Strontium isotopes, age, and tectonic setting of the Argentine Precordillera and southern Laurentia

The Argentine Precordillera generally is interpreted as a fragment of continental crust and sedimentary cover that was rifted from the Ouachita embayment of southern Laurentia in Cambrian time. Synrift sedimentary successions in the northern part of the Precordillera and in the Birmingham graben of southern Laurentia include thick evaporite deposits. Analyses of strontium isotopes indicate that the evaporites are of late Early Cambrian age, consistent with biostratigraphic correlations. Variations in strontium ratios indicate that these Early Cambrian salinas had a persistent base of marine water and that contributions from meteoric runoff varied through time and with location. The stratigraphic and isotopic data suggest a depositional setting of restricted circulation within a system of extensional fault blocks. The salinas occupied graben blocks that were connected to marine circulation and that intermittently received meteoric runoff carrying fine detritus and radiogenic strontium from horst blocks of continental crust. The evaporites in the Precordillera show less meteoric influence than the evaporites in the Birmingham graben, which was far inboard from the continental margin of southern Laurentia. These relationships are consistent with continental rifting in Early Cambrian time, extension of continental crust, and deposition of synrift sediment in graben blocks that were partially open to marine circulation.