Valid HTML 4.01! The Garnet Zone

Garnet first appears within mafic horizons of the Phe Formation in association with amphibole, plagioclase, epidote, quartz, carbonates and titanite. These horizons have a centimetric to decimetric thickness and their contact with the surrounding rocks of the pelitic sequence is gradual. These «metabasic» strata most probably have a volcano-sedimentary origin but might also result from the metamorphic transformation of marls. The amphiboles have an elongated acicular texture and are often grouped in radiated aggregates. Their chemical composition reveals that they belong to the calcic amphibole group and plot in a field between tschermakites and ferrotschermakites. As their AlIV content is greater than 1.0, they correspond to aluminotschermakites (or aluminoferrotschermakites) after the classification of Leake et al. (1997). The garnets are often found as inclusions within the amphiboles and have a composition of alm40-70, grs10-30, sps5-20, prp3-10 and adr3-4. The Plagioclases have a wide range of composition, their percentage of anorthite varying between 40-80%. The absence of chlorite indicates that these rocks have reached temperatures above 550°C and their mineralogical composition is typical of epidote-amphibolite facies.

Garnet then appears in the metapelitic horizons in close contact with the metabasites and finally as a commonly found mineral within the metapsammitic-metapelitic rocks of the Phe formation. The texture of these rocks also changes from phyllites in the biotite zone to micaschists in the garnet zone, showing a mylonitic texture defined by alternating quartz-rich and mica-rich layers. The typical mineral assemblage in these rocks is:

garnet + biotite + chlorite + muscovite + plagioclase + quartz ± epidote ± tourmaline

The garnets have a composition of alm75, grs5, sps5, prp10 and adr5, and the anorthite percentage in the plagioclases varies between 20 and 30%. The stability of Grt + Bt + Chl + Ms + Qtz + H2O is limited by the discontinuous reactions Cld + Bt + H2O = Grt + Chl and Grt + Chl = St + Bt + H2O in the MnKFMASH system. As chloritoid is absent in the studied area, the lower limit of the stability field given above represents the minimal conditions at which garnet might appear. However, in our samples garnet most probably appears because of the continuous reaction Chl + Ms + Qtz = Grt + Bt + H2O

The aspect of the biotites changes as the metamorphic conditions inside of the garnet zone increase. In the pelitic rocks close to the mafic horizons, biotite still preserves the same poikiloblastic aspect as in the biotite zone. With increasing metamorphic grade, deformation becomes more intense and the biotites, together with muscovite, are regrouped as flakes into beds that define the main foliation (S4).

The garnets are often heavily clouded, such as to sometimes appear nearly opaque in plane polarized light. Some of the garnets display an atoll texture with biotite and muscovite grains in their core. Most of the garnets do however contain inclusion trails of quartz and opaque minerals. These inclusion trails either form straight lines or helicitic structures within the garnet poikiloblasts. The garnets with straight inclusion trails are restricted to the upper structural part of the garnet zone, whereas the garnets with sigmoidal inclusion trails appear in the lower structural part of the garnet zone. Depending on their structural position, the garnets are thus either post-tectonic or syn-tectonic with respect to D3. All the garnets are then moulded by a later external mylonitic foliation (S4) and show asymmetrical strain shadows.

The study of the deformation structures in the mylonitic rocks from the garnet zone is very interesting as it reveals that these rocks have undergone two major phases of ductile deformation showing opposite sense of shear. The oldest phase of deformation is preserved as inclusion trails within the poikiloblastic garnet grains, and shear sense criteria for this phase are given by the syn-tectonic «spiral-Si» garnets (Fig 5.12). These spirals of inclusion show a rotation of up to 180° and systematically indicate a top to the S-SW sense of shear. This feature reveals unequivocally that the growth of these garnets is contemporaneous with ductile, top to the S-SW, shearing associated with the burial of the HHCS below the TH (D3). In the matrix, the shear sense criteria associated with this early tectonic event are totally obliterated by a later ductile deformation which shows an opposite, top to the NE, sense of shear. Shear sense criteria related to this second event are given by asymmetrical strain shadows (sigma-type tails) around the garnet porphyroblasts (Fig 5.13) and the development in the matrix of a C/S fabrics followed by later C'-type shear bands. This intense late deformation is clearly caused by the top to the NE extensional movement along the Zanskar Shear Zone (D4).


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©Pierre Dèzes