RB/SR AND SM/ND AGE CONSTRAINTS ON THE TIMING OF FABRIC DEVELOPMENT IN ECLOGITES FROM BENEATH THE SAMAIL OPHIOLITE COMPLEX, OMAN

The Oman ophiolite sits as the highest allochthonous structural unit on top of the originally passive Arabian continental margin. The occurrence of eclogite (metabasalts) in the As Sifah subwindow as part of a metamorphosed, rifted passive margin sequence is a strong constraint for any scenario for ophiolite emplacement and for the tectonic setting before the disruption of the passive margin. Rb/Sr and Sm/Nd analyses data constrain the timing of fabric development and metamorphism in these polydeformed rocks. Obtaining isochrons from eclogite is difficult because of multiple mineral growth events and the tendency to produce two point isochrons, phengite + rock or garnet + rock for Rb/Sr and Sm/Nd, respectively. By examining both the isochron plot and daughter-product isotope evolution plot (e.g. ¹⁴³Nd/¹⁴⁴Nd vs. time), it is possible to evaluate the reliability of the age determinations from a real age and model age perspective. Pooled phengite analyses give a Rb/Sr age of approximately 75 Myr consistent with growth during exhumation. The inferred initial Sr ratio is greater than 0.710 consistent with the secondary isochron origin of the phengite array. Assuming growth from a basaltic protolith, the most radiogenic phengite gives a model age approaching 100 Myr. In contrast, garnets from three different eclogites within the mafic schist yield internal isochron ages >108 Myr. Pooled garnet data give an apparent age about 115 Myr. These results complement previous ⁴⁰Ar/³⁹Ar ages to indicate that the high pressure/low temperature metamorphism of the platform is not solely due to the load of the allochthonous ophiolite (igneous age 95 Myr). The >30 Myr time interval for metamorphism leaves two scenarios involving nascent subduction of the Arabian passive margin.