5.3.2 Metamorphism in the Paleozoic-Mesozoic (Tethyan) series
Low grade metamorphic conditions characterize most of the Tethys Himalaya sedimentary formations forming the Zanskar synclinorium. Several studies (Berthelsen, 1953; Garzanti and Brignoli, 1989; Spring, 1993; Girard, 1998; Steck et al. 1993 and 1998) however showed that the metamorphic grade increases from diagenetic conditions in the upper structural units forming the central part of the Zanskar synclinorium to reach lower amphibolite facies conditions in the lower structural units both towards the Nyimaling - Tso-Morari area to the north and in the Zanskar-Lahul-Spiti region to the south. Towards the southern rim of this synclinorium, several tectonic units have been individualized. These units are from north to south: the Kharnag, Zara and Marang-La Units (Steck et al., 1993), the Zumlung Unit (Baud et al., 1982), the Zangla Unit (Baud et al., 1982), the Chumik Unit (Spring and Crespo-Blanc, 1992) and finally the Kenlung-Serai Unit (Steck et al., 1993). Each of these units is bounded by thrust planes sometimes reactivated as normal faults and corresponds to a different structural level of the Nyimaling-Tsarap Nappe. The Kharnag Unit with its non-metamorphic grade at Lun (Steck et al., 1993) represents the highest exposed structural level, and the Padum-Kenlung-Serai Unit with its Lower greenschist to lower amphibolite facies metamorphism (Garzanti and Brignoli, 1989 and Spring, 1993) represents the lowest structural unit of this nappe. In the studied area, only the equivalents of the base of the Zangla Unit, part of the Chumik Unit and the Kenlung-Serai Unit are exposed.
The sedimentary rocks forming the Tethys Himalaya in the investigated area are generally only weakly metamorphosed. Macroscopic and microscopic investigations of both the Paleozoic or Mesozoic formations in most of the studied area rarely reveal the presence of metamorphic minerals. The only rocks which systematically yield reliable microscopic information on the metamorphic grade of the Tethys Himalaya are the effusive basaltic Panjal Traps and the mafic dikes intruding the underlying Paleozoic formations. The location of the mafic samples is shown on (Fig 5.5). The magmatic texture of these rocks is generally well preserved and some relict primary augite crystals are still present in some samples. Actinote-tremolite and chlorite are however the most common mafic minerals now occurring in these rocks. The shape of the magmatic plagioclases is still recognizable although they have all been replaced by low-grade neoformed minerals. The vesicles in the Panjal traps are filled with an association of carbonates, euhedral epidote and acicular actinote-tremolite crystals. The metamorphic mineral assemblage observed in all the mafic rocks between the Phirtse-La and Purne is systematically:
albite + actinote-tremolite + chlorite + epidote + carbonates
X-ray powder diffraction analyses of eight mafic samples confirmed this assemblage and did not reveal the presence of any supplementary mineral phase. Such a mineral assemblage is typical of the lower greenschist facies or epizone metamorphic conditions.
Only one sample collected east of Chumik-Marpo in the Chumik Unit seems to indicate lower metamorphic grade as it does only contain albite + chlorite + epidote + carbonates ± quartz ± hematite. This confirms the anchizone grade of the Chumik Unit in this area as proposed by Spring (1993).
Two other mafic samples collected in the Kenlung Serai Unit at the footwall of the Sarchu Fault, one southwest of Chumik Marpo and the other at the entrance of the Kamirup valley, are characterized by the presence of metamorphic hornblende. The presence of this mineral is indicative of lower amphibolite grade. This observation is in agreement with the interpretation of Steck et al.(1993) and Spring (1993) that the metamorphic grade south of the Sarchu Fault reaches lower amphibolite facies between Chumik Marpo and Sarchu.
In order to try to detect a possible variation of the metamorphic gradient within the Tethys Himalaya, 17 samples were selected for X-ray powder diffraction analyses on clay minerals and determination of illite «crystallinity». These samples were taken either from the dolomitic Thidsi Member of the Karsha Formation or from the micritic horizons of the Lipak Formation and one sample comes from the Hanse Formation. The extraction and analytical method follows that of Holtzapffel (1985) and of Moore and Reynolds (1989) for carbonaceous rocks and is similar to the one used by Spring (1993) in the adjacent region of Sarchu.
Only 14 samples proved to be suitable for the determination of the illite «crystallinity» index (I.C.) which is obtained by measuring in mm. the width at half-height of the illite 10Å reflection on both air dried and glycolated slides (Kübler, 1968).
