DID LATE ARCHEAN TO PALEOPROTEROZOIC SHEARING OVERTURN THE ISUA BELT OF SW GREENLAND? CONSTRAINTS FROM STRUCTURE AND U-PB GEOCHRONOLOGY

Both Early- and Mid-Archean orthogneisses and igneous rocks surrounded by the Isua supracrustal belt, SW Greenland, are commonly strongly deformed at their western edge, suggesting the structure of the neighboring greenstone belt is post-Early Archean. Throughout most of the central gneiss exposure, S2 foliation in Early Archean tonalitic gneiss and metagranite has a domal map pattern, parallel to the enclosing greenstone belt. Contacts and foliation in the greenstone belt, however, are vertical at the southwest part of the belt (the bottom of the "u"), and are "overturned" (steeply southeast-dipping-) on the belt’s western limb. Undeformed, ca. 3.45 Ga Tarssartoq dikes (dolerite) cut the S2 gneiss foliation. Within 1 km of the greenstone belt’s western limb, though, anastomosing, steeply SE-dipping, reverse-motion shear zones cut both gneisses and Tarssartoq dikes. East of these shear zones, S2 in the gneisses dips moderately northwest; between shear zones, S2 dips steeply northwest to southeast. Shear zones microstructures--Kfs and Pla subgrains, recrystallized quartz ribbons, and Hbl fabrics--indicate deformation at epidote-amphibolite to amphibolite facies. Sheared dikes are locally Chl-bearing, with Chl disseminated through the matrix or, alternatively, occurring in discrete bands that cut Hbl fabrics. Stringers of Ttn euhedra parallel all fabrics related to shearing: S and C surfaces in the tonalites, Hbl fabrics and later chloritic shears in the dolerites. Preliminary Ttn data from a mylonitic tonalite yield an age of ca. 2600 Ma, whereas a sheared dolerite with chloritic bands that cut Hbl fabrics yields an age of ca. 2125 Ma. Two other sheared samples had high proportions of common Pb, and yield discordant spectra that suggest included Early to Mid Archean components. These data suggest a complex deformation history on the west edge of the central gneisses. The pervasive nature of the Late Archean-Paleoproterozoic reverse-motion shearing makes all structure to the west suspect. We suggest that the steep SE dips in the west limb of the greenstone belt are linked to this shearing, and possibly related to motion along the Ataneq fault further west. The domal structure everywhere else in the central gneisses and surrounding greenstone belt was thus the original Early Archean structure.