GEOCHEMICAL ANALYSIS OF META-IGNEOUS ROCKS, OAXACA MEXICO: TECTONIC IMPLICATIONS

Oaxaquia is the Precambrian – Paleozoic terrane, underlying the Mesozoic and Cenozoic rocks in Mexico. (Keppie, D et al. 2002). The Oaxacan Complex contains exposures of Proterozoic (Grenville-aged) meta-sedimentary and meta-igneous rock from Oaxaquia, located to the southwest of the city of Oaxaca. The Oaxacan Complex contains three units the Huitxo, which includes the 1012Ma Anorthosite-Mangerite-Charnokite-Granitic suite (AMCG suite), the El Catrin a migmatic orthogniess dated at 1350Ma and the El Marquez dated at 1140Ma (Solari et al., 2002). The Oaxacan Complex experienced two tectnothermal events, the Olmecan orogeny, at 1106 +/- 6Ma and the Zapotecan orogeny between 980 and 1104Ma (Keppie et. al., 2001). Based on published geochemical and structural data from the Oaxacan Complex, Keppie et al. (2002) believe that the Olmecan and Zapotecan orogenies were driven by continent – continent collision, arc continent collision involving a juvenile arc and Oaxaquia or an Andean type orogeny (Keppie and Ortega-Gutierrez et. al. 1999). Samples collected from the AMCG suite in the Oaxacan Complex all show pervasive alteration due to a hydration event that post-dates the last deformational event. Common secondary minerals include calcite, and muscovite after plagioclase and biotite after pyroxene. Work done on major and trace element chemistries of meta-igneous rocks in the Guchicovi Complex (a proposed extension of the Oaxaquia terrane) suggest they are rift related intrusives with back arc basin or within plate characteristics. Geochemical data from the Guchicovi complex suggests that they formed in a volcanic arc environment; although some samples have within plate characteristics. Typically AMCG suites are associated with an-orogenic environments (Weber and Hecht, 2003). Preliminary analysis of the trace and REE geochemistry of meta-igneous units from the Oaxaca Complex suggest that these magmas originated in a volcanic arc setting. Continuing research is being done to determine the degree of alteration these rocks have undergone in an effort to deduce the potential effect of this alteration on their REE and trace element chemistries. These data will be used to see if the use of geochemical methods for the determination of tectonic environment may be adversely affected by this alteration event.