1 Classification, petrogenesis and tectonic setting of

1 Classification, petrogenesis and tectonic setting of boninites ANTHONY J. CRAWFORD,

TREVOR J. FALLOON & DAVID H. GREEN

Abstract Based o n maj or-element compositions, boninites are divided into two classes, low-Ca and high-Ca suites. High-Ca boninites, exemplified by the Upper Pillow Lavas of the Troodos Ophiolite in Cyprus, always have Si0 2 < 56010 and CaO/Ah03 > 0.75. T hey generally contain olivine as a phenocryst phase, and may cry stallize both high-Ca and low-Ca pyroxenes as phenocrysts or microphenocrysts; their high-CaO nature inhibits clinoenstatite crystalliz­ atio n. Low -Ca boninites are subd ivided into three types. T ype 1 are characterized by very low CaO ( < 6010), high Si02 ( > 58010) and total alkalis > 2010; they have low CaO/Ah03 < 0.5 and low FeO· ( < 6010); characteristic phenocrysts are olivine up to F0 94 and low-Ca pyroxenes, including both ortho- and clinoenstatite. Very refractory Cr-spinel (Cr# = 0.80-0.98) is always pre sent. T he best exam ple of Type 1 boninite is the Nepoui suite from New Caledonia . Type 2 low -C a boninites also ha ve very low CaO /A h 0 3 ( < 0.55) and low FeO · « 7010) but ha ve higher total alkalis and lower Si02 contents than T ype 1 boninites. They crystallized at lower temperatures (1150-1200° C) than T ype 1 boninites, and trace-element data su ggest that they may represent lower-degree partial me lts of th e sa me refractory harzburgitic sources which yielded Type 1 lavas . T hey usually carry olivine phenocrysts ± orthopyroxene ± clin opyro xene phenocrysts. The Setouchi (SW Japan) and Baja California suites are the best-documented Type 2 low-Ca boninites . Type 3 low-Ca boninites have higher CaO/Ah03 values (mainly 0.5-0.75) th an other low-Ca suites, and are compositionally intermediate between low-Ca Type 1 and high-Ca boninites. They crystallize oli vine and chromite, fo llowed by clinoenstatite and orthopyroxene. As in all boninite suites , plagioclase is restricted to the groundmass of evolved, more slowly cooled samples. The typ e locality Bonin Island and Cape Vogel suites are representa­ tive o f this group .

C LASS IFICAT IO N. P ETR OG ENESI S A ND T EC TO N IC SETT ING

Bo nin ite sources were considera bly mo re refracto ry than so urce s for MO RB o r typical ar c ba salt s, owin g to one or more prior melt extraction events. Ho wever, inva sio n o f these d epleted har zburg ites by hydrous fluid s, tho ught to be d eri ved from t he subd ucted slab, led to enrichments in LILE including LRE E a nd Zr , and im po rta nt ly al so in Na 20 and Si0 2. The Nd - Sr isotopic com po sitio n o f the bonin ites generated from such enriched ref racto ry per­ idoti te is co ntrolled by the bu lk Nd-Sr iso topic composition of the ocean-cru st sectio n yielding t he hydro us fluid s. If th is cru st is yo ung and relatively sedi ment -f ree, hydrous fluid s d er ived from t he slab wiII ha ve Nd - Sr iso topic signatur es litt le di fferent from MOR B. If, however , th e slab contains a compo nent o f recycled ancient crust (e.g. pelagic sedime nts ), t he resultant isot o pic signatures in bo ninites gene ra ted ar e mu ch more radi ogenic in Sr and hav e lower £N d than th e fo rmer case . T wo key ingredients for bo ninite production a re a supply of hyd ro us fluid s into refractory perid otite to lo wer solidu s tem perat ures and permi t parti al melting, a nd a mechan ism fo r t he maintenance o f very high te mperat ures (I 150- I350 °C) a t sha llow levels « 50 km) in the subd uctio n-zone environ­ ment. W e suggest that a spect ru m of boninitic magmas from initial high-Ca boninit es to T ype 3, then T ype I, boninites ma y be generated sequentially fro m sup ra-su bd uction-zo ne peridotite by continued influx of hyd ro us fluids, and pa rt ial me lting of increasingly refractory but increasingl y hydrous (and Na-, Si- an d LILE-rich) har zburgites. The so urce of the high temperatu res req uired to permi t co nti n ued m elting in this fashion is either a MORB-source d iapir rising beneath the arc during in cipient backarc opening , or a subd ucted active sp rea di ng centre . In the latter ca se, the subducted spreading ridge may be ' resur rected ', to yield 'backarc-basin-type' tholeiites "overl yin g the most refractory boninites, but in a forearc setting. Such a scenario is hypoth esized to explai n the sequence of magma types in severa l key boninite- and tho leiite -bear ing op hiolites .

1. 1

Classificatio n o f bo ninites

1.1. 1 Intr oduction Volcanic sui tes he rein considered as bo ninit ic a re those in which t he vo lu­ met ricall y d omi nant lavas eit her ha ve> 53010 Si0 2 and Mg #. > 0. 6, or are demo nst ra bly derived fro m pa rent a l magmas meeting t hese com positional requ ireme nts. Magmas wit h boninitic a ffinities have been erupted throughout Ear th histo ry a nd a re important components of some Archaean greenstone belts , many Palaeozoic and Mesozoic 'ophiolites', forearc regions of certain • T hr o ugh out this volume, unle ss indicat ed otherwise, Mg # signifies Mg/ (Mg + Fe 2 • l, where Fe2 - is calc ulated as tota l Fe.

2

A . 1. CRAWFORD ET A L .

modern or Tertia ry intra-oceanic arcs, and also in several continental-margin convergent plate boundary settings (e.g. Baja California and SW Japan). The economic importance and potential of boninitic rocks are emphasized by the fact that two of the world's largest platinum-group element deposits, the Bushveld and Stillwater Complexes, had parental magmas with boninitic affinities . A ma jor problem co nfro nting successful reporting of petrological-geo­ chem ical stu dies of boninites and related rocks lies in the inadequate classifi­ cation and nomenclature presently applied to such lavas. This is highlighted by the plethora of names used in th e recent literature for lavas and lava suites which meet the a bove chemical requirement for boninite. These terms include bo ninite , transitional boninite, high-Mg andesite, marianite, sanukitoid, low-Ti op hiolitic basalt, komatiitic basalt, basaltic kornatiite, siliceous high-Mg basalt, low-Ti quartz tholeiite, and magnesian quartz tholeiite. Two reasons fo r the existence of such confused nomenclature for boninites an d related lavas may be identified. Fir st, owing to the relatively recent emergence and recognition of boninites as a distinct magma series, existing classificatio n schemes for volcanic rocks are entirely inadequate when applied to boninitic lavas. For example, in the system recommended by the lUGS Subcommission on the Systematics of Igneous Rocks (LeMaitre 1984), Bonin (Ogasa wara) Islands (type location) boninites are classified as basaltic andesites. Secondly, within the suites herein considered as boninites and related rocks, there exist pronounced variations in wholerock compositions, modal phenocryst mineralogy and textures. These variations reflect the continuum, in nature, of petrogenetically important intensive variables (P, T, X H,O, degree of source deple tion in 'basaltic' components and so on) op erative in th e upper-mantle so urces of such magmas. For example, in a supra­ subduction-zo ne setting at a nominal temperature of 1300°C and under H20-undersaturated conditions, magma segregation at increasing depths from 20 to 100 km would generate a spectrum of primary magma compositions 'ranging from high-Si, high-Mg boninitic compositions to low-Si, high-Mg picritic or olivi ne tholeiitic end-members. Classification or assignment of lava co mpositio ns fa lling close to these end-member extremes will be relatively simple, but those falling part-way along the spectrum will generate nomen­ clatural problems. Any attempt to subdivide a nd assign names to primary magmas (or their de rivatives following crystal- liq uid fractionation) constituting this spectrum of compositions must necessarily erect arbitrary and artificial compositional limits on rock types defined. Ideally, such a classification scheme would successfully relate phenocryst mineralogy and crystallization sequences to wholeroc k com positio ns, so that specificatio n of one particular rock name would allow certain petrogenetic co nclusio ns to be drawn about that particular lava, given no further information. In reality, however, numerous factors (e.g . variable cooling rat es, cry stal fractio natio n (e. g. sinking of oliv ine), magma 3

C LASSI FICA TI O N , PET ROG ENESIS A N D T ECT ONI C SETT I NG

mixing , cont ami nat ion, post-eruptive alt eration) contri ve to com plicate and reduce the effectiveness of such an ideal scheme, fo rcing compromi se. As a prelud e to pro viding a wo rkable classificati on scheme fo r bo ninit es and related rocks, we have collated petrogra ph ic, minera l ch emical an d major­ element chem ical da ta fo r eight suites which we believe to be representative of the spectrum o f na tural co mpos itions o f bo ninites and rela ted rocks. Very brie f descripti o ns of the occurrence of each suite a re given be low; pet rographic charac teristics an d mine ra l chemi stry ar e discussed further on , following a review of the co mpositio nal va ria bilit y of bo ninit e mag mas.

1.1.2 Boninite suites used in the classification SUIT E I. BON I N (OG A SA\\'A RA) ISL ANDS

Altho ugh first described and named last cent ury (Kiku chi 1890, Pe tersen 1891), the type locat io n bonin ites have onl y recentl y been car efu lly docu­ mented (Kuro da & Shiraki 1975, Kuroda et 0/. 1978, Shiraki et 0/. 1978, 1980, Ko ma tsu 1980, Umino 1985 , 1986). On Chichi-jima, boninites and related bronzite a ndesites a nd dacit es o f Eocene age are overlain by Oligoce ne a nd Lowe r Mioc ene lim estones . Th ey ar e do minantl y pillowe d flows an d volcanic breccias and ha ve been described in deta il by Umino (1985, 1986). SUITE 2. CA PE VOGEL , PA PU A N EW GU INE A

T hese bo ninites were described by Dallw itz et 0/ . (1966) , Da llwitz (1968), J enner (1981) and Wal ker & Cameron (1983). T hey fo rm part of the poorly o utcro pping Dabi Volca nics, which a lso co nta ins ba salts transition a l to arc tholeiites (J enner 1981), and are of late Palaeocene age (Walker & McDouga ll 1982). T hey for m pa rt of a series of allo ch thonous thrust sheets which include the Papu an ult ramafic belt th at form ed in an int ra-oceanic arc bu t were overthrus t ont o th e nor thern ma rgin of th e Au stra lian Plate d uring arc-co n­ tinent co llision (J aques & Rob inso n 1977). SUITE 3.

MA RIANA T RENC H

Pr imitive boninit es of probable Eo cene age hav e been d redged fro m the inner wa ll o f th e Ma riana T ren ch at Dm itri Mendelev Sit e 1403 south of Guam (Dietrich et 0/ . 1978, Shara ski n et 0/. 1980, Crawford et 0/. 1981) and severa l other locatio ns furt her no rth along the trench wall (Bloomer 1987, Bloo mer & Ha wkins 1987). These are almost certainly co rrelat es o f the Bonin Island and DSD P Hole 458-Guam boninite occurrences. SUI T E 4. NEW CALE DON I A

Two dist inct suites of boninites occur on New Caledonia, one of Permo­ T riassic age, the other earl y Cretaceous-Middle Tertiary (Cameron 1989). The 4

A. J. CRAWFORD £T AL.

latter , which ou tcrop aro und Nepoui, have been described by Samesh ima

et al. (1983), Cam piglio et al. (1986) and Camero n (1989) and are unusual

high-Si, very low-Ca bo nin ites. The three anal yses included in th is data set are

from Cam ero n et al. (1983) , though we note that more extensive new da ta

presente d herein by Camero n (1989) conform to the general compositional

trends and interpret ation we have deri ved fro m the three published analyses.

SU ITE 5. UPP ER P ILLO W LA VAS O F T HE T ROO DOS O P HI O LITE , C YP RUS

Desp ite ea rly contro versy regard ing the origin and tectonic sett ing o f eruption

of Upper C ret aceous lavas fo rming th e volcanic carapace of the Troodos

Ophio lite (see review by Robinson et al. 1983), recent stu di es ha ve arrived at

so me co nsensus of opinion. These lavas include a lower sequence of evo lved

arc tholeiitic andesites and dacites overl ain by a series of more pr im iti ve lavas

tha t includes picrites and boninites (Robinson et al. 1983, McC ulloch &

Camer o n 1983, Cameron 1985, Thy et al. 1985, Rautenschlein et al. 1985,

Flo wer & Levine 1987, Ro gers et al. 1989), suggesting a supra-subduction­

zone setting in a primitive intra-oceanic arc or incipient backarc bas in .

SUITE 6. MA RIA NA FOREARC AND G UAM

Relative ly evo lved late Eocene-early Oli gocene boninite series lavas were

drilled fro m DSDP Hole 458 in th e Mariana fo rea rc , wh ere th ey overlie arc

th o leiites (Meijer 1980 , Wood et al. 1981, Kushiro 1981, Bougault et al. 1981,

Natland 1981, Hickey & Frey 1982) . Similar lavas outcrop on Guam in the late

Midd le Eocene Facpi Formation and the late Eocene-early Oligocene Alutorn

Formatio n (Reagan & Meijer 1984, Hickey-Vargas & Reagan 1987); the latter

are associ at ed wit h arc tholeiite basalt s.

SUITE 7. SETOUC H I VOL CA NIC BELT, SW J AP AN

A n unu sual ma gm a ser ies ra nging from basalt to high-Si, high-Mg lavas

(sanu kitoids) occ ur s along the Seto In lan d Sea (Seto uchi) regio n of SW J apa n .

These lavas have been described by Ta tsurni (198 1, 1982, 1983), Tat su rni et al.

(1983), Tatsu rni & Ishizaka (1981, 1982a, b), Ishiza ka & Carlson (1983) and

Tatsumi & Maruyama (1989) and were eru pt ed d uring a b rief magmatic event

at appr oximat ely 13 Ma .

SUIT E 8. BAJ A CALI FO RNIA

Sa und ers et al. (1987) and Roger s & Sa u nd ers (1989) describe a co m posi­

tionall y di verse suite of lavas occu rring in Baja Ca liforn ia that ra nges in age

from 12 to 4 Ma and incl udes a spect rum o f rock types fro m basalt to high -Si,

high-Mg lav as wit h boninitic a ffinities; the latter were terme d ' bajaites' by

Saunders et al. (1987).

5

CL ASSIFICA T IO • P ET ROGENESIS AN D T ECTO NIC SETT ING

1.2

Ch emical su bd ivisio n of the boninite spectrum

As a first ste p towa rds esta blishing the existe nce of composition ally (pet ro­ gen etica lly?) distinct magm a groups within the bo ninite spectru m , we ha ve plott ed major -eleme nt che mica l variation s a nd Ca O!A b 03 of the eight selected suites ver sus Mg # in Figur es 1.1 an d 1.2. Suites I to 6 a re cha racterized by abundan t lavas with > 10010 MgO , whereas suites 7 and 8 la vas mor e freq ue ntly have < 10010 MgO . Important differen ces between the suites are evident , especiall y fo r Si0 2, CaO, FeO * and Na20 co ntents . Fr om Figures 1. 1 a nd 1.2, we d istingu ish two d istinct lineag es of boninitic magmas, low-Ca bon inites a nd high- Ca bo ninit es . Low-Ca bo ninites are fur ther sub­ -. S _0 2 6 4 • 62

I

4' C

58 56

e.

54

0.50

0 . 60

0 .1 0

0 .6 5

-a9'>'.

FeO >7.5-t. CaQlAI,03- 0 70 -1_0 Total alkali! 0.7 and CaO/ Na20 values from 4 to 10, a nd crystallize liqu idu s olivine and ma gne siochrom ite with maxim um Fo an d Cr # val ues o f 92 a nd 0. 85 respectively. High CaO contents at high Mg # relative to ot her bon inites inhibit clinoensta tit e cr ystalli zation but permit crystallizatio n o f micro ph en ocrystal ca lcic pyro xen e, altho ugh au gite appea r­ a nce invari ably fo llo ws th at of olivine an d low-Ca pyrox enes. P lagioclase crysta llizes only as a lat e-sta ge ph ase in th e gro und mass of mo re slow ly co o led lavas. T he residuum fo llow ing segregation o f th is magma mu st be harzburgitic, and have co ns idera bly less clinopyro xene and lower CaO/Ab03 than th e so urce of the high-Ca boninites. To partiall y me lt this residuum a nd yield a further mag ma ba tch un der an hyd ro us o r very low H 20 content co nditio ns req uires pro hib itively high temperatures, probably well in excess o f 1400 °C.

1.5.2

Type 1 and Type 3 low-Ca bon inites

Such a re fra cto ry so urce may, however, melt further if its solidus tem pe ratur e is lowered by influx of relati vely la rge vo lumes o f hydrous fluid s. We argue that Types I a nd 3 lo w-C a bon inites a re genera ted in this manner, with Type I lavas being produced from mo re re fr acto ry harzburgitic so urc es th an Ty pe 3 boninites, as ind icated by their lower CaO/A b 0 3 and lower CaO and FeO · co ntent s. If, as we clai m , low-Ca boni nites a re deri ved by hydrous parti al melting o f residual harzburgite s left after segregation of high-Ca bon initic magm a, th en the high al kali (N a 20 in particular) contents of th e low-Ca bo ninites relative to Troo dos high -Ca bonini tes requires expla nation . Both Na20 a nd K20 conten ts sho uld de creas e fur ther with inc reased (secon d- o r third-stage) pa rt ial melting, to va lues 'well belo w those shown by the high-Ca bon inites. In fact , they are notab ly higher , especi ally for the T ype I low-Ca bo ninit es. T he high Na20 T ype I lo w-Ca bo nin ites a lso co ntai n signi ficantly higher Si0 2 co ntents than Type 3 boninites or high-Ca bo ninites. A relatively simple expla natio n of the high Si02 contents of Type 1 low- Ca boninites involv es inco ngruent melti ng of en stat ite . As is obvious from Figu res 1.8 and 1.9, and a s discu ssed by Va n der Laan et al. (1989), mo st primitive bo ninite comp ositio ns, whether of high-Ca or low -Ca type, plot ou tside the Al kemade triangles defined by th e dom inant mantle melting assemblage Ol-Opx-Cpx. It is concluded, therefo re, that inco ngru ent melting of enstatite (to yield forste ritic oli vine and a mo re silicic liq uid ) must ha ve been involved in th e generatio n o f boninitic magmas. Van der Laan et al. (1989) argue that the exte nt of this low-pressure, peritectic style o f partial melting will be enhanced by increased H 2 0 contents. Thus, since petrogenesis of low-Ca boninites involves significantly more H 20 th an is involved in production of high-Ca 22

A . J . CR A WFO RD ET A L.

bo ninites , it is to be expected that incr eased extents of peritectic melting for th e more H 20-rich sources result in production of more Si02-rich magmas. By implication , T ype 1 low-Ca boninites must have had more H20 involved in their generatio n th an the less Si0 2-rich Type 3 boninites. Such an explanatio n seems intuitively reasonable, but does not provide an obvious explanation for the high Na-O contents o f Type 1 boninites. Experim ental st ud ies by Kus hiro et al. (1968), Kushiro (1972), Nakamura & Kushiro (1974) and Ryabchikov et al. (1982 ) demonstrated co nclusively tha t Si02 and Na20 can be effectively transported in the upper mantle by high-temperature aqueous fluids. The solubility of Si0 2 increases with increas­ ing pressure and temperature, and at 15 kbar substantial amounts of Si02 co mponent exis t in the aqueous fluid coexisting with enstatite and forsterite. At co nstant temperature, the solubility of Na20 increases with decreasing pr essure. We believe that both Si0 2 and the alkalis were introduced into the refractory perido tite sources of boninitic lavas in the hydrous fluids which initiated partial melting. G reater vo lumes of fluid for a given vo lume of peridotite are required to produce partial melting of increasingl y refractory so urces, so that the more refractory Type 1 boninites have higher SiOz, Na20 (2-2. 5010 Na 20) and lower CaO/Na20 < 2.5 than Type 3 low-Ca boninites (0.7- 1.4010 and 4-10 respectively) at any Mg e . This interplay between the degree of depletion o f a peridotite so urce and the amount of hydrous fluids req uired to initiate partial fusion is perhaps the key factor in understanding the co mpositio na l va riatio n of boninite suites, and is shown diagrammatically in Figure 1.4. If high-temperature aqueous fluids are capable of controlling to a significa nt degree the major-element composition of boninites, then th e trace-element and isotopic signa tures of boninites demand explanation in the same petrogenetic scenario.

J.5.3

Type 2 (alkali) low-Ca boninites

As noted abo ve , the Seto uc hi and Baja T ype 2 lo w-Ca boninites share man y imp o rt ant majo r-element co m pos itio na l fea tures (high Na 20, lo w FeO· and CaO , and CaO/AhOJ < 0.5 5) with Type 1 low-Ca boninites. This suggests t hat t hey a re a lso derived from a very refractory pe ridotite so urce. Their Si0 2 contents at Mg # = 0.70-0.7 5 ar e variable (56- 60010), but are low er than those of Type 1 boninit es by at least 2010. In contrast, th eir Na 20 contents over this sa me Mg # range are 1010 (Setouchi) to 3010 (Baja) higher, and their AhO J co nte nts are nearly 4010 higher than in Type 1 10w-CaO boninites (Figs 1.1 & 1.3). These d ifferences ar e unlikely to be due to differences in either th e amo unt o r the co mpo sitio n of th e aq ueous fluids responsible for initi at ing meltin g in the re frac tory mantle so urces o f these lavas. A more reasonable explanatio n is that th e parent magmas of th e Type 2 low-Ca boninites wer e generate d by much lower degrees of partial melting than wer e involved in pro du ction of the T ype 1 boninites. 23

C LASS IF ICATION . PETROGENE SIS A ND T ECTON IC SETTING

Melting expe rime nts on a near-aph yric Type 2 low-Ca bon inite from the Seto uchi belt (T at sumi 1981) under H 20-undersaturated (8U1o H 20) co nd itio ns sho wed th at o livine and ortho pyro xene can coe xist on the liq uidu s of the co m positio n used (9.6U1o MgO ) at II kb ar and II 10°C; at lower, mo re real istic, H 20 co nt ents th is co mposition wo uld be in eq uilibrium with harz­ burgite at lo wer pr essures still, but at higher temperatures. Th ese results ind icate that th e liq uid us temperatures of Type 2 low-Ca boninites a re at lea st 50- 100°C lo wer than for Type I and Ty pe 3 boninites ( > 1225 °C; Van de r Laa n et 01. 1989, Um ino & Kushiro 1989), supporting the hypot hesis th at Type 2 bo ninites may have been genera ted by lo wer degr ees of partial melting than ot her low-Ca bo ninites. T he relatively high co ntents of K20, Ti02 and LI LE in both suites of Type 2 boni nites (T atsurn i & Ishizaka 1982a, b, Saunders et 01. 1987. Ro gers & Sa unders 1989) further suppo rt this hyp othesis. So me o f the co m positio nal varia bility of the Seto uchi and Baja la vas (e.g . in Si0 2, K20 an d LILE ) might be explained by limited cru stal contamination during a scent , sin ce both suites wer e er upted through relative ly thick co nt inent a l-ty pe crust, wh ereas ot her boninite suites were erupted through thin ocea nic-type crust and had less opportunity fo r pooli ng , a ssimil ation and fr actio natio n. However , both Ishi za ka & Ca rlson (1983) and Saunders et 01. (1987) ru le o ut crustal co ntamination as ha vin g played a significant role in the compositio nal evolu tion o f Setouchi and Baja Type 2 low- Ca boni nites. We note , however , that data in T ables 3 and 4 o f Ishizaka & Ca rlso n (1983) show th at 5% crustal co nt a minatio n (by Sari Grano dio rit e, tak en to be rep resent a­ tive of the lower crust o f SW Japan) o f a primitive Type 2 bo ninite mag ma at the high ENd- lo w 87Sr/ 86Sr end of th e Setouch i spectru m can pr odu ce a res ulta nt magma with Nd-Sr isotopic ra tio s still well within the range reported by th ese au thors for Seto uc hi low-Ca boni nites (ENd = + 1.8 to - 2.5; initial 87Sr/ 86Sr = 0.70487-0.70537).

