Pétrologie du massif volcanique du Guilliz (Maroc oriental). Cristallisation fractionnée, mélanges de magmas et transferts de fluides dans une série shoshonitique

Abstract

The Guilliz volcano is composed of upper miocene and pliocene suites (8.0–4.9 Ma) of shoshonitic and alkaline affinity. The shoshonitic suite is comprised of absarokites, shoshonites, latites and trachytes. Lavas with intermediate compositions are characterized by the presence of biotite and K-feldspar.

Alkaline lavas are, for the most part, hawaiites. The hawaiites are found as aphanitic inclusion in the shoshonitic series. Amphibole bearing absarokites and shoshonites erupted in the pliocene have compositions similar to that of the hawaiite inclusions.

Electron microprobe analyses of oxides, olivines, pyroxenes, amphiboles, biotites, feldspars and glasses indicate a multi-stage evolutionary process. Shoshonitic lavas appear to differentiate through crystal fractionation, leading to a zoned magma chamber. An attempt has been made to calculate the mineral assemblages and the proportion involved in the fractionation process.

The existence of an episode of magma mixing is evidenced by geological observations, and corroborated by chemical evolution of the lavas. The mixing occurs between shoshonitic and hawaiitic liquids. The mixing liberates a K-rich vapor phase from the hawaiitic magma, and this vapor contributes to the K-enrichment of the liquids present in the upper parts of the magma chamber.