A geochemical characterization of the lower part of the Miocene shield-building lavas on Gran Canaria

Sammanfattning: Ocean islands offer a great potential to provide information about mantle processes and the composition of the mantle. Gran Canaria is located in the centre of the Canary Island Archipelago and is well suited for such studies. A comprehensive study of the lower Miocene shield-building lavas is performed for this thesis. The samples come from Barranco de Tasártico. The study includes analysis of bulk rock compositions as well as analyses of the phenocryst and opaque phases. The shield-building lavas range from picro-basalt to basalt and basanite (SiO2=44.5-49.7 wt%; Na2O+K2O=2.3-4.6 wt%). MgO contents vary significantly (5.1-16.0 wt%). The rather high TiO2 content (2.8-4.3 wt%) is normal for ocean island basalt. The studied lavas are generally enriched in LILE and HFSE (e.g. Sr=358-664 ppm, Ba=ll4-251, Zr=199-377 ppm, Nb=29-57 ppm, Ta=l.8-3.5 ppm). Transition metals like Ni (19-514 ppm) and Cr (24-1059 ppm) vary significantly and are mainly controlled by clinopyroxene and olivine fractionation. The sampled rocks have a high abundance of clinopyroxene, olivine and plagioclase phenocrysts. Clinopyroxene phenocrysts range between augitic and diopsidic compositions. End-member calculations suggest Tschermak substitution and aegirine in all analysed crystals. No jadeite component could be identified. The composition of olivine ranges between Fo79 and Fo88. Alteration products are iddingsite (Fe=27.7-48.1 wt%, Mg=l.9-9.7 wt%) and serpentine (Mg=5.0-19.7 wt%, Fe=l.l-24.4 wt%). Labradorite (An50 to An70) is the common plagioclases in the studied thin sections. Bivariate plots for the major elements versus MgO indicate two different fractionation trends. At MgO contents >8 wt%, clinopyroxene and olivine control the variation trends (clinopyroxene:olivine ratio 65:35). After the onset of plagioclase fractionation at 8 wt% MgO, the trends are controlled by clinopyroxene, olivine and plagioclase fractionation (clinopyroxene:olivine:plagioclase ratio 66:22:12). The strong enrichment in Nb and Ta ([Nb/La]n=l.l-1.4; [Ta/La]n=l.2-1.5) as well as depletion in K and Pb ([K/La]n=0.6-0.9; [Pb/La]n=O.l-0.5) are characteristic for mafic HIMU-type magmas. The ratios La/Nb (0.72-0.88) versus Ba/Nb (3.4-5.3) suggest that the shield-building lavas from Gran Canaria could be generated by mixing of three mantle components: HIMU, DMM and EM. A relatively moderate enrichment in light over heavy REEs ([La/Lu]n=8.9-12.1) suggests the exclusion of garnet as a residual phase in the source. Since no negative Eu anomalies are disclosed, the effect of plagioclase fractionation is small. Obtained Zr/Y ratios (9. 7 -11.8) argue against peridotite as the only source for the lavas. High Zr/Y ratios are most easily explained by significant garnet among the residual phases, which is inconsistent with the REE results. Garnet peridotite or eclogite as source rocks would result in garnet among the residual phases. The high Zr/Y ratios are thus ambiguous. One possible explanation is fractionation of Zr and Y at the time of, or prior to, formation of the source material. The impact of clinopyroxene on the Zr/Y ratio could not be determined. This ratio increases slowly with fractionation. There are no data allowing the most primitive ratio to be determined. This together with low Ba/Nb (3.4-5.3) and high K/La (217-292) ratios suggests that the derivation from a single source, containing varying proportions of garnet is not possible. The general depletion in K, Rb and Ba displayed in the normalized multi-element diagram suggests a derivation from recycled oceanic crust. Since earlier published εNd values (3.9-5.8) are higher than expected for undepleted OIB, a significant impact of depleted lithospheric crust on the source material composition is proposed. Thus, the results imply the derivation from a heterogeneous source. This source probably resembles recycled subducted eclogitic oceanic crust (<2 Ga) which has been stirred into or reacted with ambient peridotitic mantle. Primary magmas formed within the garnet stability field. Previous investigators have suggested that they are picritic and have formed at temperatures between 1500 and 1600°C at 3 GPa. After migration into magma reservoirs, fractional crystallization of clinopyroxene, olivine and small amounts of plagioclase affected the magma composition. The crystallization of the phenocrysts phases occurred at high temperatures and low pressures. Calculated temperatures associated with large errors indicate that olivine phenocrysts crystallized at temperatures between 1172 and 1218°C. The composition of clinopyroxene phenocrysts indicates that the differentiation occurred at low pressures since no jadeite component could be identified. Appearing opaque phases are ilmenite and chromium-spinel. They show a distinct distribution within the studied stratigraphy with ilmenite being the dominant opaque phase in the lower parts and chromium-spinel dominating the upper sampled parts of the stratigraphy. End-member calculations for the spinel show the following spinel components: magnetite, ulvöspinel, chromite, magnesium-aluminium spinel and magnesioferrite.