J.5.4

Coexistence oj low-Ca and high-Co boninites

We have argued a bo ve th at the ma jor-elem ent compositiona l spectrum of bon inites is generated by a subtle in ter play bet ween degr ee of deplet io n ('refractoriness') of the source peridotite, the composition and amount of hydrous fluid s which initiate melti ng o f otherw ise refr actory per ido tite , and the ambient temperature at the site of partial melti ng (Fig . 1.4). Indi vidu al bo ninite suite s clea rly belong d o mi nant ly to either high -Ca or low -Ca suites (Fig . 1.3). Howe ver , a ver y important observation to ta ke into acco unt in models for boninite petrogenesis is th at , in man y boninite sequ ences, bot h low-Ca and high- Ca boninites co exist, albeit th at one or th e other ty pe may be volumet rica lly fa r more signi ficant than the other. For example, Umi no (1986) sho wed th at whereas our T ype 3 low-Ca boninites dominate the Chichi-jima lava pile, sparse high -Ca boninites (hi s Type 4 petrographic gro up o f 24

A . J. CR AW FO RD ET A L.

Chichi-j ima lavas ) do occur in this seq uence. These high-Ca boninites have micro ph enocrysts of ca lcic pyr oxene and bronzite, and as exp ect ed-show lo wer Si0 2 and higher CaO (10010 versus 7-8 010 at Mg# - 0.72) content s than low-Ca boni nites in the same sequence. Similarly at Cape Vogel, while th e dominant lavas are Type 3 low-Ca boninites with CaO/ Ah03 values o f 0.5 4- 0.60, several lavas repo rted by Dall witz (1968) an d Jenner (1981) ha ve values around 0.66-0.71, and at any Mg # notably lower Si0 2 and higher CaO (again by around 2010) than the dominant lavas in the seq uence. T hese higher- Ca lavas, althou gh still plo tting as low- Ca boninites in Figures 1.1 and 1.3, trend strongly to war ds high-Ca boninite compositions. Also from the Cap e Vogel sequence is at least o ne sa m ple (173 of Walker & McD ou gall 1982) with significantly higher Si0 2 (64.2010) and lower CaO (4.5010) at Mg # = 0.75 than other Cape Vogel boninites; th is sa m ple shows stro ng affinities to Type I low-Ca boninites. This compositional va riatio n within single bonini ie seq uen­ ces serves to emphasize the co nt inuum existing in nature between th e series de fined here . Relative ages of lower-Ca and higher-Ca boninites where they coexist at Cape Vogel and Chichi-jima are unknown. However, in the Cambrian Heathcote and Mount Wellington greenstone belts in SE Australia, tim e relationships between coexisting boninite suites can be determined with certainty (Crawford et al. 1984, Crawford & Cameron 1985, Crawford & K eays 1987). In bot h greenstone belts there is an upward pa ssage fro m higher-Ca , less refractory boninites at the base of th e succ essio n to Type I low-Ca, exceptionally refractory clinoenstatite-bearing lava s at the top. Th e latter have microphenocrysts of clinoenstatite growing around reacted and resorbed phenocrysts of olivine (Fo 94 ) , and chromite (Cr # = 0.97) oc curs as ph enocrysts and inclusions in olivine and clinoenstatite (Crawford 1980). Using the olivi ne--chromite geothermometer of O'Neill & Wall (1987), thes e highly refractory lavas had liquidus temperatures fr o m 1330 to 1350°C , whereas tho se lower in the seq uence p ro bably cr ystalli zed around 1200- 1250°C. T his is co m patible wit h a pet rogenetic model in whic h succes­ 'sive ba tches o f progressively lower-C a, mo re refractory boninites are deri ved fro m incr ea singly refractory ha rz burgitic so urces fo llo wing repeated or con­ tinuous in flux o f so lid us-lo wer ing hydro us fluids. In both the Heathcot e an d Mo unt Wellington greensto ne belts, the upp ermost, most re fracto ry bo nin ites hav e the lowest initial CNd values and stro ngest LREE enr ichment (Nelson et al. 1984). If the hydrous fluids init iating melting are also th e LREE tr an sporting agent (H ickey & Frey 1982, Cameron et al. 1983), then th e stro ngest LREE enrichm ent and lo west CNd in the most re fractory (in terms o f Ti 0 2 o r H REE levels) lavas at the to p of th e seq uen ce indicate tha t great er water/ro ck ratios were required to melt increasingly refractory residue s o f successive melting event s. Th is ca nno t happen within a single diapir via seq uent ial or dynamic partial melting, since th e majorit y o f LILE will be remov ed fro m the diapir du ring th e first major melt extract ion. Sub seq uent 25

C LASS I FICA T IO N . PET ROGENESIS A N D T ECTON IC SET T IN G

melts will be increasingly depl et ed in LILE , th e op pos ite o f th at o bserved in th e bo ninite seq uences described ab o ve.

1.6

Petrogenesis of boninites: a summary

1.6.1 Overview In summary, we believe th at bo nin ites are derived from so urce per idot ites mo re refra ctory than resid ua fro m MO RB generation . To melt th is de pleted perido tit e requ ires either higher temperatures than those required for the prior melting event, o r the in flux o f solid us-lowering hydrou s fluids. If onl y very sma ll volumes o f hydr ou s fluids invade th is peridotit e, the n high- Ca boninites such as the T roo dos Upper P illo w Lavas suite ma y be generated at p < IO kba r a nd 1250- 1350°C. The residuum fro m thi s melting event will not melt fur ther with out input o f larger volumes of hyd rous fluids (a nd extra heat?) than were invol ved in high -Ca boninite prod uction. If bot h high temperatures and a supply of hyd rous fluids to th e site o f partial melting can be ma int ain ed , then furt her partial meltin g (at pres sure s probably fr om 2 to 8 kba r) will generate first Type 3 low-Ca boninites and th en Type I low-Ca bon inites. Further access o f hydrous fluid is required to generate the latt er from th e very refra ctory residue s of the T ype 3 bo ninite-prod ucing melting event. T he vol ume of T ype I low-Ca boninites generated would be exp ected to be very sma ll relative to volumes of hig h-Ca and Type 3 low-Ca bon inites. Partial meltin g o f a very refracto ry perid otite so urce at higher pr essures (?8- 15 kba r) an d lo wer temperatures (lIOO- I I 50°C) will generat e lower­ deg ree Ty pe 2 low-Ca bo ninit e pa rtia l melts, su ch as the Baj a a nd Seto uch i bo ninites. Cr ustal thicknesses under bo th Baja Ca lifo rn ia and SW Japan dem and upper-ma ntle magma segregation at pr essures o f at least IO kba r.

1.6.2 Ident ity oj the enriched and depleted source components in boninite genesis All publ ished boninite petrogenetic scheme s agree that the source of these ma gma s mu st be a depleted, refractory perido tite which is enriched in LILE and so me other elements (e.g. Si, Na , Zr ) by a hyd ro us fluid prior to or during pa rt ial meltin g which generated the boninitic ma gma (e.g . Sun & Nesbitt 1978, Craw ford er 0/ . 1981, Hickey & Frey 1982, Tatsumi 1982, Cam eron et 0/ . 1983, Bloomer 1987, Beccaluva & Serri 1988). TH E DEPL ET ED CO M PON ENT

From co nsiderat ion s o f ma jor-element chemistry and phase relationships o f

26

A. 1. CRA WFORD ET AL .

eight suites o f boninites , we hav e sho wn that the source peridotites of high-Ca bonin ites may be re fra cto ry residua from prior melting events involving extraction o f MORB parent magmas. Clinopyroxene was present in th ese so urces prio r to melting (see also Beccaluva & Serri 1988) , as indicated by the rela tively high CaO / Ah03 (0 .7-1.0) and high Sc contents (av. 40 ppm at Mg # = 0 .70 for T roodo s Upper Pillow Lavas) of high-Ca boninites; it was, ho wever, elim ina ted during generation of th e high-Ca boninite parent magma . Low-Ca bo ni nites have low to very low CaO/Ah03, ranging from 0.45-0.7 for T ype 3 lavas to 0.28-0 .5 for Type 1 lo w-Ca boninites. This sug gests onl y a very min or ro le for clinopyro xen e in gen esis of the Type 3 low-Ca boninites (Sc = 30- 35 ppm at Mg # = 0 .70) . and th at clinopyroxene might have been ab sent fr om the so urces of Type 1 low-Ca boninites (Sc = 20-25 ppm at Mg # 0.70; Cameron 1989). Note that the Setouchi Type 2 low-Ca boninites ha ve a n average Sc abundance of 22 ± 4 ppm (Tatsumi & Ishizaka 1982b ); thi s, to gether with their low CaO contents and low CaO/Ah03 values, suppo rts t he a bove mo del that they are derived from very refractory. clinopyroxene-free harzburgitic sources similar to sources of type 1 low-Ca boninites. TH E ENRICHE D COM PONENT (S)

A detailed dis cussion of the nature and origin of the one or more enriched co mp onents involved in boninite genesis is beyond the scope of this chapter. We wish to stress several points . however, wh ich must be taken into account in att empting to identify' these components and their ro le in boninite petro­ genesis. (1) We are co nvince d that sign ificant amounts of at least Na-O and SiO z were t ra nspo rt ed int o 10w-Ca boninite sources by hydrous fluids which initiated partial melting. If high-temperature aqueous fluids dissol ve and t ra nsport SiO z, it is highly likely th at they will also transport LILE , including REE (T ats um i et al. 1986), which may resid e largely on gra in boundaries in upper -mantle perido tit es (e.g. Fr ey & Green 1974). Kyser et al. (1986) and Do bson & O ' Ne il (1987) have sho wn t hat H 20 inv ol ved in boninite gen esis is pr o bab ly derived fr o m altered, su bd ucted ocea nic lithosphere. Ther e is stro ng evidence in so me su ites (e.g . Pb iso topes in C hichi-jima a nd Victorian Cam bria n (H owqua ) bonin ites; Dobson & Tilton 1989; B. L. Gulson pers. comm . 1987) of some LILE enrichment derived from an ancient crustal compo nent (?sed iment s) in the dehyd rating slab. As shown by Nelson et al. (1984), this subducted sedimen ta ry component must ha ve had £"Nd values at least as lo w as - 8. However , deta iled t race-element and isotopic studies of bo ninites (J enn er 198 1, H ickey & Fr ey 1982, Cameron et al. 1983, McCulloch & Cameron 1983, Sha raskin et al. 1983 , Bloomer 1987, Hickey-Vargas & Reagan 1987, Bee­ caluv a & Serri 1988, Cameron 1989, Falloon et al. 1989, Hic key-Vargas 1989, Dobso n & Tilto n 1989) have all suggested th at t he enriched compo nent is 27

