Download Chemical Geology. 20(1977) 325-

Chemical Geology. 20(1977) 325--343
325
© ~sevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
G E O C H E M I C A L DISCRIMINATION O F D I F F E R E N T MAGMA SERIES
AND T H E I R D I F F E R E N T I A T I O N PR O D U CT S USING IMMOBILE
ELEMENTS
J.A. WINCHESTER and P.A. FLOYD
Department of Geology, Univermity of Keels, Keele. Staff~ ST5 5BG (Great Britain)
(Received August 18, 1976; revised and accepted October 18, 1976)
ABSTRACT
Winchester, J.A. and Floyd, P.A., 1977. Geochemical discrimination of different malpa8
series and their differentiation products using immobile elements. Chem. Geol., 20:
325--343.
The abundance and distribution of selected minor and trace elements (Ti. Zr, Y, Nb,
Ce, Ga and So) in fresh volcanic rocks cam be used to clses/fy the differentiation products
of subalkaline and alkaline magma series in a ndmflsr manner to methods using normative
or major-element indices. A number of variation diagrams may be used to distinguish
common volcanic rock types in terms of the above elementL
As these elements are immobile during po0t-cous~idation alteration and metamorphic
processes, this method of rock-type elmmification may, when applied to metsvolcanic
rocks, prove more reliable than the commonly used methods that utilize major elements,
some of which are known to be mobile.
INTRODUCTION
Chemical criteria c o m m o n l y used t o classify fresh or slightly altered
volcanic r o ck s all use d e m e n t s k n o w n t o be mobile during metamorphism.
T h er ef o r e, t h e y c a n n o t be reliably utilized t o i dent i fy and classify volcanic
rocks which have been m e t a m o r p h o s e d o r extensively altered. T h e aim o f
this p ap er is t o establish a chemical means o f discriminating d i f f e r e n t volcanic
r o c k t y p e s and magma series which m a y be applied t o m e t a m o r p h o s e d or
extensively altered rocks.
This ap p r o ach necessarily involves m uc h simplification, and is only int e n d e d as a b r o ad outline o f t he distribu/~on o f immobile trace elements in
d i f f e r e n t volcanic r o c k types. It is n o t i n t e n d e d t o discuss t he relative origins
o f the rocks and their relationships, n o r t he reasons f o r the variations in the
trace-element contents. This s t udy a t t e m p t s t o p r o d u c e a simple graphical
representation o f i m m obi l e e l e m e n t c o n c e n t r a t i o n s in volcanic rocks which
can be used to recognize t he original igneous r o c k t y p e where m e t a m o r p h i s m
or alteration has obscured it.
326
The m e t h o d proposed utilizes the seven minor and trace elements, Ti, Zr,
Y, Nb, Ce, Ga and Sc, which are generally considered to remain inert during
secondary alteration processes, including spflitization, submarine alteration
or metamorphism (Frey et ai., 1968; Cann, 1970; Kay et ai., 1970; Elliott,
1973; Pearce and Cann, 1973; Field and Eiliott, 1974; Herrmann et al.,
1974; Pearce, 1975; Ferrara et al., 1976). These elements are referred to
below ss "immobile" elements.
In this paper it is intended to set up, using f~esh volcanics a geochemical
grid which may subsequently be used as a basis for plotting altered and
metamorphosed rocks. In a subsequent paper (Floyd and Winchester, 1978)
we intend to illustrate the usefulness of this approach applied to altered and
metamorphosed volcanics, ~ o w i n g that the original volcanic rock type and
the magma series it belonged to may still be recognized by this method in
spite of subsequent extensive alteration.
The concentrations of the seven immobile elements and their ratios vary
systematically with differentiation in magma series (see Fig.l), and as a
result different rock types and magma series may be distinguished by their
concentrations or ratios of these elements. In this paper the immobile trace
elements are used to discriminate firstly between the main magma series,
and secondly between their varied differentiation products. No attempt has
been made to distinguish different tectonic regimes of magma emplacement.
CLARIFICATION OF ROCK SERIES
The classification presented in terms of immobile elements must clearly
correspond to well-known and documented schemes that are in general use.
