Komatiite Volcanism: Lithospheric Controls on the Earth’s Hottest Melts and Links to Crustal Growth in the Archean

David Robert Mole1, Marco L Fiorentini2, Nicolas Thebaud3, Campbell McCuaig3, Kevin F Cassidy3, Chris L Kirkland4, Sandra Romano4, Michael Doublier4, Elena A. Belousova5 and Steve John Barnes1, (1)CSIRO, Perth, Australia, (2)The University of Western Australia, School of Earth Sciences, Crawley, WA, Australia, (3)University of Western Australia, Crawley, WA, Australia, (4)Geological Survey of Western Australia, Perth, Australia, (5)Macquarie University, Department of Earth and Planetary Sciences, Sydney, Australia

Contact First Author: David Robert Mole; david.mole@csiro.au

Previously Published Material: These findings were reported in the Proceedings of the National Academy of Sciences in July 2014, issue 111 (28), pages 10083-10088.

Abstract ID#: 33105

 

English Abstract:
Major periods of komatiite activity, possibly on the scale of Phanerozoic large igneous provinces (LIPs), occurred in the Archean Yilgarn Craton of Western Australia in two regions during two distinct periods. The oldest known deposits (3.0-2.8 Ga) occur in the south of the Southern Cross Domain (SCD). The youngest (~2.7 Ga) are predominantly found in the Kalgoorlie Terrane (KT). This study aimed to investigate whether variations in lithospheric architecture could explain the location and timing of komatiite magmatism. An integrated program of U-Pb and Lu-Hf analyses were performed on granitoids and felsic volcanics from the south-central Yilgarn Craton, Western Australia. These data have been plotted as time-resolved Lu-Hf maps which record the changing architecture of the craton in space and time.

The 3.1-2.8 Ga Lu-Hf time-slice shows that the south-central Yilgarn lithosphere (encompassing the Forrestania, Lake Johnston and Ravensthorpe greenstone belts) was relatively juvenile (εHf>0) at this time. The region to the north (central SCD), encompassing the Marda greenstone belt, shows an evolved signature (εHf<0). The greenstone belts formed over juvenile crust contain thick komatiite sequences which comprise high Mg lavas, thick cumulate zones and channelized flows, indicative of rapid, voluminous emplacement of extremely primitive magma (Barnes 2006a, 2006b). In contrast, the Marda greenstones are dominated by a thick, mafic succession, with minimal komatiite.

The younger, 2.8-2.6 Ga time-slice indicates that the west Yilgarn has cratonised into a more homogenous, evolved terrane (Youanmi Terrane; YT), whereas the KT has a juvenile signature. This contrast forms an isotopic discontinuity (paleo-craton margin?) between the two terranes, and results in a ~700 km, N-S trending belt of komatiites on the juvenile (KT) side of the isotopic boundary. These komatiites represent some of the hottest magmas ever erupted on Earth (FoOl92-94). In contrast, there is no known coeval mafic-ultramafic volcanism in the YT.

These results not only explain the timing and location of major komatiite eruptions but also why some regions do not have significant komatiite sequences at all. They also indicate that the evolution of the lithosphere and its subsequent architecture form a major control on komatiite volcanism.

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