ALBERT

Description: The 14 Ma caldera-forming composite ignimbrite P1 on 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 of 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.

Description: The 14.1 Ma old composite ignimbrite cooling unit P1 (45 km3) on Gran Canaria comprises a lower mixed rhyolite-trachyte tuff, a central rhyolite-basalt mixed tuff, and a slightly rhyolite-contaminated basaltic tuff at the top. The basaltic tuff is compositionally zoned with (a) an upward change in basalt composition to higher MgO content (4.3–5.2 wt.%), (b) variably admixed rhyolite or trachyte (commonly 〈5 wt.%), and (c) an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments. The basaltic tuff is divided into three structural units: (I) the welded basaltic ignimbrite, which forms the thickest part (c. 95 vol.%) and is the main subject of the present paper; (II) poorly consolidated massive, bomb- and block-rich beds interpreted as phreatomagmatic pyroclastic flow deposits; and (III) various facies of reworked basaltic tuff. Tuff unit I is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias, radial sheet-like distribution around the central Tejeda caldera, thickening in valleys but also covering higher ground, and local erosion of the underlying P1 ash. A gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding; the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature. A similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections. In proximal sections the base is dense where it was emplaced on hot felsic P1 tuff. The intensity of welding, especially at the base, and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets. The flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse. Model calculations suggest eruption through low pyroclastic fountains (〈1000 m high) with limited cooling during eruption and turbulent flow from an initial temperature of 1160°C. The large volume of 26 km3 of erupted basalt compared with only 16 km3 of the evolved P1 magmas, and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption. It is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic P1 magma chamber and its ring-fissure conduits. Evolution of the entire P1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude. The basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma, but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c. 10 km diameter roof block, which followed a decrease in magma chamber pressure during low viscosity basaltic outflow.