The main rock series are divided into: (a) alkaline; and (b) subalkaline
groups (Chayes, 1965; Wflkiuson, 1968) with the former comprising the alkali
olivine-basalt series (Tilley, 1950) and shoshonite association (Joplin, 1965,
1968), and the latter the tholeiitic series (Kennedy, 1988) and the calc-alkah
association. Following Wilkinson (1968) it is considered that the high-alumina
b , ~ I t s (Kuno, 1960) are representative of the basic members of the calcalkali association.
On the d i s ~ z m s (see Figs. 1--10) broad distinctions are made between
degrees of alkalinity (alkaline, subaikaline) and differentiation products
(basic, intermediate, acid). Because of insufficient immobile element data,
no distinct/on is made between the products of the tholefitic suite and the
calc-alkali association. Hence, the suballmline group illustrated here contains
both tholefitic and high-alumina basalts, although the andealte and rhyolite
fields were delimited using rocks restricted to the calc-aikali association only.
Similarly, the rocks belonging to the shoshonitic association have n o t been
plotted on the diagrams, and thus the alkaline group refers to the differentiation products of alkali olivine-basalt only. However, the few analyses obteined of shoshonitic rocks indicated that the characteristic Zr/TiO2 ratio at
327
least was indistinguishable from that of the alkali olivine-basalt suite, in view
of the relatively sodic or potassic nature of the alkali olivine-basalt series
(Tilley and Muir, 1964) and the various names applied to each lineage, an
attempt was made to distinguish between them using immobile elements.
For example, although the volcanic suites of the Tristan Islands (Baker et 81.,
1964) and Gough Island (Le Maitre, 1962) are generally considered to be
representative of the potassic lineage, with high K20/Na20 ratios throughout,
no clear immobile element differences were noted between them and the
more sodic lineages.
ROCK NOML-'NCLATURE
Immobile element data shows some overlap between different rock types
in most of the accompanying diagrams (see Figs.2, 4, 6, 8--10). This may be
expected partly because the natural rock series form a chemical continuum,
and partly because the application of a geochemical criterion as a means of
distinguishing rock groups classified according to their mineral content must
allow for some slight inconsistencies. In addition, there is some variation in
the rock nomenclature used by different workers, and for this reason, groups
of related igneous rock types have, for simplicity, been collected together
under single headings (Table I).
TABLE I
Grouping of rock-~pas used in discrimination diagr.,m and their respective symbols
Subalkaline
Mildly alkaline
Strongly alkaline
•
•
X
rhyolite
comendite
pantellerite
alkali rhyolite
trachyle
phonolitic trachyte
nepheline traehyte
rhyodacite
daeite
•
÷
-
andasite
hifh-K andasite
low-K andes/re
low-Si02 andeslte
basaltic ande~te
trachyandesite
latite
benmareite
phonolite
trachytic phonolite
tholeiite
olivine-basalt
alkali olivine-basalt
traehybasalt
hawaiite
mugearite
basanite
trachybaasnite
nephelinite
nepheline---hawaiite
nepheline--mugearite
high-Al basalt
328
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330
Rocks described as latite in the literature are grouped with trachyandesites
as no analyses of the intermediate m e m b e ~ of the shoshonite association
(also termed latlte) are included. Rocks with extreme compositions, such as
carbonatites, or members of the lamprophyre suite have not been included
in the scheme.
The distinc~ve nature of the variously grouped rock types in terms of
absolute abundances and ratios is shown in Table II. Although this compilation only uses the data plotted in the diagrams presented here, and is numerically restricted for some groups, progressive trends and differences can be
seen within any one rock series.
SOURCES OF DATA
To illustrate how different volcanic rock wpes may be graphically discriminated, using the immobile dements (Ti, Zr, Y, Nb, Ce, Ga and Sc), analyses obtained from a wide variety of geographical locations and tectonic
settings have been plotted on the immobile dement-pair diag~ms (see
Figs.l--10). However, as all the d e m e n t pairs include a trace dement, none
of the many published collections of major-dement analyses of volcanic
rocks coultl be used. Also, many collected analyses which included some
trace d e m e n t s contained determinations of only some of the immobile trace
elements, and thus could not be used on some of the diagrams. Therefore. a
relatively small proportion of the total number of analyses published include
analyses of all the elements required for this study.
The data upon which the conclusions of this work are based, therefore,
derive from only a few hundred anelyses (Table HI). Consequently, the field
boundaries delimited in the following diagrams must be viewed firstly as
marking gradations/changes, and secondly as being subject to limited adjust
ment as more data becomes available.
DISCRIMINATION DIAGRAMS
It has been known for a long time that trace-element concentrations change
in a regular and predictable manner with progressive differentiation (Wager
and Mitchell, 1951; Nockolds and Allen, 1954). The accompanying diagrams
(see F i p l - - l O ) show the systematic variation of selected immobile-element
ratios and abundances for various rock groups. They, therefore, represent
indices of progressive msgmafic evolution in a similar manner to standard
ntajor~lement indices (e.g., mafic index). In addition, the diagrams also
demonstrate that a discrimination may be made between the products of
subalim]ine (mainly calc-alkali) and alkaline di~erentiation trends. In a previous paper (Floyd and Winchester, 1975), it was shown that discrimination
could be made between fresh alkaline snd subalkaline b ~ ] t s by plotting
their Zr/P2Os ratio against either t h e k TiO2 content or Nb/Y ratio. In fresh
volcanic rocks the Zr/TiO2 ratio has been applied as a means of discrimination
331
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332
between different rock types in preference to the Zr/P~Os ratio because P 15
generally more mobile than the other elements discussed here, as exemplified
by its behaviour in a weathering regime (Vogel, 1975).
Si02--Zr/P~Os
Initially the Zr/TiO2 ratio was plotted against SiO2 content (Figs.l, 2),
which can be used as a rough measure of the degree of differentiation. With
the progressive differentiation of a basic magma, the Zr/TiO2 ratio increases,
and reflects the overall decline of TiO2 content in non-basaltic differentiated
rocks. This ratio may thus be used as a relatively sensitive differentiation
index in place of SiOz. However, a m o r e marked increase in the Zr/TiOz ratio
occurs in the more alkaline magma series, reflecting the strong concentration
of Zr in alksline rocks. For example, phonolites from the Dunedin Volcano
characterktic.~lly have a conskierably higher Zr/TiO2 ratio than the basaltic
rocks with which theT are a~ociated, while by c o n t n ~ t dacitic rocks from
Tonga have Z~/TiO2 ratios only slightly higher than those of the basahic
andesites with which they are ~r~mciated ( F i ~ l ) . T h i n the amount of increase of the Zr/Ti02 ratio in proportion to Si02 increase may be applied as
an index of a/kalinitT of a rock suize.
With increasing SiO2 conr~nl the Zr/TiO2 ratios of alkaline and subalkaline
suites diverge, and alkaline volcanic rock types plot apart from the s u ~ k a l m t
76.
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- T r a c h y a n d ~ t e , Benmar~:e
• Thllehtc. High Albmma BI.~III
• I~i~lni.be "r.~cl'yba~ln, te
~ d ~
-'rrachyte
Alkali Basalt
Phonolite
i
om
Zr/~02
.
C~
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Fig.1. SiOs--ZriTiOi diagram showing the relative distribution of the differentiation producti f~om various volcanic centres.
333
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rock types (Fig.l). By using different symbols to denote the various rock
types (used consistently on all the accompanying diagrams), the different
rock types were found to plot in distinct fields, and approximate boundaries
to these fields can be drawn (Fig.2).
