Southeastern Section–56th Annual Meeting (29–30 March 2007)

Paper No. 1
Presentation Time: 1:20 PM


KEPPIE, John Duncan1, RAMOS-ARIAS, Mario Alfredo1, MORALES-GAMEZ, Miguel1, DOSTAL, Jaroslav2, NANCE, Richard Damian3, MILLER, Brent V.4, MURPHY, James Brendan5, ORTEGA-RIVERA, Amabel6 and LEE, James W.K.7, (1)Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Mexico, 04510, Mexico, (2)Geology Department, Saint Mary's Univiversity, 923 Robie Street, Halifax, NS B3H 3C3, Canada, (3)Department of Geological Sciences, Ohio University, Athens, OH 45701, (4)Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843, (5)Department of Earth Sc iences, St. F.X. Univ, Antigonish, NS B2G 2W5, Canada, (6)Instituto de Geología, Universidad Nacional Autonoma de Mexico, Estación Regional del Noroeste, Apartado Postal 1039, Hermosillo, 83000, Mexico, (7)Department of Geology, Queens University, Kingston, ON K7L 3NG, Canada,

Ordovician mafic igneous rocks have been recognized in several widely dispersed areas of the Acatlán Complex in southern Mexico, viz. Xayacatlán, Acatlán and Patlanoaya in the eastern, central and northern Acatlán Complex, respectively. The Xayacatlán Formation has been inferred to represent a fragment of Iapetus oceanic lithosphere that underwent eclogitic metamorphism during subduction followed by obduction and dehyration melting at ~440 Ma. Re-examination of the igneous rocks in the type area indicates that they consists of a N-S trending dike swarm that was originally comprised of layered, continental tholeiitic gabbroic intrusions emplaced at 442 ± 1 Ma (concordant U-Pb zircon, TIMS). These dikes were converted to amphibolitic gneiss, cooling through ~550 °C by ~434 Ma (40Ar/39Ar hornblende plateau age) and were variably affected by a greenschist facies tectonothermal event between ~250-200 Ma. This latter event is synchronous with Permo-Triassic dextral deformation that also affected the Lower Permian Tecomate Formation. Slightly older amphibolites just south of Acatlán are intruded by (?)comagmatic, ~460 Ma megacrystic granite, and are tholeiitic andesites that are heavily contaminated by continental crust. Continental tholeiitic intrusions in the Patlanoaya area are also inferred to be comagmatic with a ~461 Ma megacrystic granite. The tholeiitic character of all of these amphibolitic intrusions, together with the dike-like nature of many of them, suggests that they were intruded during extension of continental crust. Combined with data from elsewhere in the Acatlán Complex, these data indicate long-lived rifting (~480-440 Ma) along the southern margin of the Rheic Ocean, which may have started with the latest Neoproterozoic-Ediacarin separation of Avalonia and Carolinia, but continued in the Ordovician possibly in a Baja California type of tectonic regime.