A PRELIMINARY REPORT ON PILLOW LAVAS AND PEPERITES IN THE OMAN BORDER ZONE, EAST OF AL AIN, UNITED ARAB EMIRATES

Peperites are breccias consisting of a mix of exploded lava and sediment. They are thought to be the result of the intrusion of molten rock into wet, typically marine, sediments. As the hot magma intrudes the cold, water-saturated, and unconsolidated sediments the water flashes into steam. The explosive expansion brecciates and mixes the magmatic and sedimentary material. This is the first report of such an occurrence occurring in the Oman-UAE border region of Eastern Arabia. Thirteen samples have been collected from 5 locations in the Oman Mountains, east of Al Ain, UAE at Jabal Umm Bak and Jabal Dai. Material was taken from the interior and exterior of individual pillows, the peperite boundary layers, where present, and from the carbonate matrix between the pillows. Field relations of the pillow lavas, peperite, and surrounding limestone are consistent with the pillow lavas forming directly within the limestone. One of the features of the Jabel Umbak and Jabel Dai peperites and pillow lavas indicating intrusive formation is that individual pillows are often completely surrounded by carbonate material, some of which still contain evidence of pre-intrusion bedding that has been deformed by compression between pillows. The carbonate has been clearly squeezed between pillows, with peperite material often forming a boundary layer at the carbonate-pillow contact. The peperite boundaries are thin. At locations 1 and 2, where peperite is best preserved, it is less than 2 m thick. The thin peperite boundary layers were probably produced by small-scale explosive contact with the intruding magma. It is likely that the ambient pressure was great enough and cooling of the magma rapid enough to prevent large explosions. The fact that the magma also formed pillows is evidence of this. Although there appear to be some baked contacts at location 5, most of the other locations do not show significant thermal alteration, which also indicates very rapid quenching of the magma. The rapid quenching of the magma by deep, wet, unconsolidated sediments was likely produced by physical conditions similar to those attending extrusion of lava directly into water.