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
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U-Pb zircon data from three undeformed to slightly deformed, megacrystic, granitoid plutons in the northern Acatln Complex of southern Mxico has indicated that all three are part of a larger suite of late Ordovician plutons. 40 Ar 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 26 Ma and 157 12 Ma, respectively. These thermal events may be correlated, respectively, with Permo-Triassic and Jurassic tectonothermal events recorded elsewhere in the Acatln Complex. All three plutons are peraluminous with calc-alkaline affinities, characteristics that are consistent with inherited zircon ages and together suggest a source in Mesopro-terozoic calc-alkaline rocks similar to those exposed in the neighboring Oaxaca ter- rane. 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 Acatln 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 metamor-phism and thus are inferred to have remained outside the subduction zone. 2007 Geological Society of America. All rights reserved.
