Channelized metasomatism in Archean cratonic roots as a mechanism of lithospheric refertilization

Abstract

Archean cratons represent stable continental domains which form the nuclei of the Earth's continents due to their thick ( >200 km), mechanically resistant keels. Cratons and their stable roots form through melt and fluid depletion processes. However, metasomatic refertilization may occur due to processes coeval with craton construction and/or overprinting episodes. Magnetotellurics, a geophysical method measuring subsurface electrical resistivity, is sensitive to the compositional and thermal states of the lithosphere and is useful in mapping depleted and refertilized cratonic domains. Here we show the results of a 3D anisotropic inversion to image the lithospheric resistivity structure of the western Superior Craton. The resistivity model reveals widespread (500×300 km²) anisotropy with a north-south conductive axis at depths ~100-200 km, inferred to represent phlogopite-bearing channels emplaced during mantle plume activity. The results have implications for our understanding of the modification and long-term stability of cratonic lithosphere, and the imaging and interpretation of their preserved geophysical signatures.

Fig. 1. Station and phase tensor maps of the study area.

a Map of North America (gray) and the Superior Craton (dark gray). Red circles indicate inferred focal points of Proterozoic mantle plumes (after ref. ). White diamonds indicate xenolith suites (K – Kirkland Lake; T – Tamiskaming; V – Victor; KL – Kyle Lake). b Map of the western Superior (modified from ref. .) including the MT stations used in this study. Solid black lines denote craton and subprovince boundaries. Black dashed lines indicate the primary area of interest shown in panels c-e and in Fig. 2. NE – Nipigon Embayment; MCR – Mid-Continent Rift. Panels c-e show phase tensor ellipses shaded by phase-split at periods of c 160 s, d 640 s, and e 1280 s.

Fig. 2. Slices through the anisotropic resistivity model.

Plan view slices at 155 km depth and E-W slices through the preferred anisotropic model. a the E-W oriented ρ_y resistivities, b N-S oriented resistivity ρ_x, and c through the preferred isotropic resistivity model (from ref. ). East-west slices are taken through the red dashed lines in (a), (b), and (c). All slices show only the primary area of interest (dashed box in Fig. 1b).

Fig. 3. Geothermal conditions and resistivity-depth profiles.

a Approximate geothermal conditions at present-day (black line) and at 2700 Ma (red line; from ref. ) overlaid by the stability fields of graphite and phlogopite. Black dashed line indicates the estimated lithosphere-asthenosphere boundary (LAB) depth. b Volume-averaged resistivity-depth profiles within the C1 and C2 anomalies overlaid by calculated resistivity-depth profiles for lherzolite (Lhrz.) with varying water content, phlogopite (Phlg.) content, and phlogopite interconnectivity (m = 1 is perfectly connected, m = 2 is partially connected).

Fig. 4. Schematic diagram of the interpreted structure and mechanisms of emplacement.

a A ca. 1.9 Ga mantle plume impinges along the thickened, depleted lithosphere which comprises the northern cratonic core and is deflected southward. Pooling of melts at the base of the LAB results in gradual percolation of metasomatic fluids into the lithosphere forming phlogopite, possibly channeled into N-S weak zones which formed as a result of E-W compression during the Trans-Hudson Orogen. b Subsequent melting and magmatism related to the ca. 1.1 Ga mid-continent rift (MCR) overprints the phlogopite within the N-S channels and refertilizing the surrounding lithosphere.