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Ignimbrite Sequence On Gran Canaria
Published 1969 · Geology
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AbstractThe Miocene sequence of felsic extrusive rocks of about 1000 m total thickness on Gran Canaria is divided into three units:a)A lower unit of trachytic rhyolites (lavas, composite flows, ignimbrites) characterized by a phenocryst assemblage of anorthoclase (Or15–20, wt%), clinopyroxene, hypersthene (amphibole substituted for both in ignimbrites), and Fe/Ti-oxides. The commonest groundmass minerals are anorthoclase and alkali-amphibole, with minor quartz and aegirine.b)A middle unit of comenditic and pantelleritic ignimbrites characterized by anorthoclase (Or20–32) and amphibole. Phenocryst minerals restricted to individual flows are Fe/Ti-oxides (several comendites), clinopyroxene, biotite, and sphene. The commonest groundmass minerals are anorthoclase and Tiaegirine, with lesser katophorite, arfvedsonite and quartz.c)An upper unit of trachvtic and phonolitic ignimbites and lava flows (normative ne rarety exceeding 10%) with nepheline phonolite lava flows becoming increasingly abundant upwards. The ignimbrites have mostly anorthoclase (Or30-04), and biotite, with rarer Fe/Ti-oxides, hornblende, and clinopyroxene. The commonest groundmass minerals are anorthoclase, aegirine, and alkali-amphiboles, and in some flows nepheline. The change from Na-rich to K-rich anorthoclase upwards in the sequence supports the conclusion, based on over 50 new stratigraphically controlled chemical analyses that the Na2O/K2O-ratio decreases within the sequence. possibly as a result of crystal iractionation processes and this effect is independent of probable loss of Na on post-eruptive crystallization.While hydroxyl-bearing phenocryst minerals are absent from all rocks called lava in the field, they are ubiquitous in the ignimbrites, indicating the importance of Pu2o in the generation of suspension-type cruptions.Compositional gradients must have been particularly pronounced in the small magma chambers that existed beneath Gran Canaria, resulting in a wide range of compositionally zoned or mixed deposits.