Download MAJOR-ELEMENT CHEMISTRY OF PLUTONIC ROCK SUITES

Chemical Geology, 26 (1979) 217--235
217
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
M A J O R - E L E M E N T C H E M I S T R Y O F P L U T O N I C R O C K SUITES F R O M
COMPRESSIONAL AND EXTENSIONAL PLATE BOUNDARIES
WILLIAM L. PETRO*' , THOMAS A. VOGEL .2 and JOHN T. WILBAND
Geology Department, Michigan State University, East Lansing, MI 48824 (U.S.A.)
(Received December 6, 1978; accepted for publication February 19, 1979)
ABSTRACT
Petro, W.L., Vogel, T.A. and Wilband, J.T., 1979. Major-element chemistry of plutonic
rock suites from compressional and extensional plate boundaries. Chem. Geol.,
26: 217--235.
Chemical'criteria have been developed to distinguish plutonic rock suites from compressional and extensional plate margins by comparing type examples of Mesozoic and
Cenozoic plutonic suites generated at these plate boundaries. Compressional plutonic
rock suites are characterized by intermediate unimodal frequency distributions of differentiation index and normative plagioclase, calc/alkali indexes in the range of 60--64,
and distinctive patterns on AFM diagrams. Extensional plutonic rock suites are characterized by bimodal distribution of differentiation index and normative plagioclase, calc/
alkali indexes in the range of 50--56 and the presence of peralkaline rocks. These criteria are useful in determining tectonic settings of plutonic rock suites of u n k n o w n environment. No single criterion should be used to distinguish tectonic setting.
The distinguishing chemical features of compressional plutonic rock suites may
directly be related to melting and dehydration of the subducted ocean crust. Other processes that may be important are: lowering of the solidus of the overlying peridotitic
mantle wedge resulting in partial melting; fractionation of both primary melts; extensive reaction of these melts with the continental crust.
The characteristics of extensional plutonic rock suites may directly be related to
melting of anhydrous peridotitic mantle; small amounts of melting of continental crust
and a lack of mixing of the two magmas. Basic rocks from extensional suites may be
generated by smaller amounts of melting at greater depths than those from compressional suites resulting in some with alkaline affinities.
INTRODUCTION
The purpose of this paper is to develop chemical criteria, for the major
elements, to distinguish plutonic rocks generated at compressional plate
boundaries from those generated at extensional plate boundaries.
* ~ C u r r e n t address: Department of Geology and Geophysics, University of Wisconsin,
Madison, WI 53706, U.S.A.
.2 To w h o m reprint requests and comments should be addressed.
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Igneous rocks have long been divided into two groups. Joseph Iddings
(Iddings, 1892) referred to these as the alkali group and subalkali group,
whereas Alfred Harker (Harker, 1896, 1909) called these Atlantic and Pacific types and he specifically related these two great branches of igneous
rocks to the "grandest tectonic features of the globe" (Harker, 1909, p. 93).
Barth (1962) reviewed this two-fold division and emphasized the relationship
between the Pacific t y p e and orogenic activity; and the Atlantic t y p e and
the non-orogenic activity. In this paper, and consistent with Barth's (1962)
usages, orogenic suites are those associated with compressional tectonics,
whereas anorogenic suites occur in areas of minor compressional deformation and often are associated with extensional tectonics.
In m o d e m terminology, orogenic plutonic suites are generated in subduction zones at three types of compressional plate boundaries (Dickinson,
1971): (1) at ocean--ocean plate boundaries where oceanic lithosphere is
subducted beneath oceanic lithosphere; (2) at ocean--continent plate boundaries where oceanic lithosphere is subducted beneath continental lithosphere;
and (3) at continent--continent collision boundaries. In the last case, subduction and associated plutonism may be restricted by the low density of
continental lithosphere. Magmatic activity associated with continental collision, though discussed in general (Gilluly, 1971; Naylor, 1971; Dewey and
Burke, 1973), cannot adequately be evaluated due to the limited chemical
data on collision-generated igneous suites.
The anorogenic suites of interest here are generated at extensional plate
boundaries and occur c o m m o n l y at newly formed continental margins or
rift zones.
