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 d istributed south of the Trans-Mexican Volcanic Belt (TMVB) and extend to th e southern continental margin of Mexico, They represent magmatic activity t hat originated at a lime characterized by significant changes in the plate interactions in this region as a result of the formation of the Caribbean p late and the southeastward displacement of the Chortis block along the cont inental 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 rearrangeme nt. 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 batholith s and stocks of predominantly felsic composition. The second bell is inland of the first and consists of discontinuously distributed volcanic fields w ith piles of andesitic to rhyolitic flows, as well as epiclastic and pyrocl astic materials. These two belts were emplaced along a continental crust se gment constituted by a mosaic of basements with recognizable petrologic and isotopic differences. These basements originated during different tectono- thermal events developed from the Proterozoic to the Mesozoic, Major and trace element data of the SMS magmatic rocks define a clear sub-a lkaline tendency. Variations in the general geochemical behavior and in the Sr and Nd isotopic ratios indicate different degrees of magmatic different iation and/or crustal contamination. These variations, specially in the inl and Oligocene volcanic regions of Guerrero and Oaxaca slates, seem to have been controlled by the particular tectonic setting at the time of magmatism , In northwestern Oaxaca greater extension related to transtensional tecton ics produced less differentiated volcanic rocks with an apparently lower de gree of crustal contamination than those of northeastern Guerrero. The geoc hronologic data produced by us up to now, in addition to those previously r eported, 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 th e age of igneous activity, rather than a gradual northeastward migration of the locus of magmatism toward the present-day TMVB, SMS magmatic rocks exp osed to the west of the 100 degrees W meridian are dominantly Late Cretaceo us to Eocene, while those to the east range from Oligocene to Miocene, also following a southeastward age-decreasing trend. Paleocene and Eocene magma tic 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 Occiden tal (SMO). In the eastern region of the SMS the Oligocene magmatic rocks sh ow a trend that roughly defines an ESE orientation. The change in the trend of are magmatism may be the effect of the landward migration of the trench , for a given longitude, as a result of the displacement of the Chortis blo ck. The transtensional tectonic regime developed in Oligocene time in NW Oa xaca probably accentuated this trend by facilitating magma generation and a scent in these northerly regions. The geochronologic data of the SMS, in conjunction with those of the TMVB, suggest that there is a spatial and temporal magmatic gap in south central Mexico between 97 and 100 degrees W longitude during late Oligocene and mid dle Miocene time (24-16 Ma). This magmatic gap is interpreted in terms of a combination of the relatively rapid change in the subducted slab geometry after the passage of the Chortis block from a moderate to a shallow angle a nd the time needed for the mantle wedge to mature sufficiently to produce m agmas. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.