Geophysical Detection of Relict Metasomatism from an Archean (~3.5 Ga) Subduction Zone

Geophysical Detection of Relict Metasomatism from an Archean (~3.5 Ga) Subduction Zone Map of the Slave craton (exposed craton outlined in red). Inset shows the craton in center (green square), with the 62 earthquakes (red circles) used for the analysis. Crustal topology and geochemical signatures broadly subdivide the Slave craton into two distinct regions: The older (4.03-2.83 Ga) Central Slave Basement Complex to the west (brown), and isotopically juvenile rocks (2.67-2.6 Ga) to the east (blue). The lateral extent of an ultra-depleted harzburgitic layer of the mantle lithosphere has been inferred from petrological analysis of mantle xenoliths (green outline) and geochemical analyses of garnet xenocrysts (between black dashed lines). The seismic stations are from POLARIS (circles) and MIT (squares). A-A' marks the location of the seismic profile shown in Fig. 2. The lateral extent of the seismic discontinuity is indicated by blue shading, and that of the conductive anomaly is outlined in red. B-B' is the nominal projected location of the MT array.
The origin of Archean cratonic lithosphere is the subject of much debate. Geological and geochemical data are consistent with a plate tectonic origin involving assembly of predominantly oceanic terranes and micro continents into large stable cratons. Deep probing geophysical surveys of Archean cratons provide a means of understanding the processes responsible for the formation of these earliest continents, although the signals are complex and present-day cratons are only the surviving remnants of once larger entities. Here we present a unique geophysical view of structure within the Archean Slave craton, in northwestern Canada, that shows clear evidence for subduction processes frozen into Archean lithosphere. New seismic imaging results from the central Slave are synthesized with a coincident magnetotelluric model, and interpreted using petrological and geochemical constraints from mantle xenoliths. We find the most striking correlation between seismic and electrical structures ever observed in a continental setting in the form of a coincident seismic discontinuity and electrical conductor at ~100 km depth. The magnitude of both anomalies, in conjunction with the occurrence of phlogopite rich xenoliths originating at the same depth, point to a metasomatic origin. We believe that fluids were released from a subducting slab and altered the mantle directly below the base of a pre-cratonic lithosphere. Our model suggests that cratons are formed by subduction underplating and accretion of preexisting fragments, and that these processes were active as early as 3.5 billion years ago.
</p><p>Chen, C.-W., S. Rondenay, R. L. Evans, and D. B. Snyder, Geophysical detection of relict metasomatism from an Archean (~3.5 Ga) subduction zone, Science, 326, 1089-1091, 2009.
</p><p>Jones, A.G., P. Lezaeta, I.J. Ferguson, A.D. Chave, R. L. Evans, X. Garcia and J. Spratt, The electrical structure of the Slave craton. Lithos, 71, 505-527, 2003.
</p><p>Acknowledgements: This work is funded by the POLARIS consortium and NSF grant EAR-0409509 (S. R.).</p>


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