The results are presented in (Fig 5.5) and show that the I.C. values of all samples except one are comprised in a range between 1.5 and 2.0. Such values are again characteristic of epizone metamorphism or lower greenschist facies and do not show any significant variation of the metamorphic grade in the studied area. The only sample that shows a greater «crystallinity» value comes from the Hanse Formation and shows an I.C. of 2.7 indicative of upper anchizone metamorphism.
These results are in agreement with those presented by Garzanti and Brignoli (1989) for the same area. On the Basis of X-ray powder diffraction, illite and chlorite «crystallinity» and petrographic observations, these authors also obtained lower epizone values for the Cambrian to Permian sedimentary formations (Phugtal Unit) and upper anchizone values for the Triassic Formations (Zangla Unit).
Three of the seventeen selected samples did not contain illite but revealed the presence of talc and chlorite as the dominant phyllosilicates. These three samples all come from the Kenlung Serai Unit and were taken at the base of the Cambrian Karsha Formation, at the footwall of the Sarchu Fault. A close microscopic examination of these dolomitic samples also revealed the presence of talc flakes. After Bucher and Frey (1994), talc is stable in dolomites at temperatures lower than 500° when the metamorphism is of orogenic type. This observation is again in acceptance with the interpretation of Spring (1993) that the Kenlung Serai unit, which forms the footwall of the Sarchu Fault, partly reached lower amphibolite facies.
The Kamirup valley is also the only sector of the whole investigated area where we have found biotite and garnets in the pelitic horizons of the Karsha Formation (Fig 5.6and Fig 5.7). Both minerals do however only occur at the bottom of the cliffs on both sides of the valley. In these samples, a fine-scale sedimentary layering is still preserved. The sedimentary layers are cut at low-angle by an oblique slaty cleavage marked by the alignment of tiny muscovite and chlorite grains. The biotite and garnet porphyroblasts are randomly oriented and grew statically over the cleavage. The fine-grained groundmass of these rocks consists of chlorite, muscovite, quartz and some tourmaline. The metamorphic gradient then diminishes upwards, for 300 meters above the floor of the valley biotite disappears and only chlorite and muscovite are present. The illites of the Lipak Formation at the top of the mountain between the Kamirup river and Chumik Marpo have an I.C. of 0.18 indicating epizone conditions. This testifies to an upward decrease of the metamorphic gradient within the Kenlung Serai unit from lower amphibolite facies at its base (Karsha Fm.) towards lower greenschist facies at its top (Lipak Fm.).
Our metamorphic study of the Tethys Himalaya also reveals a northwestward decrease of the metamorphic gradient within the Padum-Kenlung Serai Unit from the Kamirup region towards the Tanze area. The observation that the lowermost part of the Kenlung-Serai Unit reaches lower amphibolite facies between Sarchu and Chumik Marpo whereas the same stratigraphic levels exposed along the Kurgiakh river are only of lower greenschist facies is interpreted as resulting from a diminution of the overburden between these two areas. This is supported by the fact that, as described in chapter 2, the thickness of the Paleozoic formations diminishes considerably towards the northwest.
The throw of the Sarchu Fault also seems to diminish considerably towards the northwest. From Sarchu to Chumik, there is indeed a marked metamorphic contrast between the mesozonal Kenlung-Serai Unit forming the footwall of this fault and the anchizonal Chumik Unit forming its hanging wall (Spring, 1993). Such a contrast in metamorphic grade between these two units was not observed more towards the northwest, as our study shows that all the sedimentary formations below the Zangla Unit are of epizone grade along the Kurgiakh river. Moreover, the Sarchu Fault itself disappears somewhere between Thaple and Jinshen. We thus believe that the Sarchu Fault is a rotational fault and that the Kenlung-Serai and the Chumik Unit merge into a single unit, in the region of Thaple. This unit corresponds partly to the Phugtal Unit as defined by Baud et al. (1984) but only for the area NW of Thaple. The prolongation of this unit towards the Lahul as drawn by Baud et al. (1984) is indeed erroneous as was later shown by Steck et al. (1993) and Vannay (1993). In this region, there is indeed no tectonic contact between this unit and the HHCS. Furthermore, the Phugtal Unit is not an individual nappe as proposed by Baud et al. (1984). In this study, we will define the Phugtal Zone as representing the sedimentary series (Upper Precambrian to Upper Permian) comprised between the HHCS and the Zangla Unit.