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Identification and discrimination of altered and metamorphosed volcanic rocks using immobile elements
Chemical Geology, 21 (1978) 291--306
291
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
IDENTIFICATION AND DISCRIMINATION OF ALTERED AND METAMORPHOSED VOLCANIC ROCKS USING IMMOBILE ELEMENTS
P.A. FLOYD and J.A. WINCHESTER
Department of Geology, University of Keele, Keele, Staffs. ST5 5BG (Great Britain) (Received November 4, 1976; revised and accepted February 24, 1977)
ABSTRACT Floyd, P.A. and Winchester, J.A., 1978. Identification and discrimination of altered and metamorphosed volcanic rocks using immobile elements. Chem. Geol., 21: 291--306. Altered and metamorphosed volcanic suites can be characterized in terms of magma series (alkaline, sub-alkaline) and degree of differentiation (basaltic, andesitic, etc.) using elements that are immobile (Ti, Zr, Y, Nb, Ce, Ga, Sc) during secondary alteration processes. Geochemical data on meta-volcanic suites in the greenschist, amphibolite and granulite facies have been plotted on a series of immobile-element diagrams to illustrate the method of discrimination and classification. Extrusive and high-level intrusive volcanic rocks that have undergone subsequentalteration or metamorphism, such as spilites, keratophyres, tuffs, greenstones, greenschists, ortho-amphibolites and ortho-gneisses, can be plotted. Evidence for the volcanogenic nature of the metamorphic rocks must be obtained before the diagrams (which are based on fresh volcanic rocks) can be used in a meaningful way. INTRODUCTION
In a previous paper (Winchester and Floyd, 1977) it was demonstrated that various ratios and abundances of selected minor and trace elements could be used to discriminate between associated rock types or differentiation products of alkaline and sub-alkaline volcanic rocks. Several diagrams were constructed, using combinations of elements such as Ti, Zr, Y, Nb, Ga, Ce and Sc, which exhibited both the type of alkalinity and degree of differentiation of standard magmatic rock series. As these elements are relatively immobile (Cann, 1970; Field and EUiott, 1974) during post-consolidation alteration processes (e.g., spilitic alteration, metamorphism), the constructed diagrams may prove useful in the identification of ancient volcanic suites, and their differentiation products. This approach has proved satisfactory in discriminating between sub-alkaline (tholeiites) and alkaline basaltic rocks (Floyd and Winchester, 1975; Winchester and Floyd, 1976) but has the added advantage here in that rocks of any composition may be taken without prior selection of strictly basaltic rocks -- often a difficult process in highly altered rocks.
292
ANCIENT VOLCANIC SUITES
Ancient volcanic rocks, and also some contemporary volcanics, invariably show some low-grade mineralogical and textural degradation, or may be metamorphosed so that their original nature and composition may be obscured. Where original mineralogy or texture is preserved, the volcanic origins of any altered ancient suite may be relatively clear and the diagrams (Winchester and Floyd, 1977) will identify the various rock type present and their overall character. However, where tectonism and/or higher grades of metamorphism have produced a series of schistose or gneissose rocks of variable composition that might represent metamorphosed volcanic products, the magmatic or volcanogenic parentage of the series may need to be demonstrated by comparing any trend of chemical variation with typical magmatic and sedimentary suites (cf. Evans and Leake, 1960; Leake, 1964; Van de Kamp, 1968). If magmatic, the metamorphosed series can also be characterized in terms of common volcanic rocks using the prepared diagrams. This utilization of immobile elements enables a more meaningful interpretation to be placed on ancient volcanic suites in terms of magma type and degree of differentiation. Coupled with a similar approach to identify the tectonic environment (Pearce and Cann, 1973) the immobile elements increase the potential usefulness of ancient volcanic rocks in plate-tectonic reconstructions. It should thus be possible to characterize more fully both low- and high-grade Archaean greenstone suites, as well as "spilitized" volcanics in Proterozoic and Palaeozoic eugeosynclinal sequences. The object of this paper is to demonstrate the application of the immobile-element diagrams (Winchester and Floyd, 1977) to variously altered and metamorphosed volcanic suites. SOURCES OF DATA
In general, the types of ancient volcanics that can be plotted are restricted due to the paucity of suitable trace-element data. Although there are numerous major-element analyses of spilitic rocks in the literature, comprehensive trace-element data are very limited in this group of interesting rocks. Similarly, the classic Archaean greenstone province from southern Africa (Barberton Mountainland) has few published trace-element analyses and thus an immobile-element comparison with other Archaean greenstone belts is difficult. Altered or degraded volcanic rocks as well as the variably metamorphosed equivalents of volcanic suites have been grouped into three main sections as follows: (1) low-grade volcanics (mainly spilitic assemblages), (2) Archaean greenstone suites, (3) greenschist/amphibolite assemblages. The sources of the actual data used are shown in Table I.
293
TABLE I Sources of data of altered and metamorphosed volcanic rocks plotted on discrimination diagrams Locality
Rock type
Fig.
Reference
Low-grade volcanics: Carlsberg Ridge Mid-Atlantic Ridge (6°N) Mid-Atlantic Ridge (22°N) NW Germany
spilites metagabbros greenstones spilites
1 1 1 1
spilites basaltic/andesitic volcanics acid volcanics
1
Cann (1969) Bonatti et al. (1975) Melson et al. (1968) Herrmann and Wedepohl (1970) Loeschke (1973, 1975)
1 1, 4
Kean and Strong (1975) Tremlett (1972)
basic/intermediate volcanics
3
Karawanken Mtns., Austria Notre Dame Bay, Newfoundland Lleyn peninsula, N Wales
Archaean greenstone suites: Noranda and Yellowknife, Canada E Goldfields, Western Australia
basic volcanics
Chitaldrug, India Marda Complex, Western Australia
meta-basalts intermediate/acid volcanics
W.R.A. Baragar and A.M. Goodwin (1969) and pets. commun. (1974: 2, 3 J.A. Hallberg (1972) and pets. commun. (1974: 2, 3 Naqvi and Hussain (1973 2, 4 Hallberg et al. (1976)
Greenschist/amphibolite assemblages: Tayvallich, SW Scotland greenschists/amphi- 4--6 bolites Haliburton Madoc, Canada ortho-amphibolites; 4, 5 para-amphibolites and gneisses (meta-tuffs) "Green Bed" horizon, S Scotland greenschists (meta- 4, 6 tuffs) Adirondack Mtns, U.S.A. ortho-amphibolites 5 and granulites
Wilson and Leake (1972) Van de Kamp (1968)
van de Kamp (1970) Engel and Engel (1962)
LOW-GRADE VOLCANICS
Spilitic rocks are often considered to represent the post~consolidation alteration of predominantly basaltic volcanics that have developed a low-grade (often greenschist) assemblage (Vallance, 1960, 1969; Cann, 1969). Likewise, keratophyric rocks, sometimes associated with spilitic volcanics in eugeosynclinal sequences, may be low-grade meta-rhyolites or possibly meta-dacites (Hughes, 1973). Alteration may often obscure the primary mineralogy so that the identity of the spilitic-keratophyric association is not clear, in terms of both rock type and magmatic series. If it is recognized that spilites and
294 keratophyres represent meta-volcanics then immobile-element plots, based on fresh rocks (Winchester and Floyd, 1977), should allow a better characterization to be achieved. Low-grade assemblages are not confined to ancient volcanics such as eugeosynclinal spilites, but are also found in the m o d e m ocean-floor environment. Here, rocks variously described as spilites, greenstones or meta-basalts/gabbros (Melson et al., 1968; Cann, 1969; Miyashiro et al., 1969, 1971; Bonatti et al., 1975) show similar alteration effects, although their close association with fresh material often betrays their true nature. Fig. 1 compares the distribution of modern ocean-floor meta-volcanics in terms of SiO2 content and Zr/TiO2 ratio. Unfortunately, the plotting of SiO2 values reduces the usefulness of the diagram as this c o m p o n e n t may often be mobile during alteration; as exemplified by the generally lower SiO2 c o n t e n t of some spilites plotted relative to most basalts. However, m a n y fresh (and altered) basalts have Zr/TiO2 ratios of ~<0.010 (Winchester and Floyd, 1977, figs. 2 and 6), suggesting that most spilites are indeed basaltic in composition. Spilites from NW Germany include both basaltic and andesitic types (Herrmann and Wedepohl, 1970), although this is not indicated by a marked increase in the Zr/TiO2 ratio for the so-called andesitic spilites. Based on their generally high Ti and Zr contents, Loeschke (1975) considered the Karawanken spilites to have the composition of hawaiites or mugearites. The massive spilites and pillow lavas from this area have average Zr/TiO2 ratios of 0.0077 and 0.0079, respectively, which when compared with the tabulated data of