A chronological and chemical zircon study of some pegmatite dikes and lenses from the central part (Ayoquezco-Ejutla) of the Oaxacan Complex, southern Mexico

Abstract

We carried out a geochronological and geochemical study of zircons from seven pegmatite intrusions collected from the central part (Zimatlán-Ayoquezco-Ejutla villages) of the Oaxacan Complex, southern Mexico. U-Pb ages and trace element chemistry were obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The objective of this work is to determine the time of pegmatite emplacement and its high grade metamorphism, if present, by U-Pb dating and identify its possible source and crystallization environment, using trace element concentrations in zircons. The geochronological study allowed to distinguish three main groups of pegmatites: post-tectonic, syntectonic, and pre-tectonic with respect to the granulite facies metamorphism event, which have ages in the ranges of 963 ± 7 to 977 ± 5 Ma, 980 ± 5 to 981 ± 7 Ma, and 1190 ± 7 to 1201 ± 5 Ma, respectively. The REE geochemistry in pegmatite zircons shows that the mechanism of pegmatite formation was in some cases magmatic, in others metamorphic or in between. It has been suggested before that all pegmatites of this region are “granitic” and are the result of a classical evolution of a felsic melt formed in situ during the anatexis of the Oaxacan Complex rocks. The interpretation of our chemical data indicates that the composition of the initial melt, from which each class of pegmatite was formed, can be ultramafic, alkaline or carbonatitic, and only one sample shows a granitic-like initial composition. This means that the pegmatites of the Oaxacan Complex are of diverse origin and only those of quartzfeldspar mineralogy are actually granitic in origin.