As Eric pointed out, the term skarn is purely descriptive, with no
genetic implication(s), just like "calc-silicate rock". It barely
describes a calc-silicate mineral assemblage w/hedenbergite, grandite,
wollastonite, vesuvianite, scapolite, etc. (in Mg-skarns with
forsterite, diopside, etc.) and a whole plethora of hydrated
alteration products (amphiboles, epidote, micas, other
phyllosilicates, etc.). It can form from any protolith(s) that can
provide Ca, Si, Fe, Mg, Al, etc., be that intrusive-carbonate
contacts, marl, altered basalt, impure carbonates, etc. which went
through high-temperature metamorphism, irrespective of the heat source
(magmatic, burial, regional metamorphism, etc.). The confusion made
very often is between “skarn” and “skarn deposits”. One should be
aware that many skarns are actually barren. In most cases when
mineralization is present it is actually post-skarn, mainly related to
overlapping, lower-temperature (than skarn) hydrothermal events. Thus,
depending on the scale of the system considered, no large scale
metasomatism, or mass-transfer is required to get skarn, although the
classical view was/is that skarn is formed only in contact-
metamorphic/metasomatic environments. One should bear in mind that the
region where from the term was introduced into the geological
literature more than 125 years ago – Bergslagen, Sweden – has skarns
formed in a regional metamorphic environment, with no traceable
magmatic source. It is obvious that the question why should one
use “skarn” instead of “calc-silicate rock” is on everybody’s mind.
Besides my life-long involvement w/skarns, I personally prefer “skarn”
for its brevity and because I suspect it has priority. I confess I
don’t know who coined “calc-silicate rock” or when did this happen,
but I guess it was after Törnebohm introduced the Swedish mining
term “skarn” (meaning "refuse" or "trash" in old Swedish, i.e.,
useless to ore miners) to the geological community in 1875.
To comment on Pietro's message, the term skarnoid is used in regional
metamorphic environments to describe Fe-poor calc-silicate assemblages
in relatively fine-grained rocks. However, as skarn is already used
only in descriptive sense, the use of skarnoid seems redundant.
As pointed out in my first message to Marcello, more details on
skarns, terminology, geochemistry, genesis, etc. is available in Larry
Meinert's 1992 paper "Skarns and skarn deposits", Geoscience
Canada, v. 19(4), p. 145-162. Lots of information is also available on
Larry's web site: http://www.wsu.edu/~meinert/skarnHP.html. An older
overview paper on the subject: Einaudi, M.T., Meinert, L.D., and
Newberry, R.J., 1981, Skarn deposits: Econ. Geol., 75th Anniv. Vol.,
p. 317-391.
As for the timing of carbonate (calcite or dolomite) formation in
skarns and its abundance (see Eric's earlier message on the subject),
it is true that carbonates do often occur as subordinate, late phases,
in many cases in veins cross-cutting previous assemblages. However,
there are numerous skarns in which marble is abundant and by no means
late: euhedral calc-silicates are nested in dominant marble,
suggesting they formed at the expense of the carbonate, at least syn-
if not post-carbonate recrystallization.
So much about this. I wish you all a blessed holiday season and a
great New Year, with even more exciting and challenging discussions on
the list(s)!
Stefan
Quoting Eric Essene <[log in to unmask]>:
> All,
> I do not think that the word skarn has genetic significance,
> it
> just means a rock dominated by Ca-rich minerals silicates as garnet
> and diopside. There are many skarns in the Grenville terrane and
> their origin is often unclear. Whether or not a metasomatic event
> occurred before or during regional metamorphism is unclear. The
> Canadians also map rocks called pyroxenites or clinopyroxenites,
> which are cpx-dominated rocks of obvious metasedimentary origin but
> of less obvious contact metasomatic associations in many cases.
> Why
> must one understand the origin of such rocks before naming them?
> Just use the name and if one can, try to understand their origins.
> We invest far too much time into knowing origins before we give
> names. It is much easier if the names simply represent what is.
> Of
> course, there may be transitions between marbles and skarns,
> although
> the boundaries in the field are often sharp.
> eric
>
>
>
> >Dear all,
> >maybe you could use the term skarnoid, indicating a rock
> containing
> >calc-silicates such as wollastonite, piroxene
> >(diopside-hedenbergite), garnet (grossularite-andradite), epidote,
> >scapolite. I think the term *skarnoid* does not have a genetic
> >significance, but it is a general term indicating calc-silicate
> >rocks similar to skarns but without implying necessarily a
> >metasomatic process.
