HR: 08:05h
AN: T21E-01 INVITED     [Abstracts]
TI: Geochemical Fingerprinting of `LIP Ophiolites' in the Geologic Record: Theory and Practice
AU: * Pearce, J A
EM: PearceJA@cf.ac.uk
AF: School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff, CF10 3YE United Kingdom
AB: Ophiolites were divided some 20 years ago into two major types: SSZ (Supra-Subduction Zone) Ophiolites, which have a subduction signature; and MOR (Mid-Ocean Ridge) Ophiolites which have no subduction signature. Since then, the term `ophiolite' has become used more generally to include fragments of oceanic lithosphere other than those formed at mid-ocean ridges (sensu stricto). In this context, the MOR Ophiolite sub-type can include the products of submarine volcanism at continental margins, as well as oceanic plateaus and oceanic island chains. Studies of the Pacific and Atlantic rims have revealed submarine and terrestrial outcrops where peridotites, plutonic rocks, dyke swarms and pillow lavas of this sub-type have all been exposed by accretion and extensional tectonics. Geochemical fingerprinting of non-SSZ ophiolites is thus important for recognising equivalent terranes in the Geologic Record. To fingerprint intra-oceanic LIPs, the initial aim must be to filter out samples with crustal signatures, gained either by subduction (SSZ ophiolites) or by crustal assimilation (continental edge ophiolites). Most simply, this can be achieved by use of Th-Nb systematics: LIPs lie within the `mantle array' on projections such as Th/Yb-Nb/Yb whereas upper crustal input leads to displacement above the array and some lower crustal input leads to displacement below the array. For samples plotting within the `mantle array', the theoretical key to fingerprinting of Phaneozoic (and probably Proterozoic) LIPs is likely to be the fact that LIPs require high mantle potential temperatures and so, regardless of their precise origin, first melt at greater depth than MORB mantle. The degree of melting will clearly vary according to the precise temperature, thickness of the lithospheric cap and extent of pre-conditioning of the mantle and so may not be such an effective discriminant; nonetheless, if the setting is known, it will provide a means of pinpointing more precisely the origin of the LIP. For Archaean LIPs, any fingerprint will need to be evaluated in the light of overall higher potential temperatures. This presentation examines the consequences of higher mantle potential temperatures for the preconditioning (by melt extraction during flow) and partial melting of heterogeneous mantle using the recently-published equations of Pearce ( J. Petrol. 46, 973-997). The two types of immobile, incompatible elements which are most sensitive to depth of melting are 1) Y and the HREE and 2) Zr and Hf, both of which partition more strongly into the garnet-bearing mantle residue that characterises melting at the solidus at higher temperatures and pressures. Melt extraction and fractional melting trajectories, plotted on commonly-used projections (such as Nb/Y v Zr/Y) and other potentially-useful projections (such as Nd/Yb v Zr/Yb), then provide a basis for developing a robust fingerprint for ancient LIPs. Testing of the resulting methodologies on accreted LIP terranes from the present Pacific margin and on proposed accreted LIP terranes of Phanerozoic to Archaean age illustrates that some LIPs at least can be successfully identified using this approach.
DE: 4875 Trace elements (0489)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8140 Ophiolites (3042)
DE: 8178 Tectonics and magmatism
DE: 8410 Geochemical modeling (1009, 3610)
SC: Tectonophysics [T]
MN: Fall Meeting 2005