Keppie, J. D.; Nance, R. D.; Fernandez-Suarez, J.; Storey, C. D.; Jeffries, T. E. and Murphy, J. B.
Detrital Zircon Data from the Eastern Mixteca Terrane, Southern Mexico: Evidence for an Ordovician—Mississippian Continental Rise and a Permo-Triassic Clastic Wedge Adjacent to Oaxaquia.
International Geology Review, 48(2),
The eastern part of the Mixteca terrane of southern Mexico is underlain by the Petlalcingo Group (part of the Acatlan 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; overthrust 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 similar to 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 Ala (Permian-Earl.), 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 an(] 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 similar to 850-1200 Ala, suggesting derivation from the adjacent similar to 1 Ga Oaxacan Complex. A similar to 470-640 ilia peak is limited to the Cosoltepec Formation whose source may be found in similar to 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 elastic wedge deposited in front of S-verging Permo-Triassic thrusts Oil 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.
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