A LATE ARCHEAN TERRANE BOUNDARY EXPOSED AT TIN CUP MOUNTAIN, GRANITE MOUNTAINS, WYOMING

In the Tin Cup Mountain area of the northwestern Granite Mountains, Middle Archean gneisses to the south have been juxtaposed against Late Archean granites and gneisses to the north. Preliminary field work indicates that these rocks are separated by an intense east-west trending shear zone that is at least 4 km wide. The amphibolite facies shear zone consists of mylonitized granites that are complexly interlayered with supracrustal sequences which include metabasalt, metadacite, metadiabase, and various psammitic metasediments. Samples from the supracrustal rocks reveal contrasting epsilon Nd values at 2.63 Ga of -6.5 (schist), -7.0 (metavolcanic rock), and +2.5 (schist), suggesting that rocks of different provenance are juxtaposed by this shear zone. At 2.63 Ga, a Late Archean unfoliated granite to the north of the shear zone has an epsilon Nd value of -1.1, and falls within the ca. -3 to +2 range of epsilon Nd values (at 2.63 Ga) of other granites in the Granite Mountains Batholith. A sample of ca. 3.2 Ga gneiss to the south, however, has an epsilon Nd value of -11 at 2.63 Ga. This value is consistent with highly negative epsilon Nd values (ca. -10 to -14) obtained by other workers from similar rocks elsewhere in the Sacawee block of the Granite Mountains. The contrasting ages and isotopic affinities of the granites and gneisses at Tin Cup Mountain indicate that the shear zone forms a boundary between a Late Archean terrane to the north and an isotopically distinct Middle Archean terrane to the south. Timing of the deformation near Tin Cup Mountain is yet unconstrained, however, similar but smaller-scale deformation in the eastern Granite Mountains has been constrained to 2.63-2.65 Ga. The contrasting Nd signatures found on either side of the shear zone and among the associated supracrustal rocks within the shear zone leads us to suggest that this shear zone represents the only exposed suture between the Wyoming Province and the Middle Archean Sacawee block.