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

摘要

The 20-25 m thick trachyphonolitic Ayagaures ignimbrite cooling unit [(AY); 11.8 Ma] exposed over 250 km~2 (onshore volume ca. 4.5 km~3 DRE) is the uppermost and most voluminous cooling unit of the Middle Fataga Formation (MFF), part of the Fataga Group (ca. 13.3-ca. 9 Ma) on Gran Canaria (GC), Canary Islands (28°00' N, 15°35' W). Up to 19 flow units (named b-t) subdividing the AY have been identified throughout most of the area from proximally to the caldera wall to distally as far as 14 km away. Individual flow units were distinguished from each other and logged using mainly chemical criteria. Single and/or packages of flow units (A, B and C) are tentatively interpreted to correspond to compositionally distinct magma bodies erupted from the same magma reservoir. These source-controlled flow units are interpreted to reflect successive eruptive pulses during incremental subsidence of Tejeda caldera. We subdivided AY cooling unit into four welding facies. Tentative correlation with a major syn-ignimbrite turbidite drilled during ODP Leg 157 suggests a total DRE volume of >50 km~3.rnThe cooling unit as a whole becomes less evolved upwards as shown by major elements, trace elements and REE of bulk rock and phenocrysts. All phenocryst phases, dominantly sanidine-anorthodase (up to 20 vol.%), with minor biotite, augite, titanite, haiiyne and apatite, are unzoned and show an incremental compositional zoning in the stratigraphy. The shallow level parent magma reservoir is interpreted to have undergone strong mixing prior to starting its final compositional zoning in a thermodynamically equilibrated reservoir. Compositional zoning resulted in three main bodies. This compositional and physical layering may have been triggered by rapid growth of alkali feldspar and biotite throughout the erupted part of the magma chamber. Abundant titanite and haiiyne phenocrysts in basal flow units and in a locally preserved, highly evolved fallout tephra are interpreted to reflect initial evacuation of a small volume, highly fractionated cupola. AY represents the most evolved part of a large, partially evacuated magma reservoir. Progressive downward tapping of the reservoir is interpreted to have been controlled by incremental caldera collapse. Absence of less evolved magmas suggests that the magma chamber was only partially evacuated. Incremental compositional zoning of the cooling unit, but unzoned phenocrysts and evacuation reversals show that mixing did not occur following initiation of alkali feldspar growth.