> >
> >
> >----- Original Message -----
> >From: <mailto:[log in to unmask]>Marcello Franceschelli
> >To:
> <mailto:[log in to unmask]>GEO-
[log in to unmask]
> >Sent: Thursday, December 19, 2002 1:30 PM
> >Subject: Calc-silicates
> >
> >Dear all,
> >
> > Yardley in the book "An Introduction to Metamorphic
> Petrology"
> >pag. 126 writes.........
> >....it becomes convenient for the description of metamorphosed
> >calcareous sediments
> >to divide them into two categories: marbles in which carbonates
> are
> >abundant; and calc-silicates
> >with little or no carbonate.
> >Can anyone suggest me if a rock made up of calcite ( 30%) , and
> >wollastonite+diopside+
> >grossular+ quartz (70%) can be classified as calc-silicate?
> > Thanks in advance,
> > Marcello Franceschelli
> >
> >_________________________________________
> >
> >Prof. Marcello Franceschelli
> >
> >Dipartimento di Scienze della Terra
> >Università degli Studi di Cagliari
> >Via Trentino, 51
> >I-09127 Cagliari
> >Italy
> >
> >[log in to unmask]
> >Tel: +39 -070 6757713 Fax: +39 -070 282236
> >____________________________________
>
Quoting Eric Essene <[log in to unmask]>:
> All,
> I do not think that the word skarn has genetic significance,
> it
> just means a rock dominated by Ca-rich minerals silicates as garnet
> and diopside. There are many skarns in the Grenville terrane and
> their origin is often unclear. Whether or not a metasomatic event
> occurred before or during regional metamorphism is unclear. The
> Canadians also map rocks called pyroxenites or clinopyroxenites,
> which are cpx-dominated rocks of obvious metasedimentary origin but
> of less obvious contact metasomatic associations in many cases.
> Why
> must one understand the origin of such rocks before naming them?
> Just use the name and if one can, try to understand their origins.
> We invest far too much time into knowing origins before we give
> names. It is much easier if the names simply represent what is.
> Of
> course, there may be transitions between marbles and skarns,
> although
> the boundaries in the field are often sharp.
> eric
>
>
>
> >Dear all,
> >maybe you could use the term skarnoid, indicating a rock
> containing
> >calc-silicates such as wollastonite, piroxene
> >(diopside-hedenbergite), garnet (grossularite-andradite), epidote,
> >scapolite. I think the term *skarnoid* does not have a genetic
> >significance, but it is a general term indicating calc-silicate
> >rocks similar to skarns but without implying necessarily a
> >metasomatic process.
> >
> >
> >----- Original Message -----
> >From: <mailto:[log in to unmask]>Marcello Franceschelli
> >To:
> <mailto:[log in to unmask]>GEO-
[log in to unmask]
> >Sent: Thursday, December 19, 2002 1:30 PM
> >Subject: Calc-silicates
> >
> >Dear all,
> >
> > Yardley in the book "An Introduction to Metamorphic
> Petrology"
> >pag. 126 writes.........
> >....it becomes convenient for the description of metamorphosed
> >calcareous sediments
> >to divide them into two categories: marbles in which carbonates
> are
> >abundant; and calc-silicates
> >with little or no carbonate.
> >Can anyone suggest me if a rock made up of calcite ( 30%) , and
> >wollastonite+diopside+
> >grossular+ quartz (70%) can be classified as calc-silicate?
> > Thanks in advance,
> > Marcello Franceschelli
> >
> >_________________________________________
> >
> >Prof. Marcello Franceschelli
> >
> >Dipartimento di Scienze della Terra
> >Università degli Studi di Cagliari
> >Via Trentino, 51
> >I-09127 Cagliari
> >Italy
> >
> >[log in to unmask]
> >Tel: +39 -070 6757713 Fax: +39 -070 282236
> >____________________________________
>
--
Stefan Nicolescu
Kline Geology Laboratory
Department of Geology & Geophysics
Yale University
PO Box 208109
New Haven, CT 06520-8109
USA
Phone:
+1 203 432-3169 - Office
+1 203 432-6022 - Lab
Fax:
+1 203 432-3134
Courier address:
Kline Geology Laboratory
Department of Geology & Geophysics
Yale University
210 Whitney Ave.
New Haven, CT 06511
USA
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