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

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.

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