ZIRCON PROVENANCE OF TRIASSIC (PALEOZOIC?) TURBIDITES FROM CENTRAL AND WESTERN MEXICO: IMPLICATIONS FOR THE EARLY EVOLUTION OF THE GUERRERO ARC

The Guerrero terrane has been interpreted as an oceanic arc accreted to the continent by Late Cretaceous. Although it contains evidence of a previous tectonic event (collision of oceanic-floor/continental rise deposits), it was considered by some authors as a terrane that formed far from the continental margin. New detrital zircon data suggest that the siliciclastic of the basement complexes of the Guerrero terrane have similar provenance with those deposited on the western paleocontinental margin of Mexico, suggesting that the arc was marginal. Samples were collected from four localities, two in the Sierra Madre and Central terranes, at Peñon Blanco and Pico de Teyra areas, and two from within the Guerrero terrane in Zacatecas City and Arteaga, Michoacan. The siliciclastic rocks are turbidites made up of quartz-rich sandstone and shale. The Peñon Blanco turbidites are submarine-fan deposits, and contain Late Triassic fossils. Siliciclastic turbidites at Pico de Teyra compose the matrix of an accretionary complex, with blocks of serpentinite, basalts, chert and marble, and contains Paleozoic?-Triassic fossils. Both are covered by Jurassic continental-arc and the Cretaceous calcareous platform of Sierra Madre Oriental. The turbidites at Zacatecas are Upper Triassic in age and contain MORB-type pillow lavas. Turbidites in Arteaga are also Upper Triassic, and contain volcaniclastics, chert, and blocks of gabbros and basalts. Both units are interpreted as accreted ocean floor, and are overlain by the Cretaceous Guerrero arc.

The four samples show strong similarities in zircon composition among them, and have a distinctive cluster at 260 to 300 Ma, followed by a cluster at 1.0 Ga. Other clusters at 480-650 Ma, and 800 Ma. Few grains are 1.6, 2.6 and 2.8 Ga. These data suggest that all the localities might have belonged to one major submarine fan system that spread along central-western Mexico. The possible sources are found in eastern Mexico, the Atlantic margin of the US, the Florida Peninsula and in northern South America. The stratigraphy of the basal rocks and their zircon provenance set constraints on the paleogeography of the Guerrero terrane, and indicate that during Early Mesozoic the terrane evolved near the continent, not far from its present position.