winchester e floyd 1976

Prévia do material em texto

Chemical Geology. 20(1977) 325--343 325 
© ~sevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands 
GEOCHEMICAL DISCRIMINATION OF DIFFERENT MAGMA SERIES 
AND THEIR DIFFERENTIATION PRODUCTS 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 commonly used to classify fresh or slightly altered 
volcanic rocks all use dements known to be mobile during metamorphism. 
Therefore, they cannot be reliably utilized to identify and classify volcanic 
rocks which have been metamorphosed or extensively altered. The aim of 
this paper is to establish a chemical means of discriminating different volcanic 
rock types and magma series which may be applied to metamorphosed or 
extensively altered rocks. 
This approach necessarily involves much simplification, and is only in- 
tended as a broad outline of the distribu/~on of immobile trace elements in 
different volcanic rock types. It is not intended to discuss the relative origins 
of the rocks and their relationships, nor the reasons for the variations in the 
trace-element contents. This study attempts to produce a simple graphical 
representation of immobile element concentrations in volcanic rocks which 
can be used to recognize the original igneous rock type where metamorphism 
or alteration has obscured it. 
326 
The method 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, ~owing 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 emplace- 
ment. 
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,~Its (Kuno, 1960) are representative of the basic members of the calc- 
alkali association. 
On the dis~zms (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 not been 
plotted on the diagrams, and thus the alkaline group refers to the differentia- 
tion products of alkali olivine-basalt only. However, the few analyses ob- 
teined 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 trachyle 
pantellerite phonolitic trachyte 
alkali rhyolite nepheline traehyte 
rhyodacite 
daeite 
• ÷ - 
andasite trachyandesite phonolite 
hifh-K andasite latite trachytic phonolite 
low-K andes/re benmareite 
low-Si02 andeslte 
basaltic ande~te 
tholeiite 
high-Al basalt 
olivine-basalt basanite 
alkali olivine-basalt trachybaasnite 
traehybasalt nephelinite 
hawaiite nepheline---hawaiite 
mugear i te nepheline--mugearite 
328 
u ; 
- ~ ~ ~ 
[ _ 
.i~ 
| .~ ~. ~= ~,, ,~- ~ ~-~ 
• " ,~c~ ~ , " 
m 
. . , ,0 
8 
~O ~O ~Q ~O ~O O0 ~O 
U 
0 
' ° i i i ' ] j i J 
r,c. 
329 
U 
o ~ ~ o - o 
~ ®® _ ~ ®o~ -®® ~®~ g~ ~ "~ 
1 
~ - ~ o o 
" " . ! .9 
~, .gs 
., 1.0 
330 
Rocks described as latite in the literature are grouped with trachyandesites 
as no analyses of the intermediate membe~ of the shoshonite association 
(also termed latlte) are included. Rocks withextreme 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 compila- 
tion only uses the data plotted in the diagrams presented here, and is numeri- 
cally 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 discrim- 
inated, using the immobile dements (Ti, Zr, Y, Nb, Ce, Ga and Sc), anal- 
yses 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 dement 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 dements 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 ip l - - lO) 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 pre- 
vious paper (Floyd and Winchester, 1975), it was shown that discrimination 
could be made between fresh alkaline snd subalkaline b~]ts by plotting 
their Zr/P2Os ratio against either thek TiO2 content or Nb/Y ratio. In fresh 
volcanic rocks the Zr/TiO2 ratio has been applied as a means of discrimination 
331 
c-. '~"o 
. . _ g 
eam '=" 7 , . . , 
~ . ~ = :~ , . - . - . 
, - , ca " - " Dr,,~ , ,~ 0 
. k I ~.. ~ . -~ ~ ~ 0 
,-- , i - . ,~ "~ . . ea ..~ ~.~ ..I ~ 
l l I I0 ,.-~ 
0 . v ~ • , . . .~'01~1 ~ "O. , .v L',- ~- - O¢ I~L , ,L~.~. ,L~.~. , = l -o~. I 
_ -~ ' .=o o=~o.o~.= . . , . , . . . , , .~_~a~.., , ,~=o~ 
3a 
0~ 
• ~ "o 
~ 3 - ~' " ' - • , - -a [ . - ' i a . - i a 
. .~ _1 . ~ ~ a"a .~"a ~ 
O.¢ .a ..a m ~e C ,,,, , - , 0 
i 
-3. 
o 
.E 
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 more 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 contn~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 ) . Thin the amount of in- 
crease 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 su~ka lmt 
t,/,~" , . • -'rrachyte - T rachyand~te , Benmar~:e 
"e • Alkali Basalt • Thl lehtc. High Albmma BI.~III 
401 ", Phonolite • I~i~lni.be "r.~cl 'yba~ln, te 
i . . | 
om C~ • 3 
Zr /~02 
Fig.1. SiOs--ZriTiOi diagram showing the relative distribution of the differentiation prod- 
ucti f~om various volcanic centres. 
