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摘要
The Chiquerio Formation in southern Peru records the only documented Neoproterozoic glacial episode in the entire Andean Belt. We present U-Th-Pb secondary ion mass spectrometry (SIMS) detrital zircon ages and C isotopic data from the tillite and its overlying dolomite cap, the San Juan Formation. Two prominent negative C isotopic excursions are documented: an older excursion (delta C-13 = -2 parts per thousand) in the cap-carbonate unit overlying the tillite, and a younger excursion (delta C-13 = -8 parts per thousand) in a laminated limestone unit 700 m up sequence. In both cases, delta C-13 values recover to 2 parts per thousand. U-Th-Pb SIMS detrital zircon results from the tillite (both matrix and interbedded turbiditic sandstones) indicate a restricted age distribution of 950-1300 Ma. Turbiditic dolomitic sandstones overlying the younger (-8 parts per thousand) carbon isotope excursion yield a similar 950-1300 Ma peak, but also contain grains dated as 1600-2000 Ma and 700-820 Ma. The detrital zircon geochronology and C isotope chemostratigraphy are consistent with the Chiquerio Formation being equivalent to the ca. 700 Ma Sturtian glacial. The younger negative C isotope excursion is delimited by the youngest detrital zircon (697 11 Ma) in overlying strata. A correlation with the 635 Ma Marinoan glacial is inferred, although no unequivocal glaciogenic strata have been identified. The detrital zircon data are consistent with derivation from the Proto-Andean margin, despite the Chiquerio, Formation unconformably overlying basement gneisses of the 1800-2000 Ma Arequipa-Antofalla basement (AAB), which is exotic to Amazonia. This implies the Chiquerio Formation and AAB were proximal to the proto-Andean margin during Neoproterozoic glaciation, and supports paleogeographic reconstructions that favor AAB accretion to the Amazonian craton during the 1000-1300 Ma Grenville-Sunsas orogeny.
- 出版日期2007-12
