If so, could we start thinking of a "keratophyre" due to sea-floor 
alteration?

Kees Linthout
IGBA, VU Free University Amsterdam

Op 16/07/2011 14:06, Bruce Yardley schreef:
> Just a thought, but are these textures a form of chess-board albite, which is a low-T metasomatic texture well known in the literature of sandstone diagenesis?
>
> Bruce
>
> Professor Bruce Yardley
> School of Earth and Environment
> University of Leeds
> Leeds LS2 9JT, UK
>
> Tel: +44 (0)113 3435227
> Fax: +44 (0)113 3435259
> -----Original Message-----
> From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Bottrill, Ralph (DIER)
> Sent: 16 July 2011 10:55
> To: [log in to unmask]
> Subject: Re: cataclasites and albitites
>
> Taija, it does look odd but there is definitely no microcline or Kspar in these rocks, we have done a number of XRF and Xrd analyses and some probe analyses, and there are muscovite schists in the area that are K-rich but these albitites are all very low in K (0.05-0.26% K2O) and Rb, though these microcline-like grains can make up 10% of the rocks. That is why I think they are metasomatites rather than igneous rocks, but I have not encountered anything similar. Interestingly, the ones with the highest white mica contents contain<0.1% K2O, so it may be paragonite. Some with a few% biotite contain higher K2O(~0.2%). Ron Vernon in his reply said that similar twinning can form during deformation.
> There is another distinct albite-rich suite that has more micas, and appears to grade into the schists and meta-arenites, but the purer albities do not match anything else in the area.
> regards
> Ralph Bottrill
> Mineral Resources Tasmania.
> 03 6233835
> ________________________________________
> From: Metamorphic Studies Group [[log in to unmask]] On Behalf Of Taija Torvela [[log in to unmask]]
> Sent: Friday, 15 July 2011 7:02 PM
> To: [log in to unmask]
> Subject: Re: cataclasites and albitites
>
> Ralph,
> deformation (without melting) and recrystallisation does indeed not produce cross-hatch twinning. So, either the twinning is preserved even after the removal of K, (which in my opinion is unlikely), or the mineral is in fact microcline. As you know, all micas contain K as a main element, so their presence indicates that there is at least some K in the rock. But if the amount of mica and the microcline is volumetrically very small and these minerals occur only very locally, maybe the whole-rock geochemistry might still show very low K2O (what is used analysis method and its error margin?) - especially if the analysed samples are not exactly the same as the samples from which the thin sections were made? But if the microcline and the micas occur throughout the rock, I'm afraid I have to suggest that there might be some problem with the geochemical analysis? In any case, at least to me, it seems unusual that a felsic rock like this should contain virtually no potassium.
> Best,
> Taija
>
> -----Original Message-----
> From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Bottrill, Ralph (DIER)
> Sent: 15. heinäkuuta 2011 11:00
> To: [log in to unmask]
> Subject: Re: cataclasites and albitites
>
> Hi Taija
> It does seem odd that it could be so well preserved, which is why I wondered if it was an artefact of deformation and recrystallisation. There are some traces of altered biotite and fine white mica, but it's very minor.
>
> 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
> Email: [log in to unmask]  or: [log in to unmask]
>
>
>
> -----Original Message-----
> From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Taija Torvela
> Sent: Friday, 15 July 2011 17:33
> To: [log in to unmask]
> Subject: Re: cataclasites and albitites
>
> Ralph,
> very curious indeed. I have seen some cross-hatching in plg before, but not this much. I guess if the temperature increased during deformation, the potassium in the K-feldspar can be remobilized and be replaced by Na, but I have never seen the original cross-hatching preserved in such conditions.
>
> Are there any micas in the rocks?
> Taija
>
> -----Original Message-----
> From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Bottrill, Ralph (DIER)
> Sent: 15. heinäkuuta 2011 9:22
> To: [log in to unmask]
> Subject: Re: cataclasites and albitites
>
> Taija
> I meant to add, there is practically no K-spar in these rocks, even though in hand specimen and thin section you would swear it was there - K2O is usually<0.2%.
>
> 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
> Email: [log in to unmask]  or: [log in to unmask]
>
>
>
> -----Original Message-----
> From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Taija Torvela
> Sent: Wednesday, 13 July 2011 19:05
> To: [log in to unmask]
> Subject: Re: cataclasites and albitites
>
> 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
> [log in to unmask]
>
>
> -----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
> Email: [log in to unmask]  or: [log in to unmask]
>
>
>
>
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