Dugald and all,
The two type localities of Turner for the amphibolite-granulite
transition are the NW Adirondacks and Broken Hill. Based on our
observations in the NW Adirondacks, the amphibolite - granulite transition
in metamafites appears to be prograde with pyroxenes generally rimming the
hornblendes. David Jenkins recently had a student reexamining this
transition. At Broken Hill, Neil Phillip's observations are that it is a
retrograde transition. In both localities the evidence for involvement of
melting in mafic rocks at the transition is not strong, although there are
plenty of migmatites in adjacent and even lower grade metasediments. Of
course, the mafic rocks may have lower a(H2O) or may have been dried
somewhat by migration into adjacent melt.
"At 10:54 AM 07/11/2001 +0100, Christian wrote:
>> You seem to exclude the formation of granulites without fusion??
>It is theoretically possible, but I have not seen it in the field, nor have
>I been persuaded by reading the literature. Bob Newton's case for
>infiltration of CO2 as a means of suppressing a(H2O) was hotly disputed by
>John Valley and others.
>> There are huge proterozoic terranes with basic magmatic complex in
>>granulitic conditions; In SW of Madagascar, the parageneses are 1 or 2prxs,
>>Pl, with or without Q, Brown Hb, Grt in a space of a few hundred square
>>meters. All the parageneses of the transition zones of the 3facies:
>>amphibolite, HP and IP granulite facies in the domain of the multivariant
>>(because of the variations of MG/Fe , Na/Ca ratios, asiO2, aH2O and/or
>>CO2?) subsolidus reactions Cpx+Grt+Q+V = Hb +Pl, Cpx+Grt+Q = Opx +Pl,
>>Cpx+Ox+Pl+V = Hb, The magmatic textures are sometimes preserved and there
>>no melting of these rocks.
>?? How can preservation of magmatic textures be reconciled with inferred
>absence of magma?
>> Moreover, at first time, I have speak about the transition in the
>>direction HP granulite to amphibolite facies.
>Yes this echoes my own experience. For 30 years I have been wanting to
>study a prograde transition from amphibolite to granulite facies, but none
>is to be found! In every case the granulite-facies domain appears to have
>been formerly more extensive and the amphibolite facies has encroached upon
>it during cooling. The retrograde character of this transition is easily
>explained if a small amount of unquenchable silicate liquid was the last
>phase to finish crystallizing during cooling, because the reaction by which
>it crystallized would consume anhydrous minerals and produce hydrous
>minerals. Corresponding to each of the V-consuming retrograde reactions you
>listed is a vapor-absent L-consuming reaction (e.g. Grt + Cpx + liquid = Hb
>+ Pl + Qtz). It is known from experimental studies that the V-consuming
>reactions are metastable in respect to the L-consuming reactions unless
>a(H2O) has been suppressed to less than ~0.5 by infiltration of gaseous
>species other than H2O from a hypothetical external source.
> By contrast, in the H2O-vapor-present domain on the low-grade side of the
>migmagrad it is common to find prograde textures across a reaction isograd,
>suggesting that H2O vapour can readily escape from medium-grade rocks as
>fast as it is produced.
Professor of Geology
Department of Geological Sciences
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425 E. University Ave.
University of Michigan
Ann Arbor MI 48109-1063 USA