Rift-related basalts in the 1.2–1.3 Ga granulites of the northern Oaxacan Complex, southern Mexico: evidence for a rifted arc on the northwestern margin of Amazonia

Meta-igneous granulites with estimated c. 1.3−1.2 Ga protolith ages in the northern Oaxacan Complex of southern Mexico have bimodal protoliths: rift-related basalts (probably continental tholeiites) and felsic volcanic rocks that could also have been arkosic sediments. Negative Nb, Ta and Ti anomalies in the mafic rocks are interpreted to reflect derivation from a mantle source previously enriched by subduction processes. In contrast, contemporaneous igneous country rocks in the southern Oaxacan Complex have arc affinities and the simplest interpretation suggests a south-to-north transition from arc to back-arc or intra-arc rift. Various locations in 1.2−1.0 Ga Rodinia reconstructions have been proposed for these and correlative rocks in Middle America: an autochthonous location; and as an allochthonous terrane originating either off northwestern or southern Amazonia or the Arequipa-Antofalla terrane. The collision between southern Amazonia and southern Laurentia at 1.2 Ga followed by relative northward travel (present-day coordinates) of Amazonia along eastern Laurentia appear to eliminate all but a northwestern Amazonian location for Middle America. The development of an arc complex on the northwestern margin of Amazonia would be consistent with the relative northward motion of Amazonia.