Petrogenesis of rhyolite‐trachyte‐basalt composite ignimbrite P1, Gran Canada, Canary Islands

Abstract

Abstract. The 14 Ma caldera-forming composite ignimbrite PI 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: (I) Crystal-poor to highly phyric rhyolite ( -10 km 3 ), (2) sodic trachyandesite through mafic to evolved trachyte ( -6 km 3 ), (3) Nafoor trachyandesite (<I km 3 ), and (4) basalt zoned from 5.2 to 4.3 wt% MgO (-26 km ). PI 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 PI 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" PI 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 PI 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 PI magma chamber only shortly before eruption. The lower temperature for trachyte compared to rhyolite and the strong crustal contamination of trachyte and sodic trachyandcsite support their residence along the walls of the vertically and laterally zoned PI magma chamber. The complex magmatic ...

The incrementally zoned Miocene Ayagaures ignimbrite (Gran Canaria, Canary Islands)

“…are parallel and decrease rather smoothly by almost two orders of magnitudes from light (LREE) to heavy rare earth elements (HREE). The REE pattern of AY is similar to trachytic/phonolitic lava flows on Tenerife (Wolff and Storey, 1984;Ablay et al, 1995Ablay et al, , 1998Wolff et al, 2000;Bryan, 2006) contrasting with the more depleted MREE of the Mogán Group ignimbrites (Freundt and Schmincke, 1995b;Kobberger and Schmincke, 1999;Troll and Schmincke, 2002) (Fig. 9).…”

“…Schmincke, 1969;Freundt and Schmincke, 1995b;Kobberger and Schmincke, 1999;Troll and Schmincke, 2002). Freundt and Schmincke (1995b) described stratigraphic sub-units within 3 "structural" or "lithological" units in ignimbrite P1, zoned from rhyolite to basalt.…”

“…Schmincke, 1969;Freundt and Schmincke, 1995b;Kobberger and Schmincke, 1999;Troll and Schmincke, 2002). Freundt and Schmincke (1995b) described stratigraphic sub-units within 3 "structural" or "lithological" units in ignimbrite P1, zoned from rhyolite to basalt. Troll and Schmincke (2002) described 7 "flow units", based on matrix color as well as relative abundance of 3 types of rhyolitic and trachytic fiamme, but a specific fiamme and crystal composition per flow unit was not given.…”

“…All fields have consistent slope and are composed of parallel trends. Mogán Group peralkaline rhyolites (Freundt and Schmincke, 1995b;Kobberger and Schmincke, 1999;Troll and Schmincke, 2002)] and Tenerife phonolites (Wolff and Storey, 1984;Ablay et al, 1995Ablay et al, , 1998Wolff et al, 2000;Bryan, 2006)] bulk rock compositions for comparison. Matrix and alkali feldspar ratios show the three compositional groups A, B, C (see Fig.…”

Chronostratigraphy of Gran Canaria

“…P1 The Lower Mogán Formation begins with the most widespread (>400 km 2 ) cooling unit on the entire island, an ignimbrite zoned from rhyolite with admixed trachyte in the lower part to basalt at the top, representing a bulk volume of ~45 km 3 (Freundt and Schmincke, 1995). The eruption of Pl was accompanied by the onset of the collapse of the Tejeda Caldera and local faulting of the lower part of Pl, indicating the synchroneity of ash flow eruption and caldera subsidence.…”