The relationship between tectonic setting and the chemical composition
of volcanic rock suites has been well d o c u m e n t e d (Christiansen and Lipman, 1972; Martin and Piwinskii, 1972; Pearce et al., 1977). Plutonic suites
have also been compared with volcanic suites of known tectonic setting,
which allow indirect inferences to be made about the tectonic setting of
the plutonic suites (Strong and Minatidis, 1975). However, there is a need
to establish the direct relationships between tectonic setting and plutonic
rock chemistry.
This study evaluates the use o f major elements as indicators of tectonic
settings of plutonic suites. These elements are useful because they are readily available from a variety of tectonic settings and relatively easy to obtain. Dispersed elements are not considered because a sufficiently large data
source is not available. However, Pearce and Cann (1973), Miyashiro (1975),
and Strong and Minatidis (1975) have shown that these elements are potential indicators of tectonic setting.
PREVIOUS WORK
Martin and Piwinskii (1972) first quantitatively evaluated the relationship between the tectonic setting and chemical composition of igneous rock
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suites. They distinguished orogenic and anorogenic volcanic suites on the
basis of the frequency distribution of the differentiation index (DI);
where DI = normative Q + Or + Ab + Ne + Lc + Kp (Thornton and Tuttle,
1960). They showed that the differentiation index frequency distributions
of orogenic suites are unimodal with an intermediate mode, whereas those
of anorogenic suites are bimodal with acidic--basic modes. They used AFM
diagrams to show that anorogenic suites have stronger Fe-enrichment trends
than do orogenic suites. They also showed that anorogenic, suites contain
more compositional scatter in variation diagrams of weight percent oxides
vs. differentiation index.
Christiansen and Lipman (1972) used the CaO/(Na:O + K20) ratio to
distinguish the two tectonic types of volcanic suites in the western U.S.A.
The calc/alkali index is the value of SiO2 for which the CaO/(Na20 + K20) is
equal to 1.00. This index is similar to Peacock's (1931) alkali/lime index.
They noted that orogenic volcanic suites have higher calc/alkali indexes
than anorogenic volcanic suites do.
Christiansen and Lipman (1972) also noted that the volcanic suites in
the western U.S.A. changed from unimodal (intermediate) to bimodal
(acidic--basic). They related this to the change in tectonic setting that
occurred when compression changed to extension about 30 Ma ago.
Strong and Minatidis (1975) inferred the tectonic setting of a plutonic
suite by comparing its calc/alkali ratio with those of volcanic suites pubfished by Christiansen and Lipman (1972).
Johnston et al. (1976) concluded that extensional granitic suites have
alumina undersaturation, high FeO, and low MgO and CaO, whereas compressional granitic suites have alumina oversaturation, low FeO, and high
MgO and CaO.
C H E M I C A L C H A R A C T E R I S T I C S O F P L U T O N I C SUITES
Approach and methodology
Published analyses of plutonic rocks have been selected to evaluate chemical criteria for the distinction of extensional and compressional plutonic
suites. The major source of data is a computer-based set of igneous rock
analyses compiled by Mutschler et al. (1976a, b).
" T y p e " examples of plutonic rock suites were selected on the basis of
published research on tectonic settings. The Sierra Nevada, southern California and Aleutian--Alaska batholiths were selected to represent type examples of compressional plutonic suites (Hamilton, 1969a, b; Dickinson,
1970; Lanphere and Reed, 1973; Reed and Lanphere, 1973; Marsh, 1976a,
b). Iceland, east Greenland (Tertiary) and the British Isles (Tertiary) selected to represent type examples of extensional suites (Morgan, 1971, 1972;
Brooks, 1973a, b; Einarsson, 1973).
Other suites of less well-known tectonic setting were also investigated
to evaluate the use of chemical criteria in determining the tectonic setting
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of p l u t o n i c suites. T h e following suites, suggested to be compressional,
were investigated: t h e B o u l d e r B a t h o l i t h (Kistler, 1974), t h e H e r c y n i a n
Granites o f t h e British Isles (Riding, 1974), and the S o u t h M o u n t a i n
B a t h o l i t h ( M c K e n z i e and Clarke, 1975). T h e suggested e x t e n s i o n a l suites
investigated were t h e White M o u n t a i n Magma Series (Morgan, 1971), the
Younger Granites o f Nigeria ( B o w d e n , 1970), and the K e w e e n a w a n plutonic rocks (Chase and Gilmer, 1973; Sims, 1976).