Winchester and Floyd (1977, table II) suggests they are principally alkali basalts or hawaiites, but not mugearites. As seen in Fig.1 only two samples from this area plot in the high-SiO2 alkali basalt field (= mugearite) with Zr/TiO2 ratios of ~0.015. The initial classification of the Karawanken spilites as belonging to the alkali basalt series by Winchester and Floyd (1976), according to their immobile-element ratios, is apparently not so clearly defined in Fig.1. However, SiO2 content is an unreliable guide to this c o n t e x t and as there is considerable overlap between the basaltic types (see Winchester and Floyd, 1977, fig.2) the sub-alkaline--alkaline boundary need not be definitive. The distinctive chemical nature of the "basalt" and "andesite" groups from Newfoundland, as described by Kean and Strong (i975) are discernable here (Fig.l) and also show the more differentiated character of the latter group in terms of generally higher Zr/TiO2 ratios. Partly altered magmatic rocks from the Lleyn peninsula, N Wales, are described by Tremlett (1972) as microgranites and rhyolites and as such represent high-level acid volcanics. According to their field relationships and chemistry they were divided into two age groupings, although in terms o f Zr/TiO2 (and Nb/Y ratio, Fig.4) this is not obvious and they could all represent a sequence of variously differentiated alkaline acidic rock. It is interesting to note that they are alkaline relative to the predominant sub-alkaline basaltic
295
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t[oor
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o N W Oermon A &
70
of m o d e r n
Korowonken
,olcom, ES
L[eyn De~,~siJIC ocid volconics
-
-
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/
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•
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. . . . ~÷
I~÷N
°
OlO1
Zr / TiO 2
o'1o
{~o
Fig.1. SiO2--Zr/TiO 2 diagram ~howing field of altered modern ocean-floor basaltic rocks and various ancient low-grade volcanic suites. References to plotted data in Table I and rock group subdivision from Winchester and Floyd ( 1 9 7 7 ) - this applies to all subsequent diagrams. A B = alkali basalts, hawaiites, mugearites, trachybasalts; s u b - A B = sub-alkaline basalts (tholeiitic and high-alumina); B + T B + N = basanites, trachybasanites, nephelinites; A = andesites; D + R D = dacites and rhyodacites; R -- rhyolites; T A = trachyandesites; T = trachytes; P h = phonolites; C + P = comendites and pantellerites. volcanism in this area d u r i n g the Ordovician (Hughes, 1 9 7 2 ; F l o y d et al., 1976). In general, i m m o b i l e trace e l e m e n t d a t a for spilites or greenstones are scarce. H o w e v e r , b o t h fresh and altered rocks f r o m the Mid-Atlantic Ridge (Melson e t al., 1 9 6 8 ) have similar Ga c o n t e n t s and Zr/TiO2 ratios, a n d all p l o t in the basaltic ( p r e d o m i n a n t l y tholeiitic) field o n the Ga--Zr/TiO2 diagram (Winchester and F l o y d , 1 9 7 7 , fig.7). O t h e r i m m o b i l e - e l e m e n t plots are clearly m o r e meaningful t h a n t h e one illustrated h e r e for spilites (Fig.l): and require m o r e - t r a c e - e l e m e n t d a t a t o be p u b l i s h e d o n these rocks• However, a n o t e o f c a u t i o n is r e q u i r e d f o r a n c i e n t pillow l a v a s t h a t e x h i b i t a spilitic m i n e r a l o g y and chemical d i f f e r e n c e s b e t w e e n core and rim. In some cases, b u t b y n o m e a n s all, the rims or selvages of pillows m a y s h o w a relative e n r i c h m e n t in t h e " i m m o b i l e " elements, such as Ti, Zr, Y, Ga and Sc (Vallance, 1969, table 3; F l o y d and Lees, 1973)• In these instances, t h e Zr/TiO2 ratio and t o a lesser e x t e n t , the N b / Y ratio, remain essentially c o n s t a n t (Table II), owing t o a s y m p a t h e t i c increase in each
296
TABLE
II
Distribution of some immobile elements between core (C) and rim (R) portions lavas and altered blocks (oxide in wt.%, trace elements in ppm) No*
TiO 2
Zr
Y
Nb
Ga
Sc
Zr/TiO 2
Nb/Y
Ga/Sc
0.22
of pillow
Pillow lavas: 1C
1.39
140
22
--
10
45
0.010
--
1R
2.44
270
15
--
10
45
0.011
--
0.22
2C
2.36
120
22
--
22
26
0.005
--
0.35
2R
2.70
270
22
--
10
35
0.010
--
0.29
3C
1.90
56
21
--
22
46
0.002
--
0.48
3R
2.10
70
21
--
28
46
0.003
--
0.61
4C
1.68
40
12
--
15
26
0.002
--
0.58
4R
4.08
175
55
--
45
100
0.004
--
0.45
5C
1.74
150
22
--
22
22
0.008
--
1.00
5R
15.78
1,200
125
--
22
46
0.007
--
0.48
6C
1.50
200
30
--
18
35
0.013
--
0.51
6R 7C 7R
1.31 0.56 0.84
150 260 180
20 23 33
----
18 15 22
20 ---
0.011 0.046 0.021
----
0.90 ---
8C
3.55
281
31
.
--
3.99
318
23
--
--
0.007 0.007
--
8R
--
--
9C
3.40
281
30
.
0.008
--
--
9R
. .
. -.
.
.
4.20
330
27
--
--
--
0.007
--
--
10C 10R
3.49 3.34
271 284
31 24
---
---
---
0.007 0.008
---
---
11C
280 416 238 264 142
31 33 24 26 13
-. . 31 38 17
--
--
----
0.008 0.008 0.007 0.007 0.010
--
12C 12R 13C
3.49 5.17 3.01 3.58 1.42
-1.29 1.46 1.31
-----
13R 14C
1.94 2.37
186 188
19 21
17 24
---
---
0.009 0.007
0.89 1.14
---
14R
2.34
183
20
23
--
--
0.007
1.15
--
15C
2.18
196
20
22
--
--
0.008
1.10
--
15R
2.29
196
17
23
--
--
0.008
1.35
--
16C
2.56
132
19
19
--
--
0.005
1.00
--
16R
2.75
201
18
27
--
--
0.007
1.50
--
17C
2.36
197
19
25
--
--
0.008
1.32
--
17R
2.46
198
20
24
--
--
0.008
1.20
--
17.6 17.3 15.5 15.1 15.2 14.8
-------
0.009 0.009 0.007 0.007 0.006 0.006
0.04 0.04 0.11 0.12 0.05 0.07
-------
1 1 R
-. . ----
Altered blocks: 18C 18R 19C 19R 20C 20R
1.94 1.91 1.40 1.38 1.13 1.13
186 182 104 102 74 69
41 42 29 29 23 25
1.7 1.7 3.1 3.6 1.2 1.7
*Samples numbered 1--5 from Vallance (1969); 6 from Vallance (1974); 7 from Naqvi and Hussain (1973); 8--11 from Loeschke (1975); 12--17 from Floyd and Lees (1973) a n d u n p u b l i s h e d d a t a b y P . A . F l o y d ( 1 9 7 5 ) ; 1 8 - - 2 0 f r o m H a r t e t al. ( 1 9 7 4 ) .
297
component of the ratio. Thus both core and rim values can be plotted. On the other hand, the variable nature of the Ga/Sc ratio between core and rim suggests it is less reliable and should be treated more cautiously. In general practice the immobile-element content of the core portion only should be plotted in pillow lavas. A R C H A E A N G R E E N S T O N E SUITES
It is generally assumed that Archaean greenstone suites have suffered little chemical alteration, although they now frequently exhibit assemblages characteristic of the greenschist facies of metamorphism (Glikson, 1971; Hallberg, 1972; Hart et al., 1974; Hallberg et al., 1976). Here the apparently isochemical nature of low-grade metamorphism is entirely different to thaL characterized and suffered by Palaeozoic eugeosynclinical volcanics. Based on the assumption of chemical uniformitarianism with time, the chemistry of Archaean volcanics (including many elements that are generally mobile) is often directly compared with that of modern volcanics. On this basis these ancient volcanics are interpreted as representative of the calc-alkali association and tholeiitic series typical of modern island arcs and ocean floor (Baragar and Geodwin, 1969; Hart et al., 1970; 1974; Hallberg, 1972). Irrespective of whether low-grade metamorphism was isochemical or not, the immobile-element plots (Winchester and Floyd, 1977) should provide an additional method of characterizing the products of Archaean volcanism in terms of magma type and degree of differentiation. Archaean volcanics from Australia, India and Canada are plotted on the SiO2--Zr/TiO2 diagram (Fig.2), although, as stated previously, the incorporation of SiO2 renders this plot less definitive than a plot of Ga--Zr/TiO2 (Fig.3). However, a comparison of the two diagrams (Figs.2 and 3) bring out some interesting features. The Eastern Goldfields intrusives and pillow lavas from Australia are distinctly sub-alkaline (low Ga) and principally basaltic (generally low Zr/TiO2), although a few intermediate, mainly andesitic, types do occur. This is similar to the findings of Hallberg (1972) who considered the majority of the greenstones to be tholeiitic basalts. Geochemically the Marda volcanics, also from Australia, are a calc-alkaline sequence of andesitic-dacitic--rhyolitic volcanic rocks (Hallberg etal., 1976) that have undergone mild greenschist metamorphism. Although this characterization is indicated using the SiO2 content (Fig.2) a plot using the more reliable immobile-element ratios, Zr/TiO2 and Nb/Y (see Fig.4), confirms the spread of rock types. The Chitaldrug volcanics from India include both intrusives (now amphibolites) and pillow lavas that are stated by Naqvi and Hussain (1973) to represent tholeiitic meta-basalts. As seen in Fig.2 they have generally high Zr/TiO~ ratios and plot predominantly in the alkali basalt sector. It was noticed by Winchester and Floyd (1976), utilizing basalt-type immobile-element plots, that these rocks were generally characterized as sub-alkaline, al-
298
8C
75
+ ChJtofdrug metobosolt x Marda votcanpc5 • E Goldfield greenstones (-:'fleld of ye(lowknlfe greenstones C) f~eld of Noranda greenstones
J × •
_x
RD,D
~
R x--x-.~" /
7o
.p
65
I.f)
/
ss
.j.
