Petrochemical ~ t i e s from the Tonga - Kermadec

of Volcanic Rocks I s l a n d Arc, Southwest P a c i f i c

R. N. BROTHERS Department of Geology, University of Auckland, Auckland, New Zealand

Abstract Eruptive suites from Tonga (tholeiitic), Raoul Island (tholeiitic) and Macauley Island (high-alumina) are characterised by low alkalis, an absence of andesites in the range 56-65% silica, and restricted acidity for minor glassy differentiates (SiO2 = 65-68 %). These volcanics form a chain of islands overlying a seismic zone which extends from Tonga to the central volcanic region of North Island, New Zealand where a calc-alkaline series contains basaltic, andesitic and rhyolitic members in that order of increasing abundance. Within this continental suite, tholeiitic and high-alunfina phases are recognised as closely similar to the intra-oceanic Tonga-Kermadec magma types and show petrochemical gradation into the medium-silica andesites, apparently by sialic assimilation.

Introduction Chemical analyses for volcanic rocks f r o m the Tonga-Kermadec ridge have been listed for Tonga by R:CHAm~ (1962), f o r Raoul Island by BROTHERS and SEARLE (in press), f o r Macauley Island by BROTHERS and MART:N (in press), and for an active u n d e r w a t e r seamount, Rumble III, on the s o u t h w e s t e r n extension of the ridge by BROTHERS (1967). Tonga and Raoul, the n o r t h e r n m o s t K e r m a d e c island, have closely comparable tholeiitic suites; Macauley, some 70 miles southwest of Raoul, has a buried tholeiitic group with a capping of higha l u m i n a basalt; Rumble I I I has erupted basaltic andesite. On a variety of evidence (BROTHERS and MARTIN, in press) the initiation of volcanism appears to have been progressively later f r o m n o r t h e a s t to southwest, the Tongan group being Cenozoic, Raoul having an upper Tertiary-Quaternary age, Macauley a late Pleistocene age, and Rumble III being currently active.

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These widely-spaced volcanic centres (Fig. 1) lie to the west of the Tonga-Kermadec trench and immediately above the westward dipping zone of earthquake epicentres which extends from New

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Zealand (HAMILTON and GALE, 1968) to beyond Tonga as the great oceanic fault of BENIOFF (1954-1955) or sub-crustal rift of EI~v (1958; 1964). Location of the volcanoes, in relation to SYKES' (1966, Fig. 8) cross-sections of the earthquake focal surface, shows that the Kermadec vents are situated immediately west of the strongest concentrations of shallow foci ( < 100 km depth) recorded along the ridge; below the line of these volcanoes the focal zone is about 100 km thick

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and the vents are 50 k m above the upper surface of the zone. The connection here between earthquake foci and volcanism may be either a direct one, involving m a g m a segregation actually in the seismic zone, or indirect where phase changes at that depth are only a first step towards partial melting that occurs at still higher levels. For example, KUNO (1959; 1966) has argued for direct correlation between depth of seismic foci and basalt type in Japan, with alkali basalt originating at depths greater than 200 km, tholeiitic basalt at 50-200 km, and high-alumina basalt from depths transitional between these two levels. More recently GREEN et al. (1967) experimentally examined the pressure regimes affecting fractional melting and concluded that the sloping earthquake focal zone is the site from which their upward-migrating pyrolite mantle begins to move vertically, with the onset of partial melting and fractionation delayed until significantly higher levels; on this basis they derive quartz-tholeiites at depths of 0-15 km, highalumina basalts at 15-35 km, and alkali basalts at 35-60 km. The petrological character and rock sequence in the Kermadec volcanic series could support the model proposed by GREEN et al. (1967); the m a x i m u m depth to the u p p e r surface of the seismic zone is in the order of 50 km, and high-alumina basalt follows tholeiitic in the eruptive sequence at Macauley Island. An important issue is the relationship of the Tonga-Kermadec suites to the calc-alkaline series of the central volcanic zone of New Zealand's North Island. For Tonga, no recent petrographic descriptions are available although earlier reports (e.g. DALY, 1916; JAGGAR, 1930; ALLING in HOFFMEISTER, 1932; MACDONALD,1948) refer to basalts, andesites, dacites and rhyolites within the island group. Nevertheless, the seven available chemical analyses of these rocks (RICHARD, 1962) are clearly tholeiitic in character as recognized by KtlNo (1966) and DICKINSON (1968), and as pointed out by MACDONALD(1960) assimilation of sial may be lacking within the igneous suite of the Tongan arc. The Raoul and Macauley suites appear not to be affected by contamination, very possibly because no sialic rocks are present on uparched oceanic crust that forms the bulk of the Kermadec ridge; in both islands only a minor a m o u n t of acid differentiate is found as pumice and obsidian, and basaltic rocks are the c o m m o n e s t type. The geological setting for this well defined line of volcanoes extending some 2,500 k m from Tonga to Mt Ruapehu (North Island) provides an example of primitive intra-oceanic suites which, southwestwards along their locus of generation in or above the seismic zone, have

