MID-JURASSIC TECTONOTHERMAL EVENT SUPERPOSED ON A PALEOZOIC GEOLOGICAL RECORD IN THE ACATLAN COMPLEX OF SOUTHERN MEXICO: HOTSPOT ACTIVITY DURING THE BREAKUP OF PANGEA

U-Pb and 40Ar/39Ar isotopic analyses of various minerals from the lowest structural units of the Acatlán Complex of southern Mexico indicate that low pressure migmatization (5-6 kb), polyphase deformation, and intrusion of felsic and mafic magmas all occurred between 175±3 Ma and 171±1 Ma, i.e. mid-Jurassic. These effects are superimposed upon Ordovician-Silurian and Permian tectonothermal events reflected in older age data in the northern part of the Acatlán Complex. Fractionation of olivine and bronzite in the mafic magmas was associated with assimilation of alumina producing layered lower crustal intrusions of cumulitic dunites, gabbro, and norite that are cut by appinitic dykes. Ensuing rapid cooling through 350°, 280° and 150°C by 167±2 Ma, 166±2 Ma, and 160±14 Ma, respectively, accompanied rapid exhumation (up to 10 km/my), and was followed by deposition of Middle Jurassic (Bajocian=166-160 Ma) sediments. The Jurassic uplift is consistent with the top-to-the west tectonic unroofing inferred to have occurred during the Main phase of deformation. Synchronous tectonic unroofing is also recorded in the adjacent Oaxaca terrane, 100 km to the east, where top-to-the-north, extensional shear zones occur in Paleozoic strata. This pattern of extension suggests radial tectonic unroofing in response to domal uplift (radius >100 km) like that associated with core complexes, slab windows, and hotspots. Most tectonic analyses for the Jurassic place the Acatlán Complex in the forearc region of an arc lying 600-800 km inboard of the subduction zone in Colombia, presumably in response to flat-slab subduction. Modern analogues suggest that flat-slab subduction reflects subduction of young buoyant oceanic lithosphere adjacent to either a mid-oceanic ridge, or a plume. Since core complexes are typical of arc-backarc regions, and slab windows generally produce metamorphic belts, the forearc setting and associated domal uplift suggest a plume to be the most likely cause of this Jurassic tectonothermal pulse in southern Mexico. This plume activity is synchronous with the opening of the Gulf of Mexico during the breakup of Pangea, to which it may have contributed.