OF ISLAND ARCS AND CONTINENTAL MARGINS1 …Hatherton (1968) related hornblende andesites to the "miogeosyn-clinal" environment, and Japanese authors have noted the dbsence of hornblende-bearing

American MineialogistVol. 57, pp. 887-902 (1972)

HORNBLENDES FROM CALC-ALKALINE VOLCANIC ROCKSOF ISLAND ARCS AND CONTINENTAL MARGINS1

Pnm JernS,z The Lu,na,r Ssienae Instituta, 9303 Na,ffi, Road 1,Houston, Teras 77058

A. J. R. Wurrn, Dep,artment of Geology, The Au'stralia.n NqtionalUni,u er sitg, C,anb erca, A.C .T ., Austrq.Lia"

AesrnAer

Calc-alkaline volcanic rocks occurring in island arcs and eontinental marginsdiffer in trace element chemistry but overlap in major element composition.When hornblendes from andesites of identical major element composition arecompared, those from the island a,rc environment have higher total alkali, > Alcontent, and lower Fe/Mg ratios. Chemical differences in hornblendes, the posi-tion of hornblende in the crystallization sequence, and new experimental datasuggest that the liquidus temperatures of chemically identical andesites are higherin island arcs than in continental margin areas ar' a result of difrerent I[rO con-tents. This zuggests a difference in origin of calc-alkaline rocks in island arcs andcontinental margins.

INrnooucrror*

There is a graduational sequence from tholeiites to calc-alkalineand to shoshonitic rocks a,cross the island arcs or in the stratigraphicalsequence of island arcs (Kuno, 1966; Sugimura, 1968; Dickinson andHatherton, 1967; Jake5 and White, 1969; and Gill, 1970). Jake5 andWhite (1971) suggested that two calc-alkaline associations occur inthe orogenic regions-an island arc and a continental margin (Andean)calc-alkaline association.

The principal differences of these associations are SiOz variation,trace element abundances and phenocryst mineralogy. The differencesare summarized in Table 1.

Some medium- and high-K andesites from island arcs have verysimilar or identical major element composition to rocks occurring inthe continental margins (Andean association) or intracontinentalchains (Carpathians) (c.1., McBirney, 1969). Those rocks most simi-lar in respect to major element chemical composition and mineralcomposition are hornblende andesites. In this paper we present newbulk chemical analyses of hornblende-bearing calc-alkaline volcanicrocks and analyses of their phenocrystic minerals in order to establish

tPrepared at The Lunar Science Institute, Houston, Texas, Contribution No.7 4 .

'Present address: Geologickf Ustav CSAV Suchdol 2,Pruha 6, Czechoslovakia.

887

888 P11'1'R JAKES ,4ND A. J. R. WHITE

cont inenta l marg in(Andean )

G EN DRAL IZED D I FIT EIiENCES BET'! 'TEEN CALC-ALKALI NE VOLCANI C

iiOC](S OF ISLAND ARCS AND THOSE OF CONTINENTAL MARGINS

I s l a n d a r c

) . n - i ^ q q ; n s 6 . o - 7 5 . 0 % 5 O - O - 6 6 . 0 %

i co - Fe2O^/ ,ugO h i g h e r t h a n 2 . 0 l o w e r t h a n 2 . 0

^ . u / N r 2 u u - b u - f . r l e s s t h a n 0 . 8

Trace e le idents a t

same l {2O anc l S iO,

B a , S r ,

K /Pb (230) ,l o w e r N c , B a , S r , T h ,

z r , l l /Pb (4oo) , Th /uh igher Rb,

rh/v

D l . : . r ^ ^ 1 - \ r - : f < b r o t i t e , h o r n b l e n d e ,

c l inopyroxene, o r tho-

p y r o x e n e , r a r e q u a r t z ,^ r 1 6 6 f ^ ^ r ^ i a r i t a

c l inopyroxene, o r tho-

pyroxene, hornb lende( r a r e b i o t i t e ) n o

q u a r t z , g a r n e t , a n d

c o r d i e r i t e

c ^ ^ 1 r 6 - ^ 6 ^ f

n L ! : a ^ ^ - . ( r c r

1 i - - ! i ^ -- ! ] - L d ! t r z o L r v r ,

ho rnbl ende>c1 i no-pyroxene>or chopyroxene

cI inopyroxene)horn-b I ende>c 1 i nopy roxen e

possible chemical and petrological differences between both caic-alkaline associations.

