Granitic rock nomenclature in reports of Ontario

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ONTARIO
DIVISION OF MINES
HONOURABLE
LEO BERNIER, Minister of Natural Resources
W. Q. M AC NEE, Deputy Minister of Natural Resources
Division of Mines
E.G.Pye, Director, Geological Branch
GUIDE TO
GRANITIC ROCK NOMENCLATURE
USED IN REPORTS OF THE
ONTARIO DIVISION OF MINES
By
L. D. AYRES
MISCELLANEOUS PAPER 52
1972
ODM 1972
Publications of the Ontario Division of Mines
and price list
are obtainable through the
Publications Office, Ontario Division of Mines
Parliament Buildings, Queen's Park, Toronto, Ontario
and
The Ontario Government Bookstore
880 Bay Street, Toronto, Ontario.
Orders for publications should be accompanied by cheque,
or money order, payable to Treasurer of Ontario.
Parts of this publication may be quoted if credit is given to
the Ontario Division of Mines. It is recommended that reference
to this report be made in the following form:
Ayre s , L.D.
1972:
Guide to Granitic Rock Nomenclature Used in Reports
of the Ontario Division of Mines; Ontario Div. Mines,
MP52, 14p.
CONTENTS
Page
Introduction ... . . . . . . . .
Modal Classification ..
Field Classification . ..
References Cited . . . .. . .
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TABLE
1-Classification of plutonic rocks
FIGURES
1-Potassic feldspar-albite-anorthite triangle
2-Modal analyses of granitic rocks, North Trout Lake
area . .. . . . .. . . ... .. ... . . . .. .. . ..................
...
...
...
...
l
5
10
13
GUIDE TO GRANITIC ROCK NOMENCLATURE
USED IN REPORTS OF THE
ONTARIO DIVISION OF MINES
By
L.D. Ayresl
INTRODUCTION
Many classification schemes have been proposed for granitic
rocks; granitic rocks are generally considered to be those
plutonic rocks containing more than 10 percent quartz, and a
plagioclase feldspar more sodic than An5Q.
Most classifications
are based on modal analyses of thin sections (Chayes 1956), but
they differ in parameters chosen for classification, in rock
terminology used, and in definitions of rock types.
The modal
classification scheme used by the Geological Branch, Ontario
Division of Mines, is presented in Figure l and Table l and has
been devised for the granitic rocks of Ontario, the most
abundant rocks in the Precambrian Shield of the Province.
In general, modal granitic rock classifications are based
on two parameters (1) plagioclase:potassic feldspar ratio, and
^Geologist, Ontario Division of Mines, Parliament Buildings,
Toronto. Manuscript accepted for publication by the Chief
Geologist, 22 May 1972.
- 2 -
Anorthite
Albite
Z.
3
Potassic Feldspar
Figure l-Potassic feIdspar-albite-anorthite triangle showing
critical parameters used in granitic rock modal
classification by the Ontario Division of Mines.
- 3 -
Table 1-Classification of plutonic rocks used by the Ontario
Division of Mines (prepared by S.B. Lumbers, L.D. Ayres,
and E.G. Pye).
RATIO
COMPOSITION OF
PLAGIOCLASE
POTASSIC
FELDSPAR
TOTAL
FELDSPAR
A" 0-10
* i
6?
o
1
3
2
3
1
8
1
3
A
An 10-30
An 30-50
GRANITE
ALBITE
QUARTZ MONZONITE
(OLIGOCLASE)
An 50-100
?
ANDESINE
9
QUARTZ MONZONITE
QUARTZ MONZONITE
(OLIGOCLASE)
QUARTZ
QUARTZ
GRANODIORITE
DIORITE
GABBRO
N
1-
cr
<
D
O
< i
ALBITE
GRANODIORITE
ALBITE
(OLIGOCLASE)
TRONDHJEMITE
TRONDHJEMITE
K
o
1
3
2
3
1
8
1
3
V
7
SYENITE
* i
ALBITE
(OLIGOCLASE)
ANDESINE
CALCIC
MONZONITE
MONZONITE
MONZONITE
MONZONITE
(OLIGOCLASE)
DIORITE
GABBRO
N
t-
ec.
