Dear Ralph,

some thoughts. First of all, I don't see evidence of cataclasis (brittle deformation) in the photos - maybe you have it elsewhere, but the microstructures in these photos show ductile deformation by creep. Photos 1 and 2 seem almost mylonitic to me (so yes, shearing!) although they don't show strong shape fabrics, but this might be due to the orientation of the thin section (not parallel to stretching lineation?). Note that in my vocabulary, mylonites form dominantly by ductile deformation (there are some signs of brittle deformation in photos 1 and 2 but the key word here is 'dominantly'). The rock may or may not have been a volcanic porphyrite, the deformation fabrics cannot be used as evidence either way as porphyroclasts can form by shearing in originally equigranular rocks.

In any case, the fabrics, in photos 3 and 4, imply that the rocks have been dynamically recrystallised (reorganization of the crystal lattice due to active deformation). The main mechanism here seem to me be the so-called subgrain rotation in the quartz grains, possibly with some so-called bulging recrystallisation in the feldspar grains. Both mechanisms lead to overall grain size reduction. Furthermore, the tapering twins in the lower part of the feldspar in figure 4 certainly could be the result of dynamic recrystallisation of the feldspar. 

It is however unlikely that the temperature was high enough to produce melts, since subgrain rotation in quartz and bulging in feldspars is most efficient at temperatures around 400-500 C, and in any case in higher temperatures the syn- and post-deformation recovery processes would be more likely to erase some or most of the dynamic deformation structures. Likewise, the temperature is not enough to cause grain size increase, which means that the coarser grains were already there when the deformation started (on the contrary, they might and probably have undergone grain size _reduction_). But certainly the temperature was high enough to produce recrystallisation within the feldspars, which might indeed explain the curious twinning pattern in figure 4 (are you sure the lower part of the grain is plagioclase, and not K-feldspar just lacking the cross-hatch twinning?).

If you are not familiar with deformation microstructures but would like to know more, an excellent place to start it the book Microtectonics by Passchier & Trouw (2005).

Hope this helps
All the best
Taija


************************************
Dr Taija Torvela

Lecturer
Structural geology and tectonics
University of Helsinki
Department of Geosciences and Geography
PO Box 64
FI-00014 University of Helsinki
Finland

Tel. +358 9 191 51716
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-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Bottrill, Ralph (DIER)
Sent: 11. heinäkuuta 2011 8:17
To: [log in to unmask]
Subject: cataclasites and albitites

We have been working on the metamorphic rocks hosting the Savage River magnetite deposits, and have found some disagreement about the interpretation of some albite-rich cataclastic rocks in the footwall. The metamorphics are mostly metamafics, quartzites, dolostones/magnesitestones, amphibolites and various muscovite and chlorite schists and skarns.

The albitites have a variety of textures. Some are very fine grained and grains are mostly simply twinned or untwinned, typical of metamorphic albite (e.g.  photo 1). Some have coarse grains in a fine matrix (eg. fig 2), which some have interpreted as porphyritic felsic volcanics, and others as sheared porphyroblasts or disaggregated veins. The immobile elements Ti/Zr etc suggest an origin from sodic alteration of associated pelites and other metasediments.

But there are also some small (<1m wide) cross-cutting coarse zones, interpreted by some as granitoids, which are more Zr-rich - these typically contain some quartz and show myrmekitic textures (fig. 3).
Some of the coarse plagioclase in these bodies shows microcline-like Twinning (Fig. 4), suggested by some to be plagioclase replacing microcline(?) I have wondered if these coarse zones could be be zones of recrystallised cataclasites or zones of partial melting? With remobilised Zr? The rocks locally contain calcic to sodic amphiboles and diopside, with maximum temperatures to about 600-700oC (magnetite-ilmenite).


Has anyone seen similar textures in cataclasites, or have any other thoughts on the origins of these rocks?


regards
Ralph Bottrill
Senior Geologist, Mineralogist and Petrologist Metallic Minerals & Geochemistry Mineral Resources Tasmania PO Box 56, Rosny Park TAS 7018
Phone: 61 3 6233 8359, Fax:  61 3 6233 8338
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