Table I lists t h e age, t o t a l n u m b e r o f samples, and t h e abbreviations for
each o f the suites.
TABLE I
List of suites
Suite
code* 1
Number of
plutonic
rocks
Name
Age .2
ARA
BIT
BLB
COR
EGL
ISL
KEW
NIG
PSM
SCB
SRN
WMT
138
164
106
58
61
104
86
89
74
87
302
94
Aleutian--Alaska Batholith
British Isles Tertiary
Boulder Batholith (Montana)
Hercynian Granites (British Isles)
East Greenland Tertiary
Iceland
Keweenawan Plutonic Rocks
Younger Granites of Nigeria
South Mountain Batholith (Nova Scotia)
Southern California Batholith
Sierra Nevada Batholith
White Mountain Magma Series
M--T
T
M
P
T
C
P~
M
P
M
M
M
• 1Abbreviations from Mutschler et al. (1976b).
• 2 C = Cenozoic; T = Tertiary; M = Mesozoic; P = Paleozoic; Pe = Precambrian.
Data
Data were t a k e n as weight p e r c e n t oxides and recalculated to 100 percent ( a n h y d r o u s ) . N o r m a t i v e minerals were calculated using a m o d i f i e d
C.I.P.W. n o r m c o m p u t e r program. Several p e t r o c h e m i c a l indicators were
also calculated, including the d i f f e r e n t i a t i o n i n d e x (DI), C a O / ( N a 2 0 + K20)
(calc/alkali ratio), and t y p e s o f alumina saturation.
Several t y p e s o f variation diagrams were used in an a t t e m p t t o distinguish
the t w o t y p e s o f suites. Simple variation diagrams such as the weight percent oxides vs.: SiO2, DI, or Larsen ( 1 9 4 8 ) i n d e x (1/3 SiO2 + K 2 0 - - C a O - FeO--MgO), do n o t s h o w clear differences b e t w e e n the t w o t y p e s o f suites.
F r e q u e n c y distributions o f the differc-ntiation i n d e x (Fig. 1 ) and t h e
c o m p o s i t i o n o f the n o r m a t i v e plagioclase (Fig. 2) can be used t o distinguish
the t w o t y p e s o f suites. C o m p a r i s o n o f the t y p e c o m p r e s s i o n a l suites
(Figs. 1A--C; 2A--C) with the t y p e extensional suites (Figs. 1DmF; 2 D - - F )
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Fig. 1. Frequency distributions of differentiation index: (A)---(C) type compressional
suites, (D)--(F) type extensional suites, (.G)--(I) possible compressional suites, (J)--(L)
possible extensional suites. The differentiation .index is given in class intervals of five.
F is the frequency. Suite abbreviations are listed in Table L
too
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Fig. 2. Frequency distributions of normative ~4agioclase: (A)~-(C) type compressional
suites, (D)--(F) type extensional suites, (G)--(I) possible compressional suites, (J)--(L)
possible extensional suites. PL is the composition of the normative plagioclase in percent
anorthite and is given in class intervals of five. F is the frequency. Suite abbreviations
are listed in Table I.
223
shows that the compressional suites have unimodal distributions with intermediate modes, whereas the extensional suites have bimodal distributions with acidic--basic modes. Other types of relative frequency distributions were examined (e.g., SiO2, Larsen index), b u t differentiation index
and normative plagioclase are preferred because the differences between
the two types of suites are more clearly shown.
The calc/alkali index is a useful classification of compressional and extensional suites. The calc/alkali indexes were estimated from curves prepared
b y two methods. Smooth, hand-drawn curves of CaO/(Na20 + K20) vs.
SiO: provided an estimate o f the value of SiO2 where the curve intercepts
at a ratio value of 1.00. The other method used intercepts of third-order
best-fit curves calculated from SPSS subprogram REGRESSION (Nie et al.,
1975). Third-order equations were used as the best compromise between
"goodness of fit" (r 2 ) and simplicity.
