Cordilleran Section - 108th Annual Meeting (29–31 March 2012)

Paper No. 11
Presentation Time: 12:30


KIRSCH, Moritz, Universidad Nacional Autónoma de México, Centro de Geociencias, Campus Juriquilla, Santiago de Querétaro, 76001, Mexico, KEPPIE, J. Duncan, Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, México, DF 04510, Mexico, MURPHY, J. Brendan, Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada and LEE, James K.W., Department of Geological Sciences and Geological Engineering, Queen's University, Bruce Wing 325, Miller Hall, Kingston, ON K7L 3N6, Canada,

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.