- ANelvlrcal Morrrons

Major elements Si, Ti, Al, total Fe, Mn, Mg, Ca, K, and P were determinedin rocks and mineral separatesl by X-ray spectrographic analyses using themethods of Norrish and Chappell (1967). Na was determined by flame photom-etry, and ferrous iron by titration and spectrographic methods (Kiss, 1968).trluorine wa€ determined by using a modification of the colorimetric alizarin bluemethod (Ilall and Walsh, 1969).

1A table listing results of bulk rock chemical analyses and analyses of mineralsmay be ordered as NAPS Document 01803 from Natioiral Auxiliary PublicationsService of the A.S.I.S., c/o CCM Information Corporation, 866' Third Avenue.New York, N. Y. 10022; remitting in advance, $2.00 for microfiche or $5.00 forphotocopies, payable to CCMIC-NAPS.

VOLCANIC ROCKS OF ISLAND ARCS

Some minerals were analyzed using an ARL electron microprobe (AustralianNational University) with natural and synthetic minerals as standards. The datawere corrected for matrix efrects. Additional data were obtained using an HitachiXMA-S electron microprobe (University of Kanazawa, Japan) with syntheticminerals and oxides as standards: Albee and Ray's (1970) correction factors wereapplied to these results.

Separation of minerals for analysis was made by electro-magnetic and heavyIiquid methods followed by handpicking. The purity of concentrates was higherthan 99 percent; impurities were clinopyroxene, magnetite, and apatite inter-grown with amphibole. The outer parts of homblende phenocrysts, usually inter-grown with glas or magnetite, were removed during separation and thus analysesof hornblende are more representative of the cores than of the whole crystal.

Col,rrosrrroN ox' HoRNBT,ENDE ANDESTTEs

Available chemical data on volcanic rocks from island arcs such asNew Guinea, Japan, Kamchatka, and other circum Pacifie areas showthat there is a relationship between rock composition and the presenceor absence of hornblende. In the island arcs homblende commonlyoccurs in calc-alkaline or shoshonitic rocks having SiOz greater than55 percent but rarely in rocks with SiOz less than this. Hornblende ismore cominon in rocks with higher KzO content within ,the calc-alkaline association but the presence of "ghost amphiboles" (pseudo-morphs of magnetite * clinopyroxene * plagioclase) in rocks fuomthe Marianas (Schmidt, lg57) suggests that hornblende may havealso crystallized in low-K (tholeiitic) rocks. Most hornblende-bearingrocks are high in total alkali content, have high KzOAazO ratios(0.5-1.0) and are usually highly oxidized (FezOaAeO is gredter than0.5). They are from rock associations which show little or rio "ironenrichment" in Al\{F diagrams.

Hatherton (1968) related hornblende andesites to the "miogeosyn-clinal" environment, and Japanese authors have noted the dbsenceof hornblende-bearing volcanib rocks near the recent volcanic front:both features may be attributed to their distinct rock chemistry.Factors controlling the presenee or absence of hornblende such as theretention of volatile content until late stages of crystallization makeamphibole andesites unreliable indicators of magma variation aOrossthe island arcs. Examples of hornblende-bearing and hornblende-freeandesites from island arcs show that the major element and traceelement chemical eomposition may be very similar although pheno-cryst mineralogy differs. Some authors consider hornblende andesitesin island arcs to result from crustal eontamination (Kuno, 1950). Themajor and trace element chemistry suggests that if the contaminationhypothbsis is applied, it must be applied equally to hornblende-freeandesites.

889

890 PETR ,IAKES AND A. J. R. WHITE

l\{rNnnar,ocv

Hornbl,end,es

In most calc-alkaline volcanic rocks of both island arcs and con-

tinental regions phenocrysts of hornblende from 0.5 to 5.0 mm in

length have a euhedral acicular or bladed form. Polycrystalline ag-gregates are lare and the amount of hornblende varies in different rock

types, but in rocks with a glassy matrix the amphibole forms about

50 percent of phenocrysts (the other phenocrysts are mostly plagio-

clases, and to lesser extent clinopyroxene). simple twins parallel to(100) are commonly observed.