The calc/alkali index has been calculated using samples that have a
CaO/(Na20 + K20) less than 4.00. By elimina~ing higher values, the index is
a better discriminator between the " t y p e " extensional and compressional
suites (e.g., the r 2 values for the regression of CaO/(Na20 + K20) vs. SiO2
are near 0.90). Most of the samples eliminated b y this editing have a SiO2
content less than 48%. Table II shows the effect on the Sierra Nevada Batholith suite of using 1--5 orders on the regression equation and also the effect
of removing CaO/(Na20 + K20) values greater than 4.0.
Table III presents the calc/alkali indexes for the suites studied and compares t h e m with Kistler's (1974) alkali/lime index for Phanerozoic batholiths in western North America. Values for the Hercynian and South Mountain suites were not reported because the chemical analyses available were
not in the compositional range [few samples with CaO/(Na20 + K20) less
than 4.00] to produce statistically reliable values. Examination of the t y p e
examples (Table III and Fig. 3) shows that compressional suites have calc/
alkali indexes that are high (60--64), whereas extensional suites have indexT A B L E II
Sierra Nevada B a t h o l i t h regression equations (order 1--5)
Order
Calc/alkali
r2
index
1
2
3
4
5
G r o u p No. 1
No. 2
No. 1
No. 2
67.00
66.84
66.27
65.81
64.36
63.21
61.38
61.37
61.43
61.49
0.13
0.27
0.50
0.59
0.77
0.81
0.87
0.88
0.88
0.88
G r o u p No. 1 is the entire Sierra Nevada Batholith suite, n = 302.
Group No. 2 is edited to remove CaO/ ( N a 2 0 + K 2 0 ) > 4.0, n = 291.
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TABLE III
Calc/alkali index
Suite
code
N *~
Published
value .2
Visual
estimate
Third-order
calculated
r 2 valuer or
multiple
regression
calculation
ARA
BIT
BLB
COR
EGL
ISL
KEW
NIG
PSM
SCB
SRN
WMT
135
105
105
(.3)
43
94
76
88
(.3)
78
291
93
--58
_
----.
.
64
60, 63 *4
--
61
56
58
_"
50
55
52
52
61.6
55.0
59.1
_
50.9
56.0
50.8
49.7
.
63.5
61.4
51.6
0.82
0.91
0.91
_
0.62
0.97
0.57
0.85
.
64
62
53
0.94
0.88
0.71
*~ Data edited to remove CaO/(Na20 + K20 ) values greater than 4.0 (see text).
,2 Alkali/lime index from Kistler ( 1974 ).
,3 Not determined.
,4 60 for the central SRN; 63 for the western SRN.
es t h a t are low ( 5 0 - - 5 6 ) . This indicates t h a t extensional suites are m o r e alkali-enriched with respect to Ca t h a n are c o m p r e s s i o n a l suites.
The A F M t e r n a r y diagrams for t h e " t y p e " c o m p r e s s i o n a l and e x t e n s i o n a l
suites are d i s t i n c t l y d i f f e r e n t (Fig. 4 A and B). The " t y p e " c o m p r e s s i o n
suites have less s c a t t e r along the FM-side and, in c o m p a r i s o n to the " t y p e "
extensional suites, have little scatter o f t h e data f r o m the FM-side t o w a r d s
A. In c o n t r a s t , the e x t e n s i o n a l suites have a great deal o f dispersion o f t h e
data points near t h e FM-side. Also, extensional suites, w h e r e t h e y are p o o r
in MgO, have considerable dispersion parallel to the AF-side. T h a t is, the
extensional trends t e n d t o be closer to and m o r e nearly parallel to the
alkali--iron o x i d e side at c o m p o s i t i o n s a p p r o a c h i n g the alkali apex, whereas t h e c o m p r e s s i o n a l trends t e n d t o be m o r e nearly p e r p e n d i c u l a r t o the
i r o n - - m a g n e s i u m o x i d e side f o r the entire trend.
The a m o u n t o f a l u m i n a s a t u r a t i o n is d e t e r m i n e d b y c o m p a r i s o n s o f t h e
molecular p r o p o r t i o n s o f A l 2 0 3 , N a 2 0 + K20, and CaO + Na20 + K 2 0
(Shand, 1927). Three main a l u m i n a s a t u r a t i o n t y p e s m a y be c o n s i d e r e d :
p e r a l u m i n o u s rocks have A1203 greater t h a n CaO + N a 2 0 + K 2 0 ( m o l e c u l a r ) ;
m e t a l u m i n o u s r o c k s have Al203 greater t h a n N a 2 0 + K20, b u t less t h a n
CaO + Na20 + K20; peralkaline r o c k s have A1203 less t h a n N a 2 0 + K20.
