MULTISTAGE MAGMA ASCENT BENEATH THE CANARY-ISLANDS - EVIDENCE FROM FLUID INCLUSIONS

Gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenoc rysts in basaltic rocks from Gran Canaria, La Palms, Fl Hierro, Lanzar ote and La Gomera (Canary Islands) contain abundant CO2-dominated flui d inclusions. Inclusion densities are strikingly similar on a regional scale. Histogram maxima correspond to one or more of the following pr essures: (1) minimum 0.55 to 1.0 GPa (within the upper mantle); (2) be tween 0.2 and 0.4 GPa (the Moho or the lower crust); (3) at about 0.1 GPa (upper crust). Fluid inclusions in several rocks show a bimodal de nsity distribution, the lower-density maximum comprising both textural ly early and late inclusions. This is taken as evidence for an incompl ete resetting of inclusion densities, and simultaneous formation of yo ung inclusions, at well-defined magma stagnation levels. For Gran Cana ria, pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts in the host basanites overlap at 0. 9 to 1.0 GPa, indicating that such magma reservoir depths coincide wit h levels of xenelith entrainment into the magmas. Magma chamber pressu res within the mantle, inferred to represent levels of mantle xenolith entrainment, are 0.65-0.95 GPa for El Hierro, 0.60-0.68 GPa for La Pa lma, and 0.55-0.75 GPa for Lanzarote. The highest-density fluid inclus ions in many Canary Island mantle xenoliths have probably survived in- situ near-isobaric heating at the depth of xenolith entrainment. Inclu sion data from all islands indicate pending of basaltic magmas at Moho or lower crustal depths, and possibly at an additional higher level, strongly suggestive of two main crustal accumulation levels beneath ea ch island. We emphasize that repeated magmatic underplating of primiti ve magmas, and therefore intrusive accretion, are important growth mec hanisms for the Canary Islands, and by analogy, for other ocean island s. Comparable fluid inclusion data from primitive rocks in other tecto nic settings, including Iceland, Etna and continental rift systems (Hu ngary, South Norway), indicate that magma accumulation close to Moho d epths shortly before eruption is not, however, restricted to oceanic i ntraplate volcanoes. Lower crustal pending and crystallization prior t o eruption may be the rule rather than the exception, independent of t he tectonic setting.