Tertiary arc-magmatism of the Sierra Madre del Sur, Mexico, and its transition to the volcanic activity of the Trans-Mexican Volcanic Belt

The Tertiary magmatic rocks of the Sierra Madre del Sur (SMS) are broadly distributed south of the Trans-Mexican Volcanic Belt (TMVB) and extend to the southern continental margin of Mexico. They represent magmatic activity that originated at a time characterized by significant changes in the plate interactions in this region as a result of the formation of the Caribbean plate and the southeastward displacement of the Chortis block along the continental margin of southwestern Mexico. The change from SMS magmatism to an E‐W trending TMVB volcanism in Miocene time reflects the tectonic evolution of southwestern Mexico during these episodes of plate tectonic rearrangement. The distribution and petrographic characteristics of the magmatic rocks of the SMS define two belts of NW orientation. The first is represented by the nearly continuous coastal plutonic belt (CPB), which consists of batholiths and stocks of predominantly felsic composition. The second belt is inland of the first and consists of discontinuously distributed volcanic fields with piles of andesitic to rhyolitic flows, as well as epiclastic and pyroclastic materials. These two belts were emplaced along a continental crust segment constituted by a mosaic of basements with recognizable petrologic and isotopic diAerences. These basements originated during diAerent tectono-thermal events developed from the Proterozoic to the Mesozoic. Major and trace element data of the SMS magmatic rocks define a clear sub-alkaline tendency. Variations in the general geochemical behavior and in the Sr and Nd isotopic ratios indicate diAerent degrees of magmatic diAerentiation and/or crustal contamination. These variations, specially in the inland Oligocene volcanic regions of Guerrero and Oaxaca states, seem to have been controlled by the particular tectonic setting at the time of magmatism. In northwestern Oaxaca greater extension related to transtensional tectonics produced less diAerentiated volcanic rocks with an apparently lower degree of crustal contamination than those of northeastern Guerrero. The geochronologic data produced by us up to now, in addition to those previously reported, indicate that the Tertiary magmatic rocks of the SMS range in age from Paleocene to Miocene. The general geochronologic patterns indicate a southeastward decrease in the age of igneous activity, rather than a gradual northeastward migration of the locus of magmatism toward the present-day TMVB. SMS magmatic rocks exposed to the west of the 1008W meridian are dominantly Late Cretaceous to Eocene, while those to the east range from Oligocene to Miocene, also following a southeastward age-decreasing trend. Paleocene and Eocene magmatic rocks of the western region of the SMS seem to keep a general NNW trend similar to that of the Tertiary magmatic rocks of the Sierra Madre Occidental (SMO). In the eastern region of the SMS the Oligocene magmatic rocks show a trend that roughly defines an ESE orientation. The change in the trend of arc magmatism may be the eAect of the landward migration of the trench, for a given longitude, as a result of the displacement of the Chortis block. The