CL ASSI FICA T ION . PETR OGEN ESIS A ND T ECTON IC SETT I NG

unlikely to be sim ply fluid s de rived by deh ydr a tion o f alt ered subd ucted oc ean cru st (incl ud ing sed iments ). T here is stro ng evidence fo r a seco nd enr iching co mpo nent , very sim ila r to th a t responsib le fo r enrichmen ts seen in oc ea n isla nd basa lts (Ol B) a nd in sp inel lherzolite nodules in alkali basal ts. The natu re o f th is seco nd enrichi ng co mp onent is poorly understood . Jen ner (198 1), H ick ey & Frey (1982), Hi ck ey- Va rga s & Reagan (1987) and Beccalu va & Serri (1988) suggested that it ma y be silicate part ial melts derived fro m alB so urce mantl e, whic h invaded re frac tory per ido tite in t he mantle wedge pr esumably befo re initi atio n o f subd uctio n ; subseq uently, this mantl e wit h O lb-type enr ich ment wa s furt her m odified by hyd ro us fluids d erived fro m th e sla b . (2) C lues to t he o rigin of the enri ched co m po nent(s) in bo ninite gen esis come from considerati on of th e unusual co m po sitio na l fea tures of t he Nep o ui (New Ca ledo nia ) T ype I lo w-C a bo nin ites. These were clearl y deri ved fr om a highl y refractory so ur ce perido tite, as indicated by their very low Sc, CaO and FeO * co nt ent s and low CaO/Ab03 va lues ( < 0. 5), yet th ey ha ve h igh er SiOz and Na -O cont ent s, and st ro nger LREE enrichment «La/Sm )N - 2) than an y T ype 3 low-Ca boninit es or high- Ca bon in ites (C ameron et 01. 1983, Cameron 1989). Desp ite st ro ng LREE enrichm ent , Ne poui la vas ha ve MORB-type e Nd va lues ( +8 to + 10) an d low initial 8 7 Sr/ 86Sr (0.7034-0.7035). Th is su ggests th at th e hydr ou s fluid s which introdu ced Si02, Na-O and LILE (includi ng REE an d Zr) a nd initi at ed partia l melti ng wer e derived almost enti rely from a yo ung, subd ucted MORB so urce mo dified only by limited int eractio n (a s ind ica ted by slight ly rad iog enic Sr relative to M O RB) with sea water . T here is no iso topic evide nce fo r the existence o f the hypothetical Ot Bctype enriched component in th e so ur ce o f these la va s. Therefore, it is im po rtant to note t ha t t he component in boninite genesis whic h enriches LREE (and Zr, leading to the charact eri stic low T i/Zr values ( < 50) of low-Ca bonin ites) need not be assigned a n O IB enrichme nt -type o rigin. Apparentl y, high-t em perature aq ueo us fluids t ranspo rt ing Si0 2 a nd Na20 fro m t he sla b (a nd nea r-sla b pe ridot ite s in th e m a ntle wed ge) ca n effe ctively ext ract a nd m ove L REE an d Z r fro m ba salts in subd ucted o cea nic crust , a nd int ro d uce these eleme nts into th e refracto ry boninite source. We suggest, t herefore, that tw o components may enrich the refractory ma ntle source of low-Ca boninites. O ne of these is ubiquitous, being derived via ext ractio n by sla b-der ived , superheated aq ueo us fluids of Si, Na , LILE including L RE E , a nd Zr from subducted MORB . These are t ransported upw a rds into the refra cto ry mantle wedge and in itiate partial me lt ing if a m bient wedge temperatures exceed th e solidus. If t he oceanic cru st from which these hydrous fluids were derived and equilibrated was typical MORB, fluid s in herited eNd va lues from + 8 to + 10. The second enriched co m po nent in boninite genesis had significa ntly low er e Nd( < - 8) and a P b isotopic signat ure suggesti ve of derivat ion from an ancient sedimentary component in t he slab; it also was probably LREE-enriched and had a relatively low Ti/Zr

28

A . 1. CRA WFO RD ET .4L.

( < 50?) . Invo lvement of the la tter component in the genesis of some boninite suites , such as the New Caledonian (Nepo ui) and Guam-Hole 458 suites (Meijer & H anan 1981, Hickey-Vargas 1989) was insignificant. whereas it was an imp o rtant co m po nent in th e genesis of other suites, such as the Chichi-jima and Howq ua (Victorian Cambrian ) suites . Fina lly. we no te that Hol e 458 and Guam boninit es ha ve lIl6P b/ l()..I P b high er than MORB . and clo ser to va lues for NE Pacific OIB (Hickey-Va rga s & Reagan 1987). However . as Blo omer (1987) ha s pointed out. Pb isoto pe data for altered J ur assic MORB in the W Pacific (Meijer 1976) ar e ver y similar to the Mariana for earc-Guam bon inite Pb isotope data; since it is preci sely thi s crust t hat is being su bducted arou nd th e W Pacific rim . pre- subduct ion O lB-type source enri chments in upper mantle eventually trapped in the wedge ma y not be necessary. I. 7

/ .7./

Tectonic setting of boninite generation

Physical conditions of boninite generation

Melting st ud ies of boninites summarized ea rlier in thi s chapter indicate that magm as wit h thes e high -SrOj, high-MgO characteristic s can only be extracted from peridotitic upper mantle at very sha llo w levels . certainl y sha llo wer than 50 km and probably sha llow er than 30 km. Requisite temperatures ar e > I 100°C for Type 2 low-Ca boninites, and probably> 1200 ° C for other low-Ca and high-Ca boninites. W at er is a key ingredient in low-Ca bo ninite genesis, and a small amount of water ( < 0.5OJo) is probably also pre sent in the so urces of high-Ca boninites. Under anhydrous co nd itio ns. magmas with high Si0 2 and high MgO contents approaching boninitic compositions can only be generated at P < 5 kbar, co rr esponding to cr ustal dept hs for isla nd ar c or cont inenta l sett ings.

1. 7. 2

Tectonic scenarios fo r boninite generation

Fo ld belts are produced by co nt inent-co ntin ent. co ntinent -arc or a rc-arc co llisio ns, at t he final phase o f a W ilson cycle of ocean-basin opening a nd closing. In theory, boninites in corporated in ophiolites in an y particula r fo ld belt cou ld ha ve been generated d uring anyone of the preceding three phases of th e W ilso n cycle which prod uced that foldbelt; that is, d uring (i) initial cont ine nt a l rift ing and ru pturing . (ii) ocea n-flo o r sp read ing a t a mid-ocean-ridge spread ing cent re. or (iii) subd uctio n-rela ted ma gm at ism (includ ing arc , fo rea rc and ba ckarc sett ings) as the ocean is clo sin g. Below, we co nst ruct idealized but actuali st ic scena rios for each o f t hese pre -co llisio nal stages of the Wilson cycle. For each stage , we det ermine whether the prerequ isite co nd ition s for bo ninit e generat ion might be attai ned .

29

C L ASSI FICA T IO N . PET ROGEN ESIS AN D T ECT ON IC SETT I NG

a nd we pred ict th e stra tigrap hic seq ue nce of lava s a nd asso cia ted ro cks in which th e bo ninites sho uld occur , if at al l.

/ . 7.3

Stage / : Cont inental rif ting

Magma tism acc o m pany ing co ntinenta l rifting results fro m parti al melting of o ne or mo re dia pirs of MORB-so ur ce mant le rising essentially adiabatically beneath gradua lly att en uat ing conti nent al crust. Ea rliest erupted lavas tapped fro m the d iapir(s) at relatively deep levels (100- 60 km) ma y be slightly al kaline, o r mor e fre q ue nt ly, T- ty pe (transition a l) MORB com po sitio ns (Fodo r & Vetter 1984). Magmas subseq uent ly genera ted by pa rti al melting o f the diap irs at shallo wer levels (60-40 km) approach norma l (N-) MORB co m posi­ tio ns, unt il the cont inent al crust ru ptures, a nd a stea dy-state spread ing cent re is estab lished . H yd ro us fluids a re unlikely to be presen t in the so urce a reas o f th ese a nhyd ro us, th o leiitic rift-related m agmas. Therefo re, relatively siliceo us high-M g magm as co uld be pro d uced o nly if th e dia pirs whic h yielded MO RB-ty pe lavas by part ia l melting at deep er levels were a ble to conti nue ascend ing an d ma intain their heat to levels per haps a s shallow as 10- 15 km (Dunca n & G reen 1980). In th is sit uation of seco nd -sta ge melting , relatively minor vo lume s o f strongly depleted magnesia n quartz th o leiites (51- 53070 Si0 2 ) might be genera ted , a nd resid ual refracto ry ha rzbu rgite will freeze close to the mo ho . Suc h lava s are co m pos itio na lly tra nsition a l between t holeiitic basalts and bo ninites. If d iapirism ass ociated with rifting progresses to a steady state, th en new, relatively ferti le d iapirs of MO RB-so urce mantle will co nti n uo usl y su p ply large vol umes of MORB, an d th e chance fo r th e generation or preserva tio n o f these seco nd-st age melts will be very limited . We suggest , there fo re, that high-Si, high-M g magmas will o nly be generated in rifts where supply o f MOR B-source d iapirs is aborted , a nd sprea ding j um ps to a mo re favo urab le, successfu l site. . T he idea lized seq ue nce o f ma gm a tic events accompa nying an d driving co nti ne nta l rifting pr ed icts a stra tigra phic pile in which ea rly a lkaline a nd transi tio nal lava s a re ove rlain by mo re abundan t T - to N-MO RB (Fig. I. lOa ), wh ich ma y be overl ain , in tu rn , by volumet rica lly subo rd inate strongly depleted bon initic ba sal ts (F ig. I.lOb). C hemica lly, these lavas will approach high-C a bo ninit es; Na 20 a nd Ti 0 2 co nte nts will be ver y lo w, a nd LREE in the par ent magm as will be highly depleted . A ny L1LE enr ichm en t will be limit ed to the ear liest-erupted seco nd -stage me lt lav as , which ha ve the most oppo r­ t unity to digest co nti nen ta l cru st. Sub seq uent lavas a re likely to ascen d through cond uits chemicall y ' insulated ' by ea rlier magmas, a nd may eru pt with unmodified L1LE co nt ent s. Lav as prod uced d uring th e tra nsitio nal riftin g stag e to ocea n o pen ing will be most likely cove red by thick sed imentary prism s; they can o nly be st udi ed if deep co nt inent al -margin d rilling has reached basem ent (e.g. Fodo r & Vetter

30

A. 1. CRAWFORD ET AL.

•

~

T-MORB

I£::LI

SECOND-STAGE MELTS

Figure 1.10 Hypothetical scenario to generate second-stage metals in an intracontinental rift. (A) Ascent and partial melting of a MORB diapir beneath an actively attenuating continental rift may generate voluminous T-MORB. If regional plate kinematics or local crustal mechanical fact ors ar e unfavo ura ble for the development of a steady-state sprea ding centre, the ridge may jump elsewhere in the crust. (B) An ' unsupported' diapir may cont inue to rise rapidly and partially melt a second time to yield magnesian quartz tholeiites with very low Ti02 contents and stro ngly LREE-depleted REE patterns; these second-stage melts are transitional to high-Ca bonin ites.

