Ignimbrite sequence on Gran Canaria |
| |
| Authors: | H U Schmincke |
| |
| Institution: | 1. Lohrstuhl für Pettologie, Ruhr Universit?t, Posttach 2148, 463 Bochun, Germany
|
| |
| Abstract: | The Miocene sequence of felsic extrusive rocks of about 1000 m total thickness on Gran Canaria is divided into three units: - 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.
- 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.
- 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. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
