Test of a spreading-ridge subduction model for the origin of granitoid plutons and metavolcanic complexes in the Palaeoproterozoic Pinal forearc/subduction complex of Southern Arizona, USA

The Pinal terrane of southern Arizona has been characterized as a Palaeoproterozoic forearc/subduction complex (Meijer 2014). It contains granitoid plutons and metavolcanic rocks with arc affinities that have a spatial distribution reminiscent of plutons and volcanic rocks that intruded into and erupted onto the early Cenozoic forearc/subduction complex of south-central Alaska (i.e. Chugach–Prince William terrane). The magmatic rocks that intruded into and erupted onto the Alaskan forearc/subduction complex are believed to have been produced largely as a result of a spreading-ridge subduction event (SRSE) during the early Cenozoic (Bradley et al. 2003). A characteristic of these rocks is that they were emplaced in a relatively short interval of time (10–30 million years) between the arc and associated trench. In the Pinal forearc/subduction complex of southern Arizona, there are numerous plutons and several metavolcanic complexes with arc affinities scattered over a distance of approximately 300 km perpendicular to the NE–SW Palaeoproterozoic orogenic trend. If these magmatic rocks are the result of an SRSE, they should have similar emplacement ages. New zircon age data for 16 samples from metavolcanic units and granitoid plutons in the Pinal forearc/subduction complex collected over a distance of 250 km perpendicular to the orogenic trend show a narrow range of emplacement ages that average 1650 ± 20 million years. The restricted age range and the spatial distribution of the plutons and metavolcanic rocks within this forearc/subduction complex are consistent with them having originated as a result of a Palaeoproterozoic SRSE.