Petrology and geochemistry of the crust–mantle boundary in a nascent arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific

Pirard, Cassian, Hermann, Jörg, and O'Neill, Hugh St.C. (2013) Petrology and geochemistry of the crust–mantle boundary in a nascent arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific. Journal of Petrology, 54 (9). pp. 1759-1792.

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Abstract

The Massif du Sud ophiolite, New Caledonia, SW Pacific, is one of the largest exposed ultramafic bodies on Earth. The ophiolite consists of a mantle section of ultra-depleted tectonite harzburgite, overlain by a large dunite zone, which is separated by a transition zone from the gabbros at the top of the massif. Profiles through the stratigraphy of the mantle section show complex geochemical and petrological variations. The harzburgites are characterized by a high degree of partial melting and document a complex evolution from mantle exhumation towards a supra-subduction zone environment. Olivine and spinel compositions suggest that the harzburgites are residual after boninite-like melt extraction at 5–10 kbar. The lower dunite zone is analogous to the replacive dunite channels in the harzburgite, in that it is formed by pyroxene consumption. In contrast, the upper dunite zone has chemical and textural characteristics of cumulus olivine crystallized from primitive mafic melts. The upper dunite zone grades into a transition zone with pyroxenite cumulates that are intruded by gabbro sills. The crystallization sequence and mineral compositions indicate that pyroxenites and gabbros formed from hydrous, oxidized primitive basaltic magmas at ∼1250°C and 2–4 kbar. Their geochemistry indicates that these parental melts are transitional between boninites and primitive arc magmas and carry a fore-arc basalt signature. The Massif du Sud therefore represents a crust–mantle section in a nascent arc. The cryptic transition between residual mantle rocks and crustal cumulates highlights the difficulty of estimating the thickness and average composition of an arc crust by seismic methods.

Item ID: 27537
Item Type: Article (Research - C1)
ISSN: 1460-2415
Keywords: New Caledonia ophiolite, arc crust, supra-subduction zone, crust–mantle boundary, dunite
Funders: Australian Research Council (ARC)
Date Deposited: 13 Jun 2013 05:42
FoR Codes: 04 EARTH SCIENCES > 0402 Geochemistry > 040202 Inorganic Geochemistry @ 40%
04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 60%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8499 Other Mineral Resources (excl. Energy Resources) > 849999 Mineral Resources (excl. Energy Resources) not elsewhere classified @ 100%
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