PROVENANCE AND PALEOZOIC HISTORY OF THE ACATLAN COMPLEX IN THE LA NORIA AREA OF SOUTHERN MEXICO

The Acatlán Complex of southern Mexico exposes the country's largest assemblage of Paleozoic rocks, the evolution of which has traditionally been linked to the Appalachian orogen and the closure of Iapetus. Previous studies have suggested that rocks correlated with the Tecomate Formation of inferred Siluro-Devonian age are intruded by the La Noria granite that yielded discordant U-Pb data with a lower intercept of ca. 370 Ma interpreted to date its intrusion during a Late Devonian (Mixtecan) orogeny. Recent work has shown the following sequence of events: (i) deposition of pelitic, psammitic, and volcaniclastic rocks: a psammite yielded detrital zircons with LA-ICPMS U-Pb ages in the Ordovician (peaks at ~450 and 472 Ma) and Proterozoic (major peaks at 814, 972, ~1180, and 1770 Ma; minor peaks at ~1675, ~1710, ~1825, and ~1962 Ma; and clusters at ~1025, ~1125, ~1250, ~1350, ~1410, and ~1550 Ma), whereas a volcaniclastic sample yielded zircons with ages in the Cambrian (major peaks at ~490, 517, and ~545 Ma; and a minor one at ~560 Ma) and Proterozoic (major peaks at 627, ~644, ~787, ~930, 990, 1030, and 1400 Ma; and minor peaks at ~570, 600, ~1150, ~1225, 1400, 1645, ~1740, ~1800, and 2002 Ma); (ii) intrusion of the feldspar megacrystic La Noria granite at 467±16 Ma (U-Pb SHRIMP ages on zircon) that may by synchronous with a quartz megacrystic granite; (iii) deformation and low grade metamorphism; (iv) intrusion of leucogranite. Most of the detrital zircons were likely derived from Ordovician granites in the Acatlán Complex and from Mesoproterozoic gneisses of the adjacent Oaxacan Complex, although a few may have more distant sources (e.g. Yucatan Peninsula, Avalonia, and the Amazon craton/Gondwana).