.
L
z.0
/
/I
,...: .... ,,
•. .'-
/
~
//~,#t + / ,,/
l 001
+/
/
/
~ /_ B*TB*N
Zr/TJO 2
~"C ~
"J ~oo
Fig.2. SiO2--Zr/TiO2 diagram showing distribution of Arehaean greenstones from Canada (Noranda and Yellowknife), Australia (Eastern Goldfields and Marda) and India (Chitaldrug).
though occasionally they had "alkaline" features (e.g., very high P205; Winchester and Floyd, 1976, fig.6) as seen here (Fig.2). This characterization problem can be resolved by considering Fig.3, which, using immobile elements only indicates that these amphibolites are by no means entirely basaltic but predominantly an andesitic-rhyodacitic/dacitic sequence. This explains their higher than usual Zr/TiO2 ratio and P2Os contents, and also demonstrates that they must have lost some SiO2 during amphibolite facies metamorphism to appear generally "basic" now. Calc-alkali volcanics from the Yellowknife and Noranda districts in Canada show a very wide spread of data in both Figs. 2 and 3. These plots suggest that some of the Noranda rocks may contain more alkaline basalts, unless the high Ga contents are (analytically) anomalous. GREENS CHIST/AMPHIBOLITE ASSEMBLAGES
Abundant suitable trace-element data from a number of well-documented amphibolitic suites permitted three immobile-element diagrams to be drawn (Figs.4--6). The correspondence in terms of magmatic series characterization between each diagram is good.
299
I O0
+ Ch~latdrug
melabasalts
• E GoldfieLds greenstones (j~ field of Noranda greenstones ,,~; field of Yellow knde greenslones
C÷P ÷ 0 10
R
÷
T° Ph ÷
RD+O +
+
TA I
O
+
i,.:-h
_ _ ~ ÷
+ +
~
___
÷ t++
N
0 01
+
+
--
/J
A
! .
~/5 ~
! •
/
. . /+
/
\ _
\\
0001
L
lt0
20
30 Ga
ppm)
Fig. 3. Zr/TiO~--Ga diagram showing distribution of Archaean greenstones from Canada (Noranda and Yellowknife), Australia (Eastern Goldfields) and India (Chitaldrug). Dotted lines enclose the spread of Canadian data.
Although ortho-amphiholites or amphibole-rich assemblages are broadly considered to represent meta-basaltic rocks, some exhibit a more intermediate chemistry and may be meta-andesites. Using the method and diagrams illustrated previously (Winchester and Floyd, 1977) the range of composition seen in amphibolitic suites can be satisfactorily translated into rock types• Also any change in chemistry with progressive metamorphism of basaltic rocks can be monitored by using immobile elements. For example, Engel and Engel (1962) noted that Adirondack Mountain amphibolites became "basified" with progressive metamorphism and developed a more alkaline character in the granitized areas. As will be demonstrated below, this overall change has not affected the immobile-element characterization of the high-grade amphibolites relative to those of lower grade. Although not strictly amphibolites, the Tayvallich volcanics from the SW Highlands of Scotland are included here as they have developed a variable regional schistosity together with actinolitic amphibole, unlike most of the
300 low-grade volcanics discussed previously. They are a series of metamorphosed sills and lavas (sometimes pillowed) subjected to greenschist facies metamorphism (Wilson and Leake, 1972; Graham, 1976) and are locally referred to as "epidiorites". According to Graham (1976) the least differentiated basaltic members represent a strongly tholeiitic series that has undergone extensive Ti, P and Zr enrichment during subsequent fractionation. However, using the data of Wilson and Leake (1972) over the full range of compositions sampled (including the acidic Barbreack agglomerate), the basaltic members are tholeiitic, but the suite as a whole has mildly alkaline features in terms of these elements. In all the diagrams (Figs. 4--6) the Tayvallich volcanics often plot astride the alkaline--sub-alkaline divide with the Barbreack agglomerate representing a meta-trachyte. Therefore, these plots suggest that the Tayvallich volcanics are less tholeiitic than other meta-volcanics within the Scottish Dalradian sequence as suggested by Wilson and Leake {1972), and are clearly distinguishable from the contemporary "Green Bed" horizon meta-volcanics developed in the southern Highlands. Currently the "Green Bed" greenschists and amphibolites are accepted as a series of metamorphosed sedimentary tuffs of overall basaltic composition (van de Kamp, 1970). As seen in Figs. 4 and 6 they are clearly sub-alkaline with low Nb/Y ratios (Fig.4), uniformly low Ce (Fig. 6) and contrast with the Tayvallich volcanics. The "Green Bed" meta-tuffs characteristically exhibit generally high Zr/TiO2 ratios which suggest that they include tuffs of andesitic or even dacitic composition. However, consistently high Cr contents (van de Kamp, 1970) suggest that the enhanced Zr/TiO2 ratios may also be a product of contamination with nonvolcanogenic material. The Adirondack Mountain ortho-amphibolites described by Engel and Engel (1962) are plotted in Fig.5. With increasing metamorphic grade the amphibolites from Emeryville develop pyroxene and pass into granulites near Cnlton, although as exhibited by Fig.5 there does not appear to be any marked changes in the Zr/TiO2 ratio and Ga content between the two rock types. Both amphibolites and basic granulites have retained their basaltic characters and the retention of relatively low Ga values is indicative of their sub-alkaline nature. Although high metamorphic grade does not appear to have affected the immobile-element proportions, Drury (1972, 1973) states that Y, Nb and Ce may be depleted during granulite facies metamorphism of both basic and acidic rocks due to non-compatibility with the new mineral phases. If abundances (and relative proportions) change at very high grades of metamorphism the diagrams should be treated with caution for volcanics in the granulite facies, although there appears to be no obvious change with increasing grade for rocks of different bulk compositions (Table III). The Haliburton--Madoc region of the Grenville Province, Canada, contains ortho-amphibolites and various amphibolitic and gneissose meta-sediments (van de Kamp, 1968). From a detailed geochemical study van de Kamp (1968) considered the ortho-amphibolites were Na-metasomatized tholeiitic basalts, whereas the para-amphibolites and biotite--plagioclase gneisses were meta-
301 1 O0
• • 4•
Lleyn peninsula ac+d votcan,cs Narde votcanJcs Tayvelbch votcanncs o-amphibotites "~ Ha[iburton mete-luffs J Madoc × "Green Bed" m e t e - t u f t s
\
C÷#
•
\
~.
*
\
"
•
Ph
,, \
~am•
•
+
R
.~.°.
~
~
\ \
T
\
010
• I-\
oo~ .~
~
RD*D
.
• ;. +,~:___~
~+
/
/
001 ÷
A+ •
ool
tl A
• x
0110
x
Nb/y
÷
+
Sub-AB
+
I
~ oo
I I
i
looo
Fig.4. Zr/TiO2--Nb/Y diagram showing distribution of various greenschist, amphibolite and granulite facies mete-volcanic suites.
morphosed tufts ranging from basic to acid in composition. These t w o groups of volcanics are ideally suited for study using the immobile-element plots and the range of compositions are directly compared in Figs.4--6. As seen in each of the diagrams both ortho-amphibolites and meta-tuffs are strongly sub-alkaline with l o w Nb/Y ratios, Ga and Ce contents. This substantiates van de Kamp's (1968) view that the apparently "alkaline" nature of the ortho-amphibelites (on a total alkalis--silica plot) is due to Na-metasomatism. N o t e also that the ortho-amphibolites are with few exceptions basalts, whereas the meta-tuffs are predominantly basalts and andesites, with some rhyodacites/ dacites. A better correspondence between the characterising of the metaluffs into andesites or rhyodacites/dacites (in Figs.4 and 5) could be made if this rock type boundary was made horizontal at a constant 0.025 Zr/TiO2 rather than a negative slope (as in Fig.4). This would then help to subdivide the rock types in Fig.6 where andesites and dacites are plotted together in one field.
302
10C
+ Tayvalhch volcanics . o - amphJbo[,tes] Hal, burtonmeta-tuffs ] Madoc • Emeryw[le amphlbobtes / Colton granuhtes } Adirondacks
C*P
/
01 0
4, / ~-~-~t_
. . . . RD+D
*
0
÷~
,' ,'
J
, / a
1:
÷
÷ T*Ph
~ TA
BASALTS
OQ1
0o ;.o.:,t4tto .{. , :
I
I
10
20
I0
3
~
_
_
_
_
40
Ga ( p p m.)
Fig. 5. Z r / T i O ~ - - G a variation diagram showing distribution of various greenschist, amphibolite and granulite facies meta-volcanic suites.
