T259 Frontiers in Geochronology: Ancient Systems and Planetary Environments (Posters)

Tuesday, 27 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

DEPTH PROXIES FOR HADEAN TO PALEOARCHEAN FELSIC ROCK FORMING PROCESSES AS REVEALED BY LA/YB COUPLED WITH U-PB DATING OF JACK HILLS ZIRCONS

Paper number: T259-3
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

This study presents new age and trace elemental data on 276 zircons of 4.2 to 3.2 Ga and also usessimilar data previously published on 4.4.-4.3 Ga zircons (Peck et al. 2001) from Jack Hills, Western Australia. These zircons provide a window into granitoid formation in the ancient Earth. Here we use the La/Yb ratio of these zircons to determine whole-rock equivalent ratios, (Chapman et al., 2016). Calculated whole rock (La/Yb)N ratios for 80% of our new and previously published data are >10. Such ratios are characteristic of magmatic arcs formed on thick continental crust (50 to 70 km) in the Phanerozoic (Profeta et al., 2015). However, the lives of typical plate tectonic magmatic arcs involve cyclic behavior with alternating episodes of crustal thickening and thinning at timescales of tens of million years. We hypothesize that elevated and relatively constant La/Yb ratios in these Hadean and Archean granitoids -as deduced from zircons spanning more than 1 Ga from one region- are not necessarily evidence for a continental crust that is thick and constant since the Hadean and throughout the Paleoarchean. Instead, we suggest that this is an indication that different melting processes were operative during those times and that perhaps partial melting of subducting oceanic plateaus was the dominant mechanism for making granitoids, as proposed by many others based on the rock record of the Paleoarchean (e.g. Willbold et al., 2009, Martin et al., 2014). Here we show that the same mechanism may have operated as early as the time of the oldest Jack Hills zircons.

1.Peck, W. W. H., Valley, J. J. W., Wilde, S. A. S. & Graham, C. M. C. Cosmochim. Acta 65, 4215–4229 (2001). doi: 10.1016/S0016-7037(01)00711-6

2.Chapman, J. B., Gehrels, G. E., Ducea, M. N., Giesler, N. & Pullen, A. Chem. Geol. 439, 59–70 (2016). doi: 10.1016/j.chemgeo.2016.06.014

3.Profeta, L., Ducea, M. N., Chapman, J. B., Paterson, S. R., Gonzales, S. M., Kirsch, M.,

Petrescu, L., DeCelles, P. G. Sci. Rep. 5, 17786 (2015). doi: 10.1038/srep17786

4. Willbold, M., Hegner, E., Stracke, A. & Rocholl, A. 279, 44–52 (2009). doi: 10.1016/j.epsl.2008.12.029

5.Martin, H., Moyen, J.-F., Guitreau, M., Blichert-Toft, J. & Le Pennec, J.-L. Lithos 198, 1–13 (2014). doi: 10.1016/j.lithos.2014.02.017

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-282110

© Copyright 2016 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.