TECTONICS, VOL. 15, NO. 1, PAGES, 67-83, FEBRUARY 1996 / / / Geochemical evolution and tectonic significance of boninites and tholeiites from the Koh ophiolite, New Caledonia S•bastien Meffre Departmentof Geologyand Geophysics, University of Sydney,New SouthWales, Australia Jonathan C. Aitchison Departmentof Earth Sciences,University of Hong Kong,Hong Kong Anthony J. Crawford Departmentof Geology,University of Tasmania,Hobart, Tasmania, Australia Abstract. The Central Chain ophiolites in New Caledonia event was driven by adiabatic decompressionof hot depleted are fragmentsof a supra-subductionzone (SSZ) ophiolite,now mantle residual from the production of the lower tholeiites, preservedfrom the upper layeredgabbros through to volcanics during initiation of rifting of young oceaniccrust intimately and overlain by pelagic cherts and a thick Middle Triassic to associatedwith propagation of a back arc basin spreading Upper Jurassicvolcaniclastic sequence. Most of the fragments centre. The occurrence of a thick blanket of calc-alkaline were formed by a single tholeiitic magmaticepisode, but one volcaniclastic sediments above the ophiolite indicates of these, the Koh ophiolite, was formed by two tholeiitic proximity to a maturearc and suggeststhat the Koh boninites magmatic episodesseparated by boninites.The first event in were not associated with the initiation of subduction. A close the Koh ophiolite formed cumulate gabbros, dolerites, modern analogy for the Koh ophiolite exists on the Hunter plagiogranites, and the first pillow lava sequence from a Ridge protoislandarc between southernmostVanuatu (New tholeiitic magma with strongdepletion in the light rare earth Hebrides island arc) and the Fijian islands; there, high-Ca elements(LREE) and abnormallylow TiO2 (0.5% at Mg#=60). boninites lacking positive Zr spikes occur together with low- Shortly after their eruption, these tholeiitic lavas were Ti tholeiites and more typical BABB tholeiites where the overlain by a high-Ca boninitic unit with a basal section of southern spreading centre of the North Fiji Basin is boninite pillows, flows, and brecciasand an upper sectionof propagatinginto the protoarc crust of the Hunter Ridge. boninitic dacites and tuffs. The last magmatic phaseinvolved eruption of evolved tholeiitic basalts, as pillows above the boninites and as dykes and sills intruding the older plutonic Introduction and volcanic sectionsof the ophiolite. This secondphase of Many ophiolites are believed to have formed as oceanic- tholeiitic magmatism is compositionally distinct from the type crust within western Pacific-type arc-back arc basin first and is closest to back arc basin basalts (BABB) erupted systemsabove subductionzones. Those with arc geochemical during the early rifting historyof modernback arc basins.The signatures have been termed supra-subductionzone (SSZ) boninitic volcanics belong to a high-Ca series with slightly ophiolites [Pearce et al., 1984] and often contain a lower SiO2, A1203, and TiO2 comparedto thosefrom modern geochemical stratigraphy which includes boninitic series island arc systems,and they lack the positiveZr spikerelative volcanics (BSV) as well as island arc tholeiites (IAT) and lavas to adjacentrare earth elements(REE) in normalisedelement transitional between IAT and mid-ocean ridge basalts variation patterns.These boniniteswere formed shortly after (MORB). Although some ophiolites may well representback the production of back arc basin crust representedby the arc basin crust, the forearc crust and upper mantle of modern depleted tholeiites and shortly before a second spreading event which caused 40-60% extension of the initial basin intraoceanic island arc systems may provide the best analoguesfor many ophiolites[Casey and Dewey, 1984; Stern crust and eruption of the upper tholeiites.The dominanceof and Bloomer, 1992; Taylor et al., 1992]. BABB-like tholeiites throughoutthe Central Chain ophiolites Recent studies of western Pacific island arc systems [e.g., in New Caledonia, the restricted occurrence of boninites, and Pearce et al., 1992; Taylor, 1992; Hawkins, 1994; Pearce et the stratigraphyand chemistry of the Koh ophiolite suggest al., 1994] have provided much new information on the that the boninites were erupted in responseto an exceptional characteristicsisland arc systems and on the way in which tectonic situation. We suggest that this boninite generation they evolve. However, there are still many problems associated with interpreting the tectonic setting of Copyright1996 by the AmericanGeophysical Union. ophiolites. One such problem centres on the petrogenetic scenario and tectonic events responsible for the common Paper number95TC02316. ophiolitic associationof refractory boninitic series volcanics 0278-7407/96/95TC-02316510.00 