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Evolutionary stages of crystallization of weakly peralkaline syenites: evidence from ejecta in the plinian deposits of Agua de Pau volcano (São Miguel, Azores Islands)

Published online by Cambridge University Press:  05 July 2018

F. Ridolfi ,
A. Renzulli ,
P. Santi  and
B. G. J. Upton
F. Ridolfi
Affiliation:
Istituto di Vulcanologia e Geochimica, Università di Urbino, I-61029 Urbino, Italy
A. Renzulli*
Affiliation:
Istituto di Vulcanologia e Geochimica, Università di Urbino, I-61029 Urbino, Italy
P. Santi
Affiliation:
Istituto di Vulcanologia e Geochimica, Università di Urbino, I-61029 Urbino, Italy
B. G. J. Upton
Affiliation:
Department of Geology and Geophysics, University of Edinburgh, EH9 3JW Edinburgh, UK
*
* E-mail: renzulli@geo.uniurb.it
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Abstract

Weakly peralkaline syenite ejecta within the plinian fall deposits of Agua de Pau volcano (São Miguel, Azores Islands) are divisible into (1) silica-undersaturated and (2) silica-saturated/oversaturated groups. The first group occurs in the pre-Fogo-A pyroclastic falls (15.2 ka) whereas the second occurs in the Fogo-A pumices (5 ka). The petrography indicates that the syenites of both groups crystallized at shallow depths and whole-rock geochemistry shows that they have compositions reflecting those of the Agua de Pau trachyte magmas. The syenites are inferred to be derived from solidification zones around the margins of the parental magma chamber. Mineral parageneses in both syenite groups permit distinction of three magmatic stages of crystallization: (1) an early crystallization stage; (2) a main crystallization stage during which most of the feldspar growth occurred; and (3) a late interstitial stage. Crystallization of the trachytic magmas yielded peralkaline residuals, highly enriched in volatiles, alkalis, HFSE and REE. In the silica-undersaturated syenites, the complex Zr-Ti silicate minerals, eudialyte and a låvenite-like mineral, crystallized interstitially from the residual magmas. In contrast, the zircono-silicate dalyite, and REE-rich minerals (chevkinite-(Ce), britholite-(Ce) and pyrochlore), together with thorite, were late-stage interstitial products of the silica-saturated/oversaturated syenites.

Keywords

syenitestrachytesmagma chamberpetrologyeudialytelåveniteREE-rich mineralsAgua de PauSão MiguelAzores.
Type
Research Article
Information
Mineralogical Magazine , Volume 67 , Issue 4 , August 2003 , pp. 749 - 767
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2003

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