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encountered overlying sialic crust in the New Zealand mainland. Here, in contrast to the Tonga-Kermadec area, the great volume of calcalkaline eruptive products in the North Island central volcanic district is acid in character with lesser a m o u n t s of intermediate rocks and only rare exposures of basalt. Diverse origins suggested for this calcalkaline suite include: a) contamination of olivine basalt with subsequent limited fractional crystallization, plus cumulate lavas developed f r o m the uncontaminated olivine basalt, plus an associated syntectic acid magma with certain immiscibility characteristics (STEINER, 1958; 1960); b) mixing in various proportions of two end-member magmas, one acid and the other basic, plus ancillary assimilation and fractionation (CLARK, 1960); c) a genetic relationship in a continuous petrochemical sequence, despite volume relationships, from basalt through to rhyolite with minimal contributions from wholesale fusion or assimilation of sial (LEwiS, 1968a, b); and d) crustal fusion for the rhyolites, with associated basalt m e m b e r s (HEALY, 1964; EWART et al., 1968; EWART and STIPP, 1968). Similar transitions from intra-oceanic volcanic areas to continental margins have been studied in detail by such authors as STARK (1963), SUCIMURA (1968) and KtJNo (1966), and circum-Pacific aspects of this type of lateral variation have been summarised by DICKINSON (1968). Petrochemistry The relationship of the Tonga-Kermadec analyses to those for North Island central volcanics has been examined in a series of petrochemical diagrams presented in Figs. 2-11. Some of these diagrams explore the relative distributions of main elements in the rocks and others are used to support petrographic identification of the suites; for the latter purpose Comparative material has been selected for tholeiitic and high-alumina standards from Hawaii (MACDONALD and KATSURA, 1964), Thingmuli (CARMICHAEL, 1964), Skaergaard (WAGER, 1960) and Japan (KUNO, 1968) and for calc-alkaline suites from Japan (Kt_rNo, 1968), Cascades (CARM[CHAEL, 1964) and world averages (NoCKOLDS, 1954). All of the parameters used in the diagrams have been standardized by computation from the raw analyses using a prog r a m m e sirailar to that of MCINTYRE (1963). In Fig. 2 a standard Harker-type diagram, using POLDERVAARTand PARKER'S (1964) crystallization index (C.I.) as the abscissa, compares

- - 312 - the differentiation curve for the Raoul thole]]tic suite with the distribution of oxides for Tonga (thole]it]c), Macauley (high-alumina), oo-

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oxides a g a i n s t PO~-F~WAItT a n d P~KER'S (1964) c r y s t a l l i z a t i o n i n d e x ( C . L ) . T h e curve f o r R a o u l I s l a n d = solid line (B~oTI-m~S and SE~, in press) is c o m p a r e d w i t h d a t a f o r T o n g a = solid triangles (RICI-]~]~D, 1962), M a c a u ] e y I s l a n d = crosses (BRo~HL~S a n d M _ ~ I ~ , in press), R u m -

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Rumble III (basaltic andesite) and the N e w Zealand North Island central volcanic series (calc-alkaline). Significant comparisons are: (1) The curve for silica enrichment at Raoul contains a compositional

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gap, which is matched by the Tongan rocks, between basaltic andesites (upper limit for SiO2 -- 56 %) and volumetrically minor quantities of dacite-rhyolite obsidians (SiO2 = 6 6 - 68 %); a similar but wider void in the Macauley rocks (from 50-68 % SiO2) terminates in a rhyolite pumice which for silica, as well as alumina, iron oxides and magnesia, approximates very closely the curve for the central calc-alkaline suite; the overall tenor for silica in the basaltic range is higher in the calc-alkaline rocks where, similarly, the upper limit for silica is 8 % greater than for any of the acid rocks in the oceanic series. (2) The Macauley suite, the North Island calc-alkaline series and Rumble III show generally higher contents of alumina than Raoul, but near the Raoul curve are a significant number of the more basic calc-alkaline members; the latter rocks invariably have been regarded by New Zealand writers (for example STmNER, 1958; CLARK, 1960; EWART, 1965; LEWIS, 1968a) on petrochemical evidence as cumulates, but the present study indicates that, in common with high-alumh~a basalts in the same suite, they may represent one of the distinct basic lineages for the calc-alkaline series. (3) The total iron oxides curve for Raoul, with marked middle-stage iron enrichment, contrast strongly with the calc-alkaIine trend, but shows similarities to the Tongan tholeiitic series. (4) Lime is notably higher in Tonga, Raoul and Macauley, including the high-alumina pumice differentiate; the narrow vertical range of plots for CaO in both the high-alumina and calcalkaline series reflects the presence of consistently large components for normative An in C.I. values on the abscissa. (5) Alkalis, particularly potash, are low in the Macauley and Raoul rocks where Na20/ K20 ratios lie between 7.5 and 2.6; in the calc-alkaline series some basic members approach the low potash level of the high-alumina and tholeiitic suites, but below C.I. = 45 the K20 content rises rapidly towards C.I. = 12 to 5, giving overall Na20/K20 ratios for the series over the range 11.6 to 0.76. The FMA diagrams of Figs. 3a and 3b make distinctions between the Tonga-Raoul and central volcanic suites. In Fig. 3a the basic members of all the volcanic associations are spread across the basaltic ends of KUNO'S (1968) fields for Japanese pigeonitic (P) and hypersthenic (H) series, but in the absence of a differential between alumina and alkalis no distinction is possible between Macauley high-alumina and Tonga-Raoul tholeiitic rocks. In Fig. 3b a comparison is made with differentiation trends for calc-alkaline averages from the Cascades (CARMICHAEL,1964) and for the world (NoCKOLDS,1954), as well

