U–Pb geochronology, geochemistry, and provenance of the Grenvillian Huiznopala Gneiss of Eastern Mexico

Abstract

The Huiznopala Gneiss is the smallest (∼25 km 2) of four exposures of Grenvillian granulites in eastern and southern Mexico. The Gneiss comprises three major lithologic units: (1) a main series of orthogneisses; (2) an anorthosite–gabbro complex; and (3) a layered paragneiss sequence. Thirty-one U–Pb zircon ages from seven samples all lie within a restricted range of ∼200 million years, and the geochronological results are interpreted to reflect principally a two-stage history. The earliest stage was arc magmatism that extended from ∼1200 to ∼1150 Ma. The second stage included granulite facies metamorphism, which peaked at about 725±50°C and 7.2±1.0 kbar, and perhaps emplacement of the anorthosite–gabbro complex at ∼1000 Ma. A pegmatite with an age of 988±3 Ma post-dates ductile deformation. The timing of ductile deformation and granulite facies metamorphism of the Huiznopala Gneiss corresponds closely to that of final thrusting and deformation in the Grenville Province. Thus, the Huiznopala Gneiss may be in the core of the orogen formed during the final thrusting event recognized in the Grenville orogeny. Gondwanan faunal assemblages are found in lower Paleozoic sedimentary rocks that overlie Grenvillian granulites elsewhere in Mexico. However, the Pb isotope compositions of samples from the Huiznopala Gneiss overlap with those of the basement from Laurentia (Adirondack Highlands and Lowlands, Texas), and they are distinct from those of the Gondwanan Arequipa–Antofalla craton of southern Bolivia and northern Chile. The magmas parental to the Huiznopala arc and those parental to the Laurentian crust may have shared a common isotope reservoir, and the Pb isotope compositions are consistent with the paleomagnetic results that place southern Mexico adjacent to Ontario/Quebec at about 950 Ma.