76. 
-',2L ; ' 
- . 
- . . j 
52. ,. ,~ / 
• / • ~ t e -- 3ac~e. ~y~lac,te 
! :/I • ~ d ~ # Comen~te. Pontelier,te 
333 
~[ 
• RI',YC_-.": 
I 
72- ~ ; : " / o .n . o 
R.,Y:.~C'E -~" e 7 ~ '~ ,' -' 
5,6" 3AC -E " ; ~ ' ° ° .~ °~ :31vE%31TE " 
"-'-:" / " I . I " PA%---L.Eqn'E 
~s~- P' ,'.. .... -~-_--J" :". " i . ,..,. _.~" , , -qAC,"YTE _ , 
>~ , ..,, ~-...:_=/ :'-..,- - _ 
ec . . . . . - - -- . . . . . . . . & l i ,~-'IlJL~ . . . . "1;:l: TI: ~-'lJ~ ~- - - I I ! - - - - P , - i . J P~" I- M • -a . . - -~£%. l : _ _ - - • J , / . '~ ' _ '~ . ; .--: _- _-_ =_ 
(~ ~" • ~" ' i ' / - - - : . ,~- - : _- - : : : • - 
~s6- , : . , iT=" ;~ . : - - - : - - _ : : 
$2- sus -~,z :~, , ;'. ~ ;&/ . . " - - .~ . : 
,ASA.T. 5. ; , ~ ; , - - 
• , "~ ~l~.''ki'~" un TRACFaYBASA~,T E 
I 
• • :F4 J 
! 
"3 01 . . . . . . 
Zr / Tn0 2 
Fig,9-. 8 iO2- -~r / ' r iO s diai~rmum Ihowing tee de l imi ted f ie l tk fo r common vo lean i© roc Inn. 
Symbols u in FLg.I. 
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 con- 
tent cham~t ic 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 
?E 
72 
M 
52 
,o t 
O01 
-_.. I l l 
• + 
: / - 
Y i : . "& 
? - & 
>o O. - 
• n - -. 
IJJ 
" z [ 
• 
= i j • 
• I I ; 
010 10C 
Nb/Y 
v 
Fig.3. SiO=--Nb!Y diagram showing the relative distribution of the differentiation pro( 
uecs from various volcanic eentres. Symbols as in Fig. 1. 
~" " "#e 
te 
72 F , COMEN~TE 
• A PANTELLERfrE 
RhYODACI"E - " -- % 
88 - ~AcirI~ ~ : 0 
• ,. : ~ ~ TRACI-,YTE• , , ,, , , ~ - _ 
• ": -: : ~ 
• . . . . . 
_ •• " ITRACHYANDESITE~ 
" i ." " - " ! " " ALKALI ~/~"~. . 
- = ~- =.=,,_. " . . - l~lw.%ftLT I/ "" 
= - - "~, - _ " J . . . _ - ' - - - - r 
: . . "E" " - "-. :,,'. 
S,.;B-,~.KALINE BASALT " r.~*. : " • . . e.J". BASANITE 
• := . - . p; . , ;'~..,'NEm4EUNtTE 
/ I I I I I I 
_= j.,,,.,wl I • • 
.,..50- 
C~56 L 
521- 
I 
,.8[ 
~0 ; " , 
01 . . . . 0'tC "~00 " " 
Nb/Y 
Fig.4. 8iO2--Nb/Y d/alpram showing the delimited field• for common 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~ter Island plots between the two 
other trends. 
1 00r 
O-lO 
0_ 
F-- 
r,,,i 
I 
o-oi L 
I 
I 
Ic~ 
,-1,< 
I 
m, ~ j , ,A .A I 
I 
i 
i 
! - - 
./." 
o-ol °11° Nb/Y 1'oo lO-t) 
Fig.5. Zr/TiO,--Nb/Y diagram showing the separation of magma series and their respec- 
tive 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 differen- 
tia~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. 
4 i t ' . 
4~p 
v w 
(:3 
I - - 
Im 
N • 
--. .-_ 
o 
t ...ONENDITE ,Ij'_ 
~A N'rELLERITE 
t " " _ P" ,0 ;~0" - " rE 
• ~- _. 
"~ 0 t 0 ~I "- "0 
. l amw =, - - 
%, 
t 
qI-'YOL I"E \ 11 -- "~ 
• \ " i l k " . . 
q -YOCAC-E f : TRACl-vTE 
~ACI'E ' " : - " ~ . 
