PETROLOGICAL, STRUCTURAL, GEOCHEMICAL AND GEOCHRONOLOGICAL CHARACTERISTICS OF THE CUICATECO TERRANE, TEHUANTEPEC ISTHMUS AREA , OAXACA, MEXICO

The southeastern portion of Cuicateco terrane in southern Mexico is composed of a low grade volcanosedimentary sequence. The volcanic rocks mainly correspond to amphibolites with fine-grained porphyritic to phaneritic textures made up of plagioclase, hornblende, epidote and quartz phenocrysts set in a microlitic matrix. The metasedimentary rocks are phyllites display lepidoblastic texture made of muscovite, quartz, graphite, zircon and iron oxides crystals, as well as lithic fragments. We obtained ages of individual detrital zircons by Laser Ablation Multicollector ICPMS. The youngest zircons indicate a Maastrichtian (maximum) depositional age (78 Ma)and the oldest zircons show Archean (minimum) depositional age (2800 Ma).

The rocks of the Cuicateco terrane display at least three deformational events: D1) Reclined to isoclinal F1 folding of the S0 bedding surface, with a general vergence toward northeast and producing a penetrative, moderately SW-dipping S1 axial plane foliation with parallel development of hornblende, epidote, chlorite, muscovite and quartz. D2) Moderately dipping, NE-directed thrusting of the entire sequence, and F2 folding of S1 axial surface without associated minerals. D3) Left-lateral to normal faulting, affecting the whole rock sequence. We consider that the D1 and D2 events can be associated to the Laramide Orogeny. D3 is a possible Tertiary feature, similarly to what is observed farther to W along the other fault system like Chacalapa - La Venta.

The chemical compositions of amphibolite rocks of the Cuicateco terrane are similar to calc-alcali basalts (SiO2 of 45 to 50 wt %, light REE depleted and flat HREE paterns). The trace element displays a incompatible element depleted pattern, with negative anomalies of Th and Nb as well as a light enrichment in compatible elements. A possible interpretation of these results suggests a basic volcanism in a backarc environment during the Mesozoic.