PROGRESSIVE CONSTRUCTION OF A BATHOLITH UNDER THE TILZAPOTLA-TAXCO REGION, SOUTHERN MEXICO, IN LATE EOCENE TIME

Late Eocene-early Oligocene silicic volcanic successions constitute an important part of the stratigraphic record in the north-central part of the Sierra Madre del Sur. Of special interest is the Tilzapotla-Taxco region, located within a NW-SE oriented elliptical dome structure (50 x 35 km) that encloses the Tilzapotla caldera, rhyolitic domes, several ignimbritic units, as well as medium-grained plutons and hypabyssal intrusive bodies, all of similar age. Plutonic rocks are dominantly granitic to granodioritic in composition, although monzonitic and dioritic facies occur. The spatial proximity of the extrusive and intrusive units provides an excellent opportunity to study the connection between volcanism and plutonism in the region.

Regional geochronology was carried out previously by our group in the silicic magmatic rocks, as well as in volcanic rocks intercalated with underlying tilted continental sediments; results suggest uplifting in this region during the late Eocene (~38-34 Ma) and a doming process related to the emplacement of silicic magmas prior to the Tilzapotla caldera collapse. We have obtained LA-ICPMS zircon U-Pb and Ar-Ar mineral ages for several of the different magmatic facies within the elliptical dome structure; the new age data support the interpretation of magma emplacement associated with a doming process. We propose that the development of the elliptical dome structure is related to the progressive construction of a batholith at depth under the Tilzapotla-Taxco region in late Eocene time (~39-35 Ma). Electron microscope analyses of different magmatic facies within the dome show compositional differences that apparently indicate the injection of different batches of magma. The Tilzapotla caldera, in the south-easternmost part of the dome structure, represents an episode characterized by a high rate of magma input. Caldera collapse might have been triggered by injection of mafic components identified in the southeastern part of the dome.