P e r a l u m i n o u s rocks are c h a r a c t e r i z e d b y c o r u n d u m in the n o r m , whereas
m e t a l u m i n o u s r o c k s are c h a r a c t e r i z e d b y n o r m a t i v e a n o r t h i t e ; peralkaline
rocks m a y have n o r m a t i v e acmite, s o d i u m silicate, or p o t a s s i u m silicate.
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Fig. 3. Variation diagrams for CaO/(Na~ O + K 2 O) vs. SiO 2 : (A) the Sierra Nevada Batholith suite compared with (B) type extensional
suites. The value of SiO~ where CaO/(Na 2 0 + K~ O) is equal to one for both diagrams is typical for compressionel and extensional tectonic suites respectively. Compare with Table III.
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Fig. 4. A F M ternary diagrams: (A) type compressional suites; (B) type extensional suites;
(C) possible compressional suites; and (D) possible extensional suites.
The relative frequency distributions of the alumina saturation of the
type suites are shown in Fig. 5. Comparison of type examples shows that
only extensional suites have peralkaline rocks (Fig. 5C). Compressional
suites tend to have higher frequencies of peraluminous rocks (Fig. 5A)
whereas extensional suites tend to have slightly higher frequencies of metaluminous rocks (Fig.5B). The variation in alumina saturation appears to
be due more to variations in CaO and Na20 plus K20 rather than to variations in A1203, because A1203 cannot be used to distinguish tectonic setting
using any type of A1203 variation diagram.
227
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Fig. 5. Relative frequency distributions of alumina saturation types: (A) peraluminous;
(B) metaluminous; and (C) peralkaline. F is the relative frequency in percent. Suite
abbreviations are listed in Table I.
Applications to other suites
Once the chemical criteria that characterize the Mesozoic and Cenozoic
tectonic suites have been established, they can be applied to other suites
for which the tectonic setting is less well-known. The compressional suites
that are evaluated are the Boulder Batholith, Montana; Hercynian Granites,
Great Britain, and South Mountain Batholith, Nova Scotia. The suites
which are suspected to have been intruded in extensional environments are
Keweenawan plutonic rocks; White Mountain Series, New Hampshire; and
the Younger Granites, Nigeria.
The suspected compressional suites can be compared to those of the
type sections as follows: (1) The Boulder Batholith is similar to the type
suites with respect to frequency distributions of differentiation index and
normative plagioclase (Figs. 1G and 2G) and A F M trends, but has a much
lower calc/alkali index (58) and corresponding low frequency of peraluminous rocks and lacks peralkaline rocks. (2) The Hercynian and South Mountain suites also have unimodal frequency distributions of differentiation index and normative plagioclase, but both are skewed toward the acidic side
(Figs. l I and 2I).The A F M diagrams (Fig. 4C) for these suites compare
favorably with the type suites (Fig. 4A); the frequencies of distributions
of relative alumina saturation are similar to the type suites (Fig. 5). The
calc/alkali index of these suites cannot be determined because the variation
in chemistry of the suites is not sufficient to define a statistically significant
curve.
228
The suspected extensional suites generally compare favorably with the
type suites. The calc/alkali indices for the suggested suites range from 50--56
and are similar to the type suites. The A F M ternary diagram (Fig. 4D) as
well as alumina frequency diagrams (Fig. 5A--C) are all consistent with a
postulated extensional origin for these suites. With respect to frequency
distributions of differentiation index and normative plagioclase the Keweenawan suite has well-developed acidic and basic modes (Figs. 1J and 2J)
whereas the Nigerian (Figs. 1K and 2K) and White Mountain suites (Figs. 1L
and 2L) have a unimodal acidic mode with minor basic rocks.
The comparison of suspected compressional and extensional suites with
the type examples has shown that most suites have some unique characteristics. The Boulder Batholith compares favorably with type compressional suites except that it is slightly more alkalic. This may be due to the
variation in the source rock that was melted to produce the suite (Kistler,
1974). Alternatively, the difference may be related to the depth of the subduction zone, in a manner analogous to that suggested b y Dickinson and
Hatherton (1967) for the variation in K20. The Boulder Batholith is further
to the east ("inland") of the postulated plate margin than the other batholiths in western North America.