There is some degree of disequilibrium between hornblendes and

the host rocks, the phenocrystB showing different degrees of resportionor recrystallization. ,,opacite rims" consisting of a reaction corona ofplagioclase, orthopyroxene and an abundance of fine-grained opaquephase are common. In some hornblendes there are two, three or evenmore "buried" opacite rims observable in sections oriented perpen-

dicular !,o "2" axis. Zoning is characterized by a deepening in color and

a small change in extinction angle in sorne samples. Microprobe data

show that for iron and magnesium oxides the variation does not exceed

2.wt percent and usually is less than 1.5 percent. The cores have higher

Mg, Ti, and sometimes Al, whereas rims have higher Fe and Si con-

tents.New chemical analyses of hornblendes show relatively narrow

compositional range. In all analyses, except three from silica-richvolcanic rocks of continental areas, the content of aluminum was suffi-cient to fill the Z group: in exceptional cases Fe8* has been added to

the Z group. In most of the analyses the total number of cations in Y

exceeds 5.00 and Leake (1968) claims a value of. 5.25 as a limit for

superior analysis. Because of the positive correlation of OH * F andtotal cations in the Y group (e.g., Leake 1968) and significantly lowcontent of OH * F in the hornblendes from volcanic rocks, a value of5.35 was accepted as satisfactory. The X group is eompletely filledwith Ca and Na, and exeess Na * K is attribued to occupancy of theA site.

Geologieal and petrological features mentioned in Table I wereused to subdivide the studied hornblende andesites into two groups:

a) island arc occurrencesb) continental margin (intracontinental chain) occurrences.

The Fijian, Solomon Island, East Papuan, New Zealand (Mt'

Egmont and solander Island), and western united states (Mt. shasta

VOLCANIC ROCKS OF ISLAND ARCS 891

and Mt. Mazama) are of island arc type calc-alkaline rocks, whereasthose from Mt. Tateyama and Mt. Hakusan in Japan, Neogene ande-sites from Carpathians, and Carboniferous andesites and dacites fromNew South Wales are of continental type. Only two sarnples can notbe clearly classified into mentioned groups. These are Japanese sam-ples 21 and 22 (603) which come from new still poorly studied local-ities (Yamasaki, 1970, pers. comm.) and sample 20 (1) from Mt.Elden, Arizona, which shows alkaline rather than calc-alkaline char-acteristics.

The relationship between Aliv-Siiv in hornblende and the SiOz con-tent of the host rocks is shown in Figure 1. There is some overlap ofboth SiOz and AlzOg values in most rocks of both associations, buthornblendes from island arc volcanic rocks are appreciably morealuminous (Alt" - 1.5-1.6) than those of continental areas (Ali" =1.0-1.5). Hornblendes from continental (Andean) calc-alkaline rocksare similar to those from diorites and some granitic rocks. Kostyukand Sobolev (1969) suggest an average of 1.3 Alt" in hornblendes fromdiorites and this is a typical value for Alio in hornblendes from Andeancalc-alkaline rocks. This means that dioritic and tonalitic masses(with SiOz around 62 percent) in continental areas, are comparablewith Andean andesites and dacites not only in rock composition (a.9.,Hamilton, 1969, 1970) but also in chemical composition.of horn-blendes (c.1., Dodge et al. 196,8 data on Sierra Nevada Batholith).This may suggest that diorites and tonalites are genetically moreclosely related to Andean type calc-alkaline magmas than to islandarc types.

