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GEO-METAMORPHISM  2003

GEO-METAMORPHISM 2003

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Subject:

Re: calc-silicates and skarns

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Metamorphic Studies Group <[log in to unmask]>

Date:

Fri, 3 Jan 2003 11:15:56 +0000

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The nomenclature problem we have here is the classic one of confusing
descriptive names with genetic useages, compounded by an overlap between
mineral deposit and metamorphic communities that have gone their own
separate ways using the same names for different rocks and different nmames
for the same rocks! The resulting diversity of opinion and useage hardly
helps convey useful information, and really is not acceptable.

There are sedimentary rocks that range from pure carbonates through
mixtures of carbonate with mud or sand to clastic sediments with minor
carbonates. The problem is that during metamorphism the large volume
changes associated with decarbonation readily trigger metasomatism through
transient enhanced permeability, so that many of the metamorphic
equivalents of these sediments display at least some degree of
metasomatism. For example, cm thick pure limestone is common in some
sediment sequences; in their amphibolite facies equivalents you find cm
thick calc silicate beds (and thick marble units often have a cm or so of
calc silicate at their margins). Up to the upper greenschist facies, many
carbonate sediments, especially those without dolomite, probably survive
relatively unmetasomatised, but the amphibolite facies has much more
decarbonation and metasomatism. There, equivalents of the different
calcareous sediments are marble, probably not much metasomatised, calc
schists with micas plus calc silicate minerals +/- carbonate, which may
well have undergone some metasomatic addition of silica, calc silicate
rocks, generally without free carbonate because metasomatic addition of
silica has occurred, and schists with minor Ca silicate phases.

Notwithstanding its original use, nowadays skarn generally refers to a
blatantly metasomatic rock, as Greg Dipple has pointed out, and it is also
normally associated with infiltration metasomatism rather than diffusion
metasomatism (or comparable local exchange), although the name is used in
both contexts. This is a lot of subjective judgement to be making before
you can name something! Also, not all skarns are predominantly calc
silicate, and in some literature, as you can see from Stefan's message,
skarn can mean an association of rock types that includes skarns. Confused?
I suspect that skarn is the traditional term of choice of the ore deposit
community, while metamorphic petrologists mostly deal with more mildly
modified calc-silicates in the Goldschmidt tradition. There certainly isn't
any coherent scientific logic in what we do, and the historical precedents
predate modern understanding of these rocks and fail to address what we see
now as an important distinction between rocks simply dominated by calc
silicate minerals, and thoroughgoing metasomatic rocks. To insist on the
term skarn for all rocks rich in Ca-silicate minerals and all rocks
associated with them, devalues the term totally.

 Personally, since we now recognise that most calc silicate rocks are to a
greater or lesser degree metasomatic in origin, I would prefer NOT to use
the term skarn for cases, such as the interfaces between marble and schist,
where calc silicate layers develop as a result of very local exchanges, let
alone for weakly or unmetasomatised calcareous metasediment. There is too
much diversity in calc silicate dominated rocks to lump them all together
as skarn. Skarn should imply large scale mass transfer and a small number
of phases in the rock, as Greg proposes. So I guess this means that I think
that skarn has effectively become a genetic term in much modern useage, and
if we agree that this is reasonable, it would help if we could avoid using
it for description. Instead, it could be used as a qualifier where the
metasomatic criteria were clearly met as in "calc-silicate skarn",
"wollastonite skarn". Is that possible and reasonable?

Happy New Year to you all!
Bruce




--------------------------------------------
Professor Bruce Yardley
School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK

PLEASE NOTE NEW TELEPHONE & FAX Nos:

tel: 0113 343  5227
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