Processes in magma chambers beneath spreading axes: evidence from magmatic associations in the Oman Ophiolite

Abstract

Ophiolite complexes, where complete, consist of tectonized ultramafic rocks overlain by layered gabbros and peridotites, isotropic gabbros and plagiogranites, a sheeted dyke swarm and pillowed volcanics. This pseudo stratigraphy has been interpreted as oceanic crust and upper mantle ( Dewey & Bird 1971 ; Moores & Vine 1971 ), the base of the crust being drawn at the base of the layered series ( Malpas 1978 ). In Oman, this succession is particularly well exposed in a 500 km long, 14 km thick ophiolite nappe of Upper Cretaceous age ( Glennie et al. 1973 ). Magmatic associations in its lower crustal and upper mantle portions (oceanic layers 3 and 4) are currently being studied to determine the processes of magma supply and fractionation in oceanic magma chambers. In the first section of this report we list the evidence for the greater part of the plutonic sequence of the ophiolite forming in a dynamic, long-lived, open system magma chamber beneath a spreading axis. In the second section the remainder of the plutonic sequence is described as a series of later, short-lived, possibly closed-system, off-axis plutons. A fuller treatment of magma chamber processes during the spreading event is presented elsewhere ( Smewing, in press ). The earliest and volumetrically most significant magmatic event is represented by up to 3.5 km of layered gabbros and peridotites and an overlying unit of up to 200 m of isotropic gabbros and plagiogranites intruding the base of the sheeted dyke swarm. The layered series rests with a sharp contact