The total aluminum content (Alt" and AId) is also higher in horn-blendes from island arc rocks than in those from continental areasalthough the aluminum content in the whole rocks is equal. Generallyhigher Aloi content in island arc hornblendes suggests by analog5r withexperiments by Green and Ringwood (1g68), and Holloway and Burn-ham (1972) higher pressure of their crystallization. Total aluminumin hornblendes from island arc occurrences is only slightly lower thanthat in amphiboles from lherzolite, websterite, and clinopyroxeniteinclusions (Kuno and Aoki, 1970; Aoki and Kushiro, 1968) in alkalibasalts and from clinopyroxene-hornblende inclusions in calc-alkalinerocks (Yamazaki et orZ., 1966). The diflerences if any are in octahedralaluminum rather than tetrahedral. Hornblendes from calc-alkalinerocks and their inclusions have relatively low contents of Ti, Na, andK compared with those occurring as phenocrysts or xenocrysts inalkali basalts (c./., Le Maitre, 1g6g; McBirney and Aoki, 1g6g; Best,1e70).

oo2Eo

o2

t

o!

3co

g

892 PETR JAKES AND A J. R. WHITE

58 59 60 61 62 63 61 65 66 67

w t o / o S i O 2 i n r o c k

Frc. 1. Relationship between tetrahedra.I aluminum (Alto) in hornblende andSiO, content of the host rock. Amphiboles from Continental calc-alkaline rocks(squares) generally have a lower AItv content than those from island arc calc-alkaline rocks (circles) even for rocks of the same SiOe content.

There is positive correlation between Na f K and Alt" (Fig. 2)among hornblendes from continental calc-alkaline andesites. Therange of Na * K values is very similar in both groups but slightlyhigher in island arc rocks. The positive correlation between Alio andFe3- f Al- + Ti results from charge balance and valency require-ments (Dodge et a l . ,1968).

The relationship between total iron and magnesium in hornblendesand their host rocks is shown in Figure 3. There is an increase ofFe/Mg ratio in hornblende with increasing Fe/Mg of the host rockalthough the range of Fe/Mg in hornblende is fairly constant despitelarge variations in the host. The Fe/Mg ratio tends to be higher inAndean rocks than in island arc rocks and hence hornblendes from

o

oo

O to r r O O Ov v o

VOLCANIC ROCKS OF ISLAND ARCS 893

Andean rocks have higher Fe/Mg ratios but where the two rockseries overlap in chemical composition and have the same Fe/Mgratios (1.6-2.2) hornblendes of continental type andesites have higherFe/Mg ratios. clinopyroxene crystallizes before hornblende in islandarc rocks whereas in Andean rocks hornblende (as well as plagioclase)appear first' clinopyroxenes generally have lower Fe/Mg ratios thancoexisting hornblendes so that when they crystallize first there ismarked inuease of Fe/Mg in the amphibole.

All of the hornblendes have higher Ne/K ratios than their hostrocks although the K2o content of the hornblende is dependent on the

o v e

. ^ l- : i "

I v!, r,

t - a o a

a o o a

o

9 t :

J " ^ A

I

r t

g 1 nf

5.8u)n

9 o ' 'E q . 6( l ) l

€E 5b€ . 4a . J

.>)< . L

r . l(sz.

1.0 1 .5 2 .0 2 .5A['uin hornblende structural formutae

_ Frc.2. Relationship of total alkali content and tetrahedral alumirrum.in horn-blende structural formulae in island arc volcanic rocks and continental calc-

894 PETR JAKES AND A. J. N. WHITE

38363./r

taro J ' '-?nc - ' --28

2018,l t-l . o

1.lr121008

.7 .3 . t- 5 .6 .7 .8 .9 1.0

re'i ne7 vg in hornblendeFrc. 3. The relationship of iron-magnesium ratios in hornblendes and their

host, rocks. Note that at sa.me Fe/Mg ratio in rocks hornblendes from island arcs

rocks (circles) have lower Fe/Mg ratios than hornblendes of calc-alkaline rocks

from continental areas. Symbols as in Fig. 2:

Clinopyrorenes

Hornblende-bearing calc-alkaline rocks from island arcs usually

ou)

cN

a)LLqO+

oG)

II

262/-22

ooa '

' a

3 t r^

a oO

I- o' o o o '

9

vo

I

v

v

o v

t

9

VOLCANIC ROCKS OF ISLAND ARCS 895

crysts usually does not exceed 25 percent and clinopyroxene is farmore frequent as a phenocrystic phase in island arc rocks than inAndean types.

