GEOCHRONOLOGY AND PETROGENESIS OF THE JAMILTEPEC – PUERTO ESCONDIDO BATHOLITHS: NEW INSIGHTS ON SOUTHERN MEXICO CENOZOIC MAGMATISM

The continental margin of Oaxaca, in southern Mexico, is occupied by extensive batholiths that intruded the metamorphic Xolapa Complex. These major intrusive bodies are thought to represent the roots of an extinct magmatic arc that were exposed after rapid uplift and tectonic erosion of the continental margin, coeval with the extinction of Cenozoic magmatism. The age and patterns of magmatism, as well as the causes of margin truncation in relation to the displacement of the Chortis block, have been matter of controversy.

We carried out U-Pb zircon geochronological studies in the three major batholiths forming a belt parallel to the present trench between Jamiltepec and Puerto Escondido. Although the batholiths are heterogeneous in mineralogy and chemical composition, the data display similar Oligocene crystallization ages (27-25 Ma). These are nearly consistent with the previously reported K-Ar ages in the area, suggesting rapid magmatic cooling. Another finding in this study is the recognition of plutons yielding Permian-Triassic ages emplaced in an older unnamed and poorly known metamorphic assemblage near Puerto Escondido that is also older than the Xolapa Complex.

Abundant roof pendants of the Xolapa Complex in the study area display clear cross-cutting relationships with the Cenozoic plutons. In the Jamiltepec area we dated a migmatite roof pendant in which the leucosome yields 60 Ma, whereas the host pluton yields 26 Ma. Other roof pendants near San Pedro Mixtepec show evidence of mafic and probably ultramafic rocks not yet reported in the Xolapa Complex.

These new findings suggest the occurrence of a Paleozoic assemblage between the northern Xolapa Complex and the Oaxaca Complex probably connected with Juchatengo terrane in which the imprint of Permian magmatism has been preserved. We discuss that this new evidence may contribute to the understanding of Cenozoic tectonics of southern Mexico, as well as providing new insights on the petrogenesis and evolution of the Sierra Madre del Sur magmatic province.