The Hercynian Granites and the South Mountain Batholith are consistent
with t y p e compressional suites except for the predominance of silica-rich
and alumina-oversaturated rocks. Two alternatives may be considered to
account for these differences: (1) Paleozoic subduction may have been fundamentally different from Mesozoic--Cenozoic subduction; and (2) these
batholiths may have formed b y predominantly crustal fusion caused by
crustal thickening and heating due to continental collision. The silica-rich
nature of the batholiths may be interpreted as indicating a larger crustal
c o m p o n e n t in their petrogenesis. McKenzie and Clarke (1975) describe the
South Mountain Batholith as post-tectonic, b u t generated in response to
the Acadia orogeny. Both of these hypotheses remain to be tested.
The White Mountain Magma Series and the Younger Granites of Nigeria
have data distributions which are similar to the t y p e extensional suites except that the acidic modes predominate over the basic modes in the relative
frequency distributions of differentiation index and normative plagioclase.
In the case of the White Mountain Magma Series, the basic rocks m a y lie
concealed at depth (Chapman, 1976). The Nigerian suite has no appreciable
amount of basic rocks associated with it (Wright, 1970). The lack of basic
rocks in these suites may indicate that mantle-derived magmas may have
solidified at the mantle--crust interface where granitic magmas were generated. The rise of these magmas produced suites that were dominated by granites rocks. Alternatively, the granitic magmas were generated b y heat flow
from the mantle without the emplacement of mantle-derived magmas.
Younker (1974) concluded that heat flow by conduction w i t h o u t penetration b y mantle-derived magmas is not a likely source of crustal melting.
229
Application to granite plutons
The " t y p e " compressional and extensional plutonic suites contain a complete spectrum of rock compositions. However, in many areas, granites are
the predominant rock t y p e and occasionally the only t y p e present (e.g.,
the Hercynian Granites of Great Britain and the 300-Ma plutons of the
southeastern U.S.A.). For this reason an a t t e m p t was made to distinguish
between the granites of the " t y p e " compressional and extensional suites.
T w e n t y samples were randomly selected from a SiO2 interval of 70--75
wt.% for t w o compressional suites and two extensional suites. Tukey's test
(Snedecor and Cochran, 1974) was used to compare the means for the chemical variables for each o f the four suites (Tukey's test requires equal sample
size). As is shown in Table IV there are significant differences between
TABLE
IV
Comparison
of means
of variables for SiO 2 interval 70.00--75.00%
BIT
ISL
ARA
SCB
D1.1
88.87
a
91.97
a
83.10
b
83.34
b .2
AI~O 3
Fe203
FeO
MgO
CaO
Na20
K20
TiO 2
Alkalies
13.34
1.23
1.97
0.31
1.24
3.79
4.78
0.29
8.57
a
b
a, c
a, b
a
a, b
a
a
a
13.23
1.90
0.79
0.22
0.86
4.35
4.08
0.26
8.40
a
a
d
a
a
a
a, b
a
a
14.90
0.73
1.13
0.52
2.40
4.09
2.62
0.25
6.68
b
b
c
b
b
a, b
c
a
b
13.95
1.14
1.56
0.54
2.22
3.53
3.41
0.26
6.91
a
b
a0 d
b
b .2
b
b
a
b .2
CaO
Na20+K20
FeO .3
FeO*~+MgO
0.152
a
0.104
a
0.382
b
0.342
b .2
0.907
a
0.928
a
0.791
b
0.829
b .2
Suites with the same letter show
with Tukey's test.
,1 DI = differentiation
index
,2 Variable suitable for distinction
,3 FeO = total iron as FeO.
no significant
difference
of compressional
for that variable at a = 0.05
and extensional
suites
granites from extensional and compressional tectonic environments. Variables that are significantly different in the four suites are indicated b y a different letter. Variables with the same letter cannot be used to distinguish
the suites. The most suitable variables for distinguishing granites from compressional and extensional tectonic environments are: DI, weight percent
CaO, total alkalies, CaO/(Na20 + K20) and Fetotal i(as FeO)/[Fetota 1 (as FeO)
+ MgO]. These variables should be valuable in determining tectonic environments in areas where granite plutons dominate.