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as the tholeiitic Thingmuli suite ( C A R M I C H A E L , 1964) and for Skaergaard which is regarded by KUNO (1968) as an iron-enriched highalumina example, but by BROWN and SCHAIRER (1968) as tholeiitic. Both diagrams emphasize iron enrichment in the Tonga-Raoul series, comparable with that of Skaergaard, and the generally low content of alkalis for both Raoul and Macauley, including acid differentiates.

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Petrochemical identification of Tonga, Raoul and Macauley basic rocks is best achieved in Fig. 4 using AI203 against Na20 + K20 and superposed on Kvno's (1960) basalt fields in the silica range 47.51% 55.0 %. Here, separation between the tholeiitic and high-alumina groups is clear, with the latter (all in the silica range 47.51% -50.01%) displaced slightly below KUNO'S relevant field because of characteris-

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No20+ J'(,2 0 FIG. 4 - AI20~ - total alkalis r e l a t i o n s h i p for the Tonga, Raoul, Macauley, R u m b l e I I [ a n d central volcanic series ( s y m b o l s are the s a m e as t h o s e in Fig. 3). Also s h o w n a r e Ku,~o's (1960) J a p a n e s e tholeiitic, high a l u m i n a and alkali b a s a l t fields w h e r e fields 1 = 47.51 - 50.00 % SiO2, fields 2 = 50.01 -- 52.50% SiO2, a n d fields 3 = 52.51 - 55.00 % SIO2. Within the d i a g r a m , c o m p a r a b l e b a s a l t s f r o m T o n g a - K e r m a d e c s - N o r t h I s l a n d a r e n u m b e r e d a c c o r d i n g to Kt:No's relev a n t b a s a l t field, w i t h Macauley b a s a l l s all b e l o n g i n g in field 1; all o t h e r p l o t t e d p o i n t s have SiO2 in excess of 55 %.

tically low alkali content. For the calc-alkaline suite, the dichotomy noted on the A12OJC.I. diagram (Fig. 2) among the most basic members is again expressed as two trends with apparent tholeiitic or highalumina affinities which merge near Na20 ~- K20 = 5.75 %. Serial extensions of the oceanic suites in terms of salic/femic ratios are shown in Figs. 5 and 6 along with standard curves for Skaergaard, Thingmuli, the Japanese pigeonitic and hypersthenic series and the Cascades. For normative Ab versus Mg ratio (Fig. 5) and for iron ratio against C.I. (Fig. 6) the central calc-alkaline suite has closer similarity to the continental Cascades rocks than to KuNo's (1968) H series for Japan where moderate iron enrichment is more tholeiitic

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in character. Variation in slope of the Thingmuli curve, pointing to the possibility of syntectic or hybridised derivatives towards the acid end, has already been discussed by CARMICHAEL (1964) and noted by BROWN and SCI~ZRER (1968). 100-

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Mg ratio FIG. 5 - N o r m a t i v e m o l e c u l a r f e l d s p a r (Ab) r a t i o (CARMICHAEL, 1964) p l o t t e d a g a i n s t Mg r a t i o ( G R E ~ a n d R/NGWOOO, 1968) f o r T o n g a (solid t r i a n g l e s ) , R a o u l ( l a r g e solid circles), M a c a u l e y ( c r o s s e s ) , R u m b l e I I I ( o p e n circle) a n d N o r t h I s l a n d c e n t r a l v o l c a n i c s e r i e s ( s m a l l solid circles). Also s h o w n are: c u r v e 1 = C a s c a d e s calcalkaline series; curve 2 = Japanese hypersthenic series; curve 3 = Thingmuli tholeiitic s e r i e s ; c u r v e 4 = J a p a n e s e p i g e o n i t i c series.

