Nd isotopes and the origin of Grenville-age rocks in Texas: implications for Proterozoic evolution of the United States mid- Continent region

Nd isotopic data were obtained for Precambrian Grenville-belt rocks in Texas. Almost all Precambrian igneous, metamorphic, and sedimentary rocks from the three regions document addition to North America of mantle-derived crustal materials in the 1.6-1.0 Ga interval. The exception is a quartzite from the westernmost (Franklin Mountains) exposure, which was derived from ~1.8 Ga crust of the southwestern United States. These results can be interpreted in two ways: 1) as documenting 0-20% additions of older crustal material to mantle-derived products 1.4-1.0 Ga ago; or 2) as documenting derivation of the Grenville exposures by recycling of older crustal protoliths separated from the mantle 1.6-1.3 Ga ago. If the ages represent older crustal protoliths, then they would have formed coevally with the 1.5-1.3 Ga Granite-Rhyolite Terranes of the continental interior USA. This would imply that the Granite-Rhyolite Terranes were formed during orogenic/accretionary processes in the adjoining Grenville Belt, and are not anorogenic in association, as conventionally assumed. -from Authors