8i02--Nb / Y
The Nb/Y ratio was first noted as an indicator of alkalinity in basalts by
Pearce and Cann (1973), and further studies confirmed this (Floyd and
Winchester, 19"/5); a highe~ Nb/Y ratio generally reflects the higher Nb content c h a m ~ t i c
of alkaline suites. The Nb/Y ratio was plotted against
SiO~ content (Fip.3, 4) in order to establish whether this ratio also tended
to change systematically with differentiation. Initially, three magmatic suites
were plotted: calc.alkRH~e volcanics from Mr. Ararat (Lamber~ e: al., 1974),
a transitional alkaline suite from Easter Island (P.E. Baker et al., 1974), and
an a]kA,~e series from the Dunedin Volcano (Price and Taylor, 1973)
(Fig.3). With the possible exception of the Mt. Ararat suite, the Nb/Y ratio
was found to increase only slightly with increasing SiO2 content: the ratio,
therefore, seems to reflect the alkalinity of a magma series alone. Different
rock types were found to plot in distinct fields, around which approximate
limits could be drawn (Fig.4). Only between a few rock types, notably
trachyte, phonolite and trachyandesite are the limits ill-dei'med. For all ex-
334
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010
II;
10C
Nb/Y
v
Fig.3. SiO=--Nb!Y diagram s h o w i n g the relative distribution of the differentiation pro(
uecs from various volcanic eentres. Symbols as in Fig. 1.
~"
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RhYODACI"E-"
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Nb/Y
Fig.4. 8iO2--Nb/Y d/alpram showing the delimited field• for c o m m o n volcanic rocks.
Symbols ms in Fig,1.
335
cept the most siliceous rocks a Nb/Y ratio of 0.67 satisfactorily divides
generally subalkaline magma suites from those that are alkaline. On addition
of further data this ratio was found to produce a better discrimination than
the value of 1.0 formerly used by Floyd and Winchester (1975). Peralkaline
rock types, such as phonolites, tend to have .~o/Y ratios exceeding 2.0.
Zr /~O2--Nb / Y
Since both the Nb/Y ratio and the Zr/TiO2 ratio are indices of alkalinity,
hut only the Zr/TiO= ratio represents a differentiation index, a plot of Nb/Y
against Zr/TiO2 was also found to discriminate between different volcanic
magma series and rock types. Thus, the calc-alkaline rocks from Mr..Ararat
have a low Nb/Y ratio, with the Zr/TiO2 ratio increasing from andesize to
rhyolite, while the alkaline rocks from the Dunedin Volcano have a high
Nb/Y ratio and a Zr/TiO2 ratio that increases markedly with differentiation
(Fig.5). The mildly alkaline suite from l~A~terIsland plots between the two
other trends.
1 00r
I
I
Ic~
O-lO
,-1,<
0_
F--
m,~ j,,A
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I
.A I
i
i
r,,,i
!
--
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./."
o-oi L
o-ol
°11°
Nb/Y
1'oo
lO-t)
Fig.5. Zr/TiO,--Nb/Y diagramshowing the separation of magma series and their respective differentiation products, u illustrated by three volcanic centres. Symbols as in Fig.]..
336
When the other available analyses were plotted different rock types were
found to plot in different par6i of the diqpram (Fig.6). Thus subalkaline
basalt• are c h ~
by relatively low Zr/TiO2 and Nb/Y ratios, whereas
dacites and rhyoUtes have a h/gher Zr/TiO2 ra~o while retaining a low .Nb/Y
ratio. Alkaline basic rocks, as other basic rocks, are typified by low Zr~iO=
rat/os, but have charac~ristical]y high Nb/Y ratios, while alkaline differentia~es have typically high Zr/TiO2 and Nb/Y ratios (Fig.6). Thus, major
8roupings of volcanic rock-types may be distingtdshed solely by their
characteristic proportions of selected immobile minor and trace elements.
--.
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t ...ONENDITE
~A N'rELLERITE
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Fil~& Zr/TiOz--NblY rli=-~ram ihowinll the delimited fields for eOmmnn volcanic rocks.
The Zr/TiO= ratio acts as a differentiation index and the N b l Y ratio as an alkel/nity index
S y m b o Is as in Fig, 1.
337
Ce--Zr/~02
The contents of additional trace elements have been compared in different
volcanic rock types in a similar manner. The Ce content in differentiated
rock types of the calc-alkaline association remained broadly constant, whereas in alkaline rocks Ce content was found to increase markedly with differentiation. In a disgram plotting Ce content against the Zr/TiO2 ratio, subalkeline
and alkaline trends showed a marked divergence (Fig.7).