A particular comparison has been m a d e with similar suites from Japan which are the best documented for the western Pacific. Figs. 7a and 7b show KvNo's (1968) fields for Japanese pigeonitic and hypersthenic suites derived from his two paramenters based on FeO + FezO3 (as FeO) and a solidification index (S.I.) MgO × 100/MgO + FeO -k -b Fe,O3 ÷ Na20 q- KzO; on statistical grounds KuNo regards S.I. ~ 40 as indicative of a cumulative rock. In Fig. 7a the calc-alkaline Cascades suite lies mainly within the hypersthenic field, but with comparably

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lower content of total iron oxides as already noted from Figs. 5 and 6. CARMICnAEL'S (1964) tholeiitic Thingmuli series falls within the pigeonitic field, but three of his feldspar-enriched cumulative rocks also have S.I. values lower than 40, with two of the plotted points approximating the hypersthenic field and the third lying within the tholeiite I00-

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area; C A R M I C t l A E L ' S fourth cumulative rock, an olivine and pyroxene enriched picrite (analysis 25), has S.I. = 58.2 and lies outside the diagram. It would seem that, with total iron oxides and total alkalis in the denominator of the function, the Solidification Index has limited value for the identification of cumulates in which magnetite or feldspar are important additives. In Fig. 7b the Tonga-Raoul tholeiitic suites

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and the central volcanic calc-alkaline series show good accordance with Kl:No's Japanese fields, with the Macauley high-alumina rocks in a roughly intermediate position. Three basalts from Raoul (BROTHERS and SEARLE, in press; analyses 1, 2 and 6) and two rocks from the central volcanic area (STEINER, 1958; analyses 7 and 12) would be classed as cumulates on the S.I. differential; in addition, analysis 5 from Steiner (1958) has S.[. = 53.1 and lies outside the ordinate. l

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FI6. 7 - Total iron oxides plotted against Solidification Index (KuNo, 1968) to give the fields for Japanese pigeonitic (P) and hypersthenic (H) series. In Fig. 7a are shown averages for the Cascades calc-alkaline series (open circles) and plots for the Thingmuli tholeiitic suite (small solid circles, with cumulates = solid triangles). In Fig. 7b the same Japanese P and H fields are c o m p a r e d with data f r o m Tonga (tholeiitic) = solid triangles; Raoul (tholeiitic) = large solid circles ; Macauley (high-alumina) = crosses ; Rumble III (basaltic andesite) = = open circle; and North Island central volcanics (calc-alkaline) = small solid circles.

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-~------ S. I F16. 8 - Silica p l o t t e d a g a i n s t Solidification I n d e x for J a p a n e s e p i g e o n i t i c (P1 a n d h y p e r s t h e n i c (HI fields (KuNo, 1968). Fig. 8a c o n t a i n s C a s c a d e s c a l c - a l k a l i n e a v e r a g e s ( o p e n circles) a n d t h e T h i n g m u l i tholeiitic s u i t e ( s m a l l solid c i r c l e s , w i t h c u m u l a t e s = solid t r i a n g l e s ) . I n Fig. 8b a r e s h o w n T o n g a (tholeiitie) = = solid t r i a n g l e s ; R a o u l (tholeiitic) = large solid c i r c l e s ; M a c a u l e y (higha l u m i n a ) = c r o s s e s ; R u m b l e I I I ( b a s a l t i c a n d e s i t e ) --- o p e n circle; a n d N o r t h I s l a n d c e n t r a l v o l c a n i c s (calc-alkaline) = s m a l l solid, circles.

- - 320 - The same pigeonitic and hypersthenic fields from Japan for SiO2 versus S.I. (KuNo, 1968) are given in Fig. 8a with the Thingmuli and Cascades series added, and in Fig. 8b with the Tonga, Raoul, Macauley and North Island central volcanic suites. In Fig. 8b, analysis 5 from 9" 8"

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Cascades ca/c-alkaline averages (open circles) and the Thingmuli tholeiitic series (small solid circles). Fig. 9b shows Tonga (tholeiitic) = solid triangles; R a o u l ( t h o l e i i t i c ) = large solid circles; Macauley (high-alumina) = c r o s s e s ; Rumble III (basaltic andesite) = open circle; and North Island central volc a n i c series ( c a / c - a l k a l i n e ) = s m a l l s o l i d c i r c l e s .

STEINER (1958) with SiO2 = 54.91% again plots off the diagram, but this sample and eight others from the central volcanic suite stand apart from the common trends. They are outstanding within the central volcanics as distinct variants by containing high silica relative to high S.T., by low alumina (Fig. 2) relative to alkalis (Fig. 4), and by notably low Na20 + relative to SiO2 (Fig. 9b). These similarities