Clinopyroxenes show a narrow range of composition. All are highlycalcic and substantially different from those of the pigeonitic seriesof island arcs (Aoki, 1967; Kushiro, 1960). Both Ali'and Al"t are lowas in SiO2-saturated magmas (Kushiro, 1960) but their total Al con-tent is slightly lower than in the clinopyroxenes from hornblendepyroxenite inclusions in calc-alkaline rocks (Yamazaki et a1.,1966).Alkali contents are comparable in both hornblende-bearing and horn-blende free inclusions.

Orthopgrarene

Orthopyroxene occurs as colorless, weakly pleochroic phenocrystsin hornblende-bearing calc-alkaline rocks. It is rare in rocks from

. .84

ilII

I . 8 8' r

l . l6- t . 0 2

/ . , .oo/ ' . 9 0

1 /

o l ' 2 0 / r e

/,/

,///

/ r 2 . e

2 .41a a

. )'.o,

r . 8 0. t . 6 t

2 . 6 0

.70 t

t 1 .27 .r . 3 6

o3.8 0

t 2 . o B

K i n s i r u c t u r q l f o r m u l o s o f o m p h i b o l e

Frc. 4. The relationship of tetrahedral aluminum and potassium in hornblendestructural formula. Only calc-alkaline rocks from island arcs are plotted. Numbernear each dot indicates KzO content in pa.rent rock. Right from 1.5 dashed line allparent rocks have higher content of K,O than 15 percent. Tie line connectshornblende phenocryst and hornblende in accumulative "inclusion" of the samelava.

oo

E Oo

ot

f

o!

o!co

a.

. 5

. 1

. 3

. 2

. l

896 PETR JAKES ,4ND A. J. R.I4/HITE

island arcs but it is more common in the continental association al-though this may result from the more siliceous mode of the continentaltypes. Orbhopyroxene is common in the groundmass in rocks fromboth associations.

New analyses show that the hypersthenes have a narrow composi-tion range (Fsr* - Fs+s): this is also a feature of orthopyroxenesfrom the hypersthenic series of Japan (Kuno, 1968). Calcium is loweven compared with other igneous orthopyroxenes and aluminum isalso low. The distribution coefficients of Fe and Mg between "co-existing" ortho- and clinopyroxenes in hornblende-free lavas asso-ciated with hornblende-bearing rocks in island arcs (East Papua)indicate temperatures of around 1050' if the curves of Bunch et a,Z.(1970) are used (Jake5, 1970). However for hornblende-bearing rocksthe same method gives erratic results.

O,paqwe Minerals

There is rarely any textural evidenee that magnetite is an earlycrystallizing phase in the hornblende-bearing rocks of continental areas(c./., Smith and Carmichael, 1968) but titaniferous magnetite occursin the groundmass. It occurs sornetimes associated with early clino-pyroxenes in island arc rocks. Phenoerystic Ti-riragnetites are richerin Mg, Al, and Si than groundmass magnetites (Carmichael andNicholls, 1967; Srnith and Carmichael, 1968). Temperatures derivedusing Buddington and Lindsley's (1964) method modified for micro-probe data by Carmichael (1967) range around 900"C, but theseprobably represent the final stages of crystallization.

Biotite

Biotite is rare in the calc-alkaline volcanic rocks of island arcswhere it is limited to high-K types. It is more frequent in continentaltype andesites. Biotite from hornblende-bearing rocks are characteris-tically low in aluminum and have higher Fe/Mg ratios than those ofhornblendes and clinopvroxenes.

DrscussroN

Experimental Euidence

The liquidus and near liquidus phase relations of dry calc-alkalineandesite were studied in detail by Green and Ringwood (1968) overa wide range of pressures, and by Brown and Schairer (1968) at 1atm pressure. Their data indicates that near liquidus crystalliZationis dominated at low and moderate pressures (0-10 kb) by plagioelase,at high pressures (10-18 kb) by plagioclase and clinopyroxene, and

VOLCANIC ROCKS OF ISLAND ARCS 897

at extremely high pressures (20 kb) by garnet. yoder (1969) suggestedby analogy with basalts that olivine is a liquidus phase. There i* tittt"petrographic evidence of orivine precipitation in andesites, however(58 percent SiOr).