230
ORIGIN OF COMPRESS~ONAL AND EXTENSIONAL SUITES
Compressional plutonic suites
The chemical characteristia of compressional plutonic suites is believed
to be directly related to the subduction process. Theoretically, there are
three possible regions of melting in or above subduction zones: the subducted oceanic lithosphere, overlying mantle wedge and overlying crustal
complex (oceanic or continental). Wyllie et al. (1976) have presented a
comprehensive review of the experimental data concerning primary andesitic
(calc-alkaline) liquids produced from these three regions and concluded that
the major- and trace-element content, as well as the isotopic ratios, of liquids
produced by partial melting of quartz eclogite (subducted oceanic crust)
depart significantly from erupted andesites. However, Marsh (1976a, b) concluded that the Aleutian andesites are compatible with a model of partial
fusion of subducted ocean crust (quartz eclogite).
Melting of the peridotitic mantle wedge has been attributed to lowering
of the solidus by dehydration of the down-going slab (Wyllie, 1971;
Kushiro, 1972; Anderson et al., 1976). Partial melting of peridotitic mantle
near water-saturated conditions may lead to the production of calc/alkaline
liquids (Kushiro et al., 1968; Kushiro, 1969a, b, 1972, 1974; Mysen and
Boettcher, 1975a, b, 1976) over a broad range of temperatures and pressures. However, most andesites that reach the surface are water-deficient
(Marsh, 1976a, b; Wyllie et al., 1976), and this is the main obstacle for
accepting this mechanism for the major mechanism in producing calc/alkaline compressional suites. Furthermore, water-saturated andesitic liquids,
generated on the saturated peridotite soli~ius, could not be subsequently
dehydrated or emplaced at higher levels without crystallizing unless temperatures were above solidus temperatures.
Melting of lower crustal rocks at temperatures higher than that necessary
to produce granitic melts can produce magmas of intermediate compositions
(Tuttle and Bowen, 1958; Brown, 1970, 1973; Fyfe, 1973; Presnall and
Bateman, 1973; Winkler, 1976). However, Wyllie et al. (1976) have shown
that these temperatures are unrealistic to expect in the lower crust.
If Marsh (1976a, b) is correct, calc/alkaline liquids could be produced by
partial melting of subducted ocean crust; or by modification of liquids that
may be produced by partial melting of peridotitic mantle wedge under
water-present, but water-deficient conditions (Wyllie et al., 1976). The process may be complex, involving reaction of intermediate liquids (derived
from the subducted oceanic crust) with mantle peridotite. The rising magmas, produced in the down-going slab and overlying mantle wedge, may
fractionate or may cause melting in the overlying crust. These crustal-derived
magmas may mix in varying proportions with magmas derived from below
to produce magmas of intermediate composition (Younker and Vogel, 1976).
Calc/alkaline liquids most likely have a complex origin and each plutonic
suite may be somewhat unique in its origin and chemical characteristics.
231
Extensional plutonic suites
Rifting is thought to be directly associated with rising and melting of
mantle diapirs (Morgan, 1971, 1972; Burke and Wilson, 1976). Melting in
this rising anhydrous peridotitic mantle may produce basaltic liquids
(Ringwood, 1975) and the composition of these liquids is controlled by
pressure, volatiles and the amount of melting. Alkaline liquids are thought
to be produced at greater depths than are tholeiitic liquids (Ringwood,
1975; Yoder, 1976). Melting in the presence of higher water pressures
drives the composition of the liquids toward more intermediate silica composition (Mysen and Boettcher, 1975b; Wyllie et al., 1976; Yoder, 1976);
however, in contrast to the mantle wedge overlying dehydrating subduction
zones, the mantle in extensional zones should be relatively dry. Small
amounts of melting produce alkaline liquids whereas moderate degrees of
melting tend to produce tholeiitic liquids (Ringwood, 1975; Yoder, 1976).
Liquids in extensional areas are most likely to have been produced by small
amounts of melting of a relatively anhydrous peridotitic mantle, whereas
liquids in compressional areas are produced by melting of subducted oceanic crust and overlying partially hydrated mantle.
Fractional crystallization of basaltic magmas may also be an important
factor in extensional plutonic suites. Some of the alkaline basaltic rocks in
extensional zones may represent liquids modified by high-pressure fractionation of orthopyroxenes which produces alkalic quartz-undersaturated
basaltic liquids (Ringwood, 1975; Yoder, 1976). These alkalic basaltic liquids may be fractionated at crustal levels to produce syenitic rocks.
The production of syenitic rocks by the fractionation of basaltic magmas
combined with the generation of granitic rocks by melting of crustal rocks,
may account for the coexistence of quartz-oversaturated and undersaturated rocks in some complexes (Chapman, 1976). Chapman (1976), in his
discussion of the White Mountain Magma Series, also suggested that minor
mixing of the syenitic and granitic magmas could produce small amounts of
alkali granites and quartz syenite in the same complexes.
The granitic rocks in extensional zones are iron- and alkali-enriched when
compared with granitic rocks in compressional zones. Extensional zones
may have smaller degrees of melting of crustal rocks at lower water pressures
to account for these differences. Brown and Fyfe (1970) have shown that
the initial granitic magmas formed by fusion of crustal rocks, associated
with muscovite breakdown at relatively low temperatures, are low in MgO.
Bowden (1970) suggested that the alkali-enriched rocks in Nigeria are the
initial crustal melting products, whereas less alkali-enriched rocks are products of more extensive melting.
If extensional granitic rocks represent more limited crustal melting than
do compressional granitic rocks, there may be a major thermal difference
between the two types of plate margins. The analysis of Younker (1974)
shows that higher heat flux is required for more extensive melting, and that
the heat flux may be related to the flux of mantle-derived magmas through
232
the crust. Compressional suites may therefore represent suites generated in
areas of higher heat flux or flux over more extended periods of time. Extensional suites may have a shorter time of residence over the region of high
heat flux, and after the crust moves o f f the area o f rifting (and high heat
flow), magma generation is greatly reduced.
CONCLUSIONS
The major-element composition of the plutonic suites studied can be used
to distinguish compressional and extensional tectonic settings. The two
tectonic types o f suites can be characterized b y the following criteria:
(1) Frequency distributions of differentiation index and normative
plagioclase show that unimodal (intermediate) distributions are characteristic of compressional suites, whereas bimodal (acidic--basic) distributions
are characteristic of extensional suites. Unimodal acidic distributions may
be ambiguous.
(2) Calc/alkali indexes for extensional suites are in the range 50--56,
whereas indexes for compressional suites are in the range 60--64. The intermediate range (56--60) may be ambiguous.
(3) AFM ternary diagrams of extensional suites have more scatter along
the FM-side than do compressional suites. The A-rich portion of extensional suites is characterized b y dispersion parallel to the AF-side.
(4) Peralkaline rocks are characteristic of extensional suites, whereas compressional suites tend to have higher frequencies of peraluminous rocks.
Metaluminous rocks are c o m m o n in both suites.
Consistent results from several indicators assure the best estimate of the
tectonic setting of a plutonic rock suite. Conflicting results from different
criteria can be interpreted to indicate that the petrogeneses of the suites
are different from those suites selected as type examples. Thus, these criteria may also be valuable in determining areas of unique petrogenesis.
The production o f intermediate compositions in compressional tectonic
areas may be directly related to dehydration and partial melting of subducted oceanic crust. The detailed magmatic processes are probably complex and may involve production and fractionation of melts from both the
subducted oceanic crust and overlying hydrated peridotitic mantle wedge.
These rising magmas may mix with magmas derived b y partial melting of
the continental crust. The variations in the chemistry of the compressional
plutonic suites may indicate details of these processes.
Extensional tectonic suites may be due to partial melting of anhydrous
peridotitic mantle coupled with partial melting of continental crust b y
the emplacement of these mantle-derived magmas. It appears that little
mixing of the t w o magma types has occurred.
ACKNOWLEDGEMENTS
Many ideas for this paper were generated while Thomas A. Vogel was
233
supported by NSF Grants GF-32510X and OIP 75-07943. These grants
were a d m i n i s t e r e d b y the University of South Carolina. Thanks are expressed t o D u n c a n S i b l e y f o r d i s c u s s i o n s a n d r e v i e w o f t h e m a n u s c r i p t .
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