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321

to the Tonga-Raoul tholeiitic rocks are supported petrographically by the pigeoniteobearing assemblages reported in olivine andesites by CLARK (in GRE66, 1960); however, fine comparisons of the petrochemistry reveal significant differences from the Tonga-Raoul suite since the calc-alkaline rocks contain slightly heightened values for alkalis (Figs. 9b, 10), alumina and silica (Fig. 2), and lower iron oxides (Figs. 2, 7b). Previously these particular rocks have been separately classed within the central volcanics by STEINER (1958), CLARK (in GREGG, 1960) and EWART (1965) as cumulates associated with a main trend which commenced with a high-alumina parent (for example CLARK, Fig. 16, in GREGG, 1960). The present petrochemical comparison with the TongaKermadec suites now indicates that they represent another lineage linked probably by assimilation-contamination to a tholeiitic magma within the central volcanic area of the North Island. This possibility is examined further in Figs. 9a, 9b and 10 for alkalis versus silica. In Fig. 9a these co-ordinates determine KUNO'S (1966) fields for Japanese tholeiitic, high-alumina and alkali basalts and their derivatives; also added are the geographically-distinct Icelandic tholeiitic suite and Cascades calc-alkaline series. In Fig. 9b the Tonga-Raoul tholeiitic suite occupies a field distinct from the Macauley high-alumina and the central volcanic calc- alkaline groups. At SiO2 = 5 6 - 58 % the calc-alkaline trend shows a radical change in total alkali content which, after maintaining a level near 3.0 % over a range of silica from 49.52 % to 55.73 %, then increases rapidly to 4.5 % and thereafter assumes a steady gradient. The data are presented in detail for K20 against SiO2 in Fig. 10. Here, the more basic calcalkaline members generally show slightly higher potash values than the assumed parent magma types of Macauley or Tonga-Raoul; change in slope of the calc-alkaline variation curve is pronounced at SiO2 = 57 9/o where it rises almost vertically to join the Cascade trend. It is particularly notable that this abrupt rise in K20 content takes place at the lower limit of the highly characteristic compositional gap in the Tonga-Raoul and Macauley suites where intermediate rocks are not known in the silica range 56 % to 65 %; moreover, obsidian and pumice members of these suites define the acid ends of variation curves which continue to show low contents of potash as compared with the central volcano series. This supports a deduction already suggested by BROTHERSand SEARLE(in press) and BROTHERSand MARTIN (in press) that the Raoul and Macauley suites have not been contaminated by addition of sialic material. 21

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- - 323 Discussion

Petrochemical relationships b e t w e e n the oceanic suites of TongaK e r m a d e c and the calc-alkaline series of the N o r t h Island point to several immediate inferences. (1) Both tholeiitic and high-alumina affinities can be d e m o n s t r a t e d for the low to m e d i u m silica rocks of the N o r t h Island calc-alkaline series; b u t the recognition of these parental sources should be reconciled w i t h the conclusion reached b y EWnRT and S x I v v (1968), f r o m trace element ratios, that the acid m e m b e r s of the central volcanic association are the result of wholesale crustal fusion. (2) Where they have e n c o u n t e r e d sialic rocks in the New Zealand mainland, the two parent m a g m a s have been diverted from their oceanic differentiation paths by some process of contamination, at first indicated by very slight increases in alkalis, silica and alumina; this trend is accentuated by p r o n o u n c e d accretion of alkalis, beginning in rocks with SiO2 = 56-58 %, and at this stage of contaminationdifferentiation the separate identity of the calc-alkaline path is especially conspicuous since it p r o d u c e d rocks with petrochemical characteristics (high alkalis-medium silica) which are notably absent in the Tonga-Kermadec suites across the compo.sition gap at 56-65 % SiO> (3) The data appears to supply sufficient evidence for petrochemical gradation from high-alumina and tholeiitic parents through the onset of sialic assimilation to thoroughly contaminated calc-alkaline compositions in the range SiO2 = 58-64 % which approaches that of b a s e m e n t greywackes in the N o r t h Island. It seems unnecessary to depend solely upon any theory that the medium-silica m e m b e r s of this central calc-alkaline series are the p r o d u c t s of crustal fusion, or that they m a y have a separate primary origin in the mantle. This is contrary to the views of DICKINSON and HATHERTON (1967), DICKINSON (1968) and HATHERTONand DICKINSON ( 1968, p. 4618) that ,, the e r u p t e d andesite lavas of the central volcanic region were derived w i t h o u t crustal contamination from melts that originated in the mantle ,,, an inference based on their recognition of a certain consistency b e t w e e n increase of K20 and SiO2 with increase in vertical distance to seismic zones underlying island arc volcanoes. By excluding the possibility of assimilation - contamination for andesites in the North Island central volcanics, HATHERTON and DICKINSON (1968, Figs. 1, 3) reach the interesting position of deriving m a g m a s with 0.9 % K20 - 55 % SiO2 from a depth of 105 - 115 km, or 1.45 % K20 - 60 % SiO2 from 120 - 130 k m ;