• " • , " - " ~ . • =" ~1~ NEPHEUNP'E 
• - . . . . . " ; - " , ' - - . . - " J L , , . " 
" A . . . . S" - " " / " " ' " " " ~'" - ,~mur. r . . / /= _= = =lB ._ " " 
I 
I I 
I SI.B -ALKA,.I~IE BASAL" 
I 
I 
I 
3 01 0'10 1 '0 lC O 
Nb/Y 
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 NblY ratio as an alkel/nity index 
Symbo 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, where- 
as in alkaline rocks Ce content was found to increase markedly with differen- 
tiation. 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 dia- 
grams; 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. 
°°'L 
-t 
0 - 
ZT. ° 
/ / 
"!: / 
I I 
250 30C 350 
Ce pp.m 
Fig.'/. Zr/TiO~--Ce diallram showing the relat/ve distribution of the differentiat/on 
products from various volcanic centres. Symbols as in FiK.1. 
338 
- .?=- 
I I ! 
__ lc -_ 
_% - - 
- : -~ '~OL "E 
~- ' - - _ ".= - -= A-" -~-_~ 
t 
o: _.- . . . t -~- - i . ° . f . • 
: _ . _ . : . :.--go.-. 
- ,,,---ip e Zp~. • . 
- S_- ~.A_- 
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 found in plots of Ga a~inst ZrlTiO2 ratio (Fig.9). 
Whereas alkaline rocks show a concentration of Ga with differentiaUon, 
subalkalJne rocks show little change, or possibly a slight decrease of Ga 
content. Again the main volcanic rock types group in different fields, al- 
though 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 employed as an index of 
differentiation, so that in rocks belonging to the calc-alkaline association 
339 
' / 
• /PANTB.LERITE " / 
I [] ~,. 
/ . ,= , / 
• I / ' 
I ' / 
RHYOLITE / , / 
! , I / PHONOLITE 
• [ / TRACHYTE 
I : / l l X X 
0-10 " " / / a 
J " RHYODACITE I • +4- 
: DAClTE " / t ~ ~ 
(~ A ; + 
i : " . . . . . / TRACHYANDESITE 
i ...~.~...:-,"!-, ! • • • 
OC.. ......--" . I . " • . = I " 
- - " : ; i i 1 : = I : 
' " 
=. . - ~: ;¢ ' ' " . • 
• • = . 
• " , ALKALINE 
BASALT 
SUB -ALKALINE 
I 
I i i i i i i 
5 10 15 20 25 3C 35 ¢0 &5 
Go ppm 
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; where- 
as 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 Nb/Y ratio, used as an index of alkalinity. As before, the main 
volcanic rock types plot in distinct fields: in particular there is a clear separa- 
tion between 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 I 
U IV • l J ~" 
; " " / °~OhC, . TE : j J ' 
,~=. , . I "RACi-'Y"E 
:_,:--- T.*C.YAN S,TC 
=, I I I . 
• . . . )~ . . : _ 
• .= : 
• 33"ANDL:'S'E : : • • q 
F ' ' ~-~- -~ - " " 'A.KAL' 
/ "~" == * " =' ",L -- BASALT 
- - =. f" -. 
~ '~4 ~ P ' " i 
SUS-A_KAL I~ / 
BASA=" 
J~ 
I | 
~'~ Nb/Y Ioo 
Fig.lO. GaI~--Nb/Y diagram Ihowinl the approzimte distribution of common volcanic 
rockL Note the divergence of the alkaline and sub-alkaline masrna series. 
accumulation of further analyses, some of the compositional fields plotted 
on Fig.lO may be cormidembly enlarged, although the fundamental distribu- 
tion pattern is unlikely to alter. 
CONCLU~ONS 
Figs. 1--10 illustrate that contents or ratios of 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 fthe different rock types present can 
be deduced. More ~ i~mt~y, as the elements used are immobile during 
metamorphism, these pochemlcal grids should also be applicable to-exten- 
sively altered and metamorphosed volcanics. They should thus provide a 
341 
more reliable method o f recogniz ing the original volcanic rock type where 
ext reme alterat ion or metamorph ism hss taken place. 
REFERENCES 
Appleton, J.D., 1972. Petrogenesis of potaasium-rich lavse from the Roecamonfino 
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Bailey, D.K. and MacDonald, R.J., 1970. Petrochemical variations among mildly per- 
alkaline (eomendite) obsidians from the oceans and continents. Contrib. Mineral 
Petrol., 28: 340--351. 
Baker, L, 1969. PetroloID" of the volcanic rocks of Saint Helena Island, South Atlantic. 
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Baker, P.E., 1968. PetxoloiD" of Mt. Misery Volcano, St. IQtts, West Indies. Lithos, 1: 
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