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GEO-METAMORPHISM  2001

GEO-METAMORPHISM 2001

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Subject:

Re: SCMR and Metam Facies Names

From:

Dugald Carmichael <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Thu, 15 Nov 2001 02:26:15 -0500

Content-Type:

text/plain

Parts/Attachments:

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text/plain (49 lines)

At 05:07 PM 14/11/2001 -0500, Eric wrote:
>Well, the next step is to drop the blueschist facies because blueschists
>sometimes occur in the greenschist facies.

By the same token, glaucophane-schist facies might just as well be dropped.:-)
  But with the understanding that "blueschist" in a facies context implies
"metabasite with abundant glaucophane/crossite", I would argue that
blueschists do NOT occur in greenschist facies. OK, so Turner (1968)
defined a "glaucophane-lawsonite-schist facies" that consigned
Miyashiro&Seki's glaucophane-epidote facies to greenschist facies, but
let's not forget that in 1948 Turner consigned ALL blueschists to the
greenschist facies because he doubted that they represented unusually high
P/T!
  Metabasite equilibria can be modelled reasonably well in the system
NCMASH, with the understanding that FeO and Fe2O3 are important additional
components that displace the P-T curves and expand them into zones of
finite width. What I would call epidote-blueschist facies (= "poor man's
blueschist facies" of Peter Misch and others) is bounded against
greenschist facies by Glc+Ep+Qtz+V=Act+Chl+Ab, against
pumpellyite-blueschist facies by Glc+Ep+V=Pp+Chl+Ab+Qtz, and against
lawsonite-blueschist facies by Ep+Chl+Ab+Qtz+V=Lws+Glc. This would leave
greenschist facies bounded by 6 NCMASH reactions that limit the common
metabasite assemblage Ep+Act+Chl+Ab+Qtz against assemblages with
glaucophane/crossite, pumpellyite, prehnite, wairakite, An25, and hornblende.
 I concede that a metabasite with my epidote-blueschist assemblage
Ep+Glc+Chl+Ab+Qtz might just as well be called a glaucophane greenschist
and the facies could be named accordingly. But consigning these rocks to
greenschist facies would cause confusion by making the common
greenschist-facies-metabasite subassemblage Act+Chl+Ab unstable in a
substantial part of the P-T range of greenschist facies. Alternatively,
these rocks could be assigned to a blueschist-greenschist transitional
facies.
 SCMR's paper#3, Table 1 p.9 lists "characteristic minerals and mineral
assemblages of rocks of basaltic composition in the ten main metamorphic
facies". Characteristic of "glaucophane-schist facies" are "glaucophane,
jadeitic pyroxene, epidote, lawsonite, phengite (no biotite)".
Characteristic of "greenschist facies" are "actinolite-albite-epidote,
chlorite". Neither the mineral assemblage nor any of the minerals
encompasses the bulk composition of average basalt in the NCMASH model
system. The reaction relationships between the facies thus defined are
therefore indeterminate.

Cheers, 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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