Archean lithospheric mantle beneath Arkansas: Continental growth by microcontinent accretion

The Cretaceous Prairie Creek lamproites of southern Arkansas intrude Proterozoic crust near the boundary between the 1.5-1.3 Ga Granite-Rhyolite Province and the 1.3-1.0 Ga Grenville orogen. They carry xenocrysts and rare xenoliths derived from the subcontinental lithospheric mantle (SCLM) and the deep crust. U-Pb age dating of groundmass perovskite in the Prairie Creek lamproites gives a poorly constrained Cretaceous age. U-Pb dating and in situ Sr and Nd isotope data show that perovskite micro-nodules in the Twin Knobs #2 lamprophyre are ca. 600 Ma old, and may represent samples of rift-related-alkalic magmas derived from a juvenile mantle. A lithologic section constructed from the mantle-derived xenocrysts shows a moderately depleted SCLM that has experienced a high degree of melt-related metasomatism, especially in the depth range 150 to 140 km. In situ Re-Os analysis of sulfide grains in the xenoliths yields model ages ranging up to 3.4 Ga, with major peaks at 1.4-1.5 Ga and 200-300 Ma. Early Paleoproterozoic model ages appear to reflect mixing between residual Archean high-Os sulfides and later low-Os sulfide melts. These data suggest that the SCLM beneath the Prairie Creek area formed in Archean time, and has been progressively refertilized by a series of magmatic events, which appear to correlate in time with events in the overlying crust. The Archean SCLM sampled by the lamproites may represent the mantle root of the Sabine microcontinent, which lies mainly to the south of the lamproite field and is recognizable in seismic tomography as a feature with higher shear-wave velocity (Vs) (100-175 km depth). Seismic tomography also shows several blocks with high Vs beneath the Grenville province to the east, which may represent other microcontinental blocks. These findings suggest that the growth of individual continents is significantly affected by the accretion of older microcontinental blocks, and that the extent of early continental crust therefore may be greater than generally estimated.