- - 324 - that is, a series of primary melts with silica and potash content increasing with depth of origin. Examination of this concept (HATHERTON and DICKINSON, 1968, Fig. 1 ) against the new petrochemical data from the Tonga-Kermadec arc shows that lavas from these suites with 0.45 % K20 - 55 % SiO2 would equate with a seismic zone depth at about 66 km, and for 0.50 % K20 - 60 % SiO2 at about 74 km. These depth figures are in remarkably good accordance with the position of the upper part of the seismic zone beneath the Kermadec Islands (SYKES, 1966, Fig. 8). But a serious obstacle for the DICKINSONHATHERTON idea is that andesites with SiO2 ~ 56 % are conspicuously absent from (certainly) the Kermadec Islands and (apparently) the Tonga group, despite the fact that below this arc the seismic zone extends to a depth which is sufficient, according to the theory, for the generation of such melts. A more logical deduction for the Kermadec ridge is that the apparent absence of sialic crust and the presence of a petrochemical gap at 56- 66 % silica form interconnected evidence for the importance of sialic assimilation in the genesis of andesites. (4) The silica gap in the Tonga, Raoul and Macauley rocks can be coupled with another notable feature of their chemistry; that is, the restricted maximum acidity (65.2- 68.0 % SiO2) attained by the minor acid differentiates in these suites. In contrast, the North Island calcalkaline series contains a continuous range through to 76 % SiO2 in a voluminous group of ignimbrites and rhyolites. Petrogenetic explanations for this difference between the intra-oceanic and continental acid rocks may be (a) continued assimilation-differentiation by the medium-silica middle stage melt, or (b) as seems more likely, extensive fusion of the sialic basement (EWART and STIPP, 1968). Some writers on the central volcanic rocks (CLARK, in GREC~, 1960; EWART, 1965; LEWIS, 1968a) have presented VoN WOLFF QLM diagrams for variation paths within that suite. In Figs. 11a, 11b and 11c the new results from Rao~al, Macauley and Rumble III are plotted on QLM constructions, along with analyses from Tonga, to allow extension of these earlier interpretations. In Fig. 11a curves for the Japanese H and P series (KUNO, 1968) are superposed on the spread of points for the Tonga, Kermadec and North Island suites; for the Rao.ul group of rocks, the three basalts near and subparallel to the L baseline are assumed to be cumulates. In Fig. 11b the curves for Thingmuli and Cascades show good accordance with the tholeiitic and high-alumina trends proposed for the central volcanic series, but have consistent relative offset towards L, thus pointing again to overall suppression

- - 325 of alkalis in the comparable southwest Pacific suites. Finally, Fig. l lc contains the evolutionary trends that are now suggested; curve 1 is the differentiation path for primitive Tonga and Raoul tholeiitic m a g m a ; curve 2, the calc-alkaline trend followed by contaminated m a g m a originally of Tonga and Raoul type; curve 3 is composite, including both the oceanic Macauley high-alumina suite and the calcalkaline equivalents which form the second converging branch of the North Island central volcanic series. In Fig. l lc curve 4 represents basalts, andesites and dacites from a linear group of upper Tertiary Quaternary volcanoes situated along the west coast of the North Island and lying outside the western limit o~f shallow earthquakes in the North Island seismic belt; HATHERTON (1968) proposes a possible mantle origin for some of the andesites at a depth of about 220 km near the upper surface of the westward clipping Tonga-North Island seismic zone. The QLM curve for this western suite in Fig. 11c and K20/SiO2 ratios summarised by HATIIERTON (Fig. 2, 1968) indicate a separate petrochemical identity, already recognized by Gow (1968), not closely related to phases of the Tonga-Kermadec-central volcanic series which, by contrast, are associated with the shallow portion of the Tonga-North Island epicentre belt. A noteworthy feature of the central volcanic rocks is that whilst high-alumina basalts are known, tholeiitic basalts as such have not been found and their nearest relatives are high-silica pigeonite-bearing basaltic andesites; a blurred petrochemical picture for one basaltic stem, but not for the other, in this case may be related to the level in the crust at which magma segregation has taken place. If the shallow depth-factor emphasized by GREEN et al. (1967) is applicable, then the North Island sialic crust thickness of 30-35 km (THOMSON and EvISON, 1962) could have been critical for preservation of parent magma identity; that is, tholeiitic magma fractionating at 0-15 km depth could hardly have escaped sialic contamination, while the highalumina phase at 15-35 km could have originated in part at subcrustal levels and thus not been exposed to immediate contamination. Some writers (for example MACDONALD, 1949; HOLMES, 1965) in defining the andesite line of the southwest Pacific have drawn the boundary between continental calc-alkaline suites and the oceanic series as following the eastern limit of the Tonga-Kermadec seismic zone and parallel to the trench system. On present evidence this division now seems invalid and petrochemical differences between volcanic rocks to the east (Samoa-Cook Islands) and west (Tonga-

