Petrology and magmatic affinity of the North d'Entrecasteaux Ridge, Central New Hebrides Trench, Site 828

Petrography, major and trace elements, mineral chemistry, and Sr, Nd, and Pb isotopic ratios are reported for igneous rocks drilled on the northern flank of the North d'Entrecasteaux Ridge (NDR) during Ocean Drilling Program (ODP) Leg 134 Site 828. These rocks comprise a breccia unit beneath a middle Eocene foraminiferal ooze. Both geophysical characteristics and the variety of volcanic rocks found at the bottom of Holes 828 A and 828B indicate that a very immature breccia or scree deposit was sampled. Basalts are moderately to highly altered, but primary textures are well preserved. Two groups with different magmatic affinities, unrelated to the stratigraphic height, have been distinguished. One group consists of aphyric to sparsely Plagioclase + clinopyroxene-phyric basalts, characterized by high TiO2 (~2 wt%) and low A12O3 (less than 15 wt%) contents, with flat MORB-normalized incompatible element patterns and LREE-depleted chondrite-normalized REE patterns. This group resembles N-MORB. The other group comprises moderately to highly olivine + plagioclase-phyric basalts with low TiO2 ( 15 wt%) contents, and marked HFSE depletion and LFSE enrichment. Some lavas in this group are picritic, with relatively high modal olivine abundances, and MgO contents up to 15 wt%. Both the basalts and picritic basalts of this group reflect an influence by subduction-related processes, and have compositions transitional between MORB and IAT. Lavas with similar geochemical features have been reported from small back-arc basins such as the Mariana Trough, Lau Basin, Sulu Sea, and the North Fiji Basin and are referred to as back-arc basin basalts. However, regional tectonic considerations suggest that the spreading that produced these backarc basin basalts may have occurred in the forearc region of the southwest-facing island arc that existed in this region in the Eocene.

[1]  B. Taylor,et al.  Rifting and the Volcanic-Tectonic Evolution of the Izu-Bonin-Mariana Arc , 1992 .

[2]  A. Malahoff,et al.  Petrology and geochemistry of back-arc basin basalts from Lau Basin spreading ridges at 15°, 18° and 19°S , 1992 .

[3]  L. Beccaluva,et al.  19. PETROLOGY OF BASIC IGNEOUS ROCKS FROM THE FLOOR OF THE SULU SEA1 , 1991 .

[4]  A. Crawford,et al.  Basalts of the North Fiji Basin: the generation of back arc basin magmas by mixing of depleted and enriched mantle sources , 1990 .

[5]  J. Joron,et al.  MORB alteration : rare-earth element/non-rare-earth hygromagmaphile element fractionation , 1990 .

[6]  L. Beccaluva,et al.  Clinopyroxene composition of ophiolite basalts as petrogenetic indicator , 1989 .

[7]  A. Hall.  Igneous Petrogenesis: A Global Tectonic Approach , 1989, Mineralogical Magazine.

[8]  Roderick W. Brown,et al.  Improved precision of RbSr dating of kimberlitic micas: An assessment of a leaching technique , 1989 .

[9]  M. Yuasa,et al.  Volcanism in nascent back-arc basins behind the Shichito Ridge and adjacent areas in the Izu-Ogasawara arc, northwest Pacific: evidence for mixing between E-type MORB and island arc magmas at the initiation of back-arc rifting , 1989 .

[10]  B. Wilson,et al.  Igneous Petrogenesis a Global Tectonic Approach , 1989 .

[11]  W. McDonough,et al.  Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes , 1989, Geological Society, London, Special Publications.

[12]  G. Jenner,et al.  The genesis and significance of N-MORB sub-types , 1989 .

[13]  B. Pelletier,et al.  Premiers résultats des plongées de la campagne SUBPSO 1 dans la zone de collision des rides d'Entrecasteaux et de l'arc des Nouvelles-Hébrides , 1989 .

[14]  J. Macdougall,et al.  Lau Basin basalts (LBB): trace element and SrNd isotopic evidence for heterogeneity in backarc basin mantle , 1988 .

[15]  J. Collot,et al.  Superficial Structures and Stress Regimes of the Downgoing Plate Associated with Subduction-Collision in the Central New Hebrides Arc (Vanuatu) , 1988 .

[16]  J. Sinton,et al.  Mariana Trough lavas from 18°N: Implications for the origin of back arc basin basalts , 1987 .

[17]  I. Roelandts,et al.  Sequential Inductively Coupled Plasma Determination Of Some Rare‐Earth Elements in Five French Geostandards , 1986 .

[18]  J. Alt,et al.  Hydrothermal alteration of a 1 km section through the upper oceanic crust, Deep Sea Drilling Project Hole 504B: Mineralogy, chemistry and evolution of seawater‐basalt interactions , 1986 .

[19]  M. Fisher.  Tectonic processes at the collision of the D'Entrecasteaux Zone and the New Hebrides Island Arc , 1986 .

[20]  W. White,et al.  Sediment subduction and magma genesis in the Lesser Antilles: Isotopic and trace element constraints , 1986 .

[21]  J. Hawkins,et al.  Petrology of Mariana Trough and Lau Basin basalts , 1985 .

[22]  W. E. Cameron.  Petrology and origin of primitive lavas from the Troodos ophiolite, Cyprus , 1985 .

[23]  J. Collot,et al.  Recent tectonics associated with the subduction/ collision of the d'entrecasteaux Zone in the central New Hebrides , 1985 .

[24]  H. Dick,et al.  Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas , 1984 .

[25]  M. Monzier,et al.  The D'Entrecasteaux Zone (Southwest Pacific). A petrological and geochronological reappraisal , 1983 .

[26]  Shen-su Sun,et al.  Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs , 1980, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[27]  W. Seyfried,et al.  Basalt-seawater interaction: trace element and strontium isotopic variations in experimentally altered glassy basalt , 1979 .

[28]  Robert W. Nesbitt,et al.  Geochemical characteristics of mid-ocean ridge basalts , 1979 .

[29]  A. D. Saunders,et al.  Geochemistry of basalts drilled in the North Atlantic by IPOD Leg 49: Implications for mantle heterogeneity , 1979 .

[30]  M. Mottl,et al.  Chemical exchange during hydrothermal alteration of basalt by seawater—I. Experimental results for major and minor components of seawater , 1978 .

[31]  D. Stoeser.  Igneous Rocks from Leg 30 of the Deep Sea Drilling Project , 1975 .

[32]  S. Hart,et al.  Sea floor basalt alteration: Some chemical and Sr isotopic effects , 1974 .

[33]  J. A. Philpotts,et al.  Submarine basalts: some K, Rb, Sr, Ba, rare-earth, H2O, and CO2 data bearing on their alteration, modification by plagioclase, and possible source materials , 1969 .

[34]  R. C. Reynolds.  Estimation of mass absorption coefficients by compton scattering: improvements and extensions of the method , 1967 .