Geochemistry of Paleozoic basalts from the Juchatengo complex of southern Mexico: tectonic implications

Analyses of Lower Permian or older basalts and associated dykes of the Juchatengo sequence indicate that they are rift tholeiites that formed in a continental rift or back-are tectonic setting. Age constraints include a Middle Permian fossil recovered from the tectonically overlying sediments and a cross-cutting, post-tectonic pluton dated by K/Ar on hornblende at 282 +/- 6 Ma. A location adjacent to the Oaxacan Complex or other old continental crust is suggested by (1) an epsilon Nd-i isotopic value of -8.95 and a T-DM age of 1487 Ma in the overlying sediments, which are similar to the Oaxacan Complex; (2) enrichment of incompatible elements in the lavas, suggesting old crustal contamination; and (3) the presence of Permian-Triassic calc-alkaline plutons that stitch the Juchatengo-Oaxaca boundary. The possible tectonic models depend on the age of the Juchatengo basalts, which requires future geochronological work. If the Juchatengo basalts are Permo-Carboniferous, they could have formed near the eastern edge of a back-are basin: the contemporaneous are would have rifted away to the west. Eastward migration of the are magmatism indicated by the Permian-Triassic calc-alkaline plutonism may reflect shallowing of the dip of the subduction zone, which probably also produced the deformation of the Juchatengo sequence. (C) 1999 Elsevier Science Ltd. All rights reserved.