Detrital-zircon record of major Middle Triassic–Early Cretaceous provenance shift,central Mexico: demise of Gondwanan continental fluvial systems and onset of back-arc volcanism and sedimentation |
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| Abstract: | New stratigraphic and petrographic data and zircon U–Pb geochronology from sandstones and volcanic rocks in the states of Queretaro and Guanajuato in central Mexico indicate an important provenance change between Late Triassic and latest Jurassic–Early Cretaceous time. The Upper Triassic El Chilar Complex consists of pervasively deformed, deep-marine olistostromes, and debris-flow deposits of arkosic and subarkosic composition. Detrital-zircon populations range from latest Palaeoproterozoic (1.65 Ga) to Middle Triassic (240 Ma), all predating the depositional age of the strata. The detrital-zircon populations are similar to those previously reported from turbidites of the Potosi fan complex of north-central Mexico and interpreted as derived from Grenville and Pan-African (Maya block) basement and Permo-Triassic arc of continental Mexico directly to the east of the basin. A single sample with a dominant Proterozoic population at ～1.65–1.30 Ga was likely derived either from the Rio Negro-Juruena province of the Amazonian craton or from a local source in the Huiznopala Gneiss, and indicates that El Chilar strata were likely deposited in the proximal part of a submarine-fan system separate from the Potosi fan.Uppermost Jurassic–Lower Cretaceous strata of the San Juan de la Rosa Formation unconformably overlie the El Chilar Complex and likewise consist of deep-marine olistostromes, slump deposits, debris-flow deposits, and proximal fan-channel fills, but are volcanogenic litharenites with abundant felsic and vitric volcanic lithic fragments. Detrital-zircon populations are dominated by Early Cretaceous grains (150–132 Ma) with no known sources in eastern Mexico. Abundant young grains indicate a maximum depositional age of ～134 Ma (Valanginian–Hauterivian). The San Juan de la Rosa Formation is overlain by deepwater carbonates with interbedded siliciclastic beds of the Peña Azul Formation, which contains detrital-zircon ages as young as ～130 Ma, indicating possible equivalence with similar strata of the Las Trancas Formation, with a maximum depositional age of ～127 Ma and lying to the east in the Zimapan Basin, now part of the Sierra Madre Oriental fold and thrust belt. Decreasing content of volcaniclastic strata eastward indicates a volcanic source to the west. Upper Cretaceous marine strata in the Mineral de Pozos area to the northwest in the state of Guanajuato contain litharenites with a maximum depositional age near 92 Ma, and are thus part of a younger depositional system.Composition and detrital-zircon content of the Upper Triassic and Lower Cretaceous successions in central Mexico indicates an important shift from Gondwanan continental sediment sources in the Triassic to western volcanic sources, probably on the edge of the newly opened Arperos basin, by the end of the Jurassic. This important sediment-dispersal change records the break-up of Pangea and concomitant development of arc-related sedimentary basins on the western edge of Mexico. |
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| Keywords: | U–Pb geochronology detrital zircon Mexico provenance Mesozoic |
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