Preliminary Petrologic and Microstructural Characterization of a Metamorphic Section Beneath the Samail (Oman) Ophiolite: Results from the Oman Drilling Project Hole BT1B

The ICDP Oman Drilling Project obtained cores at several locations from the Samail Ophiolite to address questions about ocean crust formation, hydrothermal alteration on the seafloor, and emplacement on land. Phase I drilling at Hole BT1B (23°21.861' N, 58°10.957' E) recovered 200m of listvenite and serpentinite above a several m-thick shear zone of cataclasites and gouge that form the Basal Thrust. Beneath the thrust the core contains 100m of sub-ophiolite metamorphics, comprising 30m of sediments above 70m of mafic volcanics. This location lacks the high-T garnet-bearing sole that is documented at several locations. The core was logged in Japan and 35 samples were selected for microstructural characterization via petrographic and XRD analysis supplemented by microprobe x-ray mapping and phase analysis (in progress).

The phyllitic metasedimentary section (n=6 samples) is thinly laminated. Cm-scale isoclinal folds with fishhook and eye structures indicate layering was transposed and shear strains were large. Petrographic and XRD analysis (microdrilling of layers, n=139) reveal common mineralogies are varied proportions of quartz, albite, white mica, chlorite, titanite and ilmenite ± epidote. Trace apatite is common, and some layers contain up to 400 μm grains. The mafic section (n=29) lacks pseudomorphs of magmatic mineralogies suggesting it is a highly distorted, possibly glassy volcanic protolith. The common mineral assemblage is green amphibole, epidote, albite, and chlorite ± stilpnomelane. Epidotes and amphiboles are compositionally zoned, and two samples contain distinctly blue-green amphibole. Veinlets (< 2 mm thick) are present in most samples and they reveal evidence for fluid in fractures. Syn- and late-kinematic veinlets are more abundant in the mafic section. Deformed veinlets are epidote, quartz ± chlorite and stilpnomelane, while late veinlets are varied combinations of calcite and quartz. Post-kinematic veinlets are commonly associated with zones of microbrecciation.

The sub-thrust metamorphics record a dynamic period of recrystallization likely related to underthrusting and a later retrograde, brittle history probably related to unroofing. The relation of metamorphism and fluid expulsion to listvenite formation in the overlying serpentinite is a matter of great interest.