EGU2020-19390
https://doi.org/10.5194/egusphere-egu2020-19390
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Evolution of crust vs. mantle contributions to continental arc granitoids within a few Myr: evidence from zircon Hf-O isotopes and high-precision U-Pb dating in the Famatinian Arc, Argentina

Julien Cornet1, Oscar Laurent1, Jörn-Frederik Wotzlaw1, Juan Otamendi2, and Olivier Bachmann1
Julien Cornet et al.
  • 1ETHZ Institute of Geochemistry and Petrology, Zurich, Switzerland
  • 2Geology Department - Universidad Nacional de Río Cuarto Río Cuarto, Cordoba, Argentina Join institution

The presence of a thick continental crust makes Earth a unique planet in the solar system. During post-Archaean times, with the onset of plate tectonics, processes by which continents form is a complex function of juvenile growth and recycling of pre-existing crust. Indeed, post-Archean mantle-derived magmas commonly intrude pre-existing, felsic continental crust. As a result, the origin of upper crustal granitoids, the most accessible products of planetary differentiation, is either accounted for by the melting of the pre-existing mid- to lower crust or the differentiation of mantle-derived mafic magmas. It is therefore critical to identify the relative contribution of these two different granite-forming processes in a given magmatic province, as well as how this relative contribution evolves over time, to assess crustal growth and/or recycling. To shed some light on this question, we used the combination of oxygen, hafnium and uranium-lead isotopic systems in zircons from granitoids of the Ordovician Famatinian Arc (Argentina) representing a typical crust-forming geotectonic setting. While the lower crustal section of Valle Fertíl, representing the basal level of the Famatinian crust, is already well studied, little is known on the timing and nature of igneous processes that built up the mid- and upper crust. 

From our study, we observe a systematic co-variation of the O and Hf isotopic signatures of zircon in the mid- to upper crustal rocks, from a clearly crustal footprint (granodiorites with zircon δ18O of ca. +8 ‰; εHft of ca. –3) to a mantle-like signature (granites and rhyolites: zircon δ18O of ca. +5 ‰; εHft of ca. +5). Moreover, the high-precision (ID-TIMS) U-Pb dating obtained from the same zircons seem to record a progressive building of the Ordovician continental crust lasting for ca. 13Myrs from 483 to 470 Myrs ago. The results overlap with published ID-TIMS U-Pb data for the Famatinian lower crust, clustering at 470 Myrs, which confirms that the Famatinian Arc was a transcrustal magmatic system ultimately fed by mantle-derived magmas. In details, the oldest granitoids (483 Myrs) show the strongest crustal Hf-O isotopic fingerprint while the younger ones define a continuous range from this end-member towards the mantle signature. These results could be explained by (i) continuous ingrowth and “self-shielding” of lower crustal mafic intrusions progressively decreasing crustal melting or contamination of ascending mafic magma from a homogenous mantle source; (ii) progressive defertilization of an enriched lithospheric mantle or a strongly slab-enriched mantle wedge. The fact that the earliest (483 Myr-old) granitoids also show a more significant crustal contribution (ASI >1.1, inherited zircon cores) supports the first scenario. In this case, the combination of Hf-O isotopic studies as well as high precision U-Pb dating for the Famatinian arc comply with a progressive building of a magmatic column where a certain amount of time is needed for the system to mature and eventually reach mantle dominated processes in the formation of granites and so, new continental crust.

How to cite: Cornet, J., Laurent, O., Wotzlaw, J.-F., Otamendi, J., and Bachmann, O.: Evolution of crust vs. mantle contributions to continental arc granitoids within a few Myr: evidence from zircon Hf-O isotopes and high-precision U-Pb dating in the Famatinian Arc, Argentina , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19390, https://doi.org/10.5194/egusphere-egu2020-19390, 2020

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Presentation version 2 – uploaded on 05 May 2020
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  • CC1: Comment on EGU2020-19390, Agustin Cardona, 05 May 2020

    Incredible work. ALtough you consider that there is a trend that can be related to the formation of new continental crust or the shield effect. You mention that there are evidences of several departures of this trend that obey to more local tectonic events. Have you explore this further, probabbly in relation to slab roll-back or other mechanisms.

    • AC1: Reply to CC1, Julien Cornet, 06 May 2020

      Dear Agustin, 

      Thanks a lot for your comment! i am sorry to reply that late but i just realised that my answer was not recorded... 

      We did not really explore any other geodynamic related mechanisms to explain such isotopic signatures. This mainly because we think that subducting a passive margin in one place and just a normal altered oceanic crust in another is sufficient to explain the isotopic story (e.g., contaminated versus not contaminated mantle wedge) 

      Do you think mechanism like slab roll back or slab break off could explain such isotopic signatures? if yes, what do you have in mind? 

      Cheers, 

      Julien Cornet

Presentation version 1 – uploaded on 05 May 2020 , no comments