Petrology and geochemistry of metaigneous rocks from a Grenvillian basement fragment in the Maya block: the Guichicovi complex, Oaxaca, southern Mexico

The Guichicovi complex is the southeasternmost exposure of Proterozoic granulites in Mexico and forms the basement at the western edge of the Maya block. Its Grenville-affinity is similar to other basement rocks in Mexico that comprise the Oaxaquia microcontinent. Several authors have interpreted Oaxaquia as a volcanic arc terrane formed similar to 1.2 Ga ago outboard the Grenville Province. This volcanic arc was intruded by igneous rocks of the anorthosite-mangerite-charnockite-granite suite (AMCG-suites) prior to collision during the Grenville orogeny and subsequent granulite facies metamorphism similar to 1.0 Ga ago. The Guichicovi complex consists of metasedimentary sequences and basic to acid metaigneous rocks. We analyzed major, trace, and rare earth elements of metaigneous rocks from the Guichicovi complex that comprise two major lithologic units: (1) the Zacatal unit composed of felsic granulites (charnockites) and orthogneisses, and (2) the Northern Guichicovi unit composed of anorthositic-tonalitic gneisses, mafic granulites (ferrodiorites), and amphibolites. These two major lithologic units can clearly be distinguished by their chemical compositions. Most of the analyzed rocks indicate a volcanic-arc tectonic environment, but there are exceptions with within-plate characteristics. The Zacatal unit may have formed by crustal anatexis within the Oaxaquia arc-terrane, partly in a within-plate environment. One of those granitic gneisses with within-plate characteristics has a 1.23 Ga protolith age, which indicates that either the Oaxaquia arc was formed earlier, or that protoliths of this granitic unit have an origin distinct to the Oaxaquia arc terrane. We consider different models to interpret the Northern Guichicovi unit. It cannot conclusively be explained by an anorthosite suite, although the ferrodiorites can. Partial melting of the lower crust at a volcanic-arc basement may have formed the anorthositic-tonalitic gneisses and ferrodioritic melts. (C) 2003 Elsevier Science B.V. All rights reserved.