K/AR DATING AND MAGNETOSTRATIGRAPHY OF CRETACEOUS AND OLIGOCENE IGNEOUS ROCKS FROM HUAJAPAN DE LEON-PETLALCINGO, MIXTECA TERRANE, MEXICO

Results of a K-Ar dating and paleomagnetic study of intrusive and volcanic units from the Huajapan de Leon-Petlalcingo area document the occurrence of magmatic activity during the Cretaceous and Oligocene in the Mixteca region, southern Mexico. The Cretaceous dioritic intrusives, Chilixtlahuaca and San Jeronimo, present normal magnetic polarity and K-Ar dates of 129 +/-5 and 130 +/- 5 Ma, and 103 +/- 5 Ma, respectively. A date on a plagioclase concentrate of 144 +/- 5 Ma and an additional date of 127 +/- 5 Ma on a sample from a nearby outcrop 2 km away were determined for the Chilixtlahuaca intrusive. The Chilixtlahuaca intrusive was emplaced during the late part of the Mesozoic magnetic polarity M-sequence, and the San Jeronimo intrusive during the Cretaceous normal superchron. The K-Ar dates for the Tertiary units fall within a narrow range 29-32 Ma. Two intrusives, Chila and La Capilla, give dates of 30 +/- 1 Ma and 31 +/- 1 Ma and present normal polarity. Five dates for four basaltic andesitic flows of the Tres Coronas sequence range from 29 +/- 1 to 32 +/- 1 Ma. The magnetic polarity is reverse. The Oligocene Chila and La Capilla intrusives and Tres Coronas volcanics correlate with volcanic units of the Ahuehuetitlan Unit and Yucudaac and San Marcos andesites to the west and southeast of the study area. They are part of a magmatic arc that developed in mid-Tertiary times in southern Mexico.