which explains the presence of staurolite in association with kyanite. Staurolite is sometimes found as inclusions within kyanite grains testifying of prograde metamorphic conditions (Fig 5.15). Both Kyanite and staurolite form poikiloblasts overgrowing S3 and moulded by S4.
Lower in the kyanite zone and as metamorphic conditions increase, the metasedimentary rocks of the Phe Fm start to acquire a gneissic texture and are characterized by the following mineral assemblage:
kyanite + garnet + biotite + muscovite + plagioclase + quartz
Staurolite is only preserved as relic inclusions within garnet poikiloblasts (up to 1 cm), which indicates that staurolite was consumed and that a second generation of garnet was formed by the reaction: St = Grt + Bt + Ky + H2O (staurolite-out isograd). The garnets also contain quartz and opaque inclusion trails which often form beautiful helicitic spirals showing a rotation angle of more than 300°. As for the earlier mentioned syntectonic garnet porphyroblasts of the garnet zone, the sigmoidal inclusion trails in the present garnets do also indicate that they grew during a ductile top to the SW shearing episode associated with the underthrusting of the HHCS below the TH (D3). The garnet and kyanite poikiloblasts are moulded by the external foliation. Shear sense criteria in the matrix (winged porphyroblasts, C/S and C'-type shear bands) systematically show top to the NE movements associated with ductile extensional shearing along the ZSZ (D4).
The lowermost part of the kyanite zone is characterized by incipient migmatisation in the paragneisses. In these rocks the mineral assemblage is: kyanite + K-Feldspar + garnet + biotite + muscovite + plagioclase + quartz.
The apparition of K-feldspar and incipient melting of these rocks could still be related to prograde metamorphism with the reaction Ms + Pl + Qtz = Kf + kyanite + melt, but as these rocks also show late metamorphic growth of sillimanite (Fig 5.16) associated with extensional shearing (D4), it is more likely that incipient migmatisation and the growth of K-feldspar (and sillimanite) are the consequence of a later retrograde decompression metamorphism and are due to the reaction Ms + Pl + Qtz = Kf + Sill + H2O. Retrograde metamorphism will be discussed later in more details.
Directly below these migmatitic gneisses is the contact with the roof of large leucogranitic intrusion, and the lowermost kyanite zone paragneisses themselves are intruded by several generations of leucogranitic dikes escaping from the top of these magmatic bodies.
A metric to decametric layer of calc-silicate rocks quite systematically occurs close to the top of the leucogranitic intrusions. These calc-silicate are banded and are formed by garnet or vesuvianite rich layers alternating with quartz rich layers. The garnets and the vesuvianite form large crystals, often of pluricentimetric size, overgrowing millimetric diopside and forsterite grains. Titanite, carbonates and sheelite are also present in these rocks. These impure marbles are boudinaged by extensional shearing along the Zanskar Shear Zone.