TIMING AND DURATION OF HIGH-P GRANULITE FACIES METAMORPHISM IN THE NORTHERN TETON RANGE, WYOMING

Kyanite gneisses from the Moose Basin region in the northern Teton Range were metamorphosed at >11 kbar and ~950°C, based upon GRAIL barometry and Ti-in zircon thermometry. At peak metamorphism associated mafic granulites melted to make garnet-bearing leucosomes. These rocks underwent near-isobaric cooling to temperatures where staurolite was stable before being decompressed into the sillimanite and cordierite stability fields. After peak metamorphism and substantial retrograde deformation the Moose Basin gneisses were intruded by the Webb Canyon gneiss, a leucogranitic gneiss that exhibits kinematic indicators indicating extensional deformation. SHRIMP, ICP-MS, and TIMS U-Pb dating of zircons from the leucosome of the mafic granulites and from the Webb Canyon gneiss constrains the timing and the duration of this metamorphism.

The leucosome from the mafic granulite yielded a large population of zircon grains, with two major morphologies -- doubly terminated prisms and faceted soccer-balls. The SHRIMP ages of the prisms are 2697 ± 5.3 Ma. The soccer-balls yield a range of ages ranging from 2675 to 2700 MA. The margins of these grains, which are distinct in CL images, yield SHRIMP ages of 2683 ± 11 Ma. SHRIMP ages of the zircon from the Webb Canyon gneiss average 2681 ± 5.9, whereas ICP-MS age for zircon from the same rock is 2674±2.8 Ma. This is within error of the minimum age of the Webb Canyon gneiss of 2673 ± 2.0 Ma reported in Frost et. al (Can J. Earth Sci., 2006)

We interpret these data to indicate that the initial melting of the mafic granulite and hence peak metamorphism of these rocks occurred around 2700 Ma. The rocks stayed hot and zircon continued to crystallize until around 2680 Ma, while the rocks underwent deformation and cooling to upper amphibolite-facies conditions. Webb Canyon gneiss was intruded during a decompression event; the last stages of crystallization of the Webb Canyon, and probably the latest extensional deformation, occurred at around 2675 Ma. This duration of ~25 million years between contraction and extension within this Archean orogenic zone is typical of Phanerozoic mountain belts and is evidence that tectonic processes operating in the late Archean were analogous to Phanerozoic tectonic styles.