HR: 13:55h
AN: V22F-02 INVITED     [PDF]
TI: A Polygenetic Origin for some Oceanic Lithosphere: Evidence from Forearc, Continental Margin and Ophiolite Mantle Sequences
AU: * Pearce, J A
EM: PearceJA@cardiff.ac.uk
AF: School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff, CF10 3YE United Kingdom
AU: Parkinson, I J
EM: i.j.parkinson@open.ac.uk
AF: Dept. of Earth Sciences, The Open University, Milton Keynes, MK7 6AA United Kingdom
AB: It is a common assumption that ophiolites and oceanic lithosphere attain their structures and compositions through partial melting of mantle in a single tectonic setting and with a simple petrogenetic relationship between all the units. There is, however, growing evidence that some oceanic lithosphere and ophiolite complexes contain a record of a polygenetic history of formation. This may be apparent in crustal units (complex lava stratigraphies or cross-cutting dykes and gabbros) but the best evidence is recorded in the chrome spinel compositions of residual mantle. Among the most effective plots is that of oxygen fugacity, calculated from accurately-determined ferric iron concentrations, against Cr-number. In the ocean basins, forearc peridotites from the Izu-Bonin Mariana, Tonga and South Sandwich systems may be of two types. In the first, both peridotites and dunites have similar oxygen fugacities and a small range in Cr-number. We interpret these as mongenetic. In the second, the peridotites have low oxygen fugacities and moderate Cr-number and trend towards dunites with high oxygen fugacities and high Cr-number. We interpret these as representing mid-ocean ridge mantle lithosphere, which existed prior to a subduction event and was subsequently invaded by subduction-related melts. The time-gap between the ridge and subduction events may be millions of years or, in the case of subduction initiation, represent a continuum. At passive continental margins, such as the Galicia margin, the origin may again be monogenetic or polygenetic. In the latter case, the mantle peridotites may exhibit a trend from low Cr-number to moderate Cr-number and decreasing oxygen fugacity. We interpret these as representing orogenic peridotite uplifted during an amagmatic extensional event and invaded by MORB magma during subsequent spreading. As with forearc peridotites, the time gap between these two events may be large or there be a continuum. A surprising number of ophiolites exhibit this polygenetic character, especially those which may be linked to subduction initiation (such as the northern Semail ophiolite, Pindos, Zambales) or to ocean opening (e.g. Western Mediterranean ophiolites, Othris, Lizard). And even in essentially monogenetic ophiolites, such as the Troodos Massif, there are subtle variations that may be related to ridge jumps or other local processes. These observations raise questions over the extent to which oceanic lithosphere really is the product of 100% extension or whether it may sometimes contain relics of a more complex history.
DE: 1025 Composition of the mantle
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 8150 Plate boundary--general (3040)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting