THE ROLE OF PARTIAL MELTING IN THE 15-MA GEOCHEMICAL EVOLUTION OF GRAN-CANARIA - A BLOB MODEL FOR THE CANARY HOTSPOT

The subaerial portion of Gran Canaria, Canary Islands, was built by th ree cycles of volcanism: a Miocene Cycle (8.5-15 Ma), a Pliocene Cycle (1.8-6-0 Ma), and a Quaternary Cycle (1.8-0 Ma). Only the Pliocene Cy cle is completely exposed on Gran Canaria; the early stages of the Mio cene Cycle are submarine and the Quaternary Cycle is still in its init ial stages. During the Miocene, SiO2 saturation of the mafic volcanics decreased systematically from tholeiite to nephelinite. For the Plioc ene Cycle, SiO2 saturation increased and then decreased with decreasin g age from nephelinite to tholeiite to nephelinite. SiO2 saturation in creased from nephelinite to basanite and alkali basalt during the Quat ernary. In each of these cycles. increasing melt production rates, SiO 2 saturation, and concentrations of compatible elements, and decreasin g concentrations of some incompatible elements are consistent with inc reasing degrees of partial melting in the sequence melilite nephelinit e to tholeiite. The mafic volcanics from all three cycles were derived from CO2-rich garnet lherzolite sources. Phlogopite, ilmenite, sulfid e, and a phase with high partition coefficients for the light rare ear th elements (LREE), U, Th, Pb, Nb, and Zr, possibly zircon, were resid ual 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 incom patible element ratios (e.g., Nb/U, Pb/Ce, K/U, Nb/Pb, Ba/Rb, Zr/Hf, L a/Nb, Ba/Th, Rb/Nb, K/Nb, Zr/Nb, Th/Nb, Th/La, and Ba/La) exhibit extr eme 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 dur ing the Miocene Cycle and differences between major and trace element concentrations and melt production rates between the Miocene and Plioc ene tholeiites suggest that the Miocene source was more fertile than t he Pliocene-Quaternary source(s). We propose a blob model to explain t he multi-cycle evolution of Canary volcanoes and the temporal variatio ns in chemistry and melt production within cycles. Each cycle of volca nism represents decompression melting of a discrete blob of plume mate rial. Small-degree nephelinitic and basanitic melts are derived from t he cooler margins of the blobs, whereas the larger-degree tholeiitic a nd alkali basaltic melts are derived from the hotter centers of the bl obs. The symmetrical sequence of mafic volcanism for a cycle, from hig hly undersaturated to saturated to highly undersaturated compositions, reflects melting of the blob during its ascent beneath an island in t he sequence upper margin-core-lower margin. Volcanic hiatuses between cycles and within cycles represent periods when residual blob or coole r entrained shallow mantle material fill the melting zone beneath an i sland.