ABSTRACT
The eastern part of the Mixteca terrane of southern Mexico is underlain by the Petlalcingo Group (part of the Acatlán Complex), and has been interpreted as either a Lower Paleozoic passive margin, or a trench/forearc sequence deposited in either the iapetus or Rheic oceans. The group, from bottom to top, consists of: (1) the Magdalena Migmatite protolith (metapsammites, metapelites, calsilicates, and marbles), which grades up into (2) the meta-psammitic Chazumba Formation; over-thrust by (3) the Cosoltepec Formation (phyllites and quartzites with minor mafic meta-volcanic horizons). The group is unconformably overlain by the Pennsylvanian–Middle Permian Tecomate Formation, which is overthrust by the ~288 Ma Totoltepec pluton and unconformably overlain by Middle Jurassic rocks. In contrast to previous inferences that the protoliths of the units (1) to (3) were early Paleozoic in age, detrital zircon LA-ICPMS ages combined with published data constrain depositional ages as follows: (i) Magdalena Migmatite protolith: post-303 Ma–pre-171 Ma (Permian– Early Jurassic); (ii) Chazumba Formation: post-239 Ma–pre-174 Ma (Middle Triassic–Early Jurassic); and (iii) Cosoltepec Formation: post-455 Ma–pre-310 Ma (uppermost Ordovician–Mississippian). Given the different ages and depositional environments of the Cosoltepec Formation versus the Chazumba Formation and Magdalena protolith, we recommend redefining the Chazumba and Magdalena as lithodemes grouped in the Petlalcingo Suite and excluding the Cosoltepec Formation. Detrital zircons in all three units show a population peak at ~850–1200 Ma, suggesting derivation from the adjacent ~1 Ga Oaxacan Complex. A ~470–640 Ma peak is limited to the Cosoltepec Formation whose source may be found in ~470 Ma plutons in the Acatlan Complex, beneath the Yucatan Peninsula, and in the Brasiliano orogens of South America. The inferred turbiditic protolith of the Chazumba Formation and Magdalena protolith suggests that it represents a clastic wedge deposited in front of S-verging Permo-Triassic thrusts on the western margin of Pangea. The mainly oceanic affinity of the basalts in the Cosoltepec Formation suggests deposition of sedimentary protoliths in a continental rise fringing Oaxaquia. These data are more consistent with deposition of the Cosoltepec Formation in the Rheic Ocean than in the Iapetus Ocean.
