The Role of Partial Melting in the 15-Ma Geochemical Evolution of Gran Canaria: A Blob Model for the Canary Hotspot

Abstract

The subaerial portion of Gran Canada, Canary Islands, was built by three cycles of volcanism: a Miocene Cycle (8-5—15 Ma), a Pliocene Cycle (1-8-60 Ma), and a Quaternary Cycle (1-8-0 Ma). Only the Pliocene Cycle is completely exposed on Gran Canaria; the early stages of the Miocene Cycle are submarine and the Quaternary Cycle is still in its initial stages. During the Miocene, SiO2 saturation of the mafic volcanics decreased systematically from tholeiite to nephelinite. For the Pliocene Cycle, SiO2 saturation increased and then decreased with decreasing age from nephelinite to tholeiite to nephelinite. SiO2 saturation increased from nephelinite to basanite and alkali basalt during the Quaternary. In each of these cycles, increasing melt production rates, SiO2 saturation, and concentrations of compatible elements, and decreasing concentrations of some incompatible elements are consistent with increasing degrees of partial melting in the sequence melilite nephelinite to tholeiite. The mafic volcanics from all three cycles were derived from CO2-rich garnet lherzolite sources. Phlogopite, ilmenite, sulfide, and a phase with high partition coefficients for the light rare earth elements (LREE), U, Th, Pb, Nb, and Zr, possibly zircon, were residual during melting to form the Miocene nephelinites through tholeiites; phlogopite, ilmenite, and sulfide were residual in the source of the Pliocene-Quaternary nephelinites through alkali basalts. 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 many cases larger than those reported for all other ocean island basalts), but these ratios correlate well with degree of melting. Survival of residual phases at higher degrees of melting during the Miocene Cycle and differences between major and trace element concentrations and melt production rates between the Miocene and Pliocene tholeiites suggest that the Miocene source was more fertile than the Pliocene-Quaternary source(s). We propose a blob model to explain the multi-cycle evolution of Canary volcanoes and the temporal variations in chemistry and melt production within cycles. Each cycle of volcanism represents decompression melting of a discrete blob of plume material. Small-degree nephelinitic and basanitic melts are derived from the cooler margins of the blobs, whereas the larger-degree tholeiitic and alkali basaltic melts are derived from the hotter centers of the blobs. The symmetrical sequence of mafic volcanism for a cycle, from highly undersaturated to saturated to highly undersaturated compositions, reflects melting of the blob during its ascent beneath an island in the sequence upper margin-corelower margin. Volcanic hiatuses between cycles and within cycles represent periods when residual blob or cooler entrained shallow mantle material fill the melting zone beneath an island.