A linked evolution for granite-greenstone terranes of the Pilbara Craton from Nd and Hf isotopes, with implications for Archean continental growth

Abstract

Granite-greenstone terranes, such as in the Pilbara Craton (Western Australia), record the growth and stabilisation of Archean (>2.5 Ga) continental crust. Yet, uncertainty surrounds the formation of the silica-rich portions of these terranes, important for preservation of continental lithosphere, specifically whether this involved reprocessing of older continental nuclei over hundreds of millions of years, or rapid differentiation of juvenile mantle additions, as preserved in the greenstone belt volcanic sequences. In part, the uncertainty reflects difficulties with establishing the isotope evolution of the early Archean mantle, critical for ascertaining the time gap between the extraction of new crust and granitic emplacement in Archean cratons. To investigate the formation of silica-rich Archean crust, we report Hf and Nd isotope data for 46 mafic–ultramafic samples and zircon Hf isotope data for 83 felsic igneous rocks from the Pilbara Craton, selected to be representative of the major magmatic episodes from 3.56 to 2.72 Ga. The strikingly similar secular isotope trends defined by mafic and felsic rocks reveal a linkage between mantle and crustal processes throughout the history of the craton. From 3.56 Ga to 3.12 Ga, craton evolution was dominated by the transformation of juvenile mantle derived inputs into felsic igneous rocks on short timescales, thus recording sustained continental growth over 400 million years. Incorporation of older crust by felsic magmas was minor during this interval. Evolution toward sub-chondritic Nd and Hf after collision of the East and West Pilbara terranes at 3.1 Ga signals greater involvement of pre-existing crust in magma generation, possibly by recycling of crustal material into mantle source regions or by enhanced reworking of older, thickened cratonic lithosphere. Mafic–ultramafic rocks provide no evidence for anomalous or strongly depleted mantle domains in the Paleoarchean. Rather, the flat zircon εHf–time array indicates extraction of continental materials from mildly super-chondritic mantle from ca. 3.6 to 3.12 Ga and attests to a distinctive style of crust production that operated in the Archean. This study highlights the importance of comparing the mantle and crustal isotope records in order to reconstruct the crustal growth rates in ancient terranes.