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FI(;. 11 - P o r t i o n of a QLM d i a g r a m w i t h p l o t t e d p o i n t s f o r T o n g a (tholeiitic)=solid triangles, Raoul (tholeiitic) = l a r g e solid circles, M a c a u l e y (highalumina) = crosses, Rumble I I I ( b a s a l t i c a n d e s i t e ) = o p e n circle, a n d N o r t h I s l a n d c e n t r a l v o l c a n i c series (calc-alkaline) = s m a l l solid circles. I n Fig. l la t h e solid line c u r v e = pigeonitic series, and the d a s h e d line c u r v e = hypersthenic series for Japan KuN0, 1968). I n Fig. l i b t h e solid c u r v e = T h i n g mull tholeiitic series, and t h e d a s h e d c u r v e = Cas c a d e s calc-alkaline a v e r a ges. Fig. l l c . T h e s a m e p o r t i o n of a QLM d i a g r a m as s h o w n in Fig. l l a , b with the same plotted points for Tonga, Raoul, Macauley, Rumble III and N o r t h I s l a n d c e n t r a l volcanic series. Curve 1 = T o n g a a n d R a o u l tholeiitic t r e n d ; c u r v e 2 = calcalkaline trend for a contaminated Tonga-Raoul suite; curve 3 = Macauley high-alumina trend and the contaminated calcalkaline equivalent. Curve 4 = North Island west c o a s t calc-alkaline t r e n d ( d a t a f r o m HENDERSON a n d GRaNOE, 1926 ; HUXTON, 1944 ; GOW, 1968).

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- - 327 - Kermadecs) of the arc may be related to oceanic crustal thicknesses; that is, high-level tholeiitic and high-alumina basalt associations on the upwarped and thickened oceanic crust of the upstanding western side, and derivatives of deeper-level alkali basalts where the oceanic crust and upper mantle have suffered extension and thinning (WOnZEL, 1966) on the downwarped eastern side of the Tonga trench (ScnoFIELg, in GILL, 1968). Since there is no record of continental andesites within the triangular oceanic area bounded by Tonga-New Zealand-New Caledonia, or along the Tonga-Kermadec ridge, any re-draughted andesite line in this vector of the southwest Pacific on present knowledge must pass east and south of Fiji, west to New Caledonia and south to New Zealand.

References BENIOFF, H., 1954, Orogenesis and deep crustal structure - Additional evidence from seismology. Bull. Geol. Soc. Am. 65, 385400. - - , 1955, Seismic evidence for crustal structure and tectonic activity. Geol. Soc. Am. Spec. Paper 62, 61-75. BLACK, P. M., 1970, Observations on White Island volcano, N e w Zealand. Bull. Volc. this issue. BROTHERS, R. N., 1967, Andesite from Rumble 1II volcano, Kermadec ridge, southwest Pacific. Bull. Volc. XXXI, 17-19. a n d MawrlN, K. R. (in press), The geology o[ Macauley Island, Kermadec group, southwest Pacific. Bull. Volc. BnOTSV.RS, R. N. and SEARLE, [7. J., 1970, The geology of Raotd Island, Kermadec group, southwest Pacific. Bull. Volc., this issue. Br~owN, G. M. and ScHalI~-FaL J, F., 1968, Melting relations of some calcalkaline volcanic rocks. Ann. Rept. Geophys~ Lab. 1966-67, 460-467. CARMICIIAEL, I. S. E., 1964, The petrology of Thingmtdi, a ?'ertary volcano in eastern Iceland. J. Petrol. 5, 435-460. CLANK, R. H., 1960, Andesitic lavas of the North Island, New Zealand. Rept. ~[nternat. Geol. Congr. 21st Session Nordern 13, 123-131. DALy, R. A., 1916, Petrography o[ the Pacific islands. Bull. Geol. Soc. Am. 27, 325-344. DICI
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Island, N e w Zealand, as indicated by a s t u d y of Sr87/Sr~ ratios, and Sr, Rb, K, U and Th abundances. Geochim. Cosmochim. Acta 32, 699-736. EWART, A., TAYLOR,S. R. and CAPP, A. C., 1968, Trace and m i n o r element geochemistry of the rhyolitic volcanic rocks, central N o r t h Island, New Zealand. Contrib. Mineral. P e t r o l 18, 76-104. GILL, E. D., 1968, Quaternary shorelines research in Australia and N e w Zealand. Aust. J. Sci. 31, 106-111. Cow, A. J., 1968, Petrographic and petrochemical studies of Mt. E g m o n t andesites. N.Z.J. Geol. Geophys. 11, 166-190. GRI~N, T. H. and RINCW00D, A. E., 1968, Genesis of the calc-alkaline igneous rock suite. Contrib. Mineral. Petrol. 18, 105-162. - - , GREEN, D. H. and RINGWOOD,A. E., 1967, The origin of high-alumina basalts and their relationships to quartz tholeiites and alkali basalts. Earth Plan. Sci. Letters. 2, 41-51. GREC.~, D. R., 1960, The geology of Tongariro subdivision. N.Z. Geol. Surv. Bull. n.s. 40. HAMILTON, R. M. and Gate, A. W., 1968, Seismicity and structure of N o r t h Island, N e w Zealand. J. Geophys. Res. 73, 3859-3876. HaTHEaTON, T., 1968, ,, Miogeosynclinal ,, andesites. Earth Plan. Sci. Letters. 4, 441-447. and DICrdNSON, W. R., 1968, Andesitic volcanism and seismicity in N e w Zealand. J. Geophys. Res. 73, 4615-46t9. HEARLY, J., 1964, Volcanic m e c h a n i s m s in the Taupo volcanic zone, N e w Zealand. N.Z. J. Geol. Geophys. 7, 6-23. HENaEaSON, J. and GRANC~, L. I., 1926, The geology of the H u n t l y - K a w h i a suddivision. N.Z. Geol. Surv. Bull. n.s. 28. HOFFMEISTER, J. E., 1932, Geology of Eua, Tonga. Bull. Bishop Mus. Honolulu. 96. HOLMES, A., 1965, Principles of Physical Geology. Nelson. HtrrroN, C. O., 1944, S o m e igneous rocks # o r e the N e w P l y m o u t h area. Roy. Soc. N.Z. Trans. 74, 125-153. JAC,c~R, T. A., 1930, The island volcano. Volc. Letter 312, I-4. Kut~o, H., 1959, Origin of Cenozoic petrographic provinces of Japan and surrounding areas. Bull. Volc. XX, 37-76. - - , 1960, High-alumina basalt. J. Petrol. 1, 121-145. - - , 1966, Lateral variation of basalt m a g m a across continental margins and island arcs. Can. Geol. Surv. Paper, 66-15, 317-335. - - , 1968, Differentiation of basalt magmas. In Basalts: The Poldervaart Treatise on R o c k s of Basaltic Composition (editor H. H. Hess), vol. 2, pp. 623-688. Interscience. LEwis, J. F., 1968a, Trace elements, variation in alkalis, and the ratio Sr~7/Sr~6 in selected rocks f r o m the Taupo volcanic zone. N . Z . J . Geol. Geophys. 11, 608-626. - - , 1968b, Tauhara volcano, Taupo zone Part I1 - mineralogy and petrology. N . Z . J . Geol. Geophys. 11, 651-684. MAClaONALI),G. A., 1948, N o t e s on Niuafo'ou. Amer. J. Sci. 246, 65-77. - - , 1949, Hawaiian petrographic province. Bull. Geol. Soc. Am. 60, 1541-1596. - - , 1960, Dissimilarity of continental and oceanic rock types. J. Petrol. 1, 172-177. and KArStrRA, T., 1964, Chemical composition of Hawaiian lavas. J. Petrol. 5, 82-133. MCINTVRE, D. B., 1963, Fortran I I program for norms and V o n W o l f f computations. Tech. Rept. 14, Geology Dept., Seaver Lab., Pomona College, Calif. -

