Exotic rifted passive margin of a back-arc basin off western Pangea: geochemical evidence from the Early Mesozoic Ayu Complex, southern Mexico

Recent detrital zircon studies of metamorphosed and polydeformed rocks of the early Mesozoic Ayu Complex in southern Mexico suggest an allochthonous origin along the western Pangean margin. Bulk-rock geochemistry of the ca. 170200 Ma ortho-amphibolites suggests a composition ranging from alkalic and transitional basalts to normalized mid-ocean ridge basalt (N-MORB) tholeiites. Rare earth element (REE) patterns of alkaline basalts (Group I) are characterized by steep negative slopes, whereas transitional basalts (Group II) show moderate light REE (LREE) enrichment. Subalkalic Group III displays slight LREE enrichment and Group IV has relatively flat REE patterns with slight depletion in LREEs. Multiple trace element plots of Group IIIIV amphibolites reveal strongly negative NbTa anomalies caused by subduction zone contamination. Initial E-Nd values (t=190Ma) of the amphibolites range from +9.01 to 2.16. Alkalic basalts have negative E-Nd values, suggesting derivation from an older subcontinental mantle source (T-DM=877 and 791 Ma). Group IIIV amphibolites have positive E-Nd values ranging from +2.31 to +9.01, indicating a transition from an older to a relatively juvenile mantle source that is typical of a back-arc setting. The geochemistry of the metasedimentary rocks suggests derivation from an acid-arc source. Chondrite-normalized REE patterns are characterized by enriched LREEs, flat HREE, and negative Eu anomalies. SmNd systematics indicate that most samples were derived from cratonic basement and plot within the Oaxacan Complex envelope with E-Nd values (t=195Ma) ranging from 5.53 to 7.65. We interpret two samples with higher E-Nd values (1.42 and +1.06) to reflect the additional influence of a more juvenile component. The amphibolites and metasedimentary rocks of the Ayu Complex document back-arc activity and are inferred to be correlative with various western Mexican TriassicJurassic mafic suites and the Potosi fan that formed along the western rifted margin of Pangea.