responses to SLTT metamorphic classification questions

Tue Apr 24 19:51:22 2001

A comment by Stephen M. Richard about

Science language for geologic-map databases

Hi team--well, back to work. I've been collating the responses to the
questions posed in the message "SLTT metamorphic, review of comments, and
questions based on those", posted 03/14/01. Only 5 responses have been
received, and here are the results thus far.
****If you have any objections to the emerging majority/consensus decision,
please get your 2 cents in before the end of the week (04/27/01).****

Summary of votes received at this point:
1.	Should texture terms (gneiss, schist, granofels, hornfels) be used in
place of ‘rock’ in root names whenever possible?
Yes: 4
ONE vote for give texture-based and rock terms equal weight

2.	Should Robertson’s definition of amphibolite be restricted to:
Amphibolite: rock that contains >50% amphibole
YES: 2 votes
NO: 2 votes; objection seems to be use of 50% as cut off; amphibolites might
contain less am-phibole, and abundance of plagioclase should also be criteria
pass: 1

3.	Should fault rock terminology (including Pseudotachylite) be revised to be
consistent with Scholtz (1990) as endorsed by Snoke and Tullis (1998)?
(we need specific definitions here)
pass: 2

4.	Should we endorse the Robertson [1999] distinction between schist and
gneiss based on the presence of layering >0.5 centimeter thick in gneiss?
NO: 3; major objects seems to be that schist-gneiss distinction is based on
schistosity/fissility in schist, not in gneiss. See comment on Gneiss-Schist
classification at bottom.
Yes (conditional): 1
Pass: 1

5.	Are leucogneiss, leucogranofels, mafic gneiss (or melanogneiss?), mafic
granofels, mafic schist allowable root names?
NO: 3. suggestion to use leucocratic, melanocratic and mafic as modifiers
[but note this begs the question of whether there should be specific classes
for leucocratic gneiss/leucogneiss etc. in the classification—smr]
Yes (sort of): 2. terms are probably useful in a terminology hierarchy for

6.	Should the definitions of marble and metacarbonate rock read:
Marble: metamorphic rock with sedimentary protolith, composed of >50%
calcsilicate or car-bonate minerals, with carbonate minerals more abundant
than calcsilicate minerals
Metacarbonate rock: metamorphic rock with sedimentary protolith, in which
modal mineral proportions can not be estimated, but rock is apparently
largely composed of carbonate or calc-silicate minerals
YES: 3
NO: 2. objection to calling a rock with (51% calcite+49% diopside) or (14%
calcite, 12% dolo-mite, 25% calcsilicate mineral, and 49% of anything) else
to be called a calcite marble

7.	Should eclogite be considered a subdivision of meta-ultramafic rock based
on modal mineralogy?
NO: 5 (see discussion at end)

8.	Should the terminology for generic metaigneous rocks be revised to change
metafelsic-rock, metamafic rock, and meta-ultramafic rock to one the
textural terms with felsic, mafic, and ul-tramafic as modifier to generate a
root name?
YES: 1
NO: 4. General dislike of use of ortho and para as root name prefixes:
redundant in some cases (pelitic paraschist), not commonly used in North
America, compositional adjectives (felsic, granitic, mafic…) more
informative and appropriate

9.	Should the term meta-ultramafite, with modifiers for grain size (e.g.,
coarse-grained meta-ultramafite) replace meta-ultramafite and
YES: 1
NO: 2. see discussion at end.
pass: 2

10.	Should migmatite be added as a root name, replacing migmatitic gneiss and
migmatitic gran-ofels (and migmatitic schist??).
Use both migmatite and magmatitic gneiss/granofels/schist: 2
YES: 1
NO: 1, Don’t Know:1. both based on difficulty of defining migmatitic gneiss
distinct from gneiss or schist
Use Both: 2

11.	Should whiteschist be included as a root name? If so, we need a
definition for the term, and to place it in the key.
YES: 3
OK with me: 1
Don’t Know: 1

12.	Should metaquartzite be included as a root name?
NO: 4
yes: 1

13.	Should soapstone be included as a root name?
Soapstone: non-schistose, massive talc rock
NO: 4
Don’t Know: 1

14.	Should monomineralic rocks be included with different names based on the
mineral constitu-ent, or should there be a root name based on texture, e.g.
monomineralic granofels, monomin-eralic hornfels, monomineralic schist.
Keep ‘traditional’ names only: 3
Name with mineral Name: 1
Terms are necessary for complete hierarchy: 1

15.	Should schist and granofels be subdivided into paraschist, orthoschist,
paragranofels and or-thogranofels based on identification in sedimentary or
igneous protolith?
NO: 4
sort of No: 1

16.	Should the root name pelite be retained as used by Robertson [1999]? If
not, what is a better term and definition?
YES: 1
NO: 4 (see #19)

17.	Should the root name phyllite be added?
phyllite: Metasedimentary (or just metamorphic?) rock possessing a silky or
lustrous sheen on foliation surfaces imparted by fine-grained (< 0.1 mm)
white mica (including muscovite, para-gonite and phengite) orientated
parallel to the foliation in the rock. Individual mica flakes can be seen
with the naked eye in contrast to slates where they cannot be distinguished,
and to schists in which the mica crystals are >1 millimeter in diameter.
YES: 5

