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I TECTONOPHYSICS

INTERNATIONAL JOURNAL OF GEOTECTONICS AND THE

GEOLOGY AND PHYSICS

OF THE INTERIOR OF THE EARTH

Tectonophysics 284 (1998) 203-219

Geochemistry and tectonic significance of basalts in the Poya Terrane,

New Caledonia

Jean-Philippe Eissen a,*, Anthony J. Crawford b, Joseph Cotten ', Sébastien Meffre b,

Hervé Bellon ', Mireille Delaune

ORSTOM UWGP 22, Centre ORSTOM de Brest, B.P. 70, 179280 Plouzané Cedex, France

Department

of Geology, Universiy of Tasmania, GPO Box 2517C, Hobart, Tas 7001, Australia

' Université de Bretagne Occidentale, Sciences de la Terre, UMR 6538, 6 avenue Le Gorgeu, BP 809, 29285 Brest Ceda, Fraizce

ORSTOM

UWGP 22, Centre ORSTOM de Bondy, 32, averzue Heiiri Varagizut, 93143 Bondy Cedex, France

Received 30 October 1996; accepted 16 June 1997

TECTONOPHY SICS

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ELSEVIER Tectonophysics 284 (1998) 203-219

TECTONOPHYSICS

Geochemistry and tectonic significance of basalts in the Poya Terrane,

New Caledonia

Jean-Philippe Eissen a,*, Anthony J. Crawford Joseph Cotten Sébastien Meffre

Hervé Bellon Mireille Delaune

a ORSTOM UUGP 22, Centre ORSTOM de Brest, B.P. 70, 29280 Plouzane' Cedex, France

Department

of Geology, University of Tasmania, GPO Box 252C, Hobart, Tas 7001, Australia Universite' de Bretagne Occidentale, Sciences de la Terre, UMR 6538,

6 avenue Le Gorgeu, BP 809, 29285 Brest Cedex, France

ORSTOM UUGP 22, Centre ORSTOM de Bondy, 32, avenue Henri Varagnat, 93143 Bondy Cedex, France

Received 30 October 1996; accepted 16 June 1997

Abstract

The Norfolk-New Caledonia Ridge represents a continental slice which drifted away from Australia during the Late

Cretaceous breakup of the eastern Gondwana margin. The presence of widespread basaltic rocks beneath the main

ophiolite nappe of New Caledonia has been long known but the origin and the age of the Poya Terrane basalts (PTB

herein) remained controversial. Recent palaeontologically determined ages date the PTB as Late Cretaceous (Campanian). New geochemical data show that two main discrete groups constitute the PTB: a MORB-like tholeiitic suite, and a more alkaline intra-plate basaltic suite distinguished mainly on immobile HFSE and REE elements. Furthermore, low ENd and

high Th/Nb relative to MORB, and weak negative Nb anomalies, reflect limited assimilation of continental crust by these

otherwise MORB-like tholeiites. Inter-PTB sedimentary rocks all have a pelagic or hemi-pelagic origin; detrital material originated from the nearby Norfolk-New Caledonia ridge basement. The PTB form a parautochthonous sheet below the

main harzburgitic nappe constituting the New Caledonian ophiolite. They are genetically unrelated to the ophiolite, and are

interpreted to be 70-85-Ma-old rift tholeiites formed during of the eastemmost continental part of Mesozoic Gondwana,

and opening the East New Caledonia Basin. The Norfolk-New Caledonia Ridge formed the western passive margin of

this new oceanic basin, but the rifted-off eastem block is less easily identified. It may form part of the basement of the

Westem Belt of the New Hebrides island arc (Vanuatu). The cessation of rifting of the eastern Australian margin around

56 Ma was followed by an eastward-directed subduction which produced boninitic melts and its associated refractory

harzburgitic mantle, in the forearc of the primitive Loyalty-d'Entrecasteaux arc. Following the major Pacific plate motion

reorganization around 42 Ma, collision of the Norfolk-New Caledonia Ridge with the forearc region of the intra-oceanic

Loyalty-d'Entrecasteaux arc around 40 Ma led first to westward thrusting of the PTB as a slice picked up from the

upper crustal section of the colliding Norfolk Ridge. Subsequent collisional tectonism led to detachment of the main

New Caledonian harzburgitic nappe

from its forearc location in the Loyalty arc, and westward emplacement of this nappe over the PTB nearby allochthon. The presence of parautochthonous sheets of basalts unrelated to immediately

overlying forearc-derived, boninite-bearing harzburgitic ophiolites is briefly discussed in the light of two other examples in

arc-continent collision settings.

O 1998 Elsevier Science B.V. All rights reserved.

Keywords: SW Pacific; New Caledonia; Cretaceous; Eocene; allochthonous tenane; geodynamic significance; MORB;

backarc basalts; intra-plate basalts; boninites; ophiolites; geochemistry * Corresponding author. E-mail: eissen@orstom.fr

0040-1951/98/$19.00

O 1998 Elsevier Science B.V. All rights reserved.

PII S 0040- 195 1 (97)OO 1 83 - 2

204

J.-l? Eissen et al. /Tectonophj

1. Introduction

New Caledonia is a microcontinental island which

drifted away from Australia during the Late Creta- ceous breakup of the eastern Gondwana margin and opening of the Tasman Sea and New Caledonia Basin (Paris, 1981; Kroenke, 1984; Mignot, 1984; Rigolot,

1989; Cluzel et al., 1994). It

is constituted by rocks which record at least two arc-continent collisions, isics 284 (1998) 203-219 the first in the early Mesozoic, the more recent in the mid-Eocene (Cluzel et al., 1994; Aitchison et al., 1995a; Meffre et al., 1996). The latter collision resulted in SSW-directed emplacement of a mas- sive harzburgite-dominated nappe probably at least

6 km thick (Fig. 1) (Avias, 1967; Guillon, 1975;

Prinzhoffer

et al., 1980). Although this sheet appears to be broadly continuous with crust of the adjacent

South Loyalty Basin to the east of New Caledonia

0 majortowns

o sampling sites Y

Northern Metamorphic Complex

Nouméa-Bourail Unit (Permian

- Jurassic sediments)

0 Pre-Permian Unit (undated metamorphosed volcanosedimentary pile)

I)

0 Central Chain Unit (disrupted ophiolitic suite and its cover)

1

Fig. 1. (a) Location of New Caledonia in the SW Pacific. DEZ = d'Etrecasteaux Zone; SLB = South Loyalty Basin. (b) Simplified

geological map of New Caledonia showing the location of the Poya Terrane basalts along the western coast between Bourail and

Koumac.

Ne = Népoui. Sample sites: G = Gomen; O =Ouaco; Ta = Taom; Bo = Boyen; V = Voh; Te = Temala; Pi = Pinjen peninsula;

K = Kone; P = Poya (including samples from a 150-m-deep drilling done by the BRGM on the Honfleur sulphide deposit).

J.-l? Eisseii et al. /Tectoiiopliysics 284 (1998) 203-219 205 (Collot et al., 1987), there are no known occur- rences of volcanics associated with the nappe on

New Caledonia, and magmatic rocks are restricted

to limited occurrences of cumulates and occasional dolerite dykes (Prinzhoffer et al., 1980; Dupuy et al.,

198lj.

The presence of widespread basaltic rocks be-

neath the New Caledonia ophiolite nappe has been long known and well documented (Routhier, 1953; Lillie and Brothers, 1970; Guillon, 1972; Guillon and Gonord, 1972; Rodgers, 1975; Parrot and Dugas,

1980; Paris, 1981; Goenke, 1984; Maurizot et al.,

1985; Black, 1993; Cluzel et al., 1994; Aitchison

et al., 1995a,b). Marine geophysical data indicate that this basaltic formation, as well as the main harzburgite nappe, extend 300 km northwestward in the basement of the northern lagoon of New Caledonia (Collot et al., 1988). Guillon (1972) first recognized that these basaltic rocks form an ex- tensive nappe, outcropping over much of the west coast of New Caledonia, where these rocks have been called the 'Formation des Basaltes de la Côte Ouest' (West Coast Basalts Formation). More recent studies (Cluzel et al., 1994; Meffre, 1995; Aitchison et al., 1995aj have confirmed that the West Coast

Basalt Formation represents the westem part of a

20-500-m-thick nappe, which extends over much of

the island as a slice beneath the main harzburgite nappe. Basalts of similar petro-geochemical charac- teristics outcrop also locally along the east coast of New Caledonia as a thin slice beneath the harzbur- gites (Cluzel et al., 1994, 1995; Meffre, 1995). This basaltic nappe has been termed the Poya Terrane (Cluzel et al., 1994; Aitchison et al., 1995a). On- going controversy surrounds the affinities, origin and tectonic significance of the Poya Terrane basalts (PTB herein).

Here, we provide new petrological, geochemical

and sedimentological data which address this prob- lem. We note that the implications of these data have more than local significance, as extensive sheets of metabasaltic rocks, almost certainly parts of large nappes, occur beneath several other large harzbur- gite-dominated, boninite-bearing ophiolite sheets in the southwest Pacific region, for example the Emo-

Kokoda metamorphics beneath the Papuan ophio-

lite (Davies and Jaques, 1984; Worthing and Craw- ford, 1996), and the Crimson Creek Formation and correlative basalts beneath the Cambrian boninite- harzburgite ophiolites in Tasmania at the base of the Lachlan Foldbelt in eastern Australia (Crawford and

Berry, 1992).

2. Poya Terrane

The Poya Terrane was emplaced as a relatively

thin thrust sheet along with but beneath the main harzburgite sheet of the New Caledonia ophiolite, between 38 and 46 Ma (Cluzel et al., 1994, 1995; Aitchison et al., 1995a; Meffre, 1995). The direction and sense of its emplacement was determined by careful examination of the geological contacts, a kinematic analysis of the normal faults, the polarity of the metamorphism which affects the PTB, which increases northward along the east coast (Cluzel et al., 1994, 1995; Meffre, 1995) and its strong link with the emplacement of the main harzburgite sheet from the north-northeast (Guillon, 1975; Prinzhoffer et al., 1980; Collot et al., 1987).

Poya Terrane sequences are dominated by

low-grade metamorphosed, often tectonized, pil- lowed basalts, with associated hyaloclastites, fine- grained tuffaceous sediment and calcareous sedi- ments known as the 'Koné facies' (Carroué, 1972), radiolarian cherts and more locally massive basalts, gabbros and dolerites. Occasional sheeted basaltic flows and serpentinites have also been observed.

The age of the Poya Terrane has been a contro-

versial issue, with arguments being presented in the literature for Eocene-Paleocene and/or Cretaceous ages (Routhier, 1953; Espirat, 1963; Coudray and Gonord, 1967; Carroué, 1972; Guillon, 1972; Cluzel et al., 1994; Aitchson et al., 1995a; Meffre, 1995).

Palaeontological ages range between

88 and 45 Ma

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