EVOLUTION OF PARENTAL MAGMAS OF MIOCENE SHIELD BASALTS OF GRAN-CANARIA (CANARY-ISLANDS) - CONSTRAINTS FROM CRYSTAL, MELT AND FLUID INCLUSIONS IN MINERALS

Picritic units of the Miocene shield volcanics on Gran Canaria, Canary Islands, contain olivine and clinopyroxene phenocrysts with abundant primary melt, crystal and fluid inclusions. Composition and crystalliz ation conditions of primary magmas in equilibrium with olivine Fo(90-9 2) were inferred from high-temperature microthermometric quench experi ments, low-temperature microthermometry of fluid inclusions and simula tion of the reverse path of olivine fractional crystallization based o n major element composition of melt inclusions. Primary magmas parenta l for the Miocene shield basalts range from transitional to alkaline p icrites (14.7-19.3 wt% MgO, 43.2-45.7 wt% SiO2). Crystallization of th ese primary magmas is believed to have occurred over the temperature r ange 1490-1150 degrees C at pressures approximate to 5 kbar producing olivine of Fo(80.6-90.2), high-Ti chrome spinel [Mg/(Mg + Fe2+) = 0.32 -0.56, Cr/(Cr + Al) = 0.50-0.78, 2.52-8.58 wt% TiO2], and clinopyroxen e [Mg/(Mg + Fe) = 0.79-0.88, Wo(44.1-45.3), En(43.9-48.0), Fs(6.8-11.0 )] which appeared on the liquidus together with olivine approximate to Fo(86). Redox conditions evolved from intermediate between the QFM an d WM buffers to late-stage conditions of NNO + 1 to NNO + 2, The prima ry magmas crystallized in the presence of an essentially pure CO2 flui d. The primary magmas originated at pressures > 30 kbar and temperatur es of 1500 1600 degrees C, assuming equilibrium with mantle peridotite . This implies melting of the mantle source at a depth of approximate to 100 km within the garnet stability field followed by migration of m elts into magma reservoirs located at the boundary between the upper m antle and lower crust. The temperatures and pressures of primary magma generation suggest that the Canarian plume originated in the lower ma ntle at depth approximate to 900 km that supports the plume concept of origin of the Canary Islands.