THE TRIASSIC–JURASSIC AYÚ COMPLEX, SOUTHERN MEXICO: DERIVATION FROM THE WESTERN MARGIN OF PANGEA-A

Metamorphosed igneous and sedimentary rocks north of Ayú (Oaxaca, southern Mexico) were previously inferred to be part of the Late Paleozoic Acatlán Complex, however, new U–Pb LA-ICP-MS ages on detrital and igneous zircons indicate a Mesozoic depositional age for the metasedimentary rocks of the Ayú Complex. The Ayú Complex comprises mainly polydeformed metapelites and minor metapsammites (Chazumba Lithodeme) and intercalated boudinaged ortho-amphibolites that have a transitional arc- to MORB tholeiitic geochemistry. The youngest detrital zircons in the Chazumba metasedimentary rocks have ages of ca. 192, 198, 214, 250, 266, and 297 Ma that are interpreted to provide an older limit of Late Triassic—Middle Jurassic for deposition. In the southern part, the metasedimentary sequence was migmatized at ca. 171 Ma, which was accompanied by the intrusion of 171–168 Ma subalkaline dikes and sheets of granodioritic, dioritic, and granitic composition. These intrusive rocks contain inherited zircon populations of ca. 260–290, 320–360, 420–480, 880–990, and 1080–1250 Ma that are also found in the Chazumba Lithodeme. Given the transitional arc–tholeiitic geochemistry of the Chazumba amphibolites, turbidite deposition is interpreted to have occurred in a back-arc environment in the Guerrero terrane adjacent to the rifted passive margin of Oaxaquia. Inferred Early–Middle Jurassic flattening of the subduction zone led to subduction erosion and underthrusting of the Chazumba Lithodeme. Subsequent Mid–Late Jurassic steepening of the subducting slab and the diachronous opening of the Gulf of Mexico led to extension and facilitated the tectonic exhumation of the Chazumba Lithodeme by normal faulting along the reactivated Providencia shear zone during the Middle–Late Jurassic. The presence of arc to back-arc assemblages in the Ayú Complex requires deposition adjacent to a subducting ocean, and thus supports a Pangea-A reconstruction in the Late Triassic-Middle Jurassic that was synchronous with the breakup of Pangea.