HUAUTLA VOLCANIC FIELD: A TRANSITION ZONE OF THE IGNIMBRITE FLARE-UP IN THE NORTH CENTRAL REGION OF THE SIERRA MADRE DEL SUR

The Huautla volcanic field (HVF) in southern Mexico, states of Morelos and Guerrero, is the most extensive field in the north central region of the Sierra Madre del Sur (SMS). It forms part of the Paleogene magmatic province of the SMS that is composed of two belts: one represented by volcanic and subvolcanic rocks in the inland regions and the other one by a plutonic belt in the coastal zone.

The north central region of the SMS is a discontinuous inland arc composed of a series of Paleogene silicic and intermediate volcanic centers. The SMS has been considered as the southern extension of the Cenozoic ignimbrites of the Sierra Madre Occidental province (SMO). However, contrasting details in the age trends and associated tectonic features are now emerging.

The HVF is interpreted as a transitional zone based on its location between a region of old Paleozoic crust to the east and a complex Mesozoic terrane to the west. Furthermore, the late Eocene-early Oligocene volcanism in this region changed from predominantly silicic in composition in the west to intermediate composition in the east (western Oaxaca).

In the HVF several volcanic stages have been recognized and grouped into three episodes based on their ages: 1.- andesitic to rhyolitic volcanism prior to that of the Tilzapotla Ignimbrite (<34.5 Ma), ranging in age from 37 to 36 Ma and corresponding to the volcanic basement; 2.- a second episode represented by ignimbrites of the Tilzapotla Caldera, ~34.5 Ma; 3.- a predominantly siliceous event with ages ranging from ~33 to 31 Ma; and 4.- a final andesitic episode ~30 Ma.

The integration of the new dates for the HVF suggests the presence of an ignimbrite flare up in this region some ~36 to 32 Ma ago. This age contrasts with that of the flare up of the SMO province in its southern sector (~24-20 Ma). In addition, discussion of the geochemical and isotopic data obtained for the HVF, which add to the understanding of the petrogenetic factors that produced this voluminous silicic volcanism, are presented