The Arequipa-Antofalla Basement, a tectonic tracer in the reconstruction of Rodinia

Loewy, Staci Lynn
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The Arequipa-Antofalla Basement (AAB), an anomalous Proterozoic block along the central Andean margin of South America, may be critical to reconstruction of the Meso-Neoproterozoic supercontinent, Rodinia. Dalziel (1994) proposed that the AAB is an allochthonous block, transferred to Amazonia from northeastern Laurentia as a result of ca. 1.0 Ga collision between the two continents during the amalgamation of Rodinia. New U/Pb geochronology and whole-rock isotope geochemistry from the AAB and northeastern Laurentia are used to test this proposed correlation. Based on U/Pb ages, growth polarity, sequence of age provinces, and whole-rock Pb isotopic signatures I conclude: 1) the AAB is a single basement block that formed through progressive growth within a larger continent, 2) neither Amazonia nor Laurentia were its parent craton, and 3) the AAB accreted to Amazonia during the ca. 1.0 Ga Sunsás Orogeny. Thus, the data do not support the proposed collision between Amazonia and northeastern Laurentia. Instead, the characteristics of the AAB are consistent with derivation from Kalahari. Moreover, critical examination of existing Pb data also reveals that the Pb signature of the isotopically distinct ca. 1.1 Ga basement of the southern and central Appalachians matches that of southwestern Amazonia. Based on these potential correlations, I propose an alternate position for Amazonia within Rodinia, adjacent to southeastern Laurentia and western Kalahari. Evaluation of Pb isotopic signatures from juvenile rock units of large Precambrian age-provinces suggests that they were derived from two isotopically distinct mantle reservoirs. These reservoirs may be the precursors of those that exist today, a normal reservoir in the northern hemisphere and the DUPAL reservoir in the southern hemisphere. If true, the Pb isotopic composition of juvenile rocks in Precambrian age-provinces may indicate the hemisphere in which the province formed. Coherency of Pb isotopic signatures across juvenile age provinces and the existence of two isotopically distinct reservoirs permit the use of Pb isotopes to correlate transferred terranes with potential parent cratons. If Pb isotopic compositions of juvenile Precambrian rocks are indicative of the hemisphere in which they formed, Pb isotopes may also be used to resolve ambiguity in polarity of paleomagnetic data.