TIMING AND PETROGENESIS OF TERTIARY SILICIC VOLCANISM IN RELATION TO STRIKE-SLIP TECTONICS IN THE NORTH-CENTRAL PART OF SIERRA MADRE DEL SUR, MEXICO

The Eocene-Oligocene volcanic rocks of the north-central part of the Sierra Madre del Sur (SMS) represent an extensive episode of silicic arc volcanism close in age and composition with that of southeastern Sierra Madre Occidental (SMO) volcanic province. Despite these affinities, the Tertiary tectonic framework for southern Mexico displays contrasting styles and controls over the distribution of the silicic volcanism, as well as differences in the timing of the peak of magmatism. North of the Mexican Volcanic Belt (MVB) the ignimbrite flare up is related to episodes of east-west extension beginning ca. 30 Ma. South of the MVB the voluminous silicic volcanism is mainly related to strike-slip tectonics that reactivated older NW and N trending structures. K-Ar dating of volcanic rocks in the north-central part of the SMS indicates a maximum episode of silicic volcanism from ca. 38 to ca. 32 Ma. The main volcanic centers related to the beginning of this episode are located in the Taxco, Tilzapotla and Huautla regions and seem to have developed in overstep zones and extension zones at the termination of lateral faults. At the final stage of this episode N-S faults were reactivated as left lateral faults. Sr (87Sr/86Sr=0.7035 to 0.7060), Nd (ENd=+6 to -1) and Pb (206Pb204Pb=18.798 to 19.000, 207Pb/204Pb=15.590 to 15.653, 208Pb/204Pb=38.440 to 38.822) geochemical signatures of the northern SMS volcanic rocks display a trend that suggests a source in the mantle wedge with variable degrees of crustal contamination. The narrow range of the Pb data points to a relatively homogeneous crustal component. This component does not seem to represent the isotopic diversity of the exposed metamorphic basement in the region (Acatlan Complex), nor the values of the Oaxaca and the Guerrero terrane basements. These facts point to a relatively homogeneous lower crust underlying the Acatlan Complex as a source of the crustal contamination. The voluminous silicic magmatism of the northern SMS represents an episode of significant contribution of wedge mantle derived mafic magmas. The strike-slip tectonics imposed by the plate interactions in front of the continental margin of southern Mexico generated discontinuous extension zones that facilitated the ascent of magma.