LAND OR SEA: A REINTERPRETATION OF ALLOCHTHONOUS METAMAFIC ROCKS OF THE XAYACATLAN FORMATION, SOUTHERN MEXICO

The Acatlan Complex of southern Mexico consists of two major thrust slices unconformably overlain by a sequence of deformed Pennsylvanian-Lower Permian metasedimentary rocks. The lower parautochthonous slice (Petlacingo Group) comprises a thick sequence of metamorphosed siliciclastic rocks derived from the Oaxacan Complex. The upper allochthonous slice (Piaxtla Group) consists of polydeformed, high-grade, mafic igneous and interlayered pelitic and siliceous metasedimentary rocks (Xayacatlan Formation) that are structurally overlain by metagranitoids and migmatites (Esperanza granitoids). The Xayacatlan Formation has been interpreted as the Lower Paleozoic vestige of either the Iapetus or Rheic Ocean. The Esperanza granitoids are inferred to be syntectonic with respect to the emplacement of the nappe and yielded an age of 440+/-14 Ma (U-Pb zircon age). The Piaxtla Group was obducted during the Late Ordovician-Early Silurian Acatecan Orogeny over a continental rise (Petlalcingo Group) bordering the South American craton. Subsequently, the Acatlan Complex was tectonically juxtaposed against Grenville-age (ca. 1 Ga) granulites of the Oaxacan Complex at its eastern margin along the north-south trending Permian Caltepec fault zone. Re-examination of the type area of the Xayacatlan Formation reveals that it consists of a mafic igneous sequence intruded at 442 ± 1 Ma (U-Pb zircon age) that underwent upper amphibolite facies metamorphism before cooling through 500-550°C by 414 ± 15 Ma (40Ar/39Ar hornblende age). This was followed by a lower greenschist facies overprint that has been dated elsewhere at 288 ± 14 Ma (K-Ar muscovite age). Although generally thoroughly recrystallized, some of the amphibolites appear to preserve igneous textures. Their major and trace elements are rather similar to modern igneous rocks, suggesting that they also preserve primary compositions. The amphibolites have typical tholeiitic characteristics, and resemble continental tholeiites rather than rocks with oceanic affinities. In contrast to earlier studies, these rocks are interpreted here as part of a plutonic complex emplaced in continental crust, possibly the underlying Oaxacan Complex. The Xayacatlan Formation is thus inferred to represent an obducted slice of this continental lithosphere.