Everest Series metamorphic cycle

Illustrating the metamorphic evolution of the lower part of the Everest Series (a.k.a. North Col Formation) in the footwall of the South Tibetan Detachment System, from samples collected by LR Wager on the 1933 British Everest expedition. Relationships between at least three fabric-forming deformation episodes and the successive growth of the index minerals garnet, staurolite, sillimanite and andalusite define a clockwise P-T path, with peak conditions in the sillimanite-muscovite zone at about 630°C, 5 kbar.
ME-149A ps  Overview of fabric in Sil-Grt-St schist ME-149, field of view about 10mm. A main foliation defined by micas wraps around porphyroblasts of garnet and staurolite which contain discordant internal inclusion trails. The main fabric can therefore be designated S2. Garnet and staurolite porphyroblasts grew over an earlier schistosity S1. ME-148 ps1  Sil-Grt-St schist ME-148, field of view about 10mm. The S2 schistosity is locally folded during an event we can designate D3. Micas are polygonised around the folds, but there is no new fabric parallel to axial surfaces. ME-148 ps1  ME-148, crossed polars, field of view about 7mm. The samples contain syn-metamorphic quartz stringers, aligned along S2. They now form lenticular boudins with recrystallised margins, indicating that they were relatively competent during the D3 deformation. ME-148 ps1  ME-148, plane polarised light, field of view about 6mm. This view clearly shows garnet with an anhedral outline, probably somewhat resorbed since its original growth, containing an inclusion fabric almost at right angles to the matrix schistosity that wraps it. ME-149 GB  ME-149, plane polarised light, field of view 4.5mm. The inclusion pattern in this staurolite poikiloblast shows lath-shaped areas free of small quartz inclusions, showing where micas were overgrown by the staurolite. In this case, the S1 fabric is sub-parallel to the wrapping external S2 schistosity.
ME-148 ps2  ME-148, plane polarised light, field of view about 2.5mm. This staurolite poikiloblast shows very clearly a discordant internal inclusion fabric S1 defined by quartz and opaque minerals, at right angles to the external wrapping S2. ME-148 ps2  ME-148, crossed polars, field of view about 2.5mm. The same staurolite poikiloblast shows in this crossed-polars view a muscovite-rich selvage at top and bottom, and the beginnings of replacement by muscovite in the strain shadow to the lower right. ME-148 ps2  ME-148, plane polarised light, field of view 2.5mm. This staurolite poikiloblast, in contrast, has a microfolded internal inclusion fabric. This indicates that the S1 fabric was in the process of being crenulated during staurolite growth, and suggests that the S2 fabric originated as a crenulation cleavage while the rocks were in the stability field of staurolite. Curved inclusion trails are not seen in garnet. ME-148 ps2  ME-148, crossed polars, field of view 2.5mm. This view shows the folded S1 trails well, and reveals a little muscovite (bright colours) invading the staurolite rim. ME-148 ps2  ME-148, plane polarised light, field of view 7mm. This swirling aggregate is composed of fibrolitic sillimanite intergrown with biotite. This is a typical habit for fibrolite, and does not necessarily imply there has been complex microfolding within the intergrowth. The main schistosity more-or-less wraps around the aggregate. When sillimanite appears, some other Al-rich mineral normally has to be consumed, and it is not uncommon for this to involve replacement of staurolite (55% Al2O3) by muscovite (35% Al2O3), as already observed. Calculated phase diagrams indicate that sillimanite would grow in this assemblage at 610°C, 5kbar (equivalent to 18km depth).
ME-149B ts  ME-149, plane polarised light, field of view 5mm. This staurolite has a more strongly developed muscovite corona, and in this case the muscovite flakes are aligned and drawn out into the extenal S2 schistosity with a curvature that indicates a dextral sense of shear. We must be careful here: the inclusion evidence for the origin of S2 implied the formation of a crenulation cleavage, i.e. a shortening along the original S1 fabric without any particular shear sense. It is quite possible that S2 has a complex history, forming initially as a spaced crenulation cleavage but becoming reactivated and intensified later during simple shear deformation. S2 would therefore be a composite fabric. This is borne out by observations elsewhere in the Everst Series. ME-149B ts  ME-149, crossed polars, field of view 5mm. This view highlights the muscovite corona around the staurolite poikiloblast and its curved transition into the external schistosity. Staurolite replacement accompanying sillimanite growth would be occuring at the peak temperature achieved by the rock (about 630°C), and so the shear deformation may reflect the beginning of exhumation of the footwall of the South Tibetan Detachment zone. ME-148 ps1  ME-148, plane polarised light, field of view 7mm. This collection of fractured staurolite porphyroblasts appears to have been pulled apart by extension along the S2 matrix foliation, and the space filled by quartz. This is likely to be coeval with the intensification of S2 and may have occurred at the time of emplacement of the S2-parallel quartz stringers. ME-148 ps1  ME-148, crossed polars, field of view 7mm. This collection of fractured staurolite porphyroblasts appears to have been pulled apart by extension along the S2 matrix foliation, and the space filled by quartz. This is likely to be coeval with the intensification of S2 and may have occurred at the time of emplacement of the S2-parallel quartz stringers. ME-148 ps1  ME-148, plane polarised light, field of view 8mm. This view shows staurolite poikiloblasts and a nodular mass of sillimanite + biotite acting as rigid objects during the D3 microfolding, which has crumpled and polygonised the mica fabric around them. We conclude that D3 occurred after the highest-grade mineral assemblage had been established, and after at least some of the shear deformation affecting S2, but before temperatures fell to the level where polygonisation of micas could not occur.
ME-148 ps2  ME-148, plane polarised light, field of view 1mm. Small andalusite crystals can be found scattered through these samples. They are anhedral but undeformed, and generally replace micas. This small grain is statically replacing biotite and muscovite in the matrix S2 foliation. ME-148 ps2  ME-148, crossed polars, field of view 1mm. Small andalusite crystals can be found scattered through these samples. They are anhedral but undeformed, and generally replace micas. This small grain is statically replacing biotite and muscovite in the matrix S2 foliation. ME-149C ts  ME-149, plane polarised light, field of view about 2mm. Here two andalusite grains (indicated in red) grow within the muscovite corona in the strain shadow around a staurolite porphyroblast. This and other evidence indicates that andalusite is the last new metamorphic mineral to grow, at a low-pressure stage on the cooling path of the rocks. Chemical evidence indicates that the last Fe-Mg exchange equilibrium, perhaps coeval with andalusite growth and post-D3 fabric annealing, occurred at 550°C. For andalusite to appear here, the pressure would have to be about 2.5 kbar.