<
O
* i
ALBITE
SYENODIORITE
SYENODIORITE
- 4 -
(2) plagioclase composition (e.g. Johannsen 1931; Brown 1952;
Lipman 1963; O'connor 1965) although a few classifications
utilize only plagioclase:potassic feldspar ratio (e.g. Nockolds
1954; Chayes 1957).
The modal classification presented here
uses both parameters with plagioclase composition being of
secondary importance.
In other classifications many of the
same rock names are used but the classifications differ in the
limits placed on the various rock types, especially in the
plagioclase-rich rocks.
The classification shown in Table l and Figure l currently
is being used in the geological reports of the Ontario Division
of Mines, and this brochure has been prepared to familiarize
users of the reports with the scheme.
In addition there are
some explanatory notes about the classification and some
comments about field classification of granitic rocks.
Table l
also gives terminology for quartz-poor plutonic rocks and for
those rocks containing plagioclase more calcic than An5Q, but
this terminology will not be considered in the following
discussion.
Volcanic rocks are generally classified on the
basis of chemical analyses, and the terminology of Irvine and
Baragar (1971) is the standard chemical classification scheme
currently used in the geological reports of the Ontario Division
of Mines.
This terminology is not directly comparable to the
modal granitic classification and volcanic rock names are thus
- 5 -
not included in Table 1.
In the field the staff of the
Geological Branch will continue to use a 2- or 3-fold sub
division of metavolcanics based on colour index (see Ayres, in
press, for a discussion of this system).
In the few areas of
Ontario where unmetamorphosed volcanic rocks are present,
classical petrographic classifications will be used, supplemented
by chemical data.
MODAL CLASSIFICATION
On the basis of plagioclase:potassic feldspar ratio, five
major rock groups, granite, quartz monzonite, granodiorite,
trondhjemite, and quartz diorite, have been defined (Figure l
and Table 1).
Most granitic rocks in Ontario appear to be
granodiorite and trondhjemite (Figure 2), and for this reason,
these two fields have a narrower feldspar ratio range than the
other fields.
Plagioclase compositions of An^o and An3Q,
corresponding to the boundaries between albite and oligoclase,
and oligoclase and andesine respectively, are then used to
subdivide some fields into sub-fields.
Except for quartz
diorite, this subdivision utilizes the plagioclase composition
(albite, oligoclase, andesine) as modifiers of the primary rock
types, and thus no new rock names are introduced (see Table 1).
The granite field is not subdivided because granite (sensu
stricto) is rare.
Because oligoclase is by far the most common
- 6 -
plagioclase in granitic rocks, the oligoclase modifier is
commonly omitted and use of quartz monzonite, granodiorite,
and trondhjemite without modifiers generally indicates an
oligoclase-bearing granitic rock.
Precise use of the classification requires thin section
analysis with determination of measured or approximate modes
and plagioclase composition.
In rocks where plagioclase
composition cannot be measured because of alteration or some
other factor, feldspar ratio determined from the mode can still
be used to place the rock into one of the major groups.
Under
these circumstances the rock name would also be used without a
plagioclase modifier.
Subdivision based on plagioclase composition is important
for two main reasons:
(1) Plagioclase composed dominantly of albite
is rare in granitic rocks and its presence
should be recognized by appropriate
nomenclature.
Where present, it is either
secondary, indicating alteration or metamor
phism, or it is primary and indicates a rock
of abnormal composition that may have economic
connotations (see e.g. Gallagher 1940).
Where
possible, primary and secondary albite should
be distinguished, and the albite modifier
- 7 -
restricted to those rocks in which the
primary plagioclase is dominantly albite.
