Petrogenesis of mixed-magma, high-grade, peralkaline ignimbrite ‘TL’ (Gran Canaria): diverse styles of mixing in a replenished, zoned magma chamber

Abstract

The Miocene (13.4 Ma) high-grade, peralkaline ignimbrite TL on Gran Canaria comprises two overlapping ignimbrite lobes, an eastern lobe which is high-grade, with rheomorphic lithofacies, and a western lobe which is extremely high-grade with lava-like lithofacies. The two lobes were erupted from different vents tapping the same magma chamber during a single eruption; where they overlap the western lobe overlies the eastern lobe [Sumner and Branney (2002) J. Volcanol. Geotherm. Res., 115, 109–138]. Three magma compositions are present: comendite, trachyte and benmoreite. Fiamme of intermediate composition also occur and magma mingling during withdrawal produced compositionally banded juvenile clasts. Both ignimbrite lobes consist of mixed and mingled comendite and trachyte plus small mafic globules of benmoreite in the western lobe. The ignimbrite lobes have a broad vertical compositional zonation with a basal dominantly comenditic zone, grading up into a mixed zone with subequal amounts of comendite and trachyte, which passes into an overlying trachyte-dominated zone; the magma chamber is inferred to have been zoned upwards from trachyte to comendite. Major and trace element compositional variations and phenocryst–whole rock relations among comendite and trachyte are scattered in a fashion consistent with mingling of, and exchange of phenocrysts between, liquids that lie along the fractionation path from trachyte to comendite. Intrusion of benmoreite magma into the chamber over a period of several months to years before the eruption produced mafic globules that equilibrated to varying degrees with the lower trachyte magma layer. This replenishment ultimately triggered the eruption. Most of the trachyte–comendite mingling probably occurred during withdrawal and eruption; some certainly took place after ignimbrite deposition, during rheomorphic flow. Magmatically heterogeneous high-grade ignimbrites that experience an episode of non-particulate flow show particularly intimate mixing/mingling relations.