GEOCHRONOLOGY OF IGNEOUS ROCKS AND ITS RELATION TO TECTONIC PROCESSES IN THE CHILPANCINGO – TIERRA COLORADA REGION, GUERRERO STATE, SOUTHERN MEXICO

Tectonic interpretations suggest that our study area in S Mexico was affected by episodes of continental truncation, subduction erosion, and changes in the geometry of the subducted slab. We document the following chronology of magmatic, structural, and stratigraphic events.

Early Cretaceous

126 Ma metavolcanics of the Chapolapa Formation are the oldest known magmatic sequences in the region. They can be correlated to 130 Ma deformed metagranitoids that intrude the Xolapa Complex; considered to form the basement unit. Both units display similar foliation, mineral lineation, green-schist metamorphic facies, and (D2) mylonitic deformation.

Late Cretaceous – Paleocene

Volcanic rocks of the Balsas Group (Agua de Obispo and Papagayo Formations) and the deformed Amatlán pluton of the Xolapa Complex show contemporary ages between 65 and 50 Ma. These dates are important to distinguish the major tectonic processes (D1, D2, D3) between 70 and 40 Ma. Agua de Obispo andesites were dated at 64.8 ± 1.3 Ma (U-Pb) and 56.3 ± 1.4 Ma (K-Ar). From ignimbrites and rhyolitic domes of the Papagayo Formation we obtained U-Pb zircon ages of 51.36 ± 0.97 Ma, 50.7 ± 1.2 Ma, and 49.89 ± 0.78 Ma. The Amatlán granite dated at 66.45 ± 0.74 Ma (U-Pb) is characterized by both mylonitic (D2) and non-deformed fabrics. The mylonitic part is foliated and lineated, similar to the Chapolapa Formation. The Amatlan pluton intrudes the Xolapa Terrane.

Eocene-Oligocene

The youngest magmatism (plutonic bodies and pyroclastic volcanics) was emplaced along structures related to a compressive event (D3), represented by folds, thrusts, and strike-slips. We obtained a K-Ar biotite cooling age of the Azinyehualco pluton of 33.7 ± 0.9 Ma, similar to the age of the Tierra Colorada granite. The volcanic rocks (36.0 ± 0.2 Ma U-Pb zircon and 30.3 ± 0.8 Ma K-Ar-Bi).are widely displayed in the structure of the Alquitrán caldera collapse.

Deformation Events

The Laramide deformation phase (D1) affected northern, central, and southern Mexico from mid to early Cretaceous, represented by N-S oriented folds. Ductile deformation (D2) is associated with a regional shear event and corresponds to a NW dipping normal fault; as a result of the exhumation of the Xolapa Terrane (63 - 59 Ma). Finally, a post-Laramide (47 to 40 Ma) Sierra Madre del Sur deformation event D3 affected both D1 and D2 phases.