The Role of Partial Melting in the 15-Ma Geochemical Evolution of Gran Canaria: A Blob Model for the Canary Hotspot |
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| Authors: | HOERNLE KAJ; SCHMINCKE HANS-ULRICH |
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| Institution: | 1Earth Sciences, University of California Santa Cruz, California 95064
2GEOMAR, Abteilung Vulkanologie, Wischhofstr 1–3, D-2300 Kiel-14, Germany |
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| Abstract: | The subaerial portion of Gran Canaria, Canary Islands, was builtby three cycles of volcanism: a Miocene Cycle (8•5–15Ma), a Pliocene Cycle (1•8–6•0 Ma), and a QuaternaryCycle (1•8–0 Ma). Only the Pliocene Cycle is completelyexposed on Gran Canaria; the early stages of the Miocene Cycleare submarine and the Quaternary Cycle is still in its initialstages. During the Miocene, SiO2 saturation of the mafic volcanicsdecreased systematically from tholeiite to nephelinite. Forthe Pliocene Cycle, SiO2 saturation increased and then decreasedwith decreasing age from nephelinite to tholeiite to nephelinite.SiO2 saturation increased from nephelinite to basanite and alkalibasalt during the Quaternary. In each of these cycles, increasingmelt production rates, SiO2 saturation, and concentrations ofcompatible elements, and decreasing concentrations of some incompatibleelements are consistent with increasing degrees of partial meltingin the sequence melilite nephelinite to tholeiite. The maficvolcanics from all three cycles were derived from CO2-rich garnetlherzolite sources. Phlogopite, ilmenite, sulfide, and a phasewith high partition coefficients for the light rare earth elements(LREE), U, Th, Pb, Nb, and Zr, possibly zircon, were residualduring melting to form the Miocene nephelinites through tholeiites;phlogopite, ilmenite, and sulfide were residual in the sourceof the Pliocene–Quaternary nephelinites through alkalibasalts. Highly incompatible element ratios (e.g., Nb/U, Pb/Ce,K/U, Nb/Pb, Ba/Rb, Zr/Hf, La/Nb, Ba/Th, Rb/Nb, K/Nb, Zr/Nb,Th/Nb, Th/La, and Ba/La) exhibit extreme variations (in manycases larger than those reported for all other ocean islandbasalts), but these ratios correlate well with degree of melting.Survival of residual phases at higher degrees of melting duringthe Miocene Cycle and differences between major and trace elementconcentrations and melt production rates between the Mioceneand Pliocene tholeiites suggest that the Miocene source wasmore fertile than the Pliocene–Quaternary source(s). We propose a blob model to explain the multi-cycle evolutionof Canary volcanoes and the temporal variations in chemistryand melt production within cycles. Each cycle of volcanism representsdecompression melting of a discrete blob of plume material.Small-degree nephelinitic and basanitic melts are derived fromthe cooler margins of the blobs, whereas the larger-degree tholeiiticand alkali basaltic melts are derived from the hotter centersof the blobs. The symmetrical sequence of mafic volcanism fora cycle, from highly undersaturated to saturated to highly undersaturatedcompositions, reflects melting of the blob during its ascentbeneath an island in the sequence upper margin-corelower margin.Volcanic hiatuses between cycles and within cycles representperiods when residual blob or cooler entrained shallow mantlematerial fill the melting zone beneath an island. |
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