The 14 Ma caldera-forming composite ignimbrite P1 and Gran Canaria (Canary Islands) represents the first voluminous eruption of highly differentiated magmas on top of the basaltic Miocene shield volcano. Compositional zonation of the ignimbrite is the result of vertically changing proportions of four component magmas, which were intensely mixed during eruption: (1) Crystal-poor to highly phyric rhyolite (~10 km3), (2) sodic trachyandesite through mafic to evolved trachyte (~6 km3), (3) Na-poor trachyandesite (<1 km3), and (4) basalt zoned from 5.2 to 4.3 wt % MgO (~26 km3). P1 basalt is composed two compositionally zoned magma batches, B2 basalt and B3 basalt. B3 basalt is derived from a mantle source depleted in incompatible trace elements compared to the shield basalt source. Basaltic magmas were stored in a reservoir probably underplating the crust, in which zoned B2 basaltic magma formed by mixing of ''enriched'' (shield) and ''depleted'' (B3) mafic melts and subsequent crystal fractionation. Evolved magmas formed in a shallow crustal chamber, whereas intermediate magmas formed at both levels.

Abundant pyroxenitic to gabbroid cumulates in P1 support crystal fractionation as the major differentiation process. On the basis of major and trace element modeling, we infer two contemporaneous fractional crystallization series: series I from ''enriched'' shield basalt through Na-poor trachyandesite to rhyolite, and series II from ''depleted'' P1 basalt through sodic trachyandesite to trachyte. Series II rocks were significantly modified by selective contamination involving feldspar (Na, K, Ba, Eu, Sr), zircon (Zr), and apatite (P, Y, rare earth elements) components; apatite contamination also affected series I Na-poor trachyandesite. Substantial sodium introduction into sodic trachyandesite is the main reason for the different major element evolution of the two series, whereas their different parentage is mainly reflected in the high field strength trace elements. Selective element contamination involved not only rapidly but also slowly diffusing elements as well as different saturation conditions. Contamination processes thus variably involved differential diffusion, partial dissolution of minerals, partial melt migration, and trace mineral incorporation.

Magma mixing between trachyte and rhyolite during their simultaneous crystallization in the P1 magma chamber is documented by mutual mineral inclusions but had little effect on the compositional evolution of both magmas. Fe-Ti oxide thermometry yields magmatic temperatures of around 850 ¿C for crystal-poor through crystal-rich rhyolite, ~815 ¿C for trachyte and ~850¿--900 ¿C for the trachyandesitic magmas. High 1160 ¿C for the basalt magma suggest its intrusion into the P1 magma chamber only shortly before eruption. The lower temperature for trachyte compared to rhyolite and the strong crustal contamination of trachyte and sodic trachyandesite support their residence along the walls of the vertically and laterally zoned P1 magma chamber. The complex magmatic evolution of P1 reflects the transient state of Gran Canaria's mantle source composition and magma plumbing system during the change from basaltic to silicic volcanism. Our results for P1 characterize processes operating during this important transition, which also occurs on other volcanic ocean islands. ¿ American Geophysical Union 1995