EMPLACEMENT OF THE LATE CARBONIFEROUS–PERMIAN ARC-RELATED TOTOLTEPEC PLUTON, ACATLÁN COMPLEX, SOUTHERN MEXICO: SYNTECTONIC INTRUSION ALONG A MID-CRUSTAL TRANSPRESSIONAL SHEAR ZONE

The ca. 306–289 Ma tholeiitic to calc-alkaline Totoltepec pluton in the eastern Acatlán Complex, southern Mexico, is part of a Carboniferous–Permian continental magmatic arc along the western margin of Pangea. The pluton is a ca. 68 km², well-exposed, composite, felsic to ultramafic intrusive suite containing a conspicuous mesoscopic fabric. Al-in-hornblende thermobarometry and ⁴⁰Ar/³⁹Ar geochronology indicate that the main, younger part of the pluton was emplaced at about 20 km depth and was rapidly uplifted, cooling to about 400 °C in 6 ± 2 Ma. The elongate pluton shape, a down-temperature continuum from magmatic to low-temperature solid-state fabrics and the parallelism between foliations defined by hornblende and feldspars-quartz-muscovite, respectively, indicate that emplacement of the pluton was tectonically controlled. Felsic dikes that cross-cut the foliation, but contain an internal foliation parallel to the dike margins provide further evidence for syntectonic emplacement. The intrusion contains a steep, fanning foliation parallel to the pluton’s long-axis. Stretching lineations are generally poorly developed, suggesting a pure flattening component of strain. However, within internal, margin-parallel high strain zones, they show a transition from subhorizontal with sinistral shear-sense indicators (rotated amphibole porphyroclasts) to steeply dipping with top-to-S shear sense indicators, suggesting forceful pluton emplacement in a left-lateral, oblique transpressional zone with triclinic symmetry. Hornblende-bearing diorite and tonalite in the southern part of the pluton exhibit a compositional and textural banding that is interpreted to have formed by a combination of steep igneous layering, layer-parallel dike injection and melt-enhanced deformation. The timing and emplacement mechanism inferred for the Totoltepec pluton is similar to that reported for syn-tectonic Late Carboniferous to Early Permian plutons along the Caltepec Fault zone that separates the Acatlán Complex from the Oaxacan Complex. Strike-slip tectonism along this fault may be associated with oblique subduction of the paleo-Pacific beneath the western margin of Pangea.