Carboniferous tholeiitic dikes in the Salada unit, Acatlán Complex, southern Mexico: a record of extension on the western margin of Pangea

Abstract

A suite of mafi c dikes intrudes polydeformed, greenschist facies, metapsammites and metapelites of the Salada unit in the eastern part of the Acatlán Complex, southern Mexico. The age of the dikes is constrained by the youngest detrital zircon in the Salada host rocks (352±3 Ma) and the Early Permian age of the overlying Tecomate formation, which is devoid of such dikes. The mafi c rocks are generally composed of amphibole, chlorite, feldspar, epidote and accessory opaque minerals. Their chemistry resembles rift-related tholeiites with ~50 wt.% SiO2 and Mg# ~0.40–0.60. Their chondrite-normalized REE patterns resemble N-type MORB with (La/Sm)n mostly ~0.5–0.6, and their mantle-normalized patterns are relatively fl at with no negative Nb anomaly and a low Th/La ratio indicating the absence of both subduction-related fl uids and crustal contamination. Their chemistry resembles N-type MORB. Their intrusive relationships with the continentally-derived clastic rocks suggests that they were emplaced in thin continental crust. Shallow-water, Mississippian fauna in the adjacent Oaxaquia terrane, with Mid-Continent (USA) affi nities, indicate that Pangea had already amalgamated by this time. In this context, the tholeiitic dikes are inferred to have formed during extension on the western margin of Pangea that was synchronous with extrusion of high-pressure rocks above an active subduction zone.