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

Abstract

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.