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5.2 The prograde regional metamorphism M1

Two different generations of biotite occur on each side of the Shingo-La. On the southern side of this pass and above de Kade orthogneiss, the metasedimentary series of the Phe Formation are characterized by the presence of tiny (< 0,1 mm) biotite flakes. These biotite are very light coloured and their presence in hand samples can only be guessed from the slightly brownish colour of the metapsammites (Fig 5.1). The biotite flakes are closely associated with chlorite and muscovite and mark an early schistosity S1. This first schistosity corresponds to the axial plane cleavage of tight isoclinal folds P1. The sedimentary bedding S0 is still well preserved although tightly folded by the P1 folds. The main schistosity S1 is crenulated by a late deformation (D3). The biotites, muscovites and chlorite flakes are bent by this late deformation. The Kade orthogneiss below these metasedimentary rocks is also affected by a metamorphism of biotite zone grade (Vannay, 1993). Towards the top of the Shingo-la the metamorphic grade however decreases progressively and from Chumik-Nakpo up to the pass, biotite is absent and S1 is only marked by chlorite and muscovite (Fig 5.2).

On the northern side of the Shingo-La, the metamorphic grade again increases from the top of the pass towards the Lakang camping ground at the bottom of the Kurgiakh valley. On this side of the pass, the biotite zone is characterized by the presence of poikiloblastic biotite grains within the phylitic horizons of the Phe formation (Fig 5.3). These rocks have a typical grey-green lustrous aspect and are spotted with the biotite poikiloblasts varying in size from one to five millimetres. These rocks still preserve a compositional layering which is interpreted as the sedimentary bedding S0 because of its asymmetrical internal nature (Passchier and Trouw, 1996). A first schistosity S1 defined by white micas, and to a lesser extend chlorite, is sub-parallel to S0 and is preserved within the microlithons associated with P2 folds. The cleavage domains associated with these P2 folds correspond to S2 and define the main schistosity in this area. The growth of the biotite poikiloblasts is contemporaneous with the development of S2, because an early stage of crenulation of S1, less tight than at present is preserved as inclusion trails of quartz and opaque minerals within the biotite grains. D2 and thus the growth of these biotites is contemporaneous with the crustal thickening associated with the emplacement of the Nyimaling Tsarap nappe. Such poikiloblastic biotites have also been described by Fuchs and Linner (1995) in the Kenlung Serai unit south of Sarchu.

The above-mentioned observations indicate that the biotites on the southern side of the Shingo-La belong to an earlier deformation (D1) than those on the northern side of the pass which are clearly contemporaneous to D2. The origin of D1 in this region is not clear: either it is associated with an Eohimalayan event or, more likely, to an early Himalayan event preceding the southwestward thrusting of the Nyimaling-Tsarap nappe. Such an early Himalayan event was evidenced by several authors (Steck et al., 1993; Vannay and Steck, 1995; Epard et al., 1995; Wyss et al. 1998) in the Lahul area south of Zanskar. These authors attribute D1 to the NE thrusting of a nappe stack which they call the Shikar Beh Nappe. Their arguments for this nappe are mainly based on the structural analyses of the NE vergent Tandi syncline and on the distribution of the regional metamorphism (Fig 5.4). The metamorphic grade reaches amphibolite facies in the Chandra valley (kyanite-staurolite zone near Khoksar) and gently decreases towards the north, such as to reach lower greenschist facies (chlorite zone) north of Darcha. From Darcha towards the north, the metamorphic grade increases again, to reach amphibolite facies (kyanite-staurolite zone) in the Sarchu region. As this metamorphic zonation affects sedimentary formations belonging approximately to a same structural level, they argue that the overburden required to reach amphibolite facies in the Chandra valley cannot be related to the SW thrusting of the Nyimaling-Tsarap nappe because the two high-grade metamorphic domain (Chandra to the south and Sarchu to the north) are separated by a low-grade domains (Darcha), and that the metamorphism in the southern domain can thus only be explained by the thrusting from the SW towards the NE of an earlier nappe (i.e. Shikar Beh). On the basis of these interpretations, we propose that the northwards decreasing metamorphic gradient associated with D1 observed when crossing the Shingo La from south to north is part of the general northwards decreasing trend of the southern (Chandra) metamorphic domain. The biotite zone south of the Shingo-La and our S1 are thus related to crustal thickening associated with the Shikar Beh Nappe. The biotite poikiloblasts overgrowing S1 on the northern side of the pass belong to the northern metamorphic domain and are related to the thrusting of the Nyimaling Tsarap Nappe from NE towards the SW.


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©Pierre Dèzes