Structural, geochemical and geochronological analysis of metasedimentary and metavolcanic rocks of the Coatlaco area, Acatlán Complex, southern Mexico

Abstract

Low-grade, siliciclastic metasedimentary and mafic metavolcanic rocks near Coatlaco, southern Mexico, form part of the Acatlán Complex, which has been interpreted as a vestige of either the Iapetus or Rheic Oceans. They form two tectonically interleaved units, the Canoas unit consisting of interbedded psammites and pelites, and the Coatlaco unit made up of interbedded quartzite and tholeiitic, within-plate, pillow basalts. Detrital zircons in these two units yielded different age population peaks: Ordovician (459 ± 14 Ma) and ca. 900–1250 Ma detrital zircons in the Canoas unit, and 357 ± 35 Ma, 563 ± 22 Ma, 837 ± 28 Ma, and 1156 ± 74 Ma in the Coatlaco unit. Whereas a source for the Paleozoic zircons may be found in the Acatlán Complex, and provenance for the ca. 900–1250 Ma zircons is likely the adjacent Oaxacan Complex, the Neoproterozoic zircons probably have a more exotic source in Amazonia. The contrasting age populations in the Canoas and Coatlaco units may be due to either (a) different ages of deposition, post-453 ± 6 Ma and post-Devonian, respectively; or (b) contemporaneous deposition in distinct environments; continental margin vs. oceanic with currents perpendicular and parallel to the margin, respectively. The Canoas unit was deformed by four phases of deformation: D 1 and D 2 produced NE–SW, steeply dipping, composite, axial planar solution cleavages (S CN1 and S CN2) with variably plunging, tight-isoclinal folds (F CN1 and F CN2) under lower greenschist facies metamorphic conditions; D 3 produced a sub-vertical, NE–SW, crenulation cleavage parallel to the axial planes of open to close folds, whose plunges show a NE–SW great circle distribution. D 4 produced small, vertically plunging chevron and kink folds (F CN4) in the pelitic rocks. The Coatlaco basalts are massive, lack deformational fabrics, and were altered under greenschist facies metamorphic conditions. The Coatlaco quartzites, in contrast, record two phases of deformation; (i) an S CL1 solution cleavage, and (ii) an S CL2 greenschist facies crenulation cleavage. The less intense deformation in the Coatlaco unit, compared to the Canoas unit, is attributed to its higher competency. Thus S CL1–2 is correlated with S CN2–3, and, as they post-date 357 ± 35 Ma (the youngest detrital zircons), they probably correlate with similar Carboniferous–Permian structures elsewhere in the Acatlán Complex. These structures have been related to the exhumation of high-pressure rocks in the Acatlán Complex and subsequent Permian dextral movements at the western end of the Variscan orogen during closure of the Rheic Ocean.