ABSTRACT The Oaxacan Complex is the largest outcrop of Grenville-age rocks in Mexico, constituting the main crustal fragment in the backbone of Oaxaquia. It is mainly composed of scarce metasediments, intruded by arc, alkalic, and tholeiitic magmas (ca. 1.3 to ca. 1.01 Ga) and later affected (ca. 0.99 Ga) by granulite-facies metamorphism. A detailed study, combining U-Pb geochronology by laser ablation–inductively coupled plasma–mass spectrometry, with in situ Hf isotopes in zircon grains, allowed comparison of the age and isotopic patterns of the Oaxacan Complex granulite rocks with those from other similar outcrops in Mexico (Huiznopala and Novillo Gneisses, Guichicovi Complex) and with the neighboring orogens such as the Grenville Province of the eastern United States and Canada, the Sveconorwegian orogen of SW Baltica, and some of the localities in which Mesoproterozoic rocks border the Amazonian craton of South America (Colombia, Peru, Brazil). Detrital zircon ages show that most metasedimentary rocks are younger than 1.4 Ga (only three samples contained zircon grains between 1.6 and 1.4 Ga), whereas U-Pb dating of igneous rocks (1245–1161 Ma) confirmed previous findings. Hf isotopes of dated zircon grains show that few crystals have negative ε Hf( t ) values, indicating a recycling component from an older crust, but most of them are moderately primitive, with ε Hf( t ) values of up to +12, and linear arrays parallel to the 176 Lu/ 177 Hf average crustal evolution model. Those Hf values are indicative of partial assimilation of an older crustal component, with Hf model ages of ca. 1.65–1.50 Ga. Comparison of these data helps to constrain possible Mesoproterozoic conjugate margins of Oaxaquia and propose a paleogeographic model in which Oaxaquia acted as the leading edge of Amazonia, together with the Colombian terranes, and received sedimentary input from different sources such as the southern Sveconorwegian orogen, the U.S.–Canada eastern Grenville Province, and some of the Mesoproterozoic belts bordering the Amazon craton.

ABSTRACT The Oaxacan Complex represents the largest outcrop of Grenvillian basement in Mexico. Broadly, it consists of pelitic gneisses, quartzofeldspathic gneisses, metasomatic calc-silicates, orthoamphibolites, and marbles, all intruded by anorthosites, orthocharnockites, and orthogneisses. The entire assemblage underwent granulite-facies metamorphism ca. 1 Ga. We studied for the first time the ultramafic rocks of the Oaxacan Complex, represented by six different samples, all corresponding to ultramafic granulites. Their igneous equivalents are orthopyroxenites, websterites, and clinopyroxenites, and they occur as metric-scale lenses or centimetric layers in paragneisses, or in mingling textures with anatectic marbles. We studied their petrography, geochemistry, geochronology, and geothermobarometry to elucidate their genesis and tectonic implications. Our samples have enriched mid-ocean-ridge basalt and oceanic-island-arc affinities, both tholeiitic and calc-alkaline. Rare earth element patterns normalized to chondritic uniform reservoir from whole rock or single minerals define two or three main groups related to their origin and metamorphic history. Based on their protoliths, these rocks can be divided into: (1) ortho-derived pyroxenites (pre–Grenvillian orogeny), the origin of which was a magmatic cumulate or mafic melt or a mantle rock that had undergone metasomatism; and (2) para-derived pyroxenites (syn- or post-Grenvillian orogeny), the origin of which was a calc-silicate rock undergoing pervasive anatectic and metasomatic processes. The geothermobarometry revealed different stages in the syn- and post-Grenvillian granulitic metamorphic history of the Oaxacan Complex. The high temperature calculated from one sample (~945 °C), in the ultrahigh-temperature metamorphic field, is probably closer to the granulitic metamorphism peak than those obtained in previous studies, although a relict igneous temperature cannot be ruled out with the present data.