5.4 The retrograde metamorphism M3
The third tectonometamorphic event that affected the studied area is related to the exhumation of the HHCS along the Zanskar Shear Zone. During this event, the HHCS has undergone a decompression metamorphism which is marked by the growth of new metamorphic assemblages. Structural analyses reveals that the growth of these minerals is either syn-tectonic or post-tectonic with respect to extensional movements along the ZSZ. Most of these retrograde minerals were observed in the pelitic rocks of the kyanite zone and in the migmatitic zone. The order of appearance of these minerals in the kyanite zone is: sillimanite > cordierite > K-Feldspar > andalusite > margarite.
Retrograde metamorphism starts with the apparition of sillimanite. In the kyanite zone this mineral occurs as fibrolite. This mineral is contained within the main foliation associated with extensional shearing and is oriented parallel to the main NE-SW direction of shear. Together with the apparition of fibrolite one can observe the breakdown of kyanite which reacts with quartz to produce muscovite. Kyanite grains, entirely rimmed by muscovite, are frequently observed within coarse grained samples associated with quartz veins. Fibrolite was never observed to grow directly from kyanite, or to replace it, but always to grow at the expense of micas.
In the migmatitic zone the breakdown of kyanite must have been complete as no relicts of this mineral were found. The sillimanite is here coarser grained than in the kyanite zone and forms tiny prismatic crystals. These sillimanite grains are often overgrown by quartz grains. Muscovite also grows in the rocks as a late retrograde phase.
5.4.2 Cordierite (Fig 5.24)
Cordierite grains were only found in the kyanite zone. This mineral usually appears at the contact between metapelites and quartz veins where it sometimes forms decimetric nodules. Individual grains of centimetric size were observed. These are light green coloured due to their partial alteration into pinnite. Typical pleochroic halos can be observed in cordierite porphyroblasts around small monazite or zircon grains. Cordierite was also found to grow at the expense of kyanite grains within metapelitic schists. These cordierite-bearing metapelites are rich in light coloured biotite which testifies to a Mg-rich composition. Staurolite is systematically absent in samples where cordierite is present, which also indicates that the presence of cordierite (or staurolite) is controlled by the chemical composition of the metapelites. From an extensive review of the literature, Newton (1983) considers that cordierite is usually not stable at pressures greater than 6 kbar in most common metapelites.
5.4.3 K-feldspar (Fig 5.25)
In kyanite grade metapelites, K-feldspar forms porphyroblasts sometimes overgrowing kyanite and sillimanite. The growth of K-Feldspar is syn-tectonic with respect to ductile normal shearing along the ZSZ. Fibrolite is found as inclusions within feldspar grains.
Large K-Feldspar crystals were also observed in pegmatitic dikes probably associated with a late pneumatolitic stage of the leucogranitic intrusions. These dikes cross-cut the kyanite zone at the base of the ZSZ without being affected by ductile extensional tectonics,. In these rocks, the size of individual K-Feldspar crystals can reach up to 10 centimetres. These K-Feldspar sometimes overgrow decimetric kyanite crystals. The significance of these kyanite-bearing pegmatites is not clear.
5.4.4 Andalusite (Fig 5.26)
No previous description of the occurrence of andalusite in Zanskar could be found in the literature. This mineral seems to occur rather rarely at the top of the HHCS as it has previously only been described in the Everest region (Lombardo et al., 1993; Pognante and Benna, 1993). In the studied area, andalusite forms pink euhedral crystals of centimetric size at the contact between quartz veins and metapelitic rocks. The occasional presence of relict kyanite grains within the andalusite porphyroblasts testifies to the polymorphic transition of andalusite after kyanite. In these coarse grained samples andalusite is associated with muscovite, chlorite, plagioclase, quartz and K-feldspar. In some cases, euhedral staurolite grains are preserved as inclusions within andalusite porphyroblasts. The epitaxic growth of andalusite over kyanite is also sometimes observed within the metapelitic schists. In kyanite-bearing pegmatites andalusite grows at the contact between K-feldspar and quartz in late replacement of kyanite. Andalusite is not affected by extensional tectonics and did thus crystallize once ductile normal shearing along the ZSZ had ceased.
Margarite was found as a late retrograde metamorphic mineral growing at the expense of andalusite. This mineral was only found in the coarse-grained lenses of metamorphic minerals forming at the contact with quartz veins.
5.4.6 Other minerals
Retrograde metamorphic minerals are also present within calcsilicates and metabasic rocks of the kyanite and migmatitic zones.
In the calcsilicate rocks of the kyanite zone, the retrograde transformation of diopside into tremolite or chlorite is frequently observed.
In the metabasic granulites of the sillimanite zone, retrograde hornblende grows at the contact between garnets and clinopyroxenes. The growth of hornblende according to the hydration reaction: Grt + Cpx + Qtz + Pl (Na-rich) > Hbl + Pl (Ca-rich), is indicative of a pressure drop at constant temperature.