Chamba Syncline: fabric development

Prograde fabric development in metasediments of the Chamba syncline in the NW Himalaya: a neat example of the changes seen with structural depth and increasing metamorphic grade from the first slaty cleavage in the lower chlorite zone to garnet-zone schist with a complex fabric developed from a crenulation cleavage.
First, five large-field images taken at the same scale, to show macroscopic changes in style with grade. Then, details of microstructure in sequence of increasing structural depth and metamorphic grade.
These photos formed the basis for an examination question in 2012. The samples were collected by J.D. Walker (D.Phil 1998).
JDW223b1-crop1  Sample 1 (JDW223b). Lower chlorite zone (lowest-grade sample). Banded silty mudstone from the core of the large-scale syncline. This is from the hinge zone and the slaty cleavage is at right angles to bedding. Note the convex bulges separated by cleavage zones on the coarse base of silty layers, progressively disrupting the original layering. Field of view 12 mm. JDW221-crop  Sample 4 (JDW221). Chlorite zone. A graphitic mica-rich phyllite. Lithological darker-lighter banding (S0) just visible at a small angle to the prominent S1 cleavage. Lying in the cleavage are pale elongate lenses cored by small albite porphyroblasts, presumably formed at a very early stage, before development of the strong phyllitic foliation. Later still, a widely spaced set of kink-like crenulation zones (local S2) has formed at a high angle to the foliation, and is now marked by rusty iron-staining. Field of view 12 mm. JDW180-crop  Sample 6 (JDW180). Upper chlorite zone metapelite. Sedimentary layering (S0) visible at the top of the photo. The obvious planar fabric is at 10-15° to compositional layering. It looks like a spaced fabric. At this scale we cannot see if it is a first cleavage, a crenulation cleavage or whatever. Is it S1 or S2? See later photos. Field of view 12 mm. JDW230-crop1  Sample 7 (JDW230). Biotite zone metapelite. Clearly a spaced cleavage with fairly broad and somewhat lenticular quartz-rich microlithons separated by clearly-defined narrow cleavage zones that concentrate opaque material - a disjunctive cleavage. There are hints that the microlithons contain microfold closures. There is no longer a regular compositional layering. The traces of fabric within microlithons suggest that we must call the dominant fabric S2. Field of view 12 mm. JDW226-1-crop  Sample 8 (JDW226). Garnet zone. No garnet in this specimen but it occurs nearby. A much coarser rock now with well-differentiated mica-rich and quartz-rich bands, both of which clearly show internal microfolding. The individual mineral alignments can be assigned to S1, and the main horizontal fabric is therefore S2, taking the form of an irregularly developed crenulation cleavage. The micas are polygonised around F2 microfolds. At a larger scale in some slides it is evident that a more regular banding of quartz-rich and mica-rich layers was established parallel to S1. Field of view 12 mm.
JDW-223b2 i43   Sample 1. Detail of slaty cleavage crossing silty layer. Field of view 3.5 mm. JDW-223b2 i45   Sample 1. Further detail at lower left of previous photo, shows narrow micas aligned in the slaty cleavage, trains of opaque oxide along the cleavage, and chlorite 'stacks' with their mineral cleavage at right angles to the fabric. Field of view 1 mm. JDW-220 i3   Sample 2 (JDW220). Lower chlorite zone. Metagreywacke. Tectonic fabric defined by chlorite and muscovite between and around clastic quartz and feldspar. Note the quartz grain broken in half (lower centre), the space created by extension now occupied by chlorite and quartz. Also note aligned fibrous sheet silicates in the space opened up by extension between larger clastic grains. Field of view 2 mm. JDW-220 i4   Sample 2 (JDW220). Lower chlorite zone. Metagreywacke. Same as previous. Note here the varied patterns of undulose extinction and subgrains in the quartz clasts. They are unsystematic, and most are of styles that are expected to form at higher temperature than the chlorite zone. So, they are probably inherited from their precursors and unrelated to the new cleavage. Field of view 2 mm. JDW-179 i1   Sample 3 (JDW179). This rock has an almost-continuous fine schistosity but there are still scattered chlorite-mica stacks with mineral cleavage at high angles. Field of view 0.5 mm.
JDW-221 i27   Sample 4 (JDW221). Chlorite zone. Detail of albite micro-porphyroblast in graphitic phyllite. The albite has an inclusion fabric, but it seems unreasonable to propose that this is an earlier tectonic fabric, given that nearby rocks show only one cleavage. We infer that this is a bedding-parallel graphitic lamination. The strain shadows contain relatively coarse graphite-free chlorite, muscovite and quartz. They most likely fill space generated by extension along the fabric. Field of view 0.8 mm. JDW-221 i28   Sample 4 (JDW221). Chlorite zone. Detail of albite micro-porphyroblast, as previous photo. Crossed polars. Field of view 0.8 mm. JDW-223c i51   Sample 5 (JDW223c). Chlorite zone. Metapelite with intense slaty cleavage. Like sample 4, this rock shows a well-spaced asymmetric crenulation, which locally has developed narrow weak cleavage zones marked by iron staining. Field of view 2 mm. JDW-180 i55   Sample 6 (JDW180). Upper chlorite zone metapelite. Now we see that this is a disjunctive spaced cleavage. Interestingly, there appears to be a systematic mineral alignment in the narrow microlithons, at an angle to the cleavage zones. So we should probably infer that the spaced cleavage is a new fabric S2, not seen in the lower grade samples. It is different in style and orientation from the spaced crenulation in samples 4 and 5, so it may not be related to this. Field of view 1 mm. JDW-180 i59   Sample 6 (JDW180). Upper chlorite zone metapelite. Detail of chlorite porphyroclast. It appears rotated, has asymmetric strain shadows, and is slightly wrapped by the S2 fabric. These may represent the last relics of the lowest-grade chlorite stacks. Field of view 0.5 mm.
JDW-230 i63   Sample 7 (JDW230). Biotite zone. Cleavage zones (S2) dominated by muscovite and biotite, microlithons by quartz, plagioclase and biotite. Mica flakes in the microlithons at high angles to the cleavage hint at the former existence of the closures of microfolds. Field of view 2 mm. JDW-226 i76   Sample 8 (JDW226). Garnet zone. Polygonised micas defining microfold closures in microlithons bounded by narrow S2 cleavage zones. Field of view 3 mm.