U-Pb geochronological constraints on the Triassic-Jurassic Ayu Complex, southern Mexico: Derivation from the western margin of Pangea-A

We present LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) U-Pb detrital and igneous zircon data of poly-deformed metamorphic and igneous rocks of the Ayu area, southern Mexico. These rocks were previously inferred to be part of the Late Paleozoic Acatlan Complex, but new age data indicate that they formed in the Mesozoic and should be placed in the newly designated Ayu Complex. The Ayu Complex comprises polydeformed metasedimentary rocks (Chazumba Lithodeme) of a turbidite-like protolith that are intercalated with boudinaged ortho-amphibolites with transitional arc- to MORB tholeiitic geochemistry. In the south, the metasedimentary sequence is affected by a ca. 171 Ma partial melting which formed the Magdalena Migmatite. Migmatization was accompanied by 171-168 Ma intrusions of granodioritic, dioritic, and granitic dikes and sheets as well as pegmatite bodies, which are characterized by 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. U-Pb (detrital zircon) dating of seven metasedimentary samples from the migmatized and unmigmatized Chazumba Lithodeme yielded youngest detrital zircons and clusters of 192, 198, 214, 250, 266, and 291 Ma, and are interpreted to reflect the Late Triassic-Middle Jurassic deposition of turbiditic rocks. The transitional arc-tholeiitic geochemistry of the Chazumba amphibolites is consistent with turbidite sedimentation in a back-arc environment along a rifted passive margin, close to a contemporaneous magmatic arc. Inferred flattening of the subduction zone led to subduction erosion during the Early-Middle Jurassic and under-thrusting of the Chazumba Lithodeme to depths equivalent to amphibolite facies metamorphism. Steepening of the subducting slab and diachronous rifting within the Gulf of Mexico contributed to extensional tectonics recorded on the Mexican mainland and facilitated the tectonic exhumation of the Chazumba Lithodeme by normal faulting along the reactivated Providencia shear zone during the Middle-Late Jurassic. More generally, the documentation of arc-back arc assemblages in the Ayu Complex requires deposition adjacent to a subducting ocean, and thus supports a Pangea-A reconstruction that was synchronous with the breakup of Pangea. (C) 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.