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NOCKOLOS, S, R., 1954, Average chemical compositions ol some igneous rocks. Bull. Geol. Soc. Am. 65, 1007-1032. POLDERVA~T, A. a n d PARKER, A. B., 1964, The crystallization index as a p a r a m e t e r o[ igneous dif[erentiation in binary variation diagrams. Amer. J. Sci. 262, 281-289. RI~, J. J., 1962, Cattdogue of the active volcanoes of the world. Part. X I I I : Kermadec, Tonga and Samoa. Internat. Ass. Volc. STARK, J. T., 1963, Petrology of the volcanic rocks of Guam. U.S. Geol. Surv. Prof. Paper. 403-C, 1-27. S'fEtNEa, A., 1958, Petrogenetic implications o¢ the 1954 Ngauruhoe lava and its xenoliths. N . Z . J . Geol. Geophys. 1, 325-363. - - , 1960, Origin of ignimbrites o[ the N o r t h Island, N e w Zealand: a n e w petrogenetic concept. N.Z. Geol. Surv. Bull. n.s. 68. SUCIMVRA, A., 1968, Spatial relations of basaltic magmas in island arcs. In Basalts: The Potdervaart Treatise on Rocks of Basaltic Composition. (cditor H. H. Hess), vol. 2, pp. 537-572. Interscience. SYKES, L. R., 1966, The seismicity and deep structure o[ island arcs. J. Geophys Res. 71, 2981-3006. THOMSON, A. A. and Evlsor;, F. F., 1962. Thickl2ess o[ the earth's crttst i~z N e w Zealand. N . Z . J . Geol. Geophys. 5, 29-45. Wa~;rR, L. R., 1960, The m a j o r element variation of the layered series of the Skaergaard intrusion and a re-estimation of the average composition of the hidden layered series and o[ the successive residual magmas. J. Petrol. l, 364-398. WORZEL, J. L., 1966, S t r u c t u r e o[ contincntal margins and development o[ ocea~#c trenches. Can. Geol. Surv. Paper. 66-15, 357-375. Manuscript received J,me, 1969