Minor amounts of albite are found in
many granitic rocks as rims about more
calcic plagioclase or in myrmekite, but the
plagioclase in these rocks is dominantly
oligoclase.
(2) Plagioclase in most granitic rocks is
zoned, ranging in composition from
oligoclase to andesine, but its bulk
composition is generally oligoclase
(Figure 2).
Plagioclase composed
dominantly of andesine is relatively rare
in granitic rocks and its presence should
be recognized.
Most andesine of granitic
rocks is in mafic-rich varieties that form
the quartz diorite field.
A plagioclase
composed dominantly of andesine in a quartz
monzonite indicates a rock of abnormal
composition that should be designated by
use of the andesine modifier.
Mafic mineral content of granitic rocks ranges from O to
slightly more than 45 percent, and the major mafic minerals
are biotite ^ chlorite) and hornblende.
In general there is
LATE BIOTITE
PHASES
Anorthite
Anorthite
HORNBLENDE-BIOTITE
PHASES
Q Phase R
* Phase Q
* Phase P
m Phase O
* Phase N
a Phase M
Potassic
Feldspar
Albite
Albite
EARLY
BIOTITE
PHASES
Anorthite
* Phase C
Albite
Figure 2-Modal analyses of granitic rocks from the North Trout
Lake Batholith (Ayres, in press) plotted on a potassic
feldspar-albite-anorthite triangle. This plot shows
the concentration of rock types in the trondhjemite
and granodiorite fields, prevalence of oligoclase,
wide compositional range of some phases, and
compositional overlap of phases.
- 9 -
a progressive increase in mafic mineral content in the series
granite-quartz monzonite-granodiorite-trondhjemite-quartz
diorite.
Where mafic mineral content is low, biotite is
generally the only mafic mineral present, but where mafic
mineral content is more than 20 percent, both hornblende and
biotite, in varying proportions, are commonly present.
Only
rarely is biotite completely absent from granitic rocks.
Mafic mineral content, which corresponds to colour index,
is not part of the modal classification scheme, but, as
discussed below, it is a critical parameter in field classifi
cation of granitic rocks.
Mafic and other varietal minerals
are useful, however, as modifiers for the primary rock types
and serve to further subdivide the granitic rocks.
Possible
modifiers include one or more of garnet, muscovite, biotite,
hornblende, and pyroxene.
Where more than one of these
modifiers is used, as for example hornblende-biotite, the
order of modifiers indicates that biotite is more abundant than
hornblende (Shaw 1957).
Of terminology used in Table l and Figure l, only
trondhjemite may be unfamiliar, and it is used in the sense
intended by Goldschmidt (1916, p.77) as a potassic feldsparpoor, relatively leucocratic, granitic rock in which plagio
clase is dominantly oligoclase.
Two common rock names,
adamellite and tonalite, used in many other classifications
- 10 -
have not been used in Table l or Figure 1.
In general,
adamellite is equivalent to quartz monzonite as used herein,
whereas tonalite has a more variable definition and may
correspond to (1) our quartz diorite field, (2) part of our
quartz diorite and trondhjemite fields, or (3) parts of our
quartz diorite, trondhjemite, and granodiorite fields,
depending on the classification used.
FIELD CLASSIFICATION
Many granitic plutons are composed of several distinct
intrusive phases, each of which represents a discrete emplace
ment of magma.
The various phases may be relatively homogeneous
in composition or have regular or irregular compositional
variation because of (1) differentiation during emplacement or
subsequent crystallization, (2) hybridization with other
granitic phases, or (3) contamination by supracrustal rocks.
Furthermore, several phases may be compositionally indisting
uishable.
threefold:
The main emphasis in mapping granitic rocks is thus
(l) recognition of intrusive phases, (2) determina
tion of the relationship between phases, and (3) measurement
of compositional variations within and between phases.
Recognition of intrusive phases can only be done in the
field utilizing textural, structural, and compositional
- 11 parameters.
