THE CHORTÍS BLOCK - SOUTHWESTERN MEXICO CONNECTION DURING THE LATE MESOZOIC - CENOZOIC: U-PB ZIRCON CONSTRAINTS

The proposed connections between basement terranes of southwestern Mexico and the Chortís Block of Central America are tested here, focussing on metamorphic units in northern Chortís, southeastern Guatemala. Two chief units are recognized: the mid- to high-metamorphic grade Las Ovejas Complex (LOC) made up of ortho-, para-gneisses, schists, amphibolites and marbles; and the tectonically juxtaposed, low- metamorphic grade San Diego Phyllite. U-Pb geochronology carried out by LA-ICPMS on zircons separated from both units reveal mayor differences between them. The high-grade metamorphism and deformation of LOC is bracketed between ~27 and ~37 Ma, which correspond to the U-Pb zircon ages of the oldest cross-cutting dikes and youngest zircons in metasedimentary samples. Las Ovejas samples share xenocrystic and detrital zircon populations of Middle-Upper Jurassic, Middle-Upper Triassic and Permo-Carboniferous age. In contrast, the San Diego Phyllite contains zircons that overlap at ~830-870 and ~1150-1220 Ma, with the youngest concordant zircons yielding Cambrian age. The contrast in population ages supports the idea that LOC and San Diego Phyllite are different lithotectonic units tectonically juxtaposed during or after their exhumation. When compared with available U-Pb zircon data for southern Mexico, LOC show similarities to samples of the Guerrero terrane, mainly the Arteaga Complex and Zihuatanejo subterrane, as well as to the southern and northern portions of the Cuicateco terrane. The San Diego Phyllite shows instead similarities with the Teloloapan subterrane of Guerrero terrane, the Mexcala Formation, as well as the low grade units of the Cuicateco terrane and, for the Grenvillian ages of the Oaxacan Complex igneous samples, as well as some of the samples belonging to the Maya Block. We propose a tectonic model in which LOC was a portion of a fringing arc located in front of the Cuicateco terrane of southwestern Mexico, removed during the Early Tertiary by the docking of the Chortís Block represented by the San Diego Phyllite, and tentatively by those units recognized elsewhere in the Chortís Block in Honduras and Nicaragua. Eocene-Oligocene metamorphism and deformation of LOC would be a result of the tectonic removal, mainly characterized by sinistral stretching and tectonic transport.