The Sierra de Juárez Complex: a new Gondwanan Neoproterozoic-early Palaeozoic metamorphic terrane in southern Mexico

Abstract

ABSTRACT Proterozoic to lower Palaeozoic metamorphic sequences are exposed in southern Mexico providing an important geological record for Rodinia and northwestern Gondwana reconstructions. The Sierra de Juárez Complex (SJC) is an enigmatic basement block in southern Mexico that recorded Jurassic dextral strike-slip mylonitic deformation, likely instigated by Pangea break-up and opening of the Gulf of Mexico. However, the origin and pre-Mesozoic evolution of this block are poorly constrained. In this paper, we present the result of fieldwork, LA-ICP-MS U-Pb zircon geochronology, and major and trace element geochemical analyses that allow us to define the following lithological units in the SJC: (1) an anorthositic-gabbroic metamorphic unit (Viguera Suite) with protolith ages of 979–976 Ma and a thermal disturbance event at ca. 920–880 Ma; (2) a MORB-like metagabbroic unit (San Agustín Orthogneiss) with a protolith age of 441.9 ± 3.6 Ma; (3) a metamorphic supracrustal sequence (Etla Unit) that includes amphibolites and pelitic schists with maximum depositional ages of 422–414 Ma and a detrital provenance sourced in Gondwanan-type terranes; and (4) metagranitic intrusions with a protolith age of 191.9 ± 1 Ma, suspected to be syntectonic with respect to the Jurassic mylonitic event. The Silurian-Devonian San Agustín Orthogneiss and Etla Unit constitute the roots of a formerly undocumented lower Palaeozoic terrane that, together with the Viguera Suite, underwent high-pressure amphibolite facies metamorphism between 398–391 Ma, likely related to contractional tectonics. Based on these data, the SJC is proposed as a Mesoproterozoic to middle Palaeozoic crustal block with a geologic evolution coupled to NW Gondwana and a potential correlation with the Grenville-aged and peri-Gondwanan-type terranes like those currently located in southeastern Mexico, Central America, and northwestern South America.