1984), or if the pa ssive margin produced during succe ssful riftin g eventually collides with an arc or continent, to be uplifted and exposed du rin g or ogenic defor matio n (Crawford & Berry 1989). Lava piles representing the rift-to-ocean stage of a Wilson cycle are known in early Proterozoic (Canada: Francis et al. 1983), late Proterozoic­ Palaeozoic (Newfoundland: Strong & Dostal 1980; Tasmania: Brown & Waldron 1982, Cr awford & Berry 1989; Gr eenland: Kalsbeek & Jep sen 1984) and Meso zoic (Bertrand et al. 1982) fold belt s. In each of these occurrences, the lava seq uence is as predicted. Early transitional or alkaline lavas pass upwa rds into a th oleiite pile which includes lower T-t ype MORB overla in by more depleted tholeiites approach ing or attaining N-MORB compositio ns. Second-stage melts co mpositionally transit ional to boninite with strongly LREE-depleted REE pa ttern s are pr esen t at several localities in late Pro­ teroz oic rift sequences in W Tasmania. On King Island, least alt ered picrit es (underlain by T-t ype MORB tholeiites) with 15-20% MgO contain around 50 070 Si02 (cf. 46070 for Baffin picri tes; Francis 1985), have 0 .2-0.3070 Ti02 and (La!Sm)N - 0 .4 (Brown & Waldron 1982) and ar e clearly second-stage melt lavas. Further so ut h, at Double Co ve, T-MORB are overlain by high ly LREE-depleted ba sal ts with around 52070 Si0 2 an d 0.3 -0.6070 Ti02 (C raw fo rd & Berry 1989). In the mid-Proterozoic Cape Smith Foldbelt in Canada (Francis et 01. 1983), simila r but somewhat less depleted lavas con stitute the Chu kotat Gro up , and overlie transitional alkaline to T-MORB tholeiites of the Povun gnituk G roup . Francis et 01. (1983) proposed that the Chuk otat lavas 31

CLASS IF ICATIO '. P ET ROG ENESIS A ND TECTO NIC SETT ING

may ha ve been ge ne rated by fu rther m elting upon co ntinued adiabatic ascent of t he di apir(s) wh ich yielded the P ov un gnit u k lava s during partial melt ing at deepe r levels. T he high Ni and Cr , a nd very low L REE co nt ents of th e T asma nia n a nd Chukotat tholeiites ru le out t he poss ib ility that the relatively high-Si cha racter o f these magnesian la va s was produced by cru stal co nt am­ inati on o f mo re typica l (i.e. no t seco nd-stage mel t) tholeiitic m agm a s. In sum ma ry, it is po ssib le that bas alts tr a n sition al to high-Ca bo ninites could be gener ate d in a co nt inental rift during aborted , earliest stages o f o cea n o pening. In such ten sio nal settings , subordi nate vo lumes of depleted mag­ nesia n q ua rtz tholeiites (Si0 2 of 50-5 3UJo) may be gener ated as a final melt inc rement extrac ted fro m a d ia pi r which con tinued to rise to shallow levels after yield ing ea rlier MOR B-t ype bas a lts during part ial m elting de eper in t he upper ma nt le (see Fig . 1.12b). Du ring the ocean clo sing ph a se of t he Wilso n cycle , it is inevita ble (exce pt perhap s during a rc-ar c co llisions) that a passive m a rgin bea ring a ri ft -generat ed basa lt pi le (which may includ e some ' second ­ stage' depleted m agne sia n q uartz tholeiite s) will coll ide with a n ar c or co nt inent al b lock , genera ting a foldbel t. Taking into account geochem ical ev idence a nd th e stratigra phic sequence o f la va s and as sociated sed ime nts , th ere is litt le likelihood that d epleted ma gn esian qua rt z tholeiit es in such rift- p ro du ced la va piles incorp o rated wit hin a fol d belt will be mi sidenti fied as bo ninit e pr odu ct s of co nver gent plate bo unda ry magmatism . .

1. 7.4

Stage 2: Ocean-floor spreading

D uring the ocea n-floo r spread ing p hase of t he Wilson cycle . steady -state ascent o f MO RB -so urce diapirs from d eeper in the mantle provid es a near -continuous sup ply o f MORB to spreading centres, with magm a segrega ­ tio n oc cu rring m ainly at d ept hs exceeding 50 km (Green et 01. 1979). Con­ tinued up wa rd ascent a nd seco nd-stage m elting o f t hese dia pirs is likel y to be very limited . since new d ia pirs are regu larly arriving at d ep th be neath t he sprea d ing cent re. H owever , MO RB wit h Si0 2 content s in excess o f 50UJo, a nd with re lative ly low Na20 and Ti02 contents a re known fro m th e oc ea n basins (see Fall oon & Green 1988). and may represent lo w-P (- IO kba r) melts. To explai n the presence of very calcic plagiocl ase mega crysts a nd xenocry­ sts , and also high-Mg # , high Ca/Na melt inclusions recorded fro m some MO RB, Duncan & G reen (1980 , 1987) proposed that low-pressur e ( < IO kbar) second-s tag e m elting of d iapirs which had al ready yielded pic ritic MORB might occur beneath sp read ing centres. C ontinued ad ia batic as cent o f the MO RB-depleted diapirs m ight occa sionall y occur, lead ing to a furt her - IOUJo pa rt ial melting and producti o n of magnesian quartz tholeiites with high C aO / N a 20 . low Ti0 2 and very depleted LILE contents. Such magmas are ra rely pr eserved . probabl y d ue to mi xing with more typical MORB in crustal magma ch a m bers . H owever , the recent discovery (Da nj ushe vsky et 01. 1987) in some MORB o f magnesian olivine-hosted melt inclusions with 52.8UJo Si0 2• 32

A . J . CRAWFORD £T AL.

0.3010 T iO z a nd C aO! NazO = 12.6 demon strates that seco nd-sta ge me lts transi­ tio nal to bon ini te co m position s do oc cur in the oc ean ba sins, as proposed by Duncan & Green (1980). Although we d o ubt that major ocean-basin crust can be incorporat ed in fo ldbelts during plate co llisio ns , t he re is no rea son wh y the process descri bed a bove fo r gener a tio ns of seco nd- stage melts cannot al so take p lac e in backa rc-basin sp read ing centres. E ven so , t heir rare , very limited occurrence in a pile o f vol umetrica lly do mina nt MORB-type la vas will render seco nd-stag e melts (with com po sitions tran siti onal to high-Ca bonini te s) gen er at ed at mid-ocea n-ri d ge o r bac karc-basin spread ing centres immed ia tely ident ifiable as such ; their misiden tifica t io n as subd uctio n-rela ted boninit es is most unlike ly.

1.7.5 Stage 3: Subduction-related magmatism A co nse ns us o f opinion exists that bo th high-Ca and lo w-Ca bon inites are products of part ial melting abo ve a subducted slab of oceanic lith osphere at co nvergent plate boundaries. However, the apparent absence of boninitic lav as in many well studied island arcs, such as the Aleutians, Vanuat u, Sunda a nd Lesser A ntilles arcs, indicates that th e req uisite condit ions fo r boninite genera tio n a re probably not met with at a ll converg ent plate bounda ries (C rawford et al. 198 1). The mi ssing ingredient in suc h settings is almost ce rta inly the req uireme nt o f very high temperatures (> I 100°C) a t very sha llo w levels in the mantle wedge ( < 50 km). All geophysica l models fo r the thermal st ru cture o f subductio n zones in isla nd arc sett ings suggest tha t am bient tem pera tures in the fo rearc wed ge at levels sha llo wer th a n 50 km sho uld be < 800 °C , and probab ly < 500 °C (e. g . Hsui & To kso z 1979). T o genera te boni nites in fo rea rcs, where th ey occ ur in th e Boni n- Maria na a rc and the N T o nga a rc, a mech ani sm capab le o f rai sin g the shallo w upper-ma ntle tem perature to > I 100°C m ust be identi fied. For th e Bon in - Ma rian a se tt ing, two differe nt mod els ha ve been pro posed to prov ide th e ext ra heat req uired to generate bonini tes a t depths sha llow er than 50 km in th e fo rea rc ma nt le wedge. T he first of these, sugges ted by Camero n et al. (1979) and expanded by M eijer (1980), en visa ges boninites as a magma type gene ra ted d ur ing t he earliest sta ges of initiation of a subd uction zone. Meijer's (1980) mode l, adopted by H ic key & Frey (1982), H a wki ns et al. (1984) a nd H ickey-V argas & Rea gan (1987) is based largely on DSDP Hole 458 in th e Ma ria na fo rea rc (late Eo cene-basa l Oligocene bo ninitic la va s o ver lying a rc tho leiite s) and on Guam (late Middle Eocene boninit es interbedded with a nd succeede d by a rc tholeiites ). The model suggests that th e cha ng e in sp rea ding direc tio n of t he P acific plat e at - 44-42 Ma result ed in initiation o f a su bduction zo ne a lo ng a fo rmer major N- S transform now broadly ma rked by the P alau-K yushu Rid ge. We st of the new trench lay an acti ve spread ing ce nt re segment o f the We st Philippine Sea , orient ated appro ximat ely NW - SE 33

C L A SSI FICA T ION, PET ROGEN ESIS A ND T ECTONI C SET T I NG

West Philippine Sea

Boninitic Lavas

· ;' D LJ ., .

.:

Abnormally Shallow Dehydration

'.

Young, Hot Lithosphere

(Seno & Maru yam a 1984) . Sub d uctio n o f o ld , co ld ocea nic cru st into this you ng , hot subsp read ing cen tre up per mantle resulted in sha llower-t ha n­ norma l de hydratio n o f altered ocean ic crust and ing ress o f wa ter into peridotite s which had yielded MORB pa ren t magmas only a short time ea rlie r (Fig. I . l l a ). Arc tholeiitic ma gmas were generated by shallo w melting of t his relatively re fractory sub-ocea nic upper mantle, and boninitic ma gmas were gen er ated from t he resid uum of th e arc th ol eiite-producing melting event. To provide a source for the extra hea t req uired to generate boninites in this setti ng af ter a rc tholeiites (in DSDP Hole 458 ), Hickey & Frey (1982) adopted the mechanism ce nt ral to Model 2, suggested by Crawfo rd et 01. (1981). Thi s mo de l invol ves heat conduction from an ascending MORB-source mantle d iapir in the earliest stages of backarc spreading. This diapir, which eventuall y supplies backarc-basin basalts that ri ft the arc in two, must initially pa ss through hyd ro us, d epl eted sub-oceanic lithosphere within the metasomatic halo of the subd ucting sla b (Fig. 1.1 I b). Boninites are produced by cont act 34

