Major and trace element, strontium isotope, and microprobe analyses, as well as a fluid inclusion study, were systematically carried out on spinel harzburgite to dunite xenoliths included in Quaternary alkali basalt and basanite lavas from Lanzarote (Canary Islands). The whole xenolith population delineates a variably depleted sequence apparent as variations in the modal abundance of clinopyroxene and orthopyroxene, as well as in the composition of ‘primary’ spinel. ‘Secondary’ spinels and pyroxenes, related to pyrometamorphic textures, show a more refractory composition than those of the ‘primary’ constituent phases. This appears to be an effect of metasomatic fluids reacting with the primary mineral assemblage. The equilibration temperatures obtained by various geothermometers for the whole xenolith population range between 1000 and 1100°C. Two-pyroxene equilibrium and the highest CO2 inclusion trapping pressures of ∼ 5 kb indicate a minimum provenance depth for the xenoliths of 15 km. This corresponds to depths below the Moho (i.e., 11 km). In terms of modal, bulk-rock, and primary mineral compositions, the xenoliths closely resemble mantle peridotites recovered from several localities in the North Atlantic, and are compatible with residua after extraction of mid-ocean ridge basalt (MORB). Thus, the resulting refractory material would have suffered long-term subsolidus equilibration at P-T conditions of ∼4–5kb and 1000–1100 °C during lateral displacement of oceanic lithosphere. Later metasomatic processes caused by strongly alkaline H2O-CO2-enriched fluids of within-plate deep lithosphere provenance caused mineralogical change and incipient melting effects. The overlap of 87Sr/86Sr ratios between the xenoliths (0•7031–0•7036) and the lavas (0•7029–0•7039) of the Canaries Archipelago lends support to the interpretation that metasomatic processes took place in recent times (Neogene) and were linked to the hot-spot magmatic system which ultimately generated the Canary Islands volcanism.
Ridge to hot-spot evolution of the Atlantic lithospheric mantle: evidence from Lanzarote peridotite xenoliths (Canary Islands).
SIENA, Franca;BECCALUVA, Luigi;COLTORTI, Massimo;
1991
Abstract
Major and trace element, strontium isotope, and microprobe analyses, as well as a fluid inclusion study, were systematically carried out on spinel harzburgite to dunite xenoliths included in Quaternary alkali basalt and basanite lavas from Lanzarote (Canary Islands). The whole xenolith population delineates a variably depleted sequence apparent as variations in the modal abundance of clinopyroxene and orthopyroxene, as well as in the composition of ‘primary’ spinel. ‘Secondary’ spinels and pyroxenes, related to pyrometamorphic textures, show a more refractory composition than those of the ‘primary’ constituent phases. This appears to be an effect of metasomatic fluids reacting with the primary mineral assemblage. The equilibration temperatures obtained by various geothermometers for the whole xenolith population range between 1000 and 1100°C. Two-pyroxene equilibrium and the highest CO2 inclusion trapping pressures of ∼ 5 kb indicate a minimum provenance depth for the xenoliths of 15 km. This corresponds to depths below the Moho (i.e., 11 km). In terms of modal, bulk-rock, and primary mineral compositions, the xenoliths closely resemble mantle peridotites recovered from several localities in the North Atlantic, and are compatible with residua after extraction of mid-ocean ridge basalt (MORB). Thus, the resulting refractory material would have suffered long-term subsolidus equilibration at P-T conditions of ∼4–5kb and 1000–1100 °C during lateral displacement of oceanic lithosphere. Later metasomatic processes caused by strongly alkaline H2O-CO2-enriched fluids of within-plate deep lithosphere provenance caused mineralogical change and incipient melting effects. The overlap of 87Sr/86Sr ratios between the xenoliths (0•7031–0•7036) and the lavas (0•7029–0•7039) of the Canaries Archipelago lends support to the interpretation that metasomatic processes took place in recent times (Neogene) and were linked to the hot-spot magmatic system which ultimately generated the Canary Islands volcanism.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
