SCLM AND CRUSTAL EVIDENCE FOR 3 GA ONSET OF PLATE TECTONICS WITH IMPLICATIONS FOR THE SUPERIOR PROVINCE

Understanding of the formation of the continents requires data from the subcontinental lithospheric mantle (SCLM) and the crust. Differences exist globally between >3.2 Ga versus <3.0 Ga crust and SCLM. This age has been proposed as a boundary between different geodynamic regimes on Earth and the start of the plate tectonic supercontinent (Wilson) cycle [1,2]. Geochronological studies of sulfide and silicate inclusions in diamonds from more than 20 kimberlites on 4 cratons characterize the SCLM through time. Diamonds >3.2 Ga contain exclusively peridotitic (harzburgitic) silicate and sulfide inclusions whereas diamonds <3.0 Ga contain inclusions that are predominantly eclogitic. Similarly, >3.0 Ga kimberlite-borne eclogite xenoliths are largely absent in the SCLM rock record, whereas they are common thereafter [1]. The lack of early eclogite implies an absence of steep slab subduction. Likewise, Archean crust records major differences across the 3.0–3.2 Ga interval. Prior to 3.2 Ga, crust grew by vertical accretion over upwelling mantle in long-lived plateaux floored by extremely depleted residual harzburgitic SCLM or via slab melting and crustal imbrication over shallow subduction zones (e.g West Greenland), whereas lateral accretion, allochthonous greenstone belt growth and calcalkaline magmatic products of mantle wedge melting emerge only after 3.2 Ga [2]. This temporal and geochemical change can be explained as the result of a step-wise change in the tectonic style from rapid mantle convection, small plates, shallow subduction, and localized recycling >3.2 Ga, followed by large plates, steep subduction, and full upper mantle recycling <3.0 Ga. These geodynamic changes had profound effects on mantle depletion, crustal growth, and geochemical cycles. The Superior Province, assembled from 3.7 to 2.6 Ga displays geological elements of both regimes [3-5]. Its formation is a critical test of the planetary applicability of this change in tectonic styles.

[1] Shirey and Richardson (2011) Science 333 434-436. DOI:10.1126Science.1206275 [2] Van Kranendonk (2011) Science 333 413-414. DOI:10.1126Science.1208766 [3] Thurston (2002) Precambrian Research 115 11-36. [4] Ayer et al (2002) Precambrian Research 115 63-95. [5] Percival et al (2006) Canadian J Earth Sci. 43 1085-1117. DOI:10.1139/E06-062