explanation of their different minerarogies. The relevant data aregeneralized in Figure b.

rocks was low, and consequently liquidus temperatures were abovethe field of hornblende stability.,The possibility that. water was in-broduced from an external source is unrikely, because this would

898

tq)

f

o

G

PETN JAKES AND A. J. E. WHITE

w e t l i q u i d iH^o

+ Z

o

, r i

T e m P e r o l u r e 4

Frc. 5. Generalized relationships of initial crystallization of hornblende andesites

from island arcs and continental areas'

cause a general decrease of liquidus temperature resulting in resorp-

tion or corrosion of clinopyroxenes for which there is no textural

evidence.In the continental type of andesites and some high-K rocks from

island arcs (Eastern Papua) the crystallizaL\on of hornblende as the

first phase on liquidus suggests that the liquidus temperatures lay

within hornblende field of stability. Hornblende is joined later by

clinopyroxene and pla gioclase.compiling older and new experimental data the approximate fields

of crystallization are drawn in Figure 5 and we suggest that most

of the continental type of calc-alkaline rocks have contained more

than 2.5 percent H2O, whereas island arc hornblende-bearing an-

desites HsO contents were probably lower than 3 percent.

If the water content is the primary feature of calc-alkaline magmas

of both types and does not result from a contamination of "islandarc andesites,, by uppermost part of continental crust to produce

eontinental type andesites, then the differences in a liquidus tem-

perature allows some speculation on their origin.comparison of experimental and natural data shows that horn-

blende is unlikely to be a residual phase of parbial melting or an

early fractio rate in the island arc andesites, but could be in con-

r l'-lo

VOLCANIC NOCKS OF ISLAND ARCS 899

tinental andesites. Using trace element evidence JakeB and White(1970) suggested that in island arcs a down-going slab of oceaniccrust is melted by the decomposition of HzO bearing phases (amphi-bole and mica). The low melting point fraction in this case containsmost of the water available and inherits sorne of the geochemicalcharacter of minerals from which it has been formed.

The geochemical character of island arc lavas-i.e., relatively highK/Rb ratios, low Ba and Sr, contrasts with relatively low K/Rbhigh Ba, Sr, etc., in continental suite of calc-alkaline rocks. We be-Iieve that the differences, although they may result from primarycharacter of source material, could also result from hornblendefractionation in source area. Hornblende precipitation can explainsuch features of continental calc-alkaline rocks as the change ofKzO/NazO ratios with increasing SiO2 content as well as relativelylow Mg contents of continental type rocks, because hornblende frac-tionates K/Na and Fe/Mg.

Sornce Materi,al

Jake5 and White (1971) pointed out that in older, highly evolvedisland arcs and in the continental margins both types of calc-alka-line volcanic are probably present. The source material for islandarc rocks has been widely discussed (i.a., Ringwood and Green, 19681Ringwood, 1969; Tatsumoto, 1969) and was considered to be a mix-ture of oceanic crust of tholeiitic (abyssal tholeiite) composition andsome associated sedimentary rocks or other (alkaline) volcanic rocks(Armstrong, 1971). In order to produce rare earth element patternsand other trace element characteristics of cale-alkaline rocks fromrelatively primitive tholeiitic rocks, the residuum after partial melt-ing should have a clinopyroxene-garnet mineralogy. These residualphases also result from the fraction necessary to produce the chemicalcharacteristics of continental andesites from a more primitive rockwith hornblende crystallization accounting for Rb, K/Na, and Mgabundances. The question of source material for continental andesitesis related to the composition of the lower crust, which we believe isthe area where the largest volume of these rocks forms. It has beensuggested that early stages of island arcs are characteized by tho-leiitic activity (Baker, 1968; JakeH and White, 1969; Gill 1970) andtha;b basement of island arc is formed by the island arc tholeiites(Jake5 and Gill, f970) in agreement with the fact that tholeiites arethe most voluminous portion of island arcs (Sugimura, 1968). It isapparent that the primitive lower part of the island arc crust is tho-leiitic and the crust, as a whole, primarily stratified with calc-alkaline

9@ PETR ]AKES AND A. J. R. WHITE

rocks in its upper part. Such lower crust has lower contents of largecations, a-!1., K, Rb, Ba, ,Sr, very primitive Sr isotope ratios (0.702-+ 0.001) and does not differ substantially from oceanic basalts (c./.,Jake5 and Gill, 1970) in major element chemistry, except that itprobably has larger proportions of Si-rich rocks. We believe that thistype of lower crust, together with associated sediments, is a sourcematerial of the "continental type calc-alkaline suite" and has garnet-clinopyroxene amphibolite mineralogy.

