Dear all,
Many thanks for your replies and some nice images. I very much appreciate that you took time to contribute to the conversation.
My question was motivated by some rocks we found just near Adelaide within the 500 Ma Delamerian Orogen, which forms part of the Ross-Delamerian system that developed on the eastern margin of Gondwana. In the Adelaide region, Delamerian-aged metamorphism is high geothermal gradient with prograde andalusite-staurolite and cordierite-bearing biotite schists. Increasing metamorphic grade culminates in migmatite with loose proximity to variably deformed granites and diorites. However within pretty nondescript migmatitic meta psammites that contain biotite-sillimanite (after andalusite) are occasional layers that contain cordierite-orthopyroxene-spinel-plagioclase-biotite. The opx occurs in felsic segregations that overprint the foliation (attached). There is nothing particularly unusual about the mineral compositions; Opx has 2-3wt% Al2O3, spinel less than 0.2wt% ZnO so it doesn't appear that the assemblage is being stabilised by an exotic bulk composition. It may be that the metamorphic grade is higher than people have previously appreciated, and that the generally quartzofeldspathic bulk compositions of the rocks produce unremarkable mineral assemblages. But it also occurred to me that perhaps the opx was an example of lower than normal-T or even sub-solidus growth.
Martin Hand
Department of Earth Sciences
University of Adelaide
Phone : +61 (0)8 831 36794
Phone : +61 (0)8 831 35324
Mobile: +61 (0)459 807942
http://www.adelaide.edu.au/geothermal/
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-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Oliver, Nick
Sent: Sunday, 11 December 2016 8:07 PM
To: [log in to unmask]
Subject: Re: [geo-metamorphism] Subsolidus prograde opx in metapelite
Hi all and Hi Martin (I missed you at the Adelaide AESC); I've seen three mini-charnockite style things like you're chasing (I think), but along with some of the theme of the feedback so far, not necessarily biotite-out, rather amphibole-out. This supports Dan Harlov's comments, and it seems the ultra low P is not needed. These outcrops could be placed together despite their geographical separation. East of Albany W Australia (not far, can find details); Fletcher Creek rock holes in Kimberley W Australia (some publications of mine) and some truly spectacular coastal exposures SW of Helsinki Finland at Kopparnas (Paul Bons may have published?). Each of these has one- or two-amphibole amphibolites (some? biotite) locally converted to (typically) 1 x 5cm 'slugs' of opx in aligned trails completely discordant to the amphibole-defined foliation. After some thought in each case, I inferred these were the dregs of the passage of low-aH2O fluids, probably ultramafic melts.
In all cases 3 to 4.5 kbar, 750C +
Cheers
Nick Oliver
HolcombeCoughlinOliverValenta
HCOVGLOBAL.COM, and
Adjunct Professor of Economic Geology, James Cook University
> On 11 Dec 2016, at 7:17 pm, "Daniel Harlov" <[log in to unmask]> wrote:
>
> Hi All,
>
> I guess this is the tail end of the discussion but Dave's email below got me interested enough to at least make a reply. While it can form as a result of partial melting, ultimately the formation and stability of Opx is a function of the H2O activity - specifically a low H2O activity - and the whole rock geochemistry. P and T are secondary considerations What this means is that it can form not only as a result of partial melting but also as result of solid state dehydration of biotite and amphiboles to Opx and Cpx due to the infiltration of low H2O activity fluids such CO2-rich fluids (on a localised scale) or concentrated NaCl-KCl brines on a more regional scale - sometimes at relatively low temperatures (and subsequently low Al contents).
>
> This is a topic which I and my co-authors explored in the attached reprint, i.e.
>
> Harlov, D.E., Johansson, L., Van Den Kerkhof, A. & Förster, H.-J., 2006. The role of advective fluid flow and diffusion during localized, solid-state dehydration: Söndrum Stenhuggeriet, Halmstad, SW Sweden. Journal of Petrology, 47, 3-33.
>
> Here abundant Opx forms as a result of sold state state dehydration due to CO2 infiltration at temperatures somewhere between 650 and 700 °C and 800 MPa. We also provide a fairly detailed bibliography on previous studies of such localised dehydration zones up to 2006. No partial melts in this case.
>
> Tschau,
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> Dan
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> On 08 Dec 2016, at 23:08, Dave Pattison <[log in to unmask]<mailto:[log in to unmask]>> wrote:
>
> Not sure of prograde met'm of pelite (would have to be super-low P,
> below ~1 kbar as noted), but the incipient charnockites of S India and
> Sri Lanka are interpreted as fracture/shear-controlled conduits of
> low-aH2O fluid infiltration that resulted in reaction of Bt, maybe Hbl
> too, to Opx without melting. Think Cliff Todd and Bernard Evans had
> some examples in Alaska (early 1990's paper?) of something similar,
> due to marble devolatilizing adjacent to Hbl or Bt-bearing gneiss. For
> pelite, I agree with Bob Tracy that looking for mafic dykes or sills
> emplaced into low grade metapelites would be the way to go. Dave
>
> From: Metamorphic Studies Group
> [mailto:[log in to unmask]] On Behalf Of Martin Hand
> Sent: Wednesday, December 07, 2016 1:03 PM
> To:
> [log in to unmask]<mailto:[log in to unmask]
> >
> Subject: [geo-metamorphism] Subsolidus prograde opx in metapelite
>
> Dear all,
>
> Has anyone come across or know of examples where opx has grown during prograde metamorphism of metapelite at subsolidus conditions?
>
>
> Martin Hand
> Department of Earth Sciences
> University of Adelaide
> Phone : +61 (0)8 831 35324
> http://www.adelaide.edu.au/geothermal/
> http://www.adelaide.edu.au/directory/martin.hand
>
> Daniel Harlov
> Section 4.3 Chemistry and Physics of Earth Materials Helmholtz-Zentrum
> Potsdam Deutsches GeoForschungsZentrum - GFZ Stiftung des öff. Rechts
> Land Brandenburg Telegrafenberg
> D-14473 Potsdam
> FR Germany
>
> international tel +49 (331) 288-1456
> international fax +49 (331) 288-1402
> email: [log in to unmask]<mailto:[log in to unmask]>
>
> http://www.gfz-potsdam.de/en/staff/daniel-harlov/
>
> http://link.springer.com/book/10.1007%2F978-3-642-28394-9
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> <Harlov et al. (2006) J Petrol 47, 3.pdf>
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