A. 1. CR AWFORD ET A L.

_

Boninitic Magmas

m ~

Island Arc Crust

D II

Hydration Zone MORB-Source Diapir

f igure 1.11 (a) Mod el for boninite genesis a fte r Meijer (1980), in which subduction of old , co ld oceanic crust into yo un g, hot sub-P hilippine Sea litho sphere initiates shallow de hydr ation o f the sla b, and boni nite gene ra tio n occurs along that sectio n of the new plat e bou ndar y still within (he region of elevated geo therms related to the West Phili ppi ne Sea sp reading cent re. (b) Model fo r boninite genesis a fter C rawford et 01. (198 1), in which, during the earlie st stages of backarc rifting and opening, a MO RB-so urce diapir intru des hydrous sub-arc peridotite and initiates limite d contact melting, genera ting bo ninit es. Er upt ions of basalts transitional between MORB a nd arc tholeiites, an d derived from the MORS-source diapir, may co ver the boninites,

melting o f this hyd rat ed sub-a rc per idotite and a re volumet rically min o r rela tive to sub seq uen t eruptions of backa rc basal ts. Bon ini tes are th en buried by volumino us eru pt io ns o f ea rly back arc-basin basalts a nd sedim entary aprons washing off the sunderi ng arc fra gme nts. Th e major difference between this model an d that of Meijer (1980) is th at boninites are not generated in th e earli est phase o f subd uctio n , bu t immediately precede backarc-basin opening; th e latter event may occur tens o f millions of years after the init iation of subductio n. Furth er consideratio n o f Models I and 2, strongly influenced by new ob servatio ns an d da ta fro m bo th modern (e. g. Crawford et al. 1986, Falloo n et al. 1989) sett ings an d ophioli tes (C rawfor d & Keays 1987, Flower & Levine 1987), suggests th at both mo dels enco unte r major probl ems and req uire significa nt mod ification befor e they can ad equately account for the petro­ genesis o f bon ini tes. 35

CLASSIF ICATI ON. P ET ROG ENES IS AND TECTO NIC SETT ING

Model I has seve ra l im po rt a nt sho rt co m ings : (a ) A rc tholei it es clea rly oc cur below bonin ites in Hol e 458, an d occur interbe dd ed with a nd a fter boninit es on G ua m . T h is suggests th a t ' norma l' a rc magmat ism wa s acti ve ea rl y in history of thi s subd uctio n zo ne , a nd bo ninite s her e cann o t be a peculiar feature ac compan yin g t he initi atio n o f subd uctio n . A return to ' no rma l' arc m agma gen era tion fo llowed bo ni nit e eruptio n . (b) If th e hea t so urce fo r bo ninit e gen erat io n in t he Maria na fo rea rc wa s ho t up pe r ma nt le be lo w th e so ut hern end o f th e W est P h ilippine Sea sp rea d ing cent re, a s suggested by Me ijer (1980), it is unli kely th a t the zo ne o f an omal ou sly ho t upper man tle extended more than a few hun dred kilomet res eit he r side o f t he spreadin g cent re, which trend ed NW a wa y fro m the ne w co nvergent pl ate bo und a ry (Fig. 1.lla). H o w­ ever, boninite s o f simila r age (mi ddle-late Eocen e) occur in the Bonin Islands so m e 1500 krn furt her north , and could not ha ve been generated in this petrogenetic scena rio . (c) T here is evidence fro m th e northern part o f th e Palau-K yushu Ridg e tha t arc la va s were er upting alo ng th e P alau - Kyu shu Ridge as ea rly as 48 Ma (Sen o 1984), ind icat ing tha t bo ni nites in the Mariana-Bonin region were no t th e first ma nifesta t io ns of arc magmati sm following init iat io n o f su bd uctio n . Mod el 2 a lso has its p ro b lem s, a lthough t hese ap pea r -less severe th an tho se not ed a bove for Mo del I: (a ) It p redi ct s that bo ni nitic magmatism is a likely precu rso r o f ba c karc­ ba sin sprea ding a nd basa lt gen eratio n . H o wever , alth o ug h it is possible that th ey a re buried by sedi me nt s a nd backarc basalt s, bo n inites re main unr eco rd ed fro m m an y a rc- bac ka rc bas in settings , suc h a s th e Man us Basin , N Fij i Ba sin, J apa n Sea, S La u Ba sin-H a vre T ro ugh a nd Scot ia Sea . (b) If this contact melting mechanism operat es as a MORB-source d iapir rises benea t h th e a rc, then the first boninitic melt s sh ould b e the most LI LE - and H20-enriched , and subsequent melts should be depleted in LI LE (if indeed the anhydrous residue of the first boninite-producing me lting event can be m elted further) . Well expo sed bonini te piles in C a m brian ophiolite s in SE Au stralia sho w th e oppo site , with the most LILE-enrich ed boninites at the top of t he pile and high-Ca bonin ites at th e base . Although we do not believe that Model 2 adequately acc ounts fo r the pet rogenesis of the best-studied modern an d a ncient low -Ca boninite suites, it is hard to en visa ge that MORB-source diapirism (- I400 °C) initiating backarc 36

A . 1. CR A WFO RD ET AL.

spreading can avoid ca using so me degree of contact melting of hydrated su b-ar c perid otites (solidus temperature - 1000-1100°C) . Limited vo lumes o f magmas co mpo sition ally transitional bet ween arc tholeiites and high -Ca bo nin ites migh t be generated in this fashion . High-Ca boninites of probable late Tert ia ry age occur in a complex tecto nic sett ing at the nort hern term ­ inat io n o f the T o nga T rench (Falloon et al. 1987, Falloon et al. 1989). One po ssible tecto nic scenario fo r genera tion o f these bon inites is th at the spread ing cent re at the very no rt h end o f th e Lau Basin bifurcat es, wit h one branch hea ding o ff in a NW d irectio n a nd the o ther tr ending NE , propagating into and tr an secting th e fo rearc of the N Tongan arc. Diapirs supp lying MO RB to th is po stulat ed NE-tr endi ng spr ead ing centr e mu st pass throu gh shallow, refracto ry and pro ba bly hyd rous subfo rea rc upper mantle, and could have been the heat so urce nece ssary to gen erate these high-C a bon init es from such mant le at depths pro babl y less than 50 km.

1.8 A new model for boninite petrogenesis We consider that neither Models I nor 2 a bo ve for the genera tio n (If bo ninites ca n be applied successfully to the type locat ion Bonin-Mariana bo ninites, o r to the well doc umented Cambrian bonin ite occurrences in SE Au stralia. Key ob servations which mu st be explained in a reali stic petrogenetic mo del fo r these boninite occu rrences include the following : (a) Bo ninit es were erupted in a relatively sho rtlived event in midd le or late Eocene along mo re than 1500 km of th e co nver gent plate boundary on the easte rn margin of the West Phi lippine Sea , and were pre ceded by ' no rmal ' arc ma gmati sm . (b) In the Camb ria n bon inite piles, lavas beco me mo re refractory upwa rds in terms of ' basa ltic' compo nent s, but at the sa me time mo re enr iched in LI LE a nd with decreasi ng £ Nd (Nelso n et al. 1984, Crawford & Ca me ro n 1985). The uppermost boninites ar e exceptio na lly de pleted lavas (chro­ mite C r # > 0.95) with pr on ou nced LREE enr ichment a nd low e Nd ( - 8). (c) Also in bo th the Victo ria n and New fo und lan d (Bett s C ove) Lo wer Pa laeozoic ophiolites, thol eiites chemica lly most similar to ea rly backa rc­ basin tho leiites occur int erb edd ed in and/or o verlying the up permost . bo ninites (Cra wfo rd & Keays 1987, Co ish & Church 1979, Co ish et al. 1982, Co ish 1989). The tho leiites are de rived from a very d ifferent so urce tha n th e bon inites. (d) In the Chichi -jima, Mar ia na fo rearc, Ca pe Vogel an d Victorian Ca m­ brian bo nini te seq uences , a range of bo ninite co mpos itio ns is recorded , fro m less refract or y high-Ca bo nin ites (o r lavas tr an sitio na l fro m high-Ca to lo w-Ca bo ninit es) to re fracto ry lo w-C a bo nin ites. 37

C LASS IF IC ATI ON . P ETR OG ENESIS AND TE CTON IC SETTING

We suggest that the most like ly mechan ism fo r generating bo ninites, and low-Ca bonin ites in part icular , is subd ucti on o f an active spr ead ing centre subpa rallel to a tr en ch front ing a n intra-oceanic ar c (Fig . 1.12a). As the hot lith osph ere on either side of the spr ea di ng centre approaches the trench , the d ip o f th e slab pro ba bly de creases a nd isother ms in the ma ntle wedge rise, ca using part ial melting o f depl eted su bfo rea rc oceanic lit hosphere a nd genera­

v

v" v . - . i-' . ­ .

.:;..I) ",..!'" /~"o ~ - :/

, ,;/ . ,

50 km

A

B

m G3 o

New Oc. anic Crua'

{ TYpe 1 Low- ca Boninil •• Type 3

o •

I.la nd Arc CnJaI

MORB B•••menl Hydroul Peridotite Solidu s

Figure 1.12 An alternative model fo r bon inite generatio n involving subduction of an active spread ing centre (A) accompan ied by rising isoth erms beneath the fo rearc a nd generat ion o f first boninites from refract or y, hyd rous sub-arc man tle. Conti nued descent of the ridge, which remains active and hot , generates more refractory boninites, but on ly if the sup ply o f wate r to the site of part ial melting is maintai ned. In a tensional forearc selli ng, tension on the ba se o f the thin man tle wedge may initia te rifting with in the forearc (B) and ' resurrection' of the spreading cent re. Backa rc-basin-typ e basalts ma y erupt on the uppermost; highly refractory boninites (C). See text for detailed discussion .

38

A. J. CRAWFO RD ET AL.

tion of high-C a boninites. Arc magmatism on the adjacent arc may shut off due to th e change in dip of th e slab , and possible interruption of the supply of hydrous fluids from the slab to the site of 'normal' arc magma generation at de pth s of -1 00 km. Harzburgite residues following high-Ca boninite extraction require further influx o f hyd rous fluid s from the slab , and pr o bably increasing temperature, to pa rtially melt. H ydrous fluids pas sing up into the wedge can transport in (and ext rac t fro m surrounding upper mantle) large amounts of 'enriched' com­ ponent(s), which are a key factor in boninite geochemistry. The final bo ninites (Typ e 1 low- Ca bo ninites) wh ich can be ext racted before the residual mantle beco mes in fusibl e ha ve th e deco upled signature of highly refractory (low TiO z, CaO , CaO/Ah03, Sc and very low HREE), and Na- , Si- and LILE-enriched source compo nent s . . The increase in temperature requi red to generate low-Ca boninites in this sett ing is pro ba bly due to arrival of the su bd ucted spreading centre beneath the fo rearc. We ar gue that MORB-type magmatism, derived from the spr eading centre as it co mme nces descent along the now very shallowly d ipping subduction zone, need not necessarily cease, thereby maintaining high temp­ era t ures at shallow levels. Diapirs risin g from around 150 km to feed the spread ing centre are unlikely to 'see' th e thin mantle wedge sliding above the 0 recent ly subducted sp reading centre, so that hot ( - 1400 q lithosphere below th e spread ing centre will co me into close proximity with multiply depleted per idotite at very shallow levels, leading to further boninite generation. MO RB-type magmas in shallow magma chambers below the subducted spr eading centre may erupt contemporaneously with and after highly depleted Type 1 low-Ca boninites, as at Howqua in Victoria (Crawford & Keays 1987). If th e fo rearc is under tensional stress, as in the modern Mariana-Bonin and Tongan arcs, co ntinued spreading on the subd ucted spread ing centre might lead to splitt ing o f the fo rea rc sliver of mantl e wedge (Fig. 1.12b), leadi ng to a 'resurrection ' o f th e spread ing centre, initiation of ' bac ka rc-type' sp reading in a fo rea rc setting, a nd th e cessa tio n o f bonini tic magmatism . This emergence o f a sub d ucted spread ing centre has bee n hypo thesized to occur, and termed 'e duct ion' by Dixo n & Farrar (1980 ). The thick th oleiite pile above bo ninites in th e Victorian Cambrian greensto ne belts a nd in th e Betts Cove ophiolite in Newfo und la nd might ha ve been produced in this manner (Crawford & Kea ys, 1987, Coish 1989). Fo llowing the prescient hypothesis of Seno (1984), we believe that subd uc­ tio n of an active spreading centre around 48-43 Ma is th e most probable explanatio n for the extensive oc currences of boni nites along the Bonin­ Ma riana fo rea rc . Seno (1984) found that the most simple way to account for regio na l geophysical-tecto nic relat ionships in the central W Pacific was to hypothesize the fo rmer existence o f a broadly NW -SE orient ated spread ing cent re which was su bducted at a NW-SE trending tr ench (roughly at th e site of the Pala u-K yushu Ridge) between 48 and 43 Ma . Figure 1.13 shows his 39