Useful identifying parameters include:
colour
index (abundance of mafic minerals); presence or absence of
muscovite, garnet, or pyroxene; hornblende:biotite ratio;
grain size, presence or absence of phenocrysts, abundance and
size of phenocrysts; composition of phenocrysts; and, to a
lesser extent, quartz content, abnormal abundances of accessory
minerals, habit of the major minerals, and structures such as
degree of foliation and presence of gneissosity.
Rock colour,
as distinct from colour index, is generally not a useful
classification parameter because it largely reflects secondary
alteration.
In fresh samples, quartz monzonite is commonly
pale pink whereas trondhjemite is pale grey, but this colour
distinction is not universal.
Feldspar ratio, the main parameter of the modal classifi
cation is not useful in field mapping because (1) feldspar
ratio in individual intrusive phases is commonly variable
(Figure 2), (2) different units may have the same feldspar
ratio (Figure 2), and (3) feldspar ratio is difficult to
determine in the field unless linked with some other parameter
such as colour index which generally, but not always, decreases
as plagioclase:potassic feldspar ratio decreases.
Once
intrusive phases have been identified and mapped, however,
feldspar ratios are useful for (1) determination of composi
tional variations in each phase, and (2) application of
- 12 -
relatively precise names, that, when combined with an
appropriate mineral modifier, categorize compositional
variations in the pluton without continual reference to
variation diagrams.
- 13 -
REFERENCES CITED
Ayre s , L.D.
In press:
Geology of the Trout Lakes area, District of
Kenora (Patricia Portion); Ontario Div. Mines, GR
(issued in 1971 as OFR5068, 272p., 37 tables, and
28 figures.
Accompanied by 2 maps).
Brown, I.C.
1952:
A Nomenclature of Igneous Rocks; CIM Trans.,
Vol.55, p.43-46.
Chayes, Felix
1956:
Petrographic Modal Analysis, An Elementary
Statistical Appraisal; John Wiley and Sons, New York,
113p.
1957:
A Provisional Reclassification of Granite; Geol. Mag.,
Vol.94, p.58-68.
Gallagher, David
1940:
Albite and Gold; Econ. Geol., Vol.35, p.698-736.
Goldschmidt, V.M.
1916:
Geologisch-petrographische Studien in Hochgebirge des
Sudlichen Norwegens; IV. Ubersicht der Eruptivgesteine
im kaledonischen Gebirge zwischen Stavanger und
Trondhjem: Videnskaba-Selskabet i Kristiania, I.
Math. - Naturw. Klasse, Vol.1, No.2, 140p.
- 14 Irvine, T.N., and Baragar, W.R.A.
1971:
A Guide to the Chemical Classification of the Common
Volcanic Rocks; Canadian J. Earth Sci., Vol.8, No.5,
p.523-548.
Johannsen, Albert
1931:
A Descriptive Petrography of the Igneous Rocks,
Volume l, Introduction, Textures, Classifications,
and Glossary; Univ. Chicago Press, Chicago, 111.,
267p.
Lipman, Peter W.
1963:
Gibson Peak Pluton:
A Discordant Composite Intrusion
in the Southeastern Trinity Alps, Northern California;
Geol. Soc. America Bull., Vol.74, p.1259-1280.
Nockolds, S.R.
1954:
Average Chemical Compositions of Some Igneous Rocks;
Geol. Soc. America Bull., Vol.65, No.10, p.1007-1032.
O'connor, J.T.
1965:
A Classification for Quartz-Rich Igneous Rocks
Based on Feldspar Ratios; p.79-84 in Geological
Survey Research 1965, Chapter B, United States Geol.
Surv., Prof. Paper 525-B, 195p.
Shaw, Denis M.
1957:
Some Recommendations Regarding Metamorphic
Nomenclature; Proc. Geol. Assoc. Canada, Vol.9,
p.69-81.