U-Pb ages and Hf isotope compositions of zircons in plutonic rocks from the central Famatinian arc, Argentina


Otamendi, Juan E.
Ducea, Mihai N.
Cristofolini, Eber A.
Tibaldi, Alina M.
Camilletti, Giuliano C.; Bergantz, George W.

The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ UeThePb and Lu eHf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472e465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf(t) units (0.3 to 8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta) sedimentary materials into calcealkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution.

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Fig. 9. (a) Plot of 176Hf/177Hf average ratios re-calculated at 470 Ma for data set from this study and published data. Data sources are: PV after Hauser et al. (2011) and FE after Bahlburg et al. (2016). As inset shows data were grouped into three subgroups according to crystallization ages. Number close to symbols denotes the percentage of each age group within the total data. Error bars are at 1s of the population in each subgroup. (b) Plot of 176Hf/177Hf ratios at 470 Ma versus Hf content for rocks and zircons used in mixing models. (c) Hf mixing models for a primitive arc magma and two metasedimentary crustal end members. The effect of variable zircon weight fractions in the crustal component on bulk mixing lines is taken into account using whole rock composition of (meta) sedimentary rocks from northwestern Argentina. Further information is described in the text. (d) Comparison of model results with the composition of plutonic zircons from this study and those taken after published data by Chernicoff et al. (2010) and Dahlquist et al. (2013).

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http://dx.doi.org/10.1016/j.jsames.2017.04.005 0895-9811/© 2017 Elsevier Ltd. All rights reserved.