MATANZAS STOCK: TRIASSIC CRUSTAL ANATEXIS IN THE MAYA BLOCK

The Guatemala Suture Zone is part of a series of tectonic sutures that record the Late Cretaceous–Paleogene North America-Caribbean collision. This suture is made up of three juxtaposed high-pressure metamorphic belts, one south and two north of the Motagua Fault System. This fault system includes the Polochic Fault and the Baja Verapaz Shear Zone, which are boundaries of the high-grade Chuacús Complex and greenschist-facies metasedimentary rocks of the Maya block. In the area between the Polochic and Baja Verapaz structures, a set of plutonic bodies with variable deformation (e.g., Rabinal, Sinanjá, and Matanzas) intrude low-grade metasedimentary rocks.

Here, we present a characterization of the Matanzas stock performed using fieldwork, petrographic, geochemical, and geochronological data. The results indicate that the Matanzas is an S-type, felsic, peraluminous granite bearing white mica and garnet. The unit is undeformed, and it clearly intrudes host metasedimentary rocks. U-Pb zircon geochronology has yielded relatively imprecise ages, probably because its composition favored the preservation of pre-existing zircon. The bulk of U-Pb ages range between 490–310 Ma. Minor age groups range ~1470–910 Ma, ~870–520 Ma, and ~280–232 Ma. A Rb-Sr isochron calculation using white mica and whole rock yielded a 237 ± 8 Ma age. We interpret this Middle–Late Triassic age as the best constraint for the time of igneous crystallization of the Matanzas granite.

We propose that the Matanzas stock was produced by anatexis from the continental crust, perhaps produced by magmas of the Permian–Triassic arc characteristic of Mexico and northern South America. Alternatively, anatexis may have been produced by crustal extension during the initial stages of Pangea breakup, as reported for the northern Andes. Furthermore, the massive character and age of the Matanzas stock proves that it was not involved in the Late Cretaceous–Paleocene subduction and collision processes recorded in the southernmost part of the Maya block.