U-Pb and 40Ar/39Ar constraints on the cooling history of the northern Oaxacan Complex, southern Mexico: Tectonic implications

To better define the cooling history of the northern Oaxacan Complex, titanite and phlogopite from metasedimentary calc-silicate and biotite from a pegmatite were collected. All these rocks were involved in the granulite-facies Zapotecan orogeny between ca. 1004 and 978 ± 3 Ma, inferred to result from underthrusting the Oaxacan Complex beneath an arc or continent. Fragments of 2 × 5 cm² titanite crystals yielded a concordant U-Pb age of 968 ± 9 Ma, whereas ⁴⁰Ar/³⁹Ar analyses of phlogopite and biotite gave ages of 945 ± 10 Ma and 856 ± 10 Ma, respectively. These ages are inferred to date cooling through 660-700 °C, 450 °C, and 300-350 °C, respectively. When combined with published ages (Sm-Nd garnet, ⁴⁰Ar/³⁹Ar hornblende, Rb-Sr biotite and whole rock, and K-Ar biotite and K-feldspar) the data define a two-stage cooling curve: (1) 8 °C/m.y. between 978 and 945 Ma, cooling through 450 °C by which time the rocks had risen through a depth of 15 km; and (2) 2 °C/m.y., which, by extrapolation, brought the rocks to the surface between 710 and 760 Ma. The first stage of exhumation is interpreted in terms of tectonic switching between steep and flat slab subduction, a result of interactions of a ridge, a plume, or an oceanic plateau with the trench. The second stage may be related to thermal relaxation of the lithosphere, ending with the breakup of Rodinia, which brought the rocks to the surface.