Incremental Compositional Zoning in the Widespread Phonolitic Ayagaures Ignimbrite in Miocene Fataga Group on Gran Canaria (Canary Islands)

The widespread phonolitic Ayagaures Ignimbrite (AI) (11.8 Ma) is a moderately to highly welded cooling unit of the Miocene Fataga Group (ca 13.3 - ca 9 Ma) on Gran Canaria (GC) (Canary Islands). Most of the up to 20 flow units have been mapped throughout the exposed area of at least 250km2. AI (average thickness 20- 25 m) dips regularly 4-5° in southern to southwestern GC and has reached the sea at many places. Its mapped volume is approximately 4.5 km3. This plus tentatively correlated ODP syn-ignimbrite deposits (Sumita and Schmincke, 1998 and unpubl.) and its probable former widespread distribution over at least western GC suggest a total erupted magma volume of >50km3. Phonolitic AI contains up to 20 vol % of dominantly anorthoclase-sanidine and minor (<1 vol %) biotite, Fe- augite, titanite, haüyne and apatite. The cooling unit is compositionally zoned becoming more mafic upwards. Trace elements and REE show significant magma reservoir zoning in both bulk rock and phenocryst composition, while major elements change little. All phenocryst species were unzoned and their composition within each flow unit extremely homogeneous. The shallow level magma reservoir is interpreted to have been compositionally zoned but thermodynamically equilibrated. Strong mixing preceded the separation of the magma chamber into several small convective layers in which one single growth event of alkali feldspar and biotite occurred. The presence of large haüyne and titanite crystals within the topmost layers of the reservoir (basal flow unit) and a locally preserved highly evolved fallout tephra document a highly fractionated volatile-rich but small-volume cupola. AI represents the most evolved part of a larger partially evacuated magma reservoir. Progressive downward tapping of the reservoir was controlled by incremental caldera collapse. Unzoned phenocrysts, incremental CU zoning and evacuation reversals show that mixing did not occur after separation into layers.