CONCLUSIONS
Based on the distribution of various elements considered to remain immobile during subsequent alteration of fresh volcanic rocks, different meta-volcanic suites have been distinguished in terms of magma series and degree of differentiation. This approach provides a reliable method of characterizing altered and metamorphosed rocks such as spilites, keratophyres, greenstones, amphibolites and some gneisses that may have undergone some chemical change during metamorphism. Before the diagrams can be used, field and petrochemical evidence is required to demonstrate that the metamorphosed or altered rocks are not only igneous but volcanic {extrusive and high-level intrusive). Recognizable volcanogenic sediments, such as tuffs, can also be plotted to obtain an indication of their bulk composition, provided that they do not contain much non-volcanogenic material.
303
C÷P 4-
~.~ OlC
÷
R+RD
+ +
I
.
iI + ..+~
.; "+/+
.
T . Oh
e
*t-
D
,
•
TA
+
t,
/
,h
~ :, %V.~(-o//B*TB 001
#i ÷
.
g~
+
÷
~,
5
Tayvalhch
volcan,cs
,, o-amphLbol,tes"~ Hat)burton,~ m e t a - t u f t s .,f Medoc "Green B e d meta-tuffs Lleyn pen,nsula aod vo[cGntcs
~oo
~o
~oo Ce ( p p m }
2 o
3oo
~se
Fig.6. Zr/TiO2--Ce variation diagram showing distribution of various greenschist, amphibolite and granulite facies meta-volcanic suites.
The immobile-element diagrams must be used with caution for (a) pillow lavas and (b) high-grade gneisses. In some cases chemical heterogeneity within pillow lavas produces considerable variation between the rim and core portion, and only the least altered part (usually the core) should be plotted. At present there appears to be no clear evidence for systematic change in immobile-element contents in granulite facies gneisses and thus they may be plotted along with meta-magmatic rocks of lower grades. The problem is more one of initially recognizing the magmatic nature of the gneisses and then demonstrating that they are of volcanic origin rather than plutonic. This may be a major problem in polydeformed high-grade metamorphic terrains. ACKNOWLEDGEMENTS
Grateful thanks are due to Drs. J.A. Hallberg, W.R.A. Baragar and S.A. Drury for allowing us to use unpublished compilations of their geochemical data.
0.007
-
0.006 --
-
1.56 13 45 104 --
7 (1, 2) + 8 (1, 2)
3 (F)
3 (E)
Drury (1974)
1.53 . 21 89 5 20 0.005 0.238
G
.
0.43 .
3 (B)
0.54 7 144 4 65 0.026 0.571
G
3 (A)
Drury (1974)
7 169 6 50 0.039 0.857
A
Gneisses*'
*' Gneiss of intermediate composition; * ~ amphibolite facies; *3 granulite facies. -
Table and (column)
References Engeland Engel (1962)
ZrfTiOz Nb/Y
1.89 14 59 145 --
TiO2 Ga Y Zr Nb Ce
1.37 . 23 89 7 21 0.006 0.304
A
A* 2
Grade
G* 3
Meta-basics
Rock type Amphibolites
0.5
0.6
0.024
144
G
-
2 (1)
2 (2)
Heier and Thoresen (1971)
0.025
128
A
Gneisses* '
Variation of average immobile-element abundances and ratios with m e t a m o r p h i c grade (TiO, in wt.% and trace elements in p p m )
TABLE III
0.57
0.56
0.029 0.454
11 165 5
G
II (1)
I (6)
Holland and L a m b e r t (1975)
0.024 0.250
12 140 3
A
Gneisses* '
0.55
0.47 8 197 5 42 0.041 0.625
G
2 (8)
1 (2)
Tarney et al. (1972)
9 178 5 43 0.032 0.555
A
Gneisses* '
O
305
REFERENCES Baragar, W.R.A. and Goodwin, A.M., 1969. Andesites and Archaean volcanism of the Canadian shield. Bull. Oreg. Dep. Miner. Ind., 65: 121--142. Bonatti, E., Honnorez, J., Kirst, P. and Radicati, F., 1975. Meta-gabbros from the MidAtlantic Ridge at 6°N: contact-hydrothermal-dynamic metamorphism beneath the axial valley. J. Geol., 83: 61--78. Cann, J.R., 1969. Spilites from the Carlsberg Ridge, Indian Ocean. J. Petrol., 10: 1--19. Cann, J.R., 1970. Rb, Sr, Y, Zr, Nb in some ocean floor basaltic rocks. Earth Planet. Sci. Lett., 10: 7--11. Drury, S.A., 1972. The chemistry of some granitic veins from the Lewisian of Coil and Tiree, Argyllshire, Scotland. Chem. Geol., 9: 175--193. Drury, S.A., 1973. The geochemistry of Precambrian granulite facies rocks from the Lewisian complex of Tiree, Inner Hebrides, Scotland. Chem. Geol., 11: 167--188. Drury, S.A., 1974. Chemical changes during retrogressive metamorphism of Lewisian granulite facies rocks from Coll and Tiree. Scott. J. Geol., 10: 237--256. Engel, A.E.J. and Engel, C.G., 1962. Progressive metamorphism of amphibolite, northwest Adirondack mountains, New York. In: Buddington Volume, Geol. Soc. Am., Bull., pp. 37--82. Evans, B.W. and Leake, B., 1960. The composition of the striped amphibolites of Connemara, Ireland. J. Petrol., 1: 337--363. Field, D. and Elliott, R.B., 1974. The chemistry of gabbro/amphibolite transitions in south Norway. Contrib. Mineral. Petrol. 47: 63--76. Floyd, P.A. and Lees, G.J., 1973. Ti--Zr characterization of some Cornish pillow lavas. Proc. Ussher Soc., 2: 489--494. Floyd, P.A. and Winchester, J.A., 1975. Magma type and tectonic setting discrimination using immobile elements. Earth Planet. Sci. Lett., 27 : 211--218. Floyd, P.A., Lees, G.J. and Roach, R.A., 1976. Basic intrusions in the Ordovician of North Wales - - geochemical data and tectonic setting. Proc. Geol. Assoc., 87: 389--400. Glikson, A.Y., 1971. Primitive Archaean element distribution patterns: chemical evidence and geotectonic significance. Earth Planet. Sci. Lett., 12: 309--320. Graham, C., 1976. Petrochemistry and tectonic significance of Dalradian metabasaltic rocks of the S.W. Scottish Highlands. J. Geol. Soc. London, 132: 61--84. Hallberg, J.A., 1972. Geochemistry of Archaean volcanic belts in the Eastern Goldfields region of Western Australia. J. petrol., 13: 45--56. Hallberg, J.A., Johnston, C. and Bye, S.M., 1976. The Archaean Marda igneous complex, Western Australia. Precambrian Res., 3: 111--136. Hart, S.R., Brooks, C., Krogh, T.E., Davis, G.L. and Nava, D., 1970. Ancient and modern volcanic rocks: a trace element model. Earth Planet. Sci. Lett., 10: 17--28. Hart, S.R.I Erlank, A.J. and Kable, E.J.D., 1974. Sea-floor basalt alteration: some chemical and Sr isotopic effects. Contrib. Mineral. Petrol., 44: 219--230. Heier, K.S. and Thoresen, K., 1971. Geochemistry of high grade metamorphic rocks, Lofoten--Vesteralen, North Norway. Geochim. Cosmochim. Acta, 35: 89--99. Herrmann, A.G. and Wedepohl, K.H., 1970. Untersuchungen an spilitischen Gesteinen der variskischen geosynkline in Nordwestdeutschland. Contrib. Mineral. Petrol., 29: 255--274. Holland, J.G. and Lambert, R.St.J., 1975. The chemistry and origin of the Lewisian gneisses of the Scottish mainland: the Scourie and Inver assemblages and sub-crustal accretion. Precambrian Res., 2: 161--188. Hughes, D.J., 1972. The petrochemistry of the Ordovician igneous rocks of the Welsh B a s i n - - a review. Proc. Geol. Soc. London, 128: 418. Hughes, C.J., 1973. Spilites, keratophyres and the igneous spectrum. Geol. Mag., 109: 513--527.
306
Kean, B.F. and Strong, D.F., 1975. Geochemical evolution of an Ordovician island arc of the central Newfoundland Appalachians. Am. J. Sci., 275: 97--118. Leake, B., 1964. The chemical distinction between ortho- and para-amphibolites. J. Petrol., 5: 238--254. Loeschke, J., 1973. Zur petrogenese pal~ieozoischer Spilite aus den Ostalpen. Neues Jahrb. Mineral. Abh., 119: 20--56. Loeschke, J., 1975. Spurelement-Daten yon pal~ioischen Spiliten aus den Ostalpen und ihre Bedeutung fur geotektonische Interpreta~ionen. Geol. Run dsch., 64: 62--74. Melson, W.G., Thompson, G. and van Andel, T.H., 1968. Volcanism and metamorphism in the Mid-Atlantic Ridge, 22°N latitude. J. Geophys. Res., 73: 5925--5941. Miyashiro, A., Shido, F. and Ewing, M., 1969. Diversity and origin of abyssal tholeiite from the Mid-Atlantic Ridge near 24 ° and 30 ° north latitude. Contrib. Mineral. Petrol., 23: 38--52. Miyashiro, A., Shido, F. and Ewing, M., 1971. Metamorphism in the Mid-Atlantic Ridge near 24 ° and 30°N. Philos. Trans. R. Soc. London, 268: 589--603. Naqvi, S.M. and Hussain, S.M., 1973. Relation between trace and major element composition of the Chitaldrug metabasalts, Mysore, India and the Archaean mantle. Chem; Geol., 11: 17--30. Pearce, J.A. and Cann, J.R., 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth Planet. Sci. Lett., 19: 290--300. Tarney, J., Skinner, A.C. and Sheraton, J.W., 1972. A geochemical comparison of major Archaean gneiss units from northwest Scotland and East Greenland. Proc. 24th Int. Geol. Congr., Montreal, Que., 1: 1 6 2 - 1 7 4 . Tremlett, W.E., 1972. Some geochemical characteristics of Ordovician and Caledonian acid intrusions of Lleyn, North Wales. Proc. Yorks. Geol. Soc., 39: 33--58. Vallance, T.G., 1960. Concerning spilites. Proc. Linn. Soc. N.S.W., 85: 7--52. Vallance, T.G., 1969. Spilites again: some consequences of the degradation of basalts. Proc. Linn. Soc. N.S.W., 94: 8--51. Vallance, T.G., 1974. Spilitic degradation of a tholeiitic basalt. J. Petrol., 15: 79--96. van de Kamp, P.C., 1968. Geochemistry and origin of meta-sediments in the Haliburton-Madoc area, southeastern Ontario. Can. J. Earth Sci., 5: 1337--1372. van de Kamp, P.C., 1970. The Green Beds of the Scottish Dalradian series: geochemistry, origin and metamorphism of mafic sediments. J. Geol., 78: 281--303. Wilson, J.R. and I~ake, B., 1972. The petrochemistry of the epidiorites of the Tayvallich peninsula, north Knapdale, Argyllshire. Scott. J. Geol., 8: 215--252. Winchester, J.A. and Floyd, P.A,, 1976. Geochemical magma type discrimination: application to altered and metamorphosed basic igneous rocks. Earth Planet. Sci. Lett., 28: 459--469. Winchester, J.A. and Floyd, P.A., 1977. Geochemicaldiscriminationof different magma series and their differentiation products using immobileelements. Chem. Geol., 20: 325--343.
291
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
IDENTIFICATION AND DISCRIMINATION OF ALTERED AND METAMORPHOSED VOLCANIC ROCKS USING IMMOBILE ELEMENTS
P.A. FLOYD and J.A. WINCHESTER
Department of Geology, University of Keele, Keele, Staffs. ST5 5BG (Great Britain) (Received November 4, 1976; revised and accepted February 24, 1977)
ABSTRACT Floyd, P.A. and Winchester, J.A., 1978. Identification and discrimination of altered and metamorphosed volcanic rocks using immobile elements. Chem. Geol., 21: 291--306. Altered and metamorphosed volcanic suites can be characterized in terms of magma series (alkaline, sub-alkaline) and degree of differentiation (basaltic, andesitic, etc.) using elements that are immobile (Ti, Zr, Y, Nb, Ce, Ga, Sc) during secondary alteration processes. Geochemical data on meta-volcanic suites in the greenschist, amphibolite and granulite facies have been plotted on a series of immobile-element diagrams to illustrate the method of discrimination and classification. Extrusive and high-level intrusive volcanic rocks that have undergone subsequentalteration or metamorphism, such as spilites, keratophyres, tuffs, greenstones, greenschists, ortho-amphibolites and ortho-gneisses, can be plotted. Evidence for the volcanogenic nature of the metamorphic rocks must be obtained before the diagrams (which are based on fresh volcanic rocks) can be used in a meaningful way. INTRODUCTION
In a previous paper (Winchester and Floyd, 1977) it was demonstrated that various ratios and abundances of selected minor and trace elements could be used to discriminate between associated rock types or differentiation products of alkaline and sub-alkaline volcanic rocks. Several diagrams were constructed, using combinations of elements such as Ti, Zr, Y, Nb, Ga, Ce and Sc, which exhibited both the type of alkalinity and degree of differentiation of standard magmatic rock series. As these elements are relatively immobile (Cann, 1970; Field and EUiott, 1974) during post-consolidation alteration processes (e.g., spilitic alteration, metamorphism), the constructed diagrams may prove useful in the identification of ancient volcanic suites, and their differentiation products. This approach has proved satisfactory in discriminating between sub-alkaline (tholeiites) and alkaline basaltic rocks (Floyd and Winchester, 1975; Winchester and Floyd, 1976) but has the added advantage here in that rocks of any composition may be taken without prior selection of strictly basaltic rocks -- often a difficult process in highly altered rocks.
292
ANCIENT VOLCANIC SUITES
Ancient volcanic rocks, and also some contemporary volcanics, invariably show some low-grade mineralogical and textural degradation, or may be metamorphosed so that their original nature and composition may be obscured. Where original mineralogy or texture is preserved, the volcanic origins of any altered ancient suite may be relatively clear and the diagrams (Winchester and Floyd, 1977) will identify the various rock type present and their overall character. However, where tectonism and/or higher grades of metamorphism have produced a series of schistose or gneissose rocks of variable composition that might represent metamorphosed volcanic products, the magmatic or volcanogenic parentage of the series may need to be demonstrated by comparing any trend of chemical variation with typical magmatic and sedimentary suites (cf. Evans and Leake, 1960; Leake, 1964; Van de Kamp, 1968). If magmatic, the metamorphosed series can also be characterized in terms of common volcanic rocks using the prepared diagrams. This utilization of immobile elements enables a more meaningful interpretation to be placed on ancient volcanic suites in terms of magma type and degree of differentiation. Coupled with a similar approach to identify the tectonic environment (Pearce and Cann, 1973) the immobile elements increase the potential usefulness of ancient volcanic rocks in plate-tectonic reconstructions. It should thus be possible to characterize more fully both low- and high-grade Archaean greenstone suites, as well as "spilitized" volcanics in Proterozoic and Palaeozoic eugeosynclinal sequences. The object of this paper is to demonstrate the application of the immobile-element diagrams (Winchester and Floyd, 1977) to variously altered and metamorphosed volcanic suites. SOURCES OF DATA
In general, the types of ancient volcanics that can be plotted are restricted due to the paucity of suitable trace-element data. Although there are numerous major-element analyses of spilitic rocks in the literature, comprehensive trace-element data are very limited in this group of interesting rocks. Similarly, the classic Archaean greenstone province from southern Africa (Barberton Mountainland) has few published trace-element analyses and thus an immobile-element comparison with other Archaean greenstone belts is difficult. Altered or degraded volcanic rocks as well as the variably metamorphosed equivalents of volcanic suites have been grouped into three main sections as follows: (1) low-grade volcanics (mainly spilitic assemblages), (2) Archaean greenstone suites, (3) greenschist/amphibolite assemblages. The sources of the actual data used are shown in Table I.
293
TABLE I Sources of data of altered and metamorphosed volcanic rocks plotted on discrimination diagrams Locality
Rock type
Fig.
Reference
Low-grade volcanics: Carlsberg Ridge Mid-Atlantic Ridge (6°N) Mid-Atlantic Ridge (22°N) NW Germany
spilites metagabbros greenstones spilites
1 1 1 1
spilites basaltic/andesitic volcanics acid volcanics
1
Cann (1969) Bonatti et al. (1975) Melson et al. (1968) Herrmann and Wedepohl (1970) Loeschke (1973, 1975)
1 1, 4
Kean and Strong (1975) Tremlett (1972)
basic/intermediate volcanics
3
Karawanken Mtns., Austria Notre Dame Bay, Newfoundland Lleyn peninsula, N Wales
Archaean greenstone suites: Noranda and Yellowknife, Canada E Goldfields, Western Australia
basic volcanics
Chitaldrug, India Marda Complex, Western Australia
meta-basalts intermediate/acid volcanics
W.R.A. Baragar and A.M. Goodwin (1969) and pets. commun. (1974: 2, 3 J.A. Hallberg (1972) and pets. commun. (1974: 2, 3 Naqvi and Hussain (1973 2, 4 Hallberg et al. (1976)
Greenschist/amphibolite assemblages: Tayvallich, SW Scotland greenschists/amphi- 4--6 bolites Haliburton Madoc, Canada ortho-amphibolites; 4, 5 para-amphibolites and gneisses (meta-tuffs) "Green Bed" horizon, S Scotland greenschists (meta- 4, 6 tuffs) Adirondack Mtns, U.S.A. ortho-amphibolites 5 and granulites
Wilson and Leake (1972) Van de Kamp (1968)
van de Kamp (1970) Engel and Engel (1962)
LOW-GRADE VOLCANICS
Spilitic rocks are often considered to represent the post~consolidation alteration of predominantly basaltic volcanics that have developed a low-grade (often greenschist) assemblage (Vallance, 1960, 1969; Cann, 1969). Likewise, keratophyric rocks, sometimes associated with spilitic volcanics in eugeosynclinal sequences, may be low-grade meta-rhyolites or possibly meta-dacites (Hughes, 1973). Alteration may often obscure the primary mineralogy so that the identity of the spilitic-keratophyric association is not clear, in terms of both rock type and magmatic series. If it is recognized that spilites and
294 keratophyres represent meta-volcanics then immobile-element plots, based on fresh rocks (Winchester and Floyd, 1977), should allow a better characterization to be achieved. Low-grade assemblages are not confined to ancient volcanics such as eugeosynclinal spilites, but are also found in the m o d e m ocean-floor environment. Here, rocks variously described as spilites, greenstones or meta-basalts/gabbros (Melson et al., 1968; Cann, 1969; Miyashiro et al., 1969, 1971; Bonatti et al., 1975) show similar alteration effects, although their close association with fresh material often betrays their true nature. Fig. 1 compares the distribution of modern ocean-floor meta-volcanics in terms of SiO2 content and Zr/TiO2 ratio. Unfortunately, the plotting of SiO2 values reduces the usefulness of the diagram as this c o m p o n e n t may often be mobile during alteration; as exemplified by the generally lower SiO2 c o n t e n t of some spilites plotted relative to most basalts. However, m a n y fresh (and altered) basalts have Zr/TiO2 ratios of ~<0.010 (Winchester and Floyd, 1977, figs. 2 and 6), suggesting that most spilites are indeed basaltic in composition. Spilites from NW Germany include both basaltic and andesitic types (Herrmann and Wedepohl, 1970), although this is not indicated by a marked increase in the Zr/TiO2 ratio for the so-called andesitic spilites. Based on their generally high Ti and Zr contents, Loeschke (1975) considered the Karawanken spilites to have the composition of hawaiites or mugearites. The massive spilites and pillow lavas from this area have average Zr/TiO2 ratios of 0.0077 and 0.0079, respectively, which when compared with the tabulated data of Winchester and Floyd (1977, table II) suggests they are principally alkali basalts or hawaiites, but not mugearites. As seen in Fig.1 only two samples from this area plot in the high-SiO2 alkali basalt field (= mugearite) with Zr/TiO2 ratios of ~0.015. The initial classification of the Karawanken spilites as belonging to the alkali basalt series by Winchester and Floyd (1976), according to their immobile-element ratios, is apparently not so clearly defined in Fig.1. However, SiO2 content is an unreliable guide to this c o n t e x t and as there is considerable overlap between the basaltic types (see Winchester and Floyd, 1977, fig.2) the sub-alkaline--alkaline boundary need not be definitive. The distinctive chemical nature of the "basalt" and "andesite" groups from Newfoundland, as described by Kean and Strong (i975) are discernable here (Fig.l) and also show the more differentiated character of the latter group in terms of generally higher Zr/TiO2 ratios. Partly altered magmatic rocks from the Lleyn peninsula, N Wales, are described by Tremlett (1972) as microgranites and rhyolites and as such represent high-level acid volcanics. According to their field relationships and chemistry they were divided into two age groupings, although in terms o f Zr/TiO2 (and Nb/Y ratio, Fig.4) this is not obvious and they could all represent a sequence of variously differentiated alkaline acidic rock. It is interesting to note that they are alkaline relative to the predominant sub-alkaline basaltic
295
i~]', f~eld • 75
oceon
metq
b@so!ts
• R
] ~ Newfobnd[ond J
group
/
•
•
spd,tes
bGs C]It group andes,re
t[oor
sp,lites
o N W Oermon A &
70
of m o d e r n
Korowonken
,olcom, ES
L[eyn De~,~siJIC ocid volconics
-
-
/ i
/
RD * D
~
C* P
/ " -$-
5s
T T
~
"°
~' so oH
,
SO o
• •
i/ , ,,
, | ~ ~tAo/bAB ~ ,2' o
•
~0
A
A
A~/ Y /
. . . . ~÷
I~÷N
°
OlO1
Zr / TiO 2
o'1o
{~o
Fig.1. SiO2--Zr/TiO 2 diagram ~howing field of altered modern ocean-floor basaltic rocks and various ancient low-grade volcanic suites. References to plotted data in Table I and rock group subdivision from Winchester and Floyd ( 1 9 7 7 ) - this applies to all subsequent diagrams. A B = alkali basalts, hawaiites, mugearites, trachybasalts; s u b - A B = sub-alkaline basalts (tholeiitic and high-alumina); B + T B + N = basanites, trachybasanites, nephelinites; A = andesites; D + R D = dacites and rhyodacites; R -- rhyolites; T A = trachyandesites; T = trachytes; P h = phonolites; C + P = comendites and pantellerites. volcanism in this area d u r i n g the Ordovician (Hughes, 1 9 7 2 ; F l o y d et al., 1976). In general, i m m o b i l e trace e l e m e n t d a t a for spilites or greenstones are scarce. H o w e v e r , b o t h fresh and altered rocks f r o m the Mid-Atlantic Ridge (Melson e t al., 1 9 6 8 ) have similar Ga c o n t e n t s and Zr/TiO2 ratios, a n d all p l o t in the basaltic ( p r e d o m i n a n t l y tholeiitic) field o n the Ga--Zr/TiO2 diagram (Winchester and F l o y d , 1 9 7 7 , fig.7). O t h e r i m m o b i l e - e l e m e n t plots are clearly m o r e meaningful t h a n t h e one illustrated h e r e for spilites (Fig.l): and require m o r e - t r a c e - e l e m e n t d a t a t o be p u b l i s h e d o n these rocks• However, a n o t e o f c a u t i o n is r e q u i r e d f o r a n c i e n t pillow l a v a s t h a t e x h i b i t a spilitic m i n e r a l o g y and chemical d i f f e r e n c e s b e t w e e n core and rim. In some cases, b u t b y n o m e a n s all, the rims or selvages of pillows m a y s h o w a relative e n r i c h m e n t in t h e " i m m o b i l e " elements, such as Ti, Zr, Y, Ga and Sc (Vallance, 1969, table 3; F l o y d and Lees, 1973)• In these instances, t h e Zr/TiO2 ratio and t o a lesser e x t e n t , the N b / Y ratio, remain essentially c o n s t a n t (Table II), owing t o a s y m p a t h e t i c increase in each
296
TABLE
II
Distribution of some immobile elements between core (C) and rim (R) portions lavas and altered blocks (oxide in wt.%, trace elements in ppm) No*
TiO 2
Zr
Y
Nb
Ga
Sc
Zr/TiO 2
Nb/Y
Ga/Sc
0.22
of pillow
Pillow lavas: 1C
1.39
140
22
--
10
45
0.010
--
1R
2.44
270
15
--
10
45
0.011
--
0.22
2C
2.36
120
22
--
22
26
0.005
--
0.35
2R
2.70
270
22
--
10
35
0.010
--
0.29
3C
1.90
56
21
--
22
46
0.002
--
0.48
3R
2.10
70
21
--
28
46
0.003
--
0.61
4C
1.68
40
12
--
15
26
0.002
--
0.58
4R
4.08
175
55
--
45
100
0.004
--
0.45
5C
1.74
150
22
--
22
22
0.008
--
1.00
5R
15.78
1,200
125
--
22
46
0.007
--
0.48
6C
1.50
200
30
--
18
35
0.013
--
0.51
6R 7C 7R
1.31 0.56 0.84
150 260 180
20 23 33
----
18 15 22
20 ---
0.011 0.046 0.021
----
0.90 ---
8C
3.55
281
31
.
--
3.99
318
23
--
--
0.007 0.007
--
8R
--
--
9C
3.40
281
30
.
0.008
--
--
9R
. .
. -.
.
.
4.20
330
27
--
--
--
0.007
--
--
10C 10R
3.49 3.34
271 284
31 24
---
---
---
0.007 0.008
---
---
11C
280 416 238 264 142
31 33 24 26 13
-. . 31 38 17
--
--
----
0.008 0.008 0.007 0.007 0.010
--
12C 12R 13C
3.49 5.17 3.01 3.58 1.42
-1.29 1.46 1.31
-----
13R 14C
1.94 2.37
186 188
19 21
17 24
---
---
0.009 0.007
0.89 1.14
---
14R
2.34
183
20
23
--
--
0.007
1.15
--
15C
2.18
196
20
22
--
--
0.008
1.10
--
15R
2.29
196
17
23
--
--
0.008
1.35
--
16C
2.56
132
19
19
--
--
0.005
1.00
--
16R
2.75
201
18
27
--
--
0.007
1.50
--
17C
2.36
197
19
25
--
--
0.008
1.32
--
17R
2.46
198
20
24
--
--
0.008
1.20
--
17.6 17.3 15.5 15.1 15.2 14.8
-------
0.009 0.009 0.007 0.007 0.006 0.006
0.04 0.04 0.11 0.12 0.05 0.07
-------
1 1 R
-. . ----
Altered blocks: 18C 18R 19C 19R 20C 20R
1.94 1.91 1.40 1.38 1.13 1.13
186 182 104 102 74 69
41 42 29 29 23 25
1.7 1.7 3.1 3.6 1.2 1.7
*Samples numbered 1--5 from Vallance (1969); 6 from Vallance (1974); 7 from Naqvi and Hussain (1973); 8--11 from Loeschke (1975); 12--17 from Floyd and Lees (1973) a n d u n p u b l i s h e d d a t a b y P . A . F l o y d ( 1 9 7 5 ) ; 1 8 - - 2 0 f r o m H a r t e t al. ( 1 9 7 4 ) .
