HR: 17:45h
AN: V22H-08    [PDF]
TI: Melt Transfer Mechanisms in the Lower Ophiolitic Crust: Examples from the Bay of Islands, Thetford-Mines, Betts Cove and Annieopsquotch
AU: * Bedard, J H
EM: jbedard@nrcan.gc.ca
AF: Geol. Survey Canada, 880 Ch. St.-Foy, Quebec, PQ G1S2L2 Canada
AU: Page, P
EM: ppage@nrcan.gc.ca
AF: INRS-ETE, 880 Ch. St.-Foy, Quebec, PQ G1S2L2 Canada
AU: Lissenberg, J
EM: johan.lissenberg@science.uottawa.ca
AF: Dept. Geol. Univ. Ottawa, 140 Louis Pasteur, Ottawa, Ont K1N6N5 Canada
AB: Field evidence from the Bay of Islands (BOI) and Annieopsquotch (AN) ophiolites (dominantly tholeiitic, boninitic subordinate) implies that `open' sill-like melt bodies were up to 30m thick, and were emplaced at all crustal levels. At BOI, melt segregation appears to have proceeded in tandem with near-pervasive high-temperature deformation, leading to dynamic recrystallization of cumulates. Comparing results of inverse trace element models to compositions of potential lavas suggests that cumulates retained low residual porosities (5-7%). Expelled Fe-H2O-rich residual magma migrated laterally and up along shear zones. At AN, 10-30m thick tabular bodies are composed either of massive granular micro-diabasic gabbro, or are zoned from coarse olivine-gabbro bases to gabbroic tops. Downward-growing feldspar dendrites in the coarser bodies, and coomb-layered apophyses issuing from their upper contacts, imply that they are sills. The granular to micro-diabasic bodies have textures similar to some AN dykes and lavas; and dykes issuing from their upper contacts also imply an intrusive origin. There is no evidence for deformation in any AN rocks, but residual melt porosities of the order of 10-15% yield inverse trace element model solutions similar to those of dykes and lavas. In contrast, at Betts Cove (BC), which is dominated by boninitic magmas, macrocyclic sequences 300+m thick that show progressive cumulus modal evolution suggest the existence of substantial open chambers. Primary size-graded beds 0.5-2m thick of harzburgite suggest deposition from episodic crystal-charged influxes of replenishing magma into an ambient melt-filled chamber. In one harzburgitic cumulate bed, cm-scale faceted orthopyroxene phenocrysts filled with olivine inclusions are aligned parallel to bedding, but abundant (20-30%) intercumulus clinopyroxene and plagioclase oikocrysts show no sign of flattening, suggesting post-compaction crystallization of a porous cumulate. Inverse trace element models from these rocks yield good matches with the dominant low-Ti boninite lavas for residual porosities of c.25%. At BC, many orthopyroxenite layers embedded within the dominant harzburgitic-lherzolitic cumulates jog up-sequence, or link up with discordant pyroxenitic dykes when traced along-strike, and may represent fossil melt-escape channels. At BC, the junction between the layered cumulates and the sheeted-dyke complex is interpreted as an extensional decollement zone which has been injected by km-scale bodies of gabbro, which can be related to slightly younger intermediate-Ti boninitic lavas. This gabbro has major, 200-m scale apophyses that penetrate the overlying sheeted dykes and underlying cumulates. Cross-cutting relationships between sheeted dykes and intrusive gabbros show that these gabbros also belong to the spreading phase. Gabbros injected into the cumulates reacted with them to form pyroxenitic reaction zones. At Thetford-Mines (TM), another boninite-related ophiolite, cumulates at the base of the crust are affected by high-temperature plastic deformation and are layered parallel to the moho. Low assumed residual melt porosities (c.10%) yield inverse trace element models that match lava compositions. The upper half of the crust is dissected into tilted (40-90 degrees) Km-scale blocks by syn-volcanic, dyke-parallel paleo-normal faults. The faults are surrounded by igneous breccias, and are injected by undeformed peridotitic and pyroxenitic intrusions, demonstrating their syn-oceanic timing and role in the transfer of melt towards the surface
DE: 1020 Composition of the crust
DE: 1065 Trace elements (3670)
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 5114 Permeability and porosity
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting