Mid-tertiary cooling ages in the Precambrian Oaxacan Complex of southern Mexico: Indication of exhumation and inland arc migration

In southern Mexico, the ∼1 Ga Oaxacan Complex is in fault contact with the Mesozoic-Cenozoic Chatino terrane. ⁴⁰Ar/³⁹Ar dating of minerals from hornblende gneiss and quartz monzonitic gneiss in the southern Oaxacan Complex collected, 10 km and 1 km north of the Oaxaca-Xolapa boundary, yielded the following data, respectively: (1) a plateau age of 584 ± 10 Ma in hornblende and a pseudoplateau age of 23 ± 3 Ma in biotite; and (2) a plateau age of 42 ± 3 Ma in biotite, and a maximum age of 36 ± 1 Ma in K-feldspar. These are inferred to date cooling through ∼500-550° C for hornblende, ∼280° C for biotite, ∼220° C for plagioclase, and ∼310-270° C for K-feldspar. The ∼582 Ma age is much younger than cooling ages from the northern Oaxacan Complex, suggesting that it records resetting during a Neoproterozoic Brasiliano tectonothermal event. On the other hand, the Tertiary cooling ages suggest reheating adjacent to either ∼40 Ma or 35-25 Ma Tertiary plutons: ∼42 Ma and ∼23 Ma biotite ages, respectively. The former was followed by rapid cooling through ∼310-270° C by ∼36 Ma and exhumation before deposition of Miocene volcanic rocks. We relate these cooling ages to northward migration of the magmatic arc during the Oligocene-Miocene as a consequence of flattening of the subduction zone due to subduction erosion.