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GEO-METAMORPHISM  2001

GEO-METAMORPHISM 2001

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Subject:

Re: amphibolite, granulite & tonalite (sl)

From:

Dugald Carmichael <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Thu, 8 Nov 2001 12:40:17 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (44 lines)

At 10:54 AM 07/11/2001 +0100, Christian wrote:
>...This transition [amphibolite to granulite facies] does not necessarily
>imply melting. Even if its often true
>in Archean provinces. There are cases where the transition amphibolite to
>granulite (and reversely) is reached by subsolidus transformations, without
>melting.
>...

True in theory, but where has such a case been persuasively documented? In
my experience (mostly Proterozoic) the migmagrad is always at distinctly
lower grade, and the transition itself always shows evidence (at least
locally) of retrograde regrowth of hornblende and/or biotite. This is what
we should expect if anatectic liquid, being unquenchable, is obliged to
crystallize across a range of T during cooling.

 In order to get a subsolidus transition from amphibolite to granulite
facies at P>~3 kbar, the activity of H2O must be suppressed to <~0.5. Bob
Newton postulated that this can be done by somehow pumping gaseous CO2 into
deep-seated rocks on a regional scale. But in all cases where the adjoining
lower-grade rocks are migmatitic (and I know of no other cases), it seems
more plausible to postulate that the transition from amphibolite to
granulite facies coincides with generation of H2O-undersaturated silicate
melt by means of vapor-absent (or CO2-rich vapor-present) melting reactions
such as Hb + Pl + Qtz = Grt + Cpx + tonalitic liquid and Bt + Pl + Qtz =
Grt + Opx + granodioritic liquid. At ~7 kbar these reactions internally
define and buffer a(H2O) at ~0.5 (Percival 1983 Am Mineral 68, 667-686). A
vapor phase may be present if gaseous species other than H2O are present,
but its composition must be adjusted by equilibration with the liquid and
hydrous solid phases so that its mol fraction of species other than H2O is
~0.5.

 A soon as the rocks begin to cool, reversal of these reactions would tend
to destroy the Grt+Px assemblages. It is therefore necessary to postulate
that a substantial part of the H2O-undersaturated liquid escapes by
intrusion to higher levels in the crust, so that it is not available for
back-reaction. Only thus can assemblages diagnostic of granulite facies be
preserved in domains that are flanked by lower-grade migmatites.

Salut, Dugald
Dugald M Carmichael Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]

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