18.	Which of the following terms should be included in the controlled word
Yes to all: 2
Yes only to Skarn and greisen: 1
No to all: 1
Pass: 1

19.	As an alternative to BGS usage of psammite, semipelite, and pelite as
root names, use psam-mitic, semipelitic, and pelitic as adjective modifiers
consistent with Robertson's triangular dia-gram.
Yes: 4
Yes (tentative): 1

Proposed definitions for schist and gneiss
The definitions for schist and gneiss must be considered in the continuum of
rocks from quartzo-feldspathic granofels with no schistosity (foliation
defined by alignment of tabular mineral grains, inde-pendent of the modal
abundance of the tabular mineral) to pure mica schists, to slate, to gneiss
and migma-titic gneiss, to pelitic hornfels or granofels containing large
percentage of mica without schistosity. Based on a compilation of
definitions of schist and gneiss, the various criteria used to distinguish
these rock types are:
Presence of compositional banding
Thickness of compositional banding
Homogeneity of the rock
Modal abundance of mica
Modal abundance of quartz+feldspar
Degree of fissility (how thinly can the rock be parted along the
Grain size
This discussion requires the definition of some key terms used in the
definition of the rock types:

Schistosity: foliation in a coarse-grained rock due to the parallel, planar
arrangement of mineral grains of the platy, prismatic or ellipsoidal types,
usually mica (Bates & Jackson, 3rd ed.); foliation defined by preferred
orientation of tabular crystals, especially of micas (Turner&Weiss, 1963).
Alternatively: secondary foliation defined by preferred orientation of
inequant fabric elements in a medium- to coarse-grained rock; individual
foliation-defining elements (e.g. mica) are visible with the naked eye
(Passchier & Trouw , 1998)
The alternative definition allows a schistosity to be defined by the grains
that are inequant due to plastic deformation—thus a pure quartz tectonite
with a grain shape fabric defined by tectonically flattened quartz grains
would have a schistosity under this definition. I prefer the older
definitions because the distinction between foliation related to
crystallographic orientation of tabular or elongate crystals (schistosity)
and foliation defined by grain shape of mineral aggregates or deformed
mineral grains (grain-shape fabric) is important.

Compositional layering: non-genetic term for an alternation of layers with
different lithological composi-tion (Passchier & Trouw , 1998)

Penetrative fabric element: a fabric element that occurs penetratively
throughout a rock at the scale of observation (Passchier & Trouw , 1998).
Penetrative: repeated at distances so small, compared with the scale of the
whole, that they can be considered to pervade the whole uniformly and be
present at every point (paraphrase from Turner&Weiss [1963]; see discussion
in this text p. 21-24).

It seems to me that the distinction of schist and gneiss is based on the
homogeneous distribution of ori-ented tabular crystals (tabular due to
crystal habit, not deformation of equant grains) in schist, resulting in a
schistosity that is penetrative on a hand-sample scale. A schist may have
compositional layering at any scale, and still be a schist. A gneiss does
not have penetrative schistosity on a hand sample scale, but may have
schistose layers separated by non-schistose layers. The boundary of gneiss
and schist in this logic is placed where the schistose layers are close
enough together that the schistosity is deemed penetrative. My
interpretation of Robertson’s [1999] definition is that the schistosity
becomes non-penetrative when at least some non-schistose layers are more
than 5 millimeter thick, the rock thus being defined to be a gneiss.
Problems: rocks in which a schistosity is present oblique to compositional

As I understand the term, marble should imply that a significant percentage
(75%?) of the rock is car-bonate mineral. This follows the definition in
Bates & Jackson (3rd edition): “a metamorphic rock consist-ing predominantly
of fine- to coarse-grained recrystallized calcite or dolomite, usually with
a granoblastic, saccharoidal texture”
Generic mafic and ultramafic metamorphic rocks
Meta-ultramafite conveys that the grain size is too small to determine
mineralogy, and the name is based mostly on color, whereas meta-ultramafic
rock conveys that the mineralogy is discernible, and the rock is indeed
composed of 90% mafic minerals. However, I don’t like these terms because
they have the connotation that the protolith was an ultramafic rock. Better
to use ultramafic hornfels for ‘meta-ultramafite’ and ultramafic granofels
for ‘meta-ultramafic-rock’. If pyrope and omphacite are both taken to be
‘mafic’ minerals, which it would appear they should be, then eclogite would
be an ultramafic granofels. In a classification scheme based on texture then
composition, eclogites would in general be a subtype of granofels,
specifically an omphacite-pyrope granofels, or ultramafic granofels.

Stephen M. Richard
Arizona Geological Survey
416 W. Congress St., #100
Tucson, Arizona, 85701
phone: (520) 770-3500.  FAX: (520) 770-3505

Context of this discussion

This page is part of a discussion of Science language for geologic-map databases:

Further discussion of responses to SLTT metamorphic classification questions (this page):