with relatively fertile tholeiitic basalts of broadly MORB or 67 68 MEFFRE ET AL.: GEOCHEMICAL EVOLLrHON OF THE KOH OPHIOLITE back arc basin basalt (BABB) affinity [Cameron et al., 1979; Pre-Cretaceousrocks can be assignedto one of three arc- Crawford and Keays, 1987; Cameron, 1989; Coish, 1989]. related terranes. The Central Chain terrane, the focus of this Another example of this boninite-tholeiite association paper, covers 15% of the island and outcropsin the mountains occurs in the Koh ophiolite in New Caledonia. This paper and on the east coast (Figure 1) [Gudrangdet al., 1975; Paris, gives new details of SSZ ophiolites from New Caledonia, 1981]. It includes a late Paleozoic to Late Jurassicbasin with a compares them with volcanic suites from modern western basal ophiolite in which boninitic and tholeiitic volcanicsare Pacific arc systems to constrain their tectonic environment, overlain by a thick volcaniclasticsedimentary sequence. This and discussesaspects of the sequenceand nature of tectono- terrane was probably formed in an intraoceanic island arc- magmatic events that produced this late Paleozoic or Early related systemaway from the Gondwanamargin. Triassic ophiolite. The T6remba terrane outcropson the mid-westerncoast and contains Late Permian calc-alkaline arc volcanics overlain by Geological Setting a sequenceof shallow water pyroclasticsand volcaniclastics The geology of New Caledoniarecords three major plate of Triassic to Jurassic age [Campbell, 1984]. The tectonic stages:island-arc convergent margin tectonicsfrom relationshipsof this arc-related terrane to the Central Chain the Late Carboniferousor Early Permian to latest Jurassic, terrane are uncertain;as the sequencesare fault bounded,they culminatingin a major accretionand obductionevent in the were deposited in different environmentsand differ in their earliest Cretaceous; mid-Cretaceousto early Eocene passive stratigraphy, tectonic evolution, and faunal content. The margin extensionaltectonics; and mid-Eoceneto Oligocene Boghen terrane is an undated, regionally metamorphosed convergentmargin tectonics[Cluzel et al., 1994; Aitchison et sequenceof deformed volcanics and sedimentaryrocks in the al., 1995]. central part of the island. Overlap sedimentsindicate that all 20øS I New•:•'- Caledonia Australia Cantaloupai Zealand 21oS New.0 50 16o 170 18 ITarøuimb%1I 1 (peddotiteultramaficterran( nappe) Koh ------ Pouebo terrane • (eclogite) ........'....:•(schist)Diahot terrane Koua I isedimentaryCretaceous to rocksEocene Pocquereux (basaltPoyaterrane nappe) 22oS Nassi .':.,'.•ß (volCentral ca nic Chainla stics terrane) • (ophiolites)CentralChain terrane •..."•T6rernba(arc volcanicsterrane & volcaniclastics) 0 km 50 I I •] (schist)Boghen terrane 164 øE 165 øE 166øE 167øE Figure 1. Geologicalmap of New Caledonia.Terrane names are from Cluzelet al. [1994] andAitchison et al. [ 1995]. Geologyis modifiedfrom Paris [1981]. MEFFRE ET AL.: GEOCHEMICAL EVOLUTION OF THE KOH OPHIOLITE 69 three terranes had amalgamated and accreted to the east by other magmatic episodes as they always occur Gondwana margin by the end of the Early Cretaceous stratigraphicallybelow the boninitic extrusive rocks and are [Aitchison et al., 1995]. intrudedby dykesof the secondtholeiitic magmatic episode. Shortly after the accretion and obduction of the pre- Tholeiitic lavas of this unit comprisethe lowest volcanic Cretaceousterranes, subsidence began in New Caledonia as a unit within the ophiolite(Figure 3). Approximately200 m of result of the breakup of the easternAustralian margin and the pillows and flows with minor pillow brecciasare intercalated opening of the Tasman Sea around 80 Ma [Aitchison et al., with subordinatered cherts.The pillows are mostly small, 1995]. A thick, deepening-upwardsequence developed on the spherical,and separatedby thin bands (1 cm) of green New Caledonian Platform, from Cretaceous sandstones hyaloclastite.Some pillows show radially arranged, elongated interbedded with basic and felsic volcanics to Paleocene and amygdalesoriented toward the pillow rims. The lowermost lower Eocene pelagic cherts and limestones. pillows are intruded by narrow dykes of dolerite and Passivesubsidence of the platform was disturbedin the mid- plagiogranite.The transition zone between the extrusivesand Eocene by collision with an island arc [Aitchison et al., the underlying plutonic rocks is less than 250 m thick and 1995], resulting in the emergenceof New Caledonia and the containsmany small dykes, as well as largerintrusions (up to obduction of a thin nappe of basalts and dolerites, overthrust 50 m in width) of gabbro,dolerite, and plagiogranitewith in turn by a mantle sequence,presently 1500 m thick, of complex relationships. Below these shallow intrusives,