In the Acatlán Complex of southern Mexico, a late Paleozoic assemblage, consisting of a gabbro-diorite-tonalite-trondhjemite suite (Totoltepec pluton) and clastic-calcareous metasedimentary rocks (Tecomate Formation), postdates collisional orogeny that resulted in the amalgamation of Pangea. This region offers a rare opportunity to examine assemblages developed at different crustal levels along the periphery of Pangea at the critical stage between amalgamation and breakup. The Totoltepec pluton consists of minor mafic-ultramafic rocks (306 ± 2 Ma; concordant U-Pb zircon analysis) that are marginal to the main mafic-felsic intrusion (289 ± 2 Ma). Geochemistry of the marginal rocks indicates an arc tholeiitic to calc-alkaline character with high large ion lithophile elements (LILEs)/high field strength elements (HFSEs), flat rare earth element (REE) patterns, and initial ɛNd values of +1.3 to +3.3. The younger Totoltepec phase exhibits a calc-alkaline trace-element geochemistry with flat to moderately fractionated light (L) REE–enriched patterns and initial ɛNd values of –0.8 to +2.6, which are also consistent with an arc environment. The Sm-Nd isotopic signature is more primitive compared to contemporaneous arc-related igneous rocks in southern Mexico, suggesting the pluton was emplaced in a less mature, outboard part of the arc, and/or along a fault conduit. The Tecomate Formation, as currently defined, is a composite of lithologically similar strata deposited in several fault-bounded basins ranging from Carboniferous to Early Permian in age. To the south of the Totoltepec pluton, the depositional age of the Tecomate Formation is tightly constrained in one section to ca. 300 Ma, but in another section, it is between ca. 288 and ca. 263 Ma. The Tecomate Formation rocks are interpreted to have been derived from a late Paleozoic arc based on (1) arc-related geochemistry, (2) ɛNd(t) values ranging from –5.6 to +0.3 (t = 288 Ma) that overlap those of the Totoltepec pluton, and (3) detrital zircons with predominantly Carboniferous–Permian ages. The Totoltepec and Tecomate units in the study area form part of a continental arc extending from Guatemala to California, which necessitates subduction of the paleo-Pacific oceanic lithosphere beneath the western margin of a Pangea-A configuration.

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