When all available data was plotted, the rock types were found to plot in
distinct fields (Fig.8). Rather more overlap occurred than in previous diagrams; for example different basaltic types and basanites were not clearly
distinguished, and the scarcity of data implies that this plot may not be
quite as reliable as former ones.
-t
0
ZT.
-
°
°°'L
//
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I
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250
30C
350
Ce pp.m
Fig.'/. Zr/TiO~--Cediallramshowing the relat/ve distribution of the differentiat/on
products from various volcanic centres. Symbols as in FiK.1.
338
- .?=-
I!
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__lc
--
-_
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.
Ce
Dpm
Fig.8. Zr/TiOF-Ce diagram showing the delimited fields for common volcanic cocks.
Symbols as in Fig.1.
Ga--Zr/Ti02
A similar pattern was f o u n d in plots of Ga a ~ i n s t ZrlTiO2 ratio (Fig.9).
Whereas alkaline rocks show a concentration of Ga with differentiaUon,
subalkalJne rocks show little change, or possibly a slight decrease o f Ga
content. Again the main volcanic rock types group in different fields, although there is considerable overlap of basaltic types, between basalt and
andesite, and between phonolite and pantellerite.
Ga/Sc--Nb / Y
Sc contents are markedly reduced with differentiation in both subalkaline
and alkaline rocks. Thus the .Ga/Sc ratio may be e m p l o y e d as an index of
differentiation, so that in rocks belonging to the calc-alkaline association
339
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,=, /
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ALKALINE
,
BASALT
SUB -ALKALINE
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5
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Fi~9. Zr/TiOz--Ga diagram showing the tentatively delimited fields for common volcanic
roelm Symbols as in Fig.l.
Ga/Sc ratios increase with differentiation from andesite to rhyolite; whereas rocks of the alkaline magma series also show an increase in the Ga/Sc ratio,
although less marked. PanteUerites, containing very low Sc contents are
distinguished by very high Ga/Sc ratios. In Fig.10 the Ga/Sc ratio is plotted
against the N b / Y ratio, used as an index of alkalinity. As before, the main
volcanic rock t y p e s plot in distinct fields: in particular there is a clear separation b e t w e e n the subalkaUne and alkaline basalts. The data on which this
diagram is based is particularly scarce, and it seems likely that, with the
340
d=
~
i
:~t-YOL T=> ..---;-,
i
I
i
U
I
IV •
:; "
,~=.,.
lJ
j
J'
~"
"
/
I
° ~ O h C , . TE
"RACi-'Y"E
T.*C.YAN
:_,:--=,
S,TC
III.
• ...)~..:_
•
•
33"ANDL:'S'E
/
F
'
:
'
~-~--~
"~"
== *
~'~4
~ P'"
" ='
- =. f"
i
.=
:
: ••
q
""
",L --
'A.KAL'
BASALT
-.
SUS-A_KALI~ /
BASA="
J~
I
~'~
|
Nb/Y
Ioo
Fig.lO. G a I ~ - - N b / Y diagram Ihowinl the a p p r o z i m t e distribution of common volcanic
rockL Note the divergence of the alkaline and sub-alkaline masrna series.
accumulation o f further analyses, some o f the compositional fields plotted
on Fig.lO may be cormidembly enlarged, although the fundamental distribution pattern is unlikely to alter.
CONCLU~ONS
Figs. 1--10 illustrate that contents or ratios o f seven trace elements are
charactm~tic of certain volcanic rock types, and that their concentrations
are strongly controUed d .urJng differentiation. By using fresh volcanic rocks
a number of pochemical grids can be devised so that ff a suite of fresh
volcanic rocks is plotted the identity o f the different rock types present can
be deduced. More ~ i ~ m t ~ y ,
as the elements used are immobile during
metamorphism, these pochemlcal grids should also be applicable to-extensively altered and metamorphosed volcanics. They should thus provide a
341
m o r e reliable m e t h o d o f r e c o g n i z i n g t h e original v o l c a n i c r o c k t y p e w h e r e
e x t r e m e a l t e r a t i o n o r m e t a m o r p h i s m h s s t a k e n place.
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