The Relationships between Spilites and Other Members of the Oman Mountains Ophiolite Suite

Abstract

In the ophiolites of the Oman Mountains, a group of primary spilitic rocks occurs in the volcanic and subvolcanic parts of the suite. The relationship of these rocks with other fresh, subvolcanic rocks suggests that they have crystallized from a hydrous residual melt that approached basaltic composition.

The mineral assemblage characteristic of spilites is also found to occur in the interstitial spaces of associated subvolcanic rocks of normal hornblende-gabbroid mineralogic composition. It is suggested that the development of the spilitic residual liquids in the interstitial “pores” of the hypabyssal rocks was essentially caused by high water pressure in the magma. Consequently, the oxygen pressure was also increased, and titanium and iron oxides crystallized instead of titanium- and iron-rich silicates. The final differentiation product was a hydrous basaltic liquid enriched in silica and having a relatively low crystallization temperature.

The spilitic liquids may have been separated from the hypabyssal rocks by a filter flow mechanism. The separated liquids then formed a large part of the subvolcanic dikes that fed the overlying extrusive pillow and breccia lavas. During the extrusion some of the silica may have diffused into sea water.

It is inferred from the associated sediments that spilitic lavas of the Oman Mountains “geosyncline” were extruded over a long period, ranging from the late Permian to late Cretaceous eras.