A mechanism forming silicic segregations from basaltic magma discovered in igneous rocks of Western Sicily

Abstract

Summary. Mechanisms forming silicic segregations from basaltic magmas are considered of primary importance when dealing with magmatic problems. However, the processes which give rise to silicic segregations from basaltic magmas are so far obscure. Fortunately, the discovery of spheroidal felsic masses in some basic rocks of Western Sicily throws light on this subject. To clarify the relationships between felsic and basic fractions particular attention has been paid to the interactions which occurred at their contact. Textural evidence indicates that the accretion mechanism of the Sicilian felsic segregations tends to obliterate the silicate liquid immiscibility effects and suggests that the formation of silicic segregations is a consequence of liquid unmixing phenomena.

Viscoelastic phase separation

“…Phase separation in magma. Recently, Lucido [191] suggested that the texture of Sicilian magma may be explained by colloidal aggregation (or phase separation). He showed that it is possible that the Sicilian magma split into two liquid portions, one (basaltic) enriched in high-charge ions like iron, magnesium and calcium and the other (felsic) enriched in elements (Si, Al, Na and K) having the tendency to concentrate in the networklike structure.…”

Critical phenomena originating magmatic rocks in western Sicily.

“…model is supported by the fact that: 1) the Sicilian ocelli are clearly richer in Si, Al, K and Na and are poorer in iron, mag nesium, calcium, manganese, titanium and phosphorus than the enclosing basaltic rock (see LuciDO, 1981). 2) at the moment of immiscibility much of the Sicilian magma was in a state of intensive (an unstable melt) (see LUCIDO, 1983). the Sicilian rock-samples exhibit regularity of spacing between the felsic clusters (Fig.…”

“…This is formed essentially by titanaugite, kaersutite, andesine, K-feldspar, analcite and zeolites. Previous studies (LucIDO, 1981(LucIDO, , 1983 Spinodal decomposition and metastable im miscibility are phenomena of considerable im portance in the glass industry (e.g., CAHN, 1968;SEWARD et al, 1968;DOREMUS, 1973;TEWHEY and HESS, 1974;LAUGHLIN and CAHN, 1975;ZDANIEWSKI, 1975;ALLEN andCAHN, 1976, LAUGHLIN, 1976). However, in contradistinction to the importance in glassy systems, these phenomena are underestimated from the geo logical point of view.…”