ZIRCON U-PB GEOCHRONOLOGY CONSTRAINTS ON THE MAGMATIC AND TECTONIC EVOLUTION OF THE XOLAPA COMPLEX, SOUTHERN MEXICO

The Xolapa complex, Sierra Madre del Sur Mountains, comprises mid-crustal arc-related gneisses of poorly resolved ages, intruded by undeformed Cenozoic calc-alkaline plutons. Twelve samples of plutonic and metaplutonic rocks were analyzed from transects across the terrane near Acapulco (western transect), Puerto Escondido (central transect) and Puerto Angel (eastern transect). The measurements were performed in-situ on single crystals of zircons, using a laser ablation multiple collector ICPMS. About 20 crystals were measured from each sample. Three gneisses and migmatites from the eastern transect yielded only Grenville-aged zircons (1080-1119 Ma). These results suggest that the samples represent Oaxacan basement, not deformed Xolapa rocks, and call upon revising the Xolapa-Oaxaca boundary at this longitude. It appears that the Xolapa complex is much less extensive there than previously thought. The central transect yielded Oligocene ages (25-32 Ma) on undeformed plutons as well as Jurassic and Permian ages on gneisses. The older ages suggest affinities between Xolapa basement and Acatlan rocks. The western transect yielded Early Cretaceous ages (140-136 Ma) on gneisses, and Paleocene (55 Ma) and Oligocene (34 Ma) ages on undeformed plutons. These and previously published data for Xolapa suggest that metamorphism and migmatization of the deformed rocks took place largely during Late Jurassic and Early Cretaceous time. Eocene and Oligocene plutons representing renewed arc-related magmatism in the area are common throughout Xolapa. However, the available U-Pb data argue against the previously proposed eastward migration of magmatism during the Oligocene, and together with a companion thermochronology study (Shoemaker et al., this volume), put limiting constraints on the timing of possible arc truncation in southern Mexico.