At 07:49 AM 16/11/2001 -0500, Eric wrote:
>All,
> Once again a rock term (hornfels) is confused with a facies name. The
>term regional facies is also unnecessarily restrictive, as contact
>metamorphism can also occur at high pressure.
I don't see either type of confusion in Bruce's message. Turner's "facies
of contact metamorphism" and "facies of regional metamorphism" were based
on a premise that later proved to be false. Presuming that the P-T ranges
of contact and regional metamorphism are separated by a finite interval of
P-T, Turner created "two classes" of facies to be distinguished by their
contact or regional setting. Thus, right from the outset, Turner's twofold
facies classification violated Eskola's facies concept. For example:
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"On the basis of petrographic and field criteria most metamorphic facies
fall into one or other of two classes - those of contact and of regional
metamorphism. This twofold division reflects a real difference between two
common environments of metamorphism. The one typically develops locally, at
relatively shallow depths, late in orogeny. The other is regional,
deep-seated, and more or less synchronous with the climax of orogeny. There
is a range of metamorphic temperatures and pressures that spans the gap
between these two environments; and corresponding transitional facies will
ultimately be established when the mineralogy of rocks so metamorphosed has
been more fully described." (Turner&Verhoogen 1960 Igneous and Metamorphic
Petrology 2nd Ed p.509).
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In compiling metapelitic mineral-assemblage data for my paper on bathozones
(1978 AmJ Sci 278, 769-797), I made no distinction between regional
metamorphism and contact metamorphism. This led to no major anomalies in
the bathozonal pattern in the northern Appalachians, but it was noteworthy
that bathozone-3 (Sil+And+St+Ms+Qtz) localities in southern Quebec were in
a well-defined contact aureole, evidently at higher P than
regional-metamorphic bathozone-2 (Grt+And+Bt) localities in Maine and Nova
Scotia and regional-metamorphic bathozone-1 (And+Kfs) localities in SE
Australia. I concluded: "Hence, if the bathograds are roughly isobaric, it
appears that the P-range of contact metamorphism must overlap that of
regional metamorphism, certainly by 1 kbar and possibly by as much as 2
kbar. Even if the bathograds are _not_ isobaric, they show that several of
the same metamorphic _facies_ can occur in both regional and contact
settings, and thus they support the recommendation of Miyashiro
(1973,[Metamorphism and Metamorphic Belts] p. 297) that no general
distinction should be made between the 'facies of regional metamorphism'
and the 'facies of contact metamorphism' (compare Fyfe,Turner&Verhoogen
1958, Turner 1968)."(ibid p.785).
I wrote this with some trepidation, because I had seen Frank Turner go
ballistic after Miyashiro suggested in an invited lecture at Berkeley that
maybe the amphibolite- and granulite-facies P-T conditions overlap one
another! I know he must have read it - he liked bathozones and discussed
them at length in the 1981 edition of his textbook. But he never responded
publicly to Miyashiro's recommendation, nor publicly or privately to my
endorsement of it.
Cheers, Dugald
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