C LASSIF ICA T ION , PE T ROGENES IS AN D T ECTO NIC SETTI NG

--, Amer ica

Eurasia

,,

-,

PACIFIC PLATE

INDIAN PLATE

48 Ma Figure I. 13 Reconstruction of the W Pa cific at 48 Ma a fter Seno (1984), showing the spreadi ng cent re forming the eastern ma rgin o f the Nort h New G uinea P late ab out to be subd ucted a long several thou sand s o f kilometres o f plate bounda ry. We arg ue that this subd ucted spr eading cent re pro vided the heat necessar y to genera te the Mari a na- Boni n boninites from sha llow, hydro us an d ref racto ry subfo rear c mant le. Subduction of pa rt s o f the same spreading system so me 10- 20 Ma earlier may have generated the Ca pe Vogel (50 Ma ) a nd New Caledo nia n (early Tertiar y) boni nites.

reconst ructi on at 48 Ma . Subduction of mor e southerly segment s o f the same spreading centre around 53 Ma may well have been responsible for the generation o f the Cape Vogel low-Ca boninites, and possibl y also those in the latest Cr etaceous-early Eocene in New Caledonia. As described by Tatsumi & Ma ruyama (1989), Saunders et 01. (1987) and Rogers &. Saunders (1989), a bro adly similar petrogenetic scenario may explain the origin of the Setouchi and Baja Type 2 low-Ca bo ninites, and similar lavas in S Chile (Isla Cook). Very young, ho t oceanic lithosphere was subducted beneath both th e former region s immediately prior to boninite generation, which occurr ed during a fairly brief (I Ma du ra tion for Setouchi) magmatic event. Magma segregation in the uppermost mantle (30-40 km) beneath th ese regions generated parental low -Ca boninites probably more alkali- and LILE-rich than Types I and 3 boninites; th is ma y be partly due to lower 40

A. J . CR A WFO RD ET A L.

amo unts of water and lower degrees of partial melting at 10-15 kbar relative to Types I and 3 magmas generated by melting in the wedge at pr essur es :$ 8 kba r. Type 2 boninites at these localities ar e rarely more magnesian than 10% MgO , pro ba bly due to the grea te r opportunity for pooling a nd fract io n­ ati on in the th ick arc crust o f SW Japan and central Mexico relative to the very th in ocean-type cru sta l thic knesses in the forearc s of the int ra-oceani c a rcs such as the Bonin-Mariana arc . Boninites con stitute onl y o ne part o f the compositional spectru m of magmas erupted in both regions following su bd uc­ tio n o f this hot lithosphere. Ba salt s, de rived at higher pre ssu res and from d ifferent sourc es th a n th e boninites, are an important co mpo nent of both the Baj a and Setouchi suites, and deserve further st udy.

1.9

Directions and problems for further study

Detailed stud ies of boninitic rocks ar e still comparatively few relat ive to th ose o n other unusua l magma suites in the geo logical record (e.g. kimberlit es, ca rbo natites and komatiites). Despite reasonable co nsensus regardi ng their or igin and tec tonic signi ficanc e, a nu m ber of outstanding prob lems still co nfront th e inte rpretatio n and understanding of boninites. Among these pro blems ar e: (a) Bonini tes frac tionate to bronzite a ndesites and daci tic composition s on Chichi-j ima . Are th ese highly evo lved boninites compositionally distinct­ ive, and ho w clo sely do they approach more typical calc-alkaline andesites a nd dacites? (b) W hat is the exact identi ty and origin of the enriched componen t(s) in boninite genesis ? Ar e the se same enr iched components in volved in typical arc basalt gen esis at depths clo ser to 100 km? (c) Ho w effectively can superheated aqueous fluids extract and tran sport LI LE , REE , high-field-stren gth elem ents such as Zr, Nb and Y, an d ra diogen ic Sr through th e upper mantl e? (d ) What is the nature of th e bas em ent upon which the ty pe location Bonin Isla nd boni nites were eru pted? Planned ba semen t drilling in several W Pacific ar cs and forearcs d uring th e next few years, using th e J OIDES Reso lut io n, sho uld clarify temporal-spatial relationships of boninite suites in th ese essent ially in situ sett ings, as sho uld more detail ed stud ies o f bo ninite-bea ring o phio lites. Also, it should be poss ible to examine modern plat e-tecto nic settings and predict locations where bo nin itic magmat ism might be pr esently or fo rmerly active. O ne such loca tion is th e so uth ern end o f th e N Fiji Basin, where an active N-S orient ated spreading centre segment in th e N Fiji Basin ab ut s the Hunter Fracture Zo ne (extend ing fro m S Van uat u to Fiji ). The latter, although poo rly un derstood, may delineate a site of fo rmer (o r incipient) su bd uctio n . 41

C LAS SI FIC AT IO N. PET ROG EN ESIS A N D TE CTON IC SETT I NG

J uxtapos ition o f ho t subspread ing cent re litho sphere and old , cold and pro ba bly hyd rat ed sub -arc upper mant le provi des the necessary ing re­ dient s fo r bonin ite generat io n alo ng tha t pa rt of th e Hunter Fracture Zo ne clo se to the end o f the spread ing centre. (el Refineme nt o f experimenta l petrolog ica l tech niqu es fo r HyOvundersatur­ ared melting o f peridotite at high tem perat ure s and low pre ssu res, a imed at avoidi ng quench problem s, will further con st rain relatio nships between so urce dep letion a nd fluid s (com posit ion s, volumes , etc.) involved in bonin ite petrogen esis. (f ) More extensive P GE dat a a re requ ired for modern and an cient bo nin itic rocks, to aid in un der stan d ing their petrogen esis and their potent ial as so urces fo r precio us-met a l mine rali zat ion . (g) Mos t boninite suites occ ur in clos e associatio n with th o leiitic basalts which seem to ra nge bet ween arc tho leiite and ba cka rc-basin tho leiite co mpos itio ns. Furt he r document ation of these tho leiites is req uir ed to de ter mine their tecto nic settings of eruption ; a bett er understanding of the ir petrogenesis will furt her con strain model s for boninite genesis.

1.10

Conclusions

We suggest a maj or-element chem istry-based classification scheme fo r bo n­ inites an d related rock s which sub d ivides th ese into high-Ca suites (CaO! A b 0 3 = 0.7- 1.0 at Mg # > 0.65) , typified by the Troodos Up per Pill ow Lavas , and co mpositio nall y more di verse low-Ca suites. The latt er are fu rther bro ken do wn int o three gro ups: Types 1 and 2 are generated from high and low degrees, respe ctively, of partia l melt ing of very de pleted , probably clinopyroxene-free har zbur gite , and co nseq uent ly have very low CaO!Ah03 values ( < 0.55), and lo w CaO and FeO· co ntents . Ty pe 1 lo w-Ca bo ninites ar e best rep resented by th e late C retaceo us-early Tertiary lavas fro m Nepoui, in New Ca ledo nia, an d Type 2 low-Ca bo ninites, with total a lkali con tents > 3070 , are known from th e Seto uchi regio n of SW Japan and Baja Califo rnia. Type 3 low -Ca boninites, which include th e type location suite fro m C hic hi­ jirna, ha ve somewhat high er CaO and FeO· content s, and higher CaO!Ab03 (0.55-0.7) than T ypes 1 and 2 suites, and were de rived from slightly less refracto ry harzburgite so urces. Refra ct ory so urces of low-Ca boninites were metasomatized by multi ­ so urce, LI LE-enriched hydr ou s fluids pr ior to o r during boninite generat io n . We co ncu r with previous studies of bonin ites in suggesting that the hydrous fluids involved in boninite genesis were derived from slab dehydration. Bo ninite radiogenic isotope signatures were controlled by those of the bul k subducted crustal sectio n with wh ich th ey equilibrated. Where th is su bducted crust was yo ung, fres h and virtuall y sed iment-free MORB, such as fo r the 42

A. 1. CRA W FO RD ET AL.

Nepo ui bo ninites , fluid ENd va lues are MORB-like. If significant volumes of old , co nti nen t-de rived sediments were also in the subducted slab, ENd valu es as unr adio genic as - 8 were carried into the mantle wedge with t he fluids initia ting bo ni nite generation fro m refractory peridotite. H igh -Ca bo nin ites, such as tho se in th e Troodos Upper Pillow La vas and in the N To ngan fo rearc, may be gene rated from deplete d sub-oceanic lit ho­ sphere by co nt act melt ing adjacent to rising MORB-source diapirs. These 1400°C dia pirs, wh ich event ually lead to arc rifting and ba ckarc-basin fo rmatio n if thei r asce nt is not a bo rted by regional changes in plate motion (e .g. arc-co ntinent co llision ), mu st pass through and locall y pa rtially melt shallow, hyd ra ted sub-ar c mantle. H igh-C a boninites in this idealized scena rio must be preceded by ' no rma l' arc lavas, and possibly follo wed by backarc­ ba sin- type ba sal ts. Low-Ca boninites are generated from more refractory so urces than high -Ca boninites, and co nseq uentl y hydro us fluids ar e required to initiate melting o f such highly de pleted sources. T hese fluids carried significant amounts o f Si, Na , LILE (including LREE) and po ssibly AI. Type 3 low-Ca bo ninites (rep resented by the Cape Vogel and Chichi-j irna suites) are deriv ed fro m so mewhat less re fracto ry har zburgites than T ype I low-Ca bonini tes , as reflected in their resp ective CaO, FeO·, Sc and HREE levels. Greater access of incompatible element s is required fo r Type I genesis and is reflected in their higher Na, Si and LILE contents. T emperatures required to gen er at e such melts at depths in the up per mantle less than 30 km are probably > 1200°C. We suggest su bductio n of an active spreading centre su bpara llel to the trench may provide th e necessary heat for low-Ca boninite generation, which is predicted to be limited to the forearc regio n o f oceanic arcs. Boninites erupted in this setting are in an optimum loc at ion for subsequent incorporation into foldbelts du ring arc-continent co llisio n . The Cape Vogel, New Caledonian a nd Victorian Cambrian boninite suites ar e all th ought to have been overt hrust o nto thi nned pass ive ma rgin co ntinental crust d uri ng su ch arc- continent coll ision s. Type 2 low-Ca bo nini tes, such as th e Seto uchi and Baja Californ ia suit es, are gen erated fr om depleted mant le similar to the re fracto ry so urces o f T ype I bon inites. H owever , th eir much high er con tents of alkalis, P20S an d LILE suggest they were generated via signi ficantl y lower degrees of partial melting o f a T ype 1 source. T yp e 2 bo ninites oc cur whe re a spreading centre or very you ng, ho t oceanic lithosph ere has attempted to underthrust an acti ve contine nt al mar gin . Greater cr ustal thicknesses from so u rce to su rface resul t in Ty pe 2 low-Ca bo ninites bein g genera lly more evo lved tha n T ype I and 3 bo ninites eru pted th ro ugh thin forearc crust. Finally, we no te that boninitic magmas were er up ted in the P reca m brian, as early as 2050 Ma (e. g. parent magmas to ult ramafic sectio n of the Bushveld Com plex). Siliceo us high-M g basalts in Archaean greensto ne belts are com­ positio nally trans it io nal between tholeiitic basalts and high -Ca bo ninites; 43

CLASS IF ICA T IO N, P ET ROG EN ES IS AN D T ECTON IC SETT ING

however, they ar e pr o bably generated by extensive fract ion ation an d co n­ tamination of perido tit ic ko mat iite mag ma s, and a re best no t classified with bon inite series mag ma s.

