Basic divisions in classification of metamorphic rocks

Mon Apr 30 15:13:38 2001

A comment by Stephen M. Richard about

Science language for geologic-map databases

Dear SLTT Metamorphic team (and other potentially intereseted parties!!)--

At this point, we're zeroing in on a draft classification based on rock
fabric and modal mineralogy.

This classification system is not a system for naming metamorphic rocks for
the purpose of geologic discourse. It is a descriptive classification system
that attempts to provide objective criteria for grouping the domain of
metamorphic rocks (including metasomatic rocks and strain-dominated
metamorphic rocks) into kinds based on fabric and mineralogical criteria
only. This philosophy departs from that of Robertson [1999], in that we are
designing not a set of names to use for particular rocks, but a set of
‘bins’ into which rocks that are similar (based on the classification
criteria) can be ‘placed’. The names we assign to the ‘bins’ are not the

Starting premises:
1.    the scheme should be as descriptive as possible
2.    a rock may be classified in more than one way
3	the scheme is for classifying rocks as metamorphic rocks, thus
interpretation of the protolith of a rock should not be used to determine
the classification of the rock
4	hand sample scale
5	rock mineralogy is described in other database fields
6.    A geologist can give a rock any name they feel is most appropriate (a
la Robertson, 1999, BGS scheme) using an uncontrolled ‘rock name’ field in
the database.

Thought experiment to check if classification is descriptive--imagine a civil
engineer or materials scientist trying to use the system

we (the NADM SLTT metamorphic rock classification team need to reach some sort of consensus or
at least a majority decision on the following points. In the discussion of
the questions, I'm presenting my preferred approach.

I. 'meta' prefix
	Should the metamorphic rock lithology classification pick-list include terms
such as meta-sandstone, meta-granite, etc?

Lets dodge this bullet. If we allow such terms then we face two giant
 First--defining exactly what they mean in descriptive terms so that e.g.
sandstone can be distinguished from meta-sandstone from quartzo-feldspathic
granofels or semi-pelitic schist.
 Second--choosing which meta-this and meta-that terms are included in the
controlled word list.
Inclusion of meta-xxxx terms is not necessary because the geologist will be
able to use the meta-xxxx name in the uncontrolled word list, and in the
lithology classification classify the rock according to what kind of
sandstone it is(was) **and** according to what kind of metamorphic rock it
is (see premise #2).
Terms such as meta-xxxx do not communicate the  ***kind of metamorphic
rock*** we're classifying, thus defeating the purpose of classification as a
metamorphic rock.

II. Basic classes of metamorphic rocks based on rock fabric.
The question is do the following 8 basic classes partition metamorphic rocks
based on descriptive criteria in a manner that different observers can agree

Note that all quantitative boundaries (%, ratios, dimensions) should be
considered open to discussion—the qualitative distinctions are the issue

1.	Rocks with fragmental fabric, in which >10% of rock is fragments bounded
by fractures. Cutoff % for definition is debatable, but need to distinguish
from type #3 below, and define at what point the fragmental fabric is
significant enough to merit classification as a different rock. (remember
that since a rock may belong to more than one class, it may meet this
criteria and also belong to one of the following classes, as well as a
protolith class....)

2.	Rocks that are too fine-grained to determine mineralogy. Criteria is that
average grain size <0.032 millimeter. This criteria is meant to separate
rocks that can be classified based on modal mineralogy from rocks too fine
grained to distinguish mineralogy. The cut-off diameter corresponds to the
sand-silt boundary of sedimentary rock classification schemes, which varies
between 0.032 mm [Robertson, 1999, BGS grain size scheme] and 0.074 mm
(Engineering grain-size scale, ASTM standard D422-63; D643-78)

3.	Phaneritic rocks with granoblastic fabric and very little or no foliation.
Very little foliation means some foliation may be present, but does not meet
the criteria for foliated. Foliated means that >= 10% of the mineral grains
in the rock are fabric elements. To be a fabric element, a mineral grain
must have an inequant crystal habit, or a mineral grain or grain aggregate
must have an inequant shape due to deformation with and aspect ratios >=
1.5:1 between the long and short axis of the deformed grain.

4.	Foliated phaneritic rocks in which at least one major constituent displays
a foliation defined by the shape of deformed mineral grains or grain
aggregates with an aspect ratio > 1.5:1, and >10% 'matrix' is present
associated with the deformed mineral grains. Matrix is an aggregate of new
mineral grains (not present in the protolith, but may be same mineral
species) that are similar in size or smaller than the mineral grains
affected by non-cataclastic deformation. Non-cataclastic deformation is
deformation in which the material continuity of the deformed volume is
maintained on the scale of observation, indicated by the absence of
thoroughgoing fractures in the volume. The definition is meant to identify a
fabric in the rock due to crystal plastic and/or other types of
non-cataclastic deformation. This definition is somewhat problematic because
some knowledge about the protolith must be known or assumed; a provision
might be made that in the absence of other knowledge, the protolith is
assumed to have consisted of equant mineral grains with a size distribution
similar to the largest grains in the metamorphic rock.

5.	Phaneritic rocks with a well developed, continuous schistosity. Here the
sticking point is the definition of ‘well developed schistosity’. I favor a
definition that >50% of rock consists of mineral grains with a tabular,
lamellar, or prismatic crystallographic habit that are oriented in a
continuous planar or linear fabric (following Bates & Jackson). Continuous
is defined on a hand sample-scale, and in quantitative terms to mean that
domains lacking the fabric are < 1 centimeter thick if they are layers, and
<5 cm in diameter if they are irregular patches, and constitute < 25% of the
rock. IUGS suggests using criteria that rock splits on scale <1 centimeter,
but this criteria depends on the tool used to do the splitting, the skill of
the operator doing the splitting , and the degree of weathering or
alteration of the rock being split, and is thus not objective and
universally applicable. The IUGS criteria could not be used to classify a
rock in thin section. The IUGS criteria also defines schistosity as due to
inequant mineral grains or grain aggregates, without specifying that their
shape is due to crystallographic habit.

6.	Foliated, phaneritic rocks without well developed, continuous schistosity
that have continuous compositional layering, > 5 mm thick. Continuous means
that layers defining the foliation can be traced for > 10 centimeter (length
of lateral continuity), and are spaced at a distance <= the average length
of lateral continuity.

7.	Foliated, phaneritic rocks without well developed, continuous schistosity
or con-tinuous compositional layering in which >50% of rock is part of
fabric elements defining foliation.

8.	Foliated, phaneritic rocks without well developed, continuous schistosity
or continuous compositional layering in which 10-50% of rock is part of
fabric elements defining foliation

Please pick 10 of your favorite metamorphic rocks and see if they reasonably
fit into this classification.

Classes 7 and 8 are 'everything-else' type classes necessary for
completeness, but I think lots of 'foliated granite' and 'slightly schistose
marble' type rocks will end up in them. We need to come up with something to
call these. Suggestions please??


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

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