Recent growth of island arcs (e.g., Sugimura, 1968) suggests thatwithin a period of 80-100 million years the crust of island arcsreaches a thickness between 2O-30 km. The load pressure on the baseof such crust will be of the order of 5-7.5 kbar, and temperatures maylocally rise to 750"-800oC if most of the heat is produced along theBenioff zone (e.9., Minear and Tdksoz,1970). Such pressure and ther-mal conditions at the base of island arc or continental margin crustpermit "wet" partial melting.

The differences in thermal regimes of the upper part, of <iown-going slab with relatively low water cont€nts and consequentlyhigher melting temperatures (1050-1100o) and rela;tively high con-tents of water and lower liquidus temperatures for "Andean-con-tinental type of calc-alkaline rocks" also explain the lack of basalticmembers in a continental suite. The a.ssumption of tholeiitic parentfor both calc-alkaline types explains low Sr isotope ratios. Intensivepiling of volcanic material together with the gradual change of thermalregime in the upper mantle and the crust above the descending slabcan account for the presence of continental type (high-K) calc-alka-line rocks in late stages of island arc evolution or its presence in thecontinental margins.

Acrrowt,nncltnnrsDr. B. W. Chappel helped substantially with X-ray spectrographic techniques;

Mr. N. G. Ware with electron probe techniques; and Messrs. W. Hibberson andA. Major with high pressure techniques. Drs. R. Williams and I. Ridley criticallyread the manuscript. This help and the financial zupport of Australian NationalUniversity, and of The Lunar Science Institute under NASA. Contract No. NSR09-051-001 are acknowledsed.

RersnpNcnsAr,nrn, A. L., eNn L. Rey (1972) Correction factors for electron probe micro-

analysis of silicates, oxides, carbonates, phosphates and srlphales. J. Geol. (inpress).

Aorr, K. (1967) Petrology and petrochemistry of latest Pliocene olivine tholeiitesof Taos Area, northern New Mexico, U.S.A. Contrib. Mineral. Petrologg,74,190-203.

in Dreiser Weiher, F,ife. Confiib. M'tneral. Petrologg,18,326-.337.

VOLCANIC ROCKS OF ISLAND ARCS 9OI

AnlrsrnoNc, R. L. (1971) rsotopic and chemical constrains on mocrels of magmagenesis in volcanic arcs. Earth plonetary Sci, Lett.12, IBZ-142.

Barnn, P. E. (1968) Comparative volcanology and petrology of the Atlanticisland-arcs. BuIl. V olcan. 32, 1g9-206.

Bnsr, M. G. (1970) Kaersutite-peridotite inclusions and kindred megacrysts inbasanitic lavaq Grand Canyon, Arizona.. Contrib. Mineral. petiotogg, zz,25-44.

Bnowx, G. II., awr J. F. Scrrarnnn (196g) Melting relation bf some calc_alkaline_ volcanic rocks. Catnegie lnst. Wash. year Book, tg66-!g67,46G,/l67.BuoorwcroN, A. F., ext D. F. Lrirrosr.ov (1g64) Iron_titanium oxide minerals and

synthetic equivalents. J. P etrol,o gy, 5, Bl0-3,52.Bunclr, T. E., K. Knrr,, exn E. Or,snx (1g20) Mineralogoy and petrology of

silicate inclusions in iron meteontes. contri,b. M,i,neral. petrol,ogg-, 2s,2gz-Bzo.caniurcHeor, r. s. E. (1962) The iron-titanium oxides of salic volcanic rocks and

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Mannscri,pt rece'i,ued, JuIg 2!, 1971; accepted lor pr,blicatinn, December Ig, 1g/1.