Acknowledgements T he ideas expressed in thi s pa per have arisen o ver th e last eight years as a result o f co nsiderable inp ut a nd stimulus fro m a number o f people; we especially ac kn owledge di scussion s with Dr s R. A . Dunc an , G . A . J enner , W. E . Camero n, S.-S. Sun , L. Beccaluva , G. Serri an d R. Varne. Ju ne Pongratz is th an ked for her help with the drafting.

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C LASSIF ICA T ION , PET ROG ENESIS AND TECTO NIC SETTING Falloon, T. J . & D. H . G reen 1988. A nh ydrous part ial melt ing o f perido tite from 8 to 35 kbars a nd the petrogenesis of MO RB. J. Petro l, Special Lithosphe re Issue, 379- 4 14. Fa lloon , T. J ., D. H . Gr een & A . J . C rawfor d 1987. Dred ged igneou s ro cks fro m th e nort hern terminat ion of the To fua mag ma tic a rc. To nga a nd adjace nt La u Basin. A I/srr. J . Earth Sci. 34, 487- 506. Falloo n, T. J .. D. H . G reen , C. J . H arton & K. L. Ha rr is 1988. The a nhydrous parti al melt ing of a fert ile and depleted peridotite from 2- 30 kba r. J. Petro l , (in pre ss). Falloon , T. J .. D. H . G reen & ~ 1. T. McC ulloc h 1989. Petrogenesis o f high- Mg and ass ociat ed lavas from the nort h Tonga T rench (t his volume). Flower , 7\ 1. F. J . & H . Levine 1987. Petro genesis o f a th oleiite- bon init e sequence fro m Ayios Ma m a s. T roodos Op hiolite: evide nce fo r sp litting of a vo lcanic arc. Cont rib . Mineral. Petrol. 97. 509- 24. Fodor, R. V. & S. K. Vetter 1984. Rift-zo ne magmatism : petro logy o f basalti c rocks tra nsitional fro m C FB to ~1 0 R B , so utheastern Brazil. Contrib. Mineral. Perrol. 88 , 307- 21. Francis, D. 1985. T he Ba ffin Bay lavas a nd the va lue o f pi crites a s ana logues o f prim ary magm as. Co ntrib . Mineral. Perrol. 89, 144- 54. Fra ncis, D.. J . Ludde n & A . Hynes 1983. Magma evolution in a Proterozoic riftin g environment. J. Perrol. 24, 556-82. Frey. F. A . & D. H . G reen 1974. T he mine ralogy, geo chemistry a nd o rigin of lherzolit e incl usion s in Victor ian basa nires. Geochim . Cosm ochim . ACTa 38, 1023-59. Frey, F. A., C. J . Su en & H . \\'. Stock ma n 1985. T he Ron da high temperatur e peridotite: geochemistry and pet rogenesis. Geochim , Cosmo chim. A cra 49, 2469- 91. G reen , D. H . 1970. T he or igin o f ba sa ltic and nephel initic ma gma s. Trans. Leics. Lit , Phil. SOC. 64. 28-54. Gr een , D. H. 1976. Eq uilibrium testin g o f 'equ ilibr ium ' part ial melt ing o f peridotite unde r water-satura ted, high-pressure co nd ition s. Can. Mineral. 14, 255- 68. G reen , D. H .. T. J . Fa lloon & W. R. Ta ylor 1987. Man tle derived magmas - role of va ria ble source perido tite a nd variable C - H - O fluid com position s. In Magma tic processes: physio­ chemical principles, B. O . Mysen (ed .), 139-5 4. Geochem . Soc . Spe c. PubI. no . I. Green . D. H .• W. D. H ibberso n & A . L. Jaq ues 1979 . Petrogene sis of mid-ocean ridge ba salts. In The Earth: irs origin, structure and evolutio n, M . W. McElhinn y (ed .), 265- 99. Londo n: Academic P ress. Ha ll, R. P & D. J . Hu ghe s 1987. Neritic dy kes o f so uthern West Greenla nd : ea rly Proterozoic boninitic magmatism . Contrib. Mineral. Perrol. 97, 169- 82. Ha rnlyn , P. R., R. R. Kea ys, W. E . Ca mero n , A . J. C rawfo rd & H. M . Wa ld ro n 1985. P recious meta ls in magne sia n low-Ti la vas: implicatio ns fo r meta llogenesis a nd sulfur sat ura tio n in pri mar y magma s. Geochim . Cosmochim , A cra 49. 1797- 81 1. Hatton, C. J . & M . R. Sha rpe 1989. Significa nce and origin of boninite-like roc ks ass ociated with the Bushveld Co mplex (th is vo lume). Ha wkins , J . W.• S. H . Bloo mer . C. A Evans & J . T. Melch ior 1984. Evolution o f intra-oceanic ar c-t rench systems . Tectonophysics 102. 175- 205. Hick ey-Vargas , R. L. 1989. Boninites and th oleiites from DSDP Site 458, Mar iana fo rea rc (t his volume). H ickey. R. & F. A . Frey 1982. Geoc hem ical ch ara cter istics of bo ninit e series volcanics: implications fo r their source. Geochim, Cosmochim. A cra 46.2099-1 15. H ick ey-Vargas, R. & M. K. Reagan 1987. Temporal va ria tion o f isoto pe and rare earth eleme nt a bunda nces in vo lca nic ro cks from G ua m: impli cati o ns for th e evolution o f th e Ma ria na Arc. Contrib. Mineral. Petrol. 97, 497- 508. Hsui, A . T. & M. N. Toksoz 1979. Th e evo lution o f therm a l structures beneath a sub duct ion zo ne. Tectonophysics 60. 43-60. Ishizaka , I. & R. \\'. Carlson 1983. Nd - Sr systematics of the Setouchi volcanic rocks, SW Japan : a clue to the orig in o f orogenic andesite. Earth Planer. Sci. Leu. 64. 327-40 .

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A . J . CRA WFO RD ET A L. Jaq ues, A . L. & D. H . Gr een 1980. An hydrou s melting of per idotite at 0-1 5 kbar pressure a nd the genesis of tholeiitic basalts. Contrib. M ineral. Petrol . 73, 287-310. Jaques, A . L. & G . P. Robinson 1977. The cont inent - island arc collis ion in no rthern Papua New Guinea . BMR J . Austr. Geol. Geoph ys. 2, 289-303. Jenner, G . A. 1981. Geochemistry o f high-Mg andesites from Ca pe Vogel, Papua New Guinea . Chem. Geoi. 33, 307-32. Jenner , G . A . 1983. Petro genesis o f high-Mg andesites: an experimental and geochemical study with emp hasis on high-Mg andesites from Cape Vogel, PNG . Ph.D. Th esis, Univ, of Tas mania, Hobart . Johnson , R. w., A. L. Jaque s, R. L. Hickey, C. O . ~IcK ee & B. W. Chappell 1985. Mariam Island, Papua New Guinea: petrology and geochem istr y o f a low T iO z basalt ic island-arc volca no. J . Petrol. 26, 283- 323. Kalsbeek, F. & H . F. Je psen 1984. The late Proterozoic Zig-Zag Dal basalt form at ion o f eastern Nort h Greenlan d . J . Petrol. 25, 644- 64. Kikuchi , Y. 1890. O n pyroxene compo nent s in certain volcan ic rocks fro m Bonin Island. J. Coli. Sci. Imp . Univ. Japan 3, 67-89. Komats u, M. 1980. Clinoenstatite in volca nic rock s from the Bonin Island s. Contr ib. Mineral. Petrol. 74, 329-38. Kuehner, S. M. 1989. Petrology and geochemistry of early Proterozoic high-Mg dykes fro m the Vestfold Hills, An tarctica (th is volume) . Kuroda, N. & K. Shira ki 1975. Bonin ite an d relat ed rocks of Chichi-jima, Bonin Island, Japan . Rep. Fac. Sci. Shizuoka Univ. 10, 145-55. Kuro da, N., K. Shira ki & H . Ura no 1978. Bonin ite as a possible calc-alkaline primary magma . Bull. Volcanol. 41, 563-75. Kushiro , I. 1972. Effect of water o n the com position of magmas for med a t high pressur es. J. Petrol. 13,311 - 34. Kushiro , I. 1981 . Petrology of high-M gO bronzite andesite resembling bon inite from site 458 near the Mari ana Tr ench . (nil. Rep. DSDP Leg 60, 731- 3. Kushiro , I., H . S. Yode r & M. Nishika wa 1968. Effect of water on the melting of enstatite . Geol. Soc. A m. Bull. 79, 1685-92. Kyser, T. K., W. E. Camero n & E. G. Nisbet 1986. Boninite petrogenesis and alt eration histor y: constra ints fro m sta ble isotope com positions of boninites from Cap e Vogel, New Caledonia and Cyprus. Contrib. Mineral. Petrol . 93, 222-6. LeMai tre, R. W. 1984. A pro posal by the lUGS Subcom mission on the Systematics o f Igneo us Rocks for a chemical classifica tio n o f volcanic ro cks based on the total alkali - silica diagra m. A ustr. J . Earth Sci. 31, 243- 56. ~IcC u lloch. M. T. & W. E. Ca meron 1983. Nd-Sr isotopic study o f primitive lavas from the Trood os Ophio lite, Cyp rus: evidence for a subductio n-related setti ng. Geology II , 727- 31. Meijer, A . 1976. Pb and Sr isotopic da ta bear ing on the ori gin of volcanic rocks from the Mariana island arc system . Geol. Soc. A m . Bull. 87, 1358- 69. Meijer, A. 1980. Primitive a rc vo lcanism and a boninire series: examples from western Pacific isla nd ar cs. In Tectonic and geologic evoluti on of south west A sian seas and islands. D. E. Ha yes (ed .), 269- 82. Am . Geophys. Unio n. Monogr, no. 23. Meijer, A. & B. Han an 1981. Pb isoto pic com position o f bo ninite a nd relat ed rocks from the Mariana an d Bon in for e-arc regions . Eos 62, 408. Na kamura , Y. & I. Kushiro 1974. Co mpo sition o f gas phase in MgzSi04-SiO z-H zO at 15 kba r. Carnegie lnst, Washington Yearb. 73, 255-8. Nat landv J. H . 1981. Cryst a l mo rpholo gies and pyroxene com po sition s in bo ninites a nd tholeiitic basa lts from Deep Sea Drilling Project Hole s 458 and 459 in the Ma ria na for earc region. lni t , Rep. DSDP Leg 60,68 1-709. Nelso n, D. R., A. 1. Crawfor d & ~1. T. ~lcCulloch 1984. Nd - Sr systema tics in Ca mbria n bo ninites and tho leiites fro m Victo ria , Au st ral ia. Contrib. Mineral. Petrol. 88, 169- 77.

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