SHRIMP U-Pb zircon geochronology of Neoproterozoic crustal granitoids(Southern Brazil):A case for discrimination of emplacement and inherited ages

摘要

SHRIMP U-Pb zircon studies on two post-collisional granitic plutons and reassessment of the data previously reported for two anatectic gneissic granites are used to assess the late Neoproterozoic history of the Florianopolis Batholith,southern Brazil.The results,supported by SEM backscattered and cathodoluminescence imagery,identify inherited zircon populations and confirm the long-lived,crustal recycling processes responsible for the accretion of the batholith.The study casts new lights on the timing of the processes involved in the generation and modification of the internal structure of distinct zircon populations,and enables discrimination to be made between inherited cores and melt-precipitated overgrowths.New dating of two post-tectonic plutons(samples 1 and 2)revealed crystals showing magmatic-textured cores sharply bounded by melt-precipitated overgrowths.The U/Pb isotopic results from both samples spread along concordia by ca.40 m.y.(sample 1)to 100 m.y.(sample 2),clustering in two closely spaced(bimodal),partially overlapping peaks.Melt-precipitated rims and homogeneous new grains,dated at ca.600 Ma,furnish the crystallisation age of the plutons.The magmatic textured cores and xenocrysts dated at ca.630-620 Ma are interpreted as inherited restitic material from supposedly short-lived(meta)granitic sources.The reassessment of previous SHRIMP data of two banded anatectic granitoids(samples 3 and 4)revealed more complex morphological patterns,in which the overgrown inherited cores are sharply bounded against large melt-precipitated rims,dated at ca.600 Ma and 59212 Ma,respectively.Major populations of magmatic-textured inherited cores dated at 2006(+-)3 Ma and 2175(+-)13 Ma characterise samples 3 and 4,respectively.The latter additionally shows metamorphic and magmatic inherited cores with a large range of ages(ca.2900-620 Ma),suggesting partial melting of metasedimentary components.The main magmatic Paleoproterozoic core populations are interpreted as inherited restite from partial melting of the adjacent(meta)tonalitic gneiss and amphibolitic country-rock(paleosome).The recognition of the(melt-precipitated)Neoproterozoic overgrowths and new crystals,and the restite provenance of the cores,supplants a previous interpretation of Paleoproterozoic magmatism(cores)and Neoproterozoic(solid-state)metamorphic overprint.As a major consequence of the former interpretation,the unit was mistakenly considered part of major Paleoproterozoic gneissic remnant within the Neoproterozoic Florianopolis Batholith/arc.