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The late- to postorogenic transition in the Apiaí domain,SE Brazil: Constraints from the petrogenesis of the Neoproterozoic Agudos Grandes Granite Batholith
Institution:1. Institute of Geological Sciences, Polish Academy of Sciences, Reasearch Centre Warsaw, Twarda 51/55, 00-818 Warsaw, Poland;2. Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, Zwirki i Wigury 93, 02-089 Warsaw, Poland;3. Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany;1. Department of Geology, Saint Mary''s University, Halifax, NS B3H 3C3, Canada;2. Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6, Canada;3. U.S. Geological Survey, Alaska Science Center, 4210 University Dr., Anchorage, AK 99508-4626, USA
Abstract:A remarkable change in the typology of granite magmatism occurred during the late evolution of the Agudos Grandes Batholith (Apiaí domain, SE Brazil), marked by the appearance of A-type granites some 10–15 Myr after the last manifestations of the compressional period. The elemental and Sr–Nd isotope geochemistry of the approximately 600 Ma, “late-orogenic”, moderately peraluminous Piedade, Serra dos Lopes, and Pilar do Sul granites shows that the middle crust was heated to approximately 820–850 °C during the latest stages of the compressional period and generated large amounts of melt through biotite dehydration–melting of gneiss protoliths. These crustal magmas either formed independent intrusions or mixed in varied proportions with raising high-K calc-alkaline magmas, as suggested by continuous trends toward more mafic granites (65–70 wt% SiO2), which have the geochemical signature of contaminated calc-alkaline magmas. The postorogenic granites occur as two subgroups with different age (∼585 and ∼565 Ma), both with the chemical fingerprints typical of A-type granites (low mg#, Ba, and Sr; high Zr, Hf, Nb, and Y), interpreted as a result of crustal melting at lower P, fO2, and a(H2O) and higher T (up to 870 °C, indicated by zircon saturation). Significant trace-element and radiogenic isotope contrasts (e.g., εNdT = −16 vs. −11) show that the two postorogenic subgroups derived from different sources. A Sr–Nd isotope signature nearly identical to the “end-member” late-orogenic crustal melts suggests that the older postorogenic granites derived from a similar (but probably less fertile) source. The less negative εNdT of the young subgroup is unmatched by known crustal sources in the Apiaí domain and may have resulted from either an infracrustal component or mobilization of an unexposed section with less crustal residence time.
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