Lithodeme El Pitayo, a Palaeozoic meta-volcanosedimentary unit of the Acatlán Complex, southern Mexico and its importance in pre- and post-Pangea tectonic evolution.

The pre-Pennsylvanian metasedimentary units (Cosoltepec Formation) of the Acatlán Complex in southern Mexico represent 70-90% of the exposed surface of this complex. The metasedimentary units mainly consist of metapelites, metapsammites and quartzites, as well as sparse pillow basalts and doleritic dikes (Ortega-Gutiérrez et al., 1978). The tectonostratigraphic evolution of these rocks remains a controversial issue in the pre- and post-Orogenic tectonics of the Acatlán Complex. As part of these units, this paper analyses El Pitayo Lithodeme, a low-grade metamorphic unit exposed at Guadalupe Allende (formerly El Pitayo) area, 20-25 km SE of Izúcar de Matamoros, Puebla State, which up to now has not been described in detail.

El Pitayo Lithodeme, located in the westernmost sector of the Complex, consists of fine-grained metapelites and metapsammites, quartzites, radiolarites, sericite-chlorite schist and interbedded metabasalts, which together suggest a deep-water depositional basin. Structurally, the sequence has a predominant N-S trending and penetrative axial plane foliation, associated with isoclinal folding, and it is bordered by faults. El Pitayo Lithodeme is overthrusted by blue schists (Casitas Lithodeme) and apparently it is juxtaposed by lateral faults with the eclogitic rocks of the Piaxtla Suite, in the westernmost sector of the Acatlán Complex.

By U-Pb geochronology of detrital zircons from metasediments, a maximal age of middle Cambrian (ca. 513 Ma, major peak age of the younger zircon grains) for the refilling of the “El Pitayo” basin was estimated, with detrital materials from Pan-African (ca. 500-605 Ma) and Grenvillian orogens (ca. 910-1325 Ma). On the other hand, a K-Ar (laser ablation) age of 234 ± 3 Ma was obtained for white micas from metasedimentary rocks, which can be interpreted as the age of low-grade metamorphism, or as a cooling age related to exhumation processes linked to the Late Triassic tectonic event during the break-up of western Pangea. Furthermore, thermobarometric analyses (chlorite geothermometer and pseudosection modelling) suggest that the greenschist facies metamorphism occurred at P-T conditions, for which no quantitative values have been estimated previously for the low-grade metamorphic rocks of the Acatlán Complex.

Thus, the El Pitayo Lithodeme, by its lithology and the structural position between high-pressure Mississippian rocks of the Piaxtla Suite at the westernmost sector the Acatlán Complex, is considered a key unit that with more detailed information will surely allow more robust correlations with other low-grade metamorphic units and will undoubtedly help to better understand about the pre- and post-Pangea tectonic evolution of the Acatlán Complex, southern Mexico.