PETROGENESIS OF RHYOLITE-TRACHYTE-BASALT COMPOSITE IGNIMBRITE P1, GRAN-CANARIA, CANARY-ISLANDS

The 14 Ma caldera-forming composite ignimbrite P1 on Gran Canaria (Can ary Islands) represents the first voluminous eruption of highly differ entiated magmas on top of the basaltic Miocene shield volcano. Composi tional zonation of the ignimbrite is the result of vertically changing proportions of four component magmas, which were intensely mixed duri ng eruption: (1) Crystal-poor to highly phyric rhyolite (similar to 10 km(3)), (2) sodic trachyandesite through mafic to evolved trachyte (s imilar to 6 km(3), (3) Na-poor trachyandesite (<1 km(3)), and (4) basa lt zoned from 5.2 to 4.3 wt % MgO (similar to 26 km(3)). P1 basalt is composed of two compositionally zoned magma batches, B2 basalt and B3 basalt. B3 basalt is derived from a mantle source depleted in incompat ible trace elements compared to the shield basalt source. Basaltic mag mas were stored in a reservoir probably underplating the crust, in whi ch zoned B2 basaltic magma formed by mixing of ''enriched'' (shield) a nd ''depleted'' (B3) mafic melts and subsequent crystal fractionation. Evolved magmas formed in a shallow crustal chamber, whereas intermedi ate magmas formed at both levels. Abundant pyroxenitic to gabbroid cum ulates in P1 support crystal fractionation as the major differentiatio n 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 int roduction into sodic trachyandesite is the main reason for the differe nt major element evolution of the two series, whereas their different parentage is mainly reflected in the high field strength trace element s. 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 mi neral incorporation. Magma mixing between trachyte and rhyolite during their simultaneous crystallization in the P1 magma chamber is documen ted by mutual mineral inclusions but had little effect on the composit ional evolution of both magmas. Fe-Ti oxide thermometry yields magmati c temperatures of around 850 degrees C for crystal-poor through crysta l-rich rhyolite, similar to 815 degrees C for trachyte and similar to 850 degrees-900 degrees C for the trachyandesitic magmas. High 1160 de grees C for the basalt magma suggest its intrusion into the P1 magma c hamber only shortly before eruption. The lower temperature for trachyt e compared to rhyolite and the strong crustal contamination of trachyt e and sodic trachyandesite support their residence along the walls of the vertically and laterally zoned P1 magma chamber. The complex magma tic evolution of P1 reflects the transient state of Gran Canaria's man tle source composition and magma plumbing system during the change fro m basaltic to silicic volcanism. Our results for P1 characterize proce sses operating during this important transition, which also occurs on other volcanic ocean islands.