Geological characterization of the Taxco and Taxco el Viejo metamorphic ensambles, Guerrero, Mexico

A combination of studies that include geological mapping, petrography, microtectonics, geochemistry and geochronology, like the one presented here, allows the understanding of individual volcanic arcs and their collective context throughout the Mexican territory. This understanding will allow the development of more precise, and therefore less speculative, geological models to explain crustal growth through volcanic arc accretion processes (e. g. Guerrero terrane) along the continental margin. The petrology and geochemistry of metavolcaniclastic (phyllites), metalava and metaignimbrite units present in Taxco and Taxco Viejo are very similar, suggesting a common origin of formation in a volcanic arc environment. In fact, this is the first time that the Taxco Greenstone is defined as a metaignimbrite. Macro-and micro-tectonic studies of metavolcanic units in Taxco and Taxco Viejo indicate very similar deformation patterns, irrespective of the nature of the protolith. These rocks present well-developed foliation with E-NE and W-SW orientation that clearly suggests more than one deformational event in the region. In this study we propose that the Pb-206/U-238 ages obtained by the SHRIMP-RG single-grain zircon technique are more reliable estimates of the time of crystallization of the Taxco (137.1 +/- 0.9 Ma) and the Taxco Viejo (135.6 +/- 1.4 Ma) metavolcanic protoliths than those obtained by the more conventional ID-TIMS (multi-grain) method. It is important to note that since both crystallization ages are the same, within analytical error, it is safe to propose a common igneous event linking the Early Cretaceous volcanism of Taxco and Taxco Viejo. This Early Cretaceous age for the metamorphosed volcanic arcs, discussed in detail in this paper, was first proposed in 2004 during a scientific meeting, and since then the classic speculation that these rocks, known as the Taxco Schist and the Taxco Viejo Greenstone, were Precambrian and Paleozoic, respectively, was proven to be incorrect. This milestone in geological knowledge helps support a hypothesis that suggested a Cretaceous and/or Jurassic age for these volcanic units present in Taxco and Taxco based on stratigraphic and paleontological constraints of volcano-sedimentary units in the Guerrero terrane. The proposed Cretaceous age for the volcanic arc rocks-combined with the influential hypothesis that suggests an allochthonous geological character for these volcanic arcs that later collided against the continental margin (e. g., Oaxaquia) during the Late Cretaceous-represents a strong alternative to the autochthonous origin that suggests an old, perhaps Precambrian, basement for the Guerrero terrane in the Taxco and Taxco Viejo areas, implying a continental volcanic arc environment for these volcanic rocks.