KAr and [formula omitted]Ar study of metamorphic rocks associated with the Oman ophiolite: Tectonic implications

Abstract

K-Ar analyses on extracted minerals are reported for a variety of metamorphic rocks associated with the Sumail ophiolite. Amphibolites lying at the sole of the ophiolite yield ages of 95–100 Ma, which are viewed as reflecting times of crystallization. High-pressure metamorphics of the Saih Hatat reveal complex results: white micas range in age from 80 to 131 Ma whereas blue amphiboles indicate ages that are systematically lower than those of coexisting white micas. Investigation of a few white micas by the 40Ar 39Ar step heating method yields rather intricate age spectra, featuring low apparent ages in the first and the last stages of gas release and high apparent ages in between. Two explanations can be equally envisaged for these convex-upward age spectra. The first is the mixing of two generations of micas, corresponding to two main metamorphisms. The first one ( M 1) is a low- to medium-temperature, high-pressure event which conceivably occurred between 130 and 114 m.y. ago. The second ( M 2) overprints M 1 and has produced rocks typical of the greenschist facies. It took place 80 m.y. ago and also affected the sole of the ophiolites. The second is the presence of excess argon in mica mixtures with complex degassing properties. Thus, the two metamorphic phases identified by microscopic inspection are not significantly different in age. They occurred in the 70–80 Ma interval. Moreover, K-Ar dates on amphibole from gabbroic dikes intersecting the peridotites suggest that they are genetically linked to the mafic part of the ophiolites. Assuming that metamorphism is a tracer of tectonic events, we view the infraophiolitic amphibolites as the result of an intraoceanic thrusting which took place near a spreading center. Nevertheless, the uncertainty as to the age of the blueschist metamorphism precludes the possibility of indicating a timetable, based on metamorphic ages, for the motion shift of Africa relative to Eurasia during the Late Cretaceous. A tentative model of the obduction of the Sumail ophiolite is proposed assuming an age of about 130 Ma for M 1. We stress differences with respect to earlier reconstructions which advocated an 80 m.y. old blueschist metamorphism.