297
component of the ratio. Thus both core and rim values can be plotted. On the other hand, the variable nature of the Ga/Sc ratio between core and rim suggests it is less reliable and should be treated more cautiously. In general practice the immobile-element content of the core portion only should be plotted in pillow lavas. A R C H A E A N G R E E N S T O N E SUITES
It is generally assumed that Archaean greenstone suites have suffered little chemical alteration, although they now frequently exhibit assemblages characteristic of the greenschist facies of metamorphism (Glikson, 1971; Hallberg, 1972; Hart et al., 1974; Hallberg et al., 1976). Here the apparently isochemical nature of low-grade metamorphism is entirely different to thaL characterized and suffered by Palaeozoic eugeosynclinical volcanics. Based on the assumption of chemical uniformitarianism with time, the chemistry of Archaean volcanics (including many elements that are generally mobile) is often directly compared with that of modern volcanics. On this basis these ancient volcanics are interpreted as representative of the calc-alkali association and tholeiitic series typical of modern island arcs and ocean floor (Baragar and Geodwin, 1969; Hart et al., 1970; 1974; Hallberg, 1972). Irrespective of whether low-grade metamorphism was isochemical or not, the immobile-element plots (Winchester and Floyd, 1977) should provide an additional method of characterizing the products of Archaean volcanism in terms of magma type and degree of differentiation. Archaean volcanics from Australia, India and Canada are plotted on the SiO2--Zr/TiO2 diagram (Fig.2), although, as stated previously, the incorporation of SiO2 renders this plot less definitive than a plot of Ga--Zr/TiO2 (Fig.3). However, a comparison of the two diagrams (Figs.2 and 3) bring out some interesting features. The Eastern Goldfields intrusives and pillow lavas from Australia are distinctly sub-alkaline (low Ga) and principally basaltic (generally low Zr/TiO2), although a few intermediate, mainly andesitic, types do occur. This is similar to the findings of Hallberg (1972) who considered the majority of the greenstones to be tholeiitic basalts. Geochemically the Marda volcanics, also from Australia, are a calc-alkaline sequence of andesitic-dacitic--rhyolitic volcanic rocks (Hallberg etal., 1976) that have undergone mild greenschist metamorphism. Although this characterization is indicated using the SiO2 content (Fig.2) a plot using the more reliable immobile-element ratios, Zr/TiO2 and Nb/Y (see Fig.4), confirms the spread of rock types. The Chitaldrug volcanics from India include both intrusives (now amphibolites) and pillow lavas that are stated by Naqvi and Hussain (1973) to represent tholeiitic meta-basalts. As seen in Fig.2 they have generally high Zr/TiO~ ratios and plot predominantly in the alkali basalt sector. It was noticed by Winchester and Floyd (1976), utilizing basalt-type immobile-element plots, that these rocks were generally characterized as sub-alkaline, al-
298
8C
75
+ ChJtofdrug metobosolt x Marda votcanpc5 • E Goldfield greenstones (-:'fleld of ye(lowknlfe greenstones C) f~eld of Noranda greenstones
J × •
_x
RD,D
~
R x--x-.~" /
7o
.p
65
I.f)
/
ss
.j.
.
L
z.0
/
/I
,...: .... ,,
•. .'-
/
~
//~,#t + / ,,/
l 001
+/
/
/
~ /_ B*TB*N
Zr/TJO 2
~"C ~
"J ~oo
Fig.2. SiO2--Zr/TiO2 diagram showing distribution of Arehaean greenstones from Canada (Noranda and Yellowknife), Australia (Eastern Goldfields and Marda) and India (Chitaldrug).
though occasionally they had "alkaline" features (e.g., very high P205; Winchester and Floyd, 1976, fig.6) as seen here (Fig.2). This characterization problem can be resolved by considering Fig.3, which, using immobile elements only indicates that these amphibolites are by no means entirely basaltic but predominantly an andesitic-rhyodacitic/dacitic sequence. This explains their higher than usual Zr/TiO2 ratio and P2Os contents, and also demonstrates that they must have lost some SiO2 during amphibolite facies metamorphism to appear generally "basic" now. Calc-alkali volcanics from the Yellowknife and Noranda districts in Canada show a very wide spread of data in both Figs. 2 and 3. These plots suggest that some of the Noranda rocks may contain more alkaline basalts, unless the high Ga contents are (analytically) anomalous. GREENS CHIST/AMPHIBOLITE ASSEMBLAGES
Abundant suitable trace-element data from a number of well-documented amphibolitic suites permitted three immobile-element diagrams to be drawn (Figs.4--6). The correspondence in terms of magmatic series characterization between each diagram is good.
299
I O0
+ Ch~latdrug
melabasalts
• E GoldfieLds greenstones (j~ field of Noranda greenstones ,,~; field of Yellow knde greenslones
C÷P ÷ 0 10
R
÷
T° Ph ÷
RD+O +
+
TA I
O
+
i,.:-h
_ _ ~ ÷
+ +
~
___
÷ t++
N
0 01
+
+
--
/J
A
! .
~/5 ~
! •
/
. . /+
/
\ _
\\
0001
L
lt0
20
30 Ga
ppm)
Fig. 3. Zr/TiO~--Ga diagram showing distribution of Archaean greenstones from Canada (Noranda and Yellowknife), Australia (Eastern Goldfields) and India (Chitaldrug). Dotted lines enclose the spread of Canadian data.
Although ortho-amphiholites or amphibole-rich assemblages are broadly considered to represent meta-basaltic rocks, some exhibit a more intermediate chemistry and may be meta-andesites. Using the method and diagrams illustrated previously (Winchester and Floyd, 1977) the range of composition seen in amphibolitic suites can be satisfactorily translated into rock types• Also any change in chemistry with progressive metamorphism of basaltic rocks can be monitored by using immobile elements. For example, Engel and Engel (1962) noted that Adirondack Mountain amphibolites became "basified" with progressive metamorphism and developed a more alkaline character in the granitized areas. As will be demonstrated below, this overall change has not affected the immobile-element characterization of the high-grade amphibolites relative to those of lower grade. Although not strictly amphibolites, the Tayvallich volcanics from the SW Highlands of Scotland are included here as they have developed a variable regional schistosity together with actinolitic amphibole, unlike most of the
300 low-grade volcanics discussed previously. They are a series of metamorphosed sills and lavas (sometimes pillowed) subjected to greenschist facies metamorphism (Wilson and Leake, 1972; Graham, 1976) and are locally referred to as "epidiorites". According to Graham (1976) the least differentiated basaltic members represent a strongly tholeiitic series that has undergone extensive Ti, P and Zr enrichment during subsequent fractionation. However, using the data of Wilson and Leake (1972) over the full range of compositions sampled (including the acidic Barbreack agglomerate), the basaltic members are tholeiitic, but the suite as a whole has mildly alkaline features in terms of these elements. In all the diagrams (Figs. 4--6) the Tayvallich volcanics often plot astride the alkaline--sub-alkaline divide with the Barbreack agglomerate representing a meta-trachyte. Therefore, these plots suggest that the Tayvallich volcanics are less tholeiitic than other meta-volcanics within the Scottish Dalradian sequence as suggested by Wilson and Leake {1972), and are clearly distinguishable from the contemporary "Green Bed" horizon meta-volcanics developed in the southern Highlands. Currently the "Green Bed" greenschists and amphibolites are accepted as a series of metamorphosed sedimentary tuffs of overall basaltic composition (van de Kamp, 1970). As seen in Figs. 4 and 6 they are clearly sub-alkaline with low Nb/Y ratios (Fig.4), uniformly low Ce (Fig. 6) and contrast with the Tayvallich volcanics. The "Green Bed" meta-tuffs characteristically exhibit generally high Zr/TiO2 ratios which suggest that they include tuffs of andesitic or even dacitic composition. However, consistently high Cr contents (van de Kamp, 1970) suggest that the enhanced Zr/TiO2 ratios may also be a product of contamination with nonvolcanogenic material. The Adirondack Mountain ortho-amphibolites described by Engel and Engel (1962) are plotted in Fig.5. With increasing metamorphic grade the amphibolites from Emeryville develop pyroxene and pass into granulites near Cnlton, although as exhibited by Fig.5 there does not appear to be any marked changes in the Zr/TiO2 ratio and Ga content between the two rock types. Both amphibolites and basic granulites have retained their basaltic characters and the retention of relatively low Ga values is indicative of their sub-alkaline nature. Although high metamorphic grade does not appear to have affected the immobile-element proportions, Drury (1972, 1973) states that Y, Nb and Ce may be depleted during granulite facies metamorphism of both basic and acidic rocks due to non-compatibility with the new mineral phases. If abundances (and relative proportions) change at very high grades of metamorphism the diagrams should be treated with caution for volcanics in the granulite facies, although there appears to be no obvious change with increasing grade for rocks of different bulk compositions (Table III). The Haliburton--Madoc region of the Grenville Province, Canada, contains ortho-amphibolites and various amphibolitic and gneissose meta-sediments (van de Kamp, 1968). From a detailed geochemical study van de Kamp (1968) considered the ortho-amphibolites were Na-metasomatized tholeiitic basalts, whereas the para-amphibolites and biotite--plagioclase gneisses were meta-
301 1 O0
• • 4•
Lleyn peninsula ac+d votcan,cs Narde votcanJcs Tayvelbch votcanncs o-amphibotites "~ Ha[iburton mete-luffs J Madoc × "Green Bed" m e t e - t u f t s
\
C÷#
•
\
~.
*
\
"
•
Ph
,, \
~am•
•
+
R
.~.°.
~
~
\ \
T
\
010
• I-\
oo~ .~
~
RD*D
.
• ;. +,~:___~
~+
/
/
001 ÷
A+ •
ool
tl A
• x
0110
x
Nb/y
÷
+
Sub-AB
+
I
~ oo
I I
i
looo
Fig.4. Zr/TiO2--Nb/Y diagram showing distribution of various greenschist, amphibolite and granulite facies mete-volcanic suites.
morphosed tufts ranging from basic to acid in composition. These t w o groups of volcanics are ideally suited for study using the immobile-element plots and the range of compositions are directly compared in Figs.4--6. As seen in each of the diagrams both ortho-amphibolites and meta-tuffs are strongly sub-alkaline with l o w Nb/Y ratios, Ga and Ce contents. This substantiates van de Kamp's (1968) view that the apparently "alkaline" nature of the ortho-amphibelites (on a total alkalis--silica plot) is due to Na-metasomatism. N o t e also that the ortho-amphibolites are with few exceptions basalts, whereas the meta-tuffs are predominantly basalts and andesites, with some rhyodacites/ dacites. A better correspondence between the characterising of the metaluffs into andesites or rhyodacites/dacites (in Figs.4 and 5) could be made if this rock type boundary was made horizontal at a constant 0.025 Zr/TiO2 rather than a negative slope (as in Fig.4). This would then help to subdivide the rock types in Fig.6 where andesites and dacites are plotted together in one field.
302
10C
+ Tayvalhch volcanics . o - amphJbo[,tes] Hal, burtonmeta-tuffs ] Madoc • Emeryw[le amphlbobtes / Colton granuhtes } Adirondacks
C*P
/
01 0
4, / ~-~-~t_
. . . . RD+D
*
0
÷~
,' ,'
J
, / a
1:
÷
÷ T*Ph
~ TA
BASALTS
OQ1
0o ;.o.:,t4tto .{. , :
I
I
10
20
I0
3
~
_
_
_
_
40
Ga ( p p m.)
Fig. 5. Z r / T i O ~ - - G a variation diagram showing distribution of various greenschist, amphibolite and granulite facies meta-volcanic suites.
CONCLUSIONS
Based on the distribution of various elements considered to remain immobile during subsequent alteration of fresh volcanic rocks, different meta-volcanic suites have been distinguished in terms of magma series and degree of differentiation. This approach provides a reliable method of characterizing altered and metamorphosed rocks such as spilites, keratophyres, greenstones, amphibolites and some gneisses that may have undergone some chemical change during metamorphism. Before the diagrams can be used, field and petrochemical evidence is required to demonstrate that the metamorphosed or altered rocks are not only igneous but volcanic {extrusive and high-level intrusive). Recognizable volcanogenic sediments, such as tuffs, can also be plotted to obtain an indication of their bulk composition, provided that they do not contain much non-volcanogenic material.
303
C÷P 4-
~.~ OlC
÷
R+RD
+ +
I
.
iI + ..+~
.; "+/+
.
T . Oh
e
*t-
D
,
•
TA
+
t,
/
,h
~ :, %V.~(-o//B*TB 001
#i ÷
.
g~
+
÷
~,
5
Tayvalhch
volcan,cs
,, o-amphLbol,tes"~ Hat)burton,~ m e t a - t u f t s .,f Medoc "Green B e d meta-tuffs Lleyn pen,nsula aod vo[cGntcs
~oo
~o
~oo Ce ( p p m }
2 o
3oo
~se
Fig.6. Zr/TiO2--Ce variation diagram showing distribution of various greenschist, amphibolite and granulite facies meta-volcanic suites.
The immobile-element diagrams must be used with caution for (a) pillow lavas and (b) high-grade gneisses. In some cases chemical heterogeneity within pillow lavas produces considerable variation between the rim and core portion, and only the least altered part (usually the core) should be plotted. At present there appears to be no clear evidence for systematic change in immobile-element contents in granulite facies gneisses and thus they may be plotted along with meta-magmatic rocks of lower grades. The problem is more one of initially recognizing the magmatic nature of the gneisses and then demonstrating that they are of volcanic origin rather than plutonic. This may be a major problem in polydeformed high-grade metamorphic terrains. ACKNOWLEDGEMENTS
Grateful thanks are due to Drs. J.A. Hallberg, W.R.A. Baragar and S.A. Drury for allowing us to use unpublished compilations of their geochemical data.
0.007
-
0.006 --
-
1.56 13 45 104 --
7 (1, 2) + 8 (1, 2)
3 (F)
3 (E)
Drury (1974)
1.53 . 21 89 5 20 0.005 0.238
G
.
0.43 .
3 (B)
0.54 7 144 4 65 0.026 0.571
G
3 (A)
Drury (1974)
7 169 6 50 0.039 0.857
A
Gneisses*'
*' Gneiss of intermediate composition; * ~ amphibolite facies; *3 granulite facies. -
Table and (column)
References Engeland Engel (1962)
ZrfTiOz Nb/Y
1.89 14 59 145 --
TiO2 Ga Y Zr Nb Ce
1.37 . 23 89 7 21 0.006 0.304
A
A* 2
Grade
G* 3
Meta-basics
Rock type Amphibolites
0.5
0.6
0.024
144
G
-
2 (1)
2 (2)
Heier and Thoresen (1971)
0.025
128
A
Gneisses* '
Variation of average immobile-element abundances and ratios with m e t a m o r p h i c grade (TiO, in wt.% and trace elements in p p m )
TABLE III
0.57
0.56
0.029 0.454
11 165 5
G
II (1)
I (6)
Holland and L a m b e r t (1975)
0.024 0.250
12 140 3
A
Gneisses* '
0.55
0.47 8 197 5 42 0.041 0.625
G
2 (8)
1 (2)
Tarney et al. (1972)
9 178 5 43 0.032 0.555
A
Gneisses* '
O
305
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306
Kean, B.F. and Strong, D.F., 1975. Geochemical evolution of an Ordovician island arc of the central Newfoundland Appalachians. Am. J. Sci., 275: 97--118. Leake, B., 1964. The chemical distinction between ortho- and para-amphibolites. J. Petrol., 5: 238--254. Loeschke, J., 1973. Zur petrogenese pal~ieozoischer Spilite aus den Ostalpen. Neues Jahrb. Mineral. Abh., 119: 20--56. Loeschke, J., 1975. Spurelement-Daten yon pal~ioischen Spiliten aus den Ostalpen und ihre Bedeutung fur geotektonische Interpreta~ionen. Geol. Run dsch., 64: 62--74. Melson, W.G., Thompson, G. and van Andel, T.H., 1968. Volcanism and metamorphism in the Mid-Atlantic Ridge, 22°N latitude. J. Geophys. Res., 73: 5925--5941. Miyashiro, A., Shido, F. and Ewing, M., 1969. Diversity and origin of abyssal tholeiite from the Mid-Atlantic Ridge near 24 ° and 30 ° north latitude. Contrib. Mineral. Petrol., 23: 38--52. Miyashiro, A., Shido, F. and Ewing, M., 1971. Metamorphism in the Mid-Atlantic Ridge near 24 ° and 30°N. Philos. Trans. R. Soc. London, 268: 589--603. Naqvi, S.M. and Hussain, S.M., 1973. Relation between trace and major element composition of the Chitaldrug metabasalts, Mysore, India and the Archaean mantle. Chem; Geol., 11: 17--30. Pearce, J.A. and Cann, J.R., 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth Planet. Sci. Lett., 19: 290--300. Tarney, J., Skinner, A.C. and Sheraton, J.W., 1972. A geochemical comparison of major Archaean gneiss units from northwest Scotland and East Greenland. Proc. 24th Int. Geol. Congr., Montreal, Que., 1: 1 6 2 - 1 7 4 . Tremlett, W.E., 1972. Some geochemical characteristics of Ordovician and Caledonian acid intrusions of Lleyn, North Wales. Proc. Yorks. Geol. Soc., 39: 33--58. Vallance, T.G., 1960. Concerning spilites. Proc. Linn. Soc. N.S.W., 85: 7--52. Vallance, T.G., 1969. Spilites again: some consequences of the degradation of basalts. Proc. Linn. Soc. N.S.W., 94: 8--51. Vallance, T.G., 1974. Spilitic degradation of a tholeiitic basalt. J. Petrol., 15: 79--96. van de Kamp, P.C., 1968. Geochemistry and origin of meta-sediments in the Haliburton-Madoc area, southeastern Ontario. Can. J. Earth Sci., 5: 1337--1372. van de Kamp, P.C., 1970. The Green Beds of the Scottish Dalradian series: geochemistry, origin and metamorphism of mafic sediments. J. Geol., 78: 281--303. Wilson, J.R. and I~ake, B., 1972. The petrochemistry of the epidiorites of the Tayvallich peninsula, north Knapdale, Argyllshire. Scott. J. Geol., 8: 215--252. Winchester, J.A. and Floyd, P.A,, 1976. Geochemical magma type discrimination: application to altered and metamorphosed basic igneous rocks. Earth Planet. Sci. Lett., 28: 459--469. Winchester, J.A. and Floyd, P.A., 1977. Geochemicaldiscriminationof different magma series and their differentiation products using immobileelements. Chem. Geol., 20: 325--343.