Ordovician calc-alkaline granitoids in the Acatlan Complex, southern Mexico; geochemical and geochronologic data and implications for the tectonics of the Gondwanan margin of the Rheic Ocean

U-Pb zircon data from three undeformed to slightly deformed, megacrystic, granitoid plutons in the northern Acatlan Complex of southern Mexico has indicated that all three are part of a larger suite of late Ordovician plutons. (super 40) Ar/ (super 39) Ar? data from hornblende and biotite show mainly disturbed spectra, but biotite from the Palo Liso and Los Hornos plutons yields plateaus with ages of 305+ or -26 Ma and 157+ or -12 Ma, respectively. These thermal events may be correlated, respectively, with Permo-Triassic and Jurassic tectonothermal events recorded elsewhere in the Acatlan Complex. All three plutons are peraluminous with calc-alkaline affinities, characteristics that are consistent with inherited zircon ages and together suggest a source in Mesoproterozoic calc-alkaline rocks similar to those exposed in the neighboring Oaxaca terrane. We interpret these granites to be related to the early Ordovician separation of peri-Gondwanan terranes from Gondwana during the opening of the Rheic Ocean. Elsewhere in the Acatlan Complex, Ordovician megacrystic granitoids of the Piaxtla Suite were subjected to high-grade metamorphism, which we infer to be related to subduction along the Gondwanan margin during the Devonian-Carboniferous. The three plutons reported here were not affected by Devono-Carboniferous metamorphism and thus are inferred to have remained outside the subduction zone.