Evolution of Archean and Proterozoic subcontinental mantle and lower crust

The