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Geology; December 2007; v. 35; no. 12; p. 1095-1098; DOI: 10.1130/G23768A.1
© 2007 Geological Society of America
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Neoproterozoic glaciation in the Proto-Andes: Tectonic implications and global correlation

David Chew1, Christopher Kirkland2, Urs Schaltegger3 and Robbie Goodhue4

1 Department of Geology, Trinity College Dublin, Dublin 2, Ireland
2 Laboratory for Isotope Geology, Swedish Museum of Natural History, S-104 05 Stockholm, Sweden
3 Department of Mineralogy, University of Geneva, Rue des Maraîchers 13, CH-1205 Geneva, Switzerland
4 Department of Geology, Trinity College Dublin, Dublin 2, Ireland

The Chiquerío 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}13C = –2{per thousand}) in the cap-carbonate unit overlying the tillite, and a younger excursion ({delta}13C = –8{per thousand}) in a laminated limestone unit 700 m up sequence. In both cases, {delta}13C values recover to 2{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{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 Chiquerío 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 Chiquerío Formation unconformably overlying basement gneisses of the 1800–2000 Ma Arequipa-Antofalla basement (AAB), which is exotic to Amazonia. This implies the Chiquerío 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.

Key Words: Neoproterozoic • Proto-Andes • cap carbonate • glacial • Amazonia • provenance






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