HR: 0800h
AN: V31C-1445    [Abstracts]
TI: Partial Melting and Contact Metamorphism of Basic Dykes in the Aureole of an Ocean Island Pyroxenitic Intrusion; Fuerteventura, Canaries
AU: * Holloway, M I
EM: MaraIsabelle.Holloway@img.unil.ch
AF: Institute of Mineralogy and Geochemistry, University of Lausanne,, BFSH2, Lausanne, CH-1015 Switzerland
AU: Bussy, F
EM: Francois.Bussy@img.unil.ch
AF: Institute of Mineralogy and Geochemistry, University of Lausanne,, BFSH2, Lausanne, CH-1015 Switzerland
AU: Hernandez, J
EM: Jean.Hernandez@img.unil.ch
AF: Institute of Mineralogy and Geochemistry, University of Lausanne,, BFSH2, Lausanne, CH-1015 Switzerland
AU: Epard, J
EM: Jean-Luc.Epard@igp.unil.ch
AF: Institute of Geology and Palaeontology, University of Lausanne, BFSH2, Lausanne, CH-1015 Switzerland
AB: The hypabyssal root zone of an ocean island volcano can be directly observed in the uplifted Basal Complex of Fuerteventura (Canary Islands). This complex records a long-lasting magmatic activity characterised by the intrusion of numerous magma batches as small plutons, dykes, dyke swarms and ring-dyke complexes of alkali-gabbros, pyroxenites, syenites and carbonatites. The high heat flow generated by the successive magma pulses feeding the overlying subaerial volcanic centres induced remarkable metasomatic and metamorphic processes, culminating with the low-pressure partial melting of basic dykes within the heterogeneous Tierra Mala gabbro-pyroxenite-syenite formation, in a contact metamorphic aureole around a layered pyroxenite-gabbro intrusion (PX1). The migmatites are characterised by a dense, zebrated network of closely spaced millimetre-wide leucocratic veins with perfectly preserved igneous textures. Only limited partial melting over a small distance (15-20m) appears to have been generated around an apophysis of PX1 (250m wide), due to a presumably limited heat input. It is thus possible to follow the evolution of this phenomenon within particular dykes with increasing distance from the intrusion. Results show that lithologies are mineralogically and chemically highly variable and behave differently in the contact aureole. Dykes of mafic, foid-microgabbros with igneous intersertal textures show increasing alteration of diopside to kaersutite with increasing proximity to the PX1 apophysis, up to 16m from the contact. Subsequently diopside crystals begin to appear smaller and recrystallised, and within the first 4m the dykes almost completely lose their small leucocratic component, leaving a framework texture dominated by sub- to euhedral diopside, Fe-Ti oxides, poikiloblastic Ti-rich biotites and minor kaersutite. This is reflected by the whole-rock SiO2, alkali, and LILE concentrations, which decrease as you approach the intrusion. Trace element contents are comparable to those of standard OIBs and there seems to be no significant change in REE composition with distance from PX1. However, dykes of diopside phyric microfoidolite that appear to have partially melted over a larger distance (up to 16m), contain trace element concentrations higher than that of common OIBs, and show a significant depletion in Si, Al, K and most incompatible trace elements, including LREE, in the anatectic samples close to the contact. As with the previous lithology, much leucocratic material was lost within the first metre of the contact, but relic diopside phenocrysts reside in abundance, and within a distance of 2m from the intrusion there is a rapid increase in kaersutite and feldspathic material (andesine and k-feldspar). Segregation of leucosomes into discrete veinlets or small pods is evident in this rock type and dykes of amphibole phyric foid-microgabbros. Previous theories of an enrichment in K, Sr, Y and REE for all migmatites relative to their protolith thus appear to have been disproved. Also, the idea that all rocks close to the PX1 intrusion are enriched in REE relative to OIBs has also been disproved by the foid- microgabbro series, which, on the contrary, are depleted in some trace-elements. Hence, we can conclude that the enrichment of some of these dykes must be due to a process unrelated, and prior to, the intrusion of PX1.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting