ORDOVICIAN CONTINENTAL THOLEIITES IN THE ACATLÁN COMPLEX, SOUTHERN MEXICO: EVIDENCE OF RIFTING ON THE SOUTHERN MARGIN OF THE RHEIC OCEAN

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.