TECTONOMAGMATIC AND PALEOZOIC EVOLUTION OF PLUTONIC ROCKS IN THE NORTHERN ACATLÁN COMPLEX, LA NORIA AREA, SOUTHERN MEXICO

The Acatlán Complex of southern Mexico exposes the country's largest assemblage of Paleozoic rocks, the evolution of which has traditionally been correlated with the Appalachian Orogen and the closure of the Iapetus Ocean. Previous studies in the La Noria area have proposed that rocks correlated with the peri-arc Tecomate Formation of inferred Siluro-Devonian age are intruded by the La Noria granite, which yielded discordant U-Pb data with a lower intercept of ~370 Ma, interpreted to date its syntectonic intrusion during a Late Devonian (Mixtecan) orogeny. However, new fossil and U-Pb SHRIMP geochronological data indicate that the Tecomate Formation is Pennsylvanian-Middle Permian. Furthermore, recent mapping has shown to expose the El Zapote Negro quartz augen granite and cross-cutting Los Malpasos leucogranite in addition to the peraluminous K-feldspar megacrystic La Noria granite now dated at ~470 Ma (U-Pb SHRIMP zircon age). Major and trace element analyzes of these granitoids indicate that they all: (i) straddle the VAG—WPG and (VAG + syn-COLG)—WPG limits in the Rb—(Y + Nb) and Y—Nb tectonic discrimination plot for granites, respectively; and (ii) show identical flat primitive mantle-normalized trace elements patterns with negative Nb, K, Sr, and Ti anomalies and positive Pb anomalies. The new geochemical and isotopic data for the La Noria granite preclude its synchronous emplacement during a Late Devonian (Mixtecan) orogeny and suggest, instead, a similar age and tectonic setting for all megacrystic granitoids in the northern part of the Acatlán Complex. Their emplacement may record development of an arc or during rifting along the formerly active margin of Gondwana on the southern flank of the Rheic Ocean. Published detrital zircon ages from the psammitic rocks with which these granitoids are in contact are as young as mid-Ordovician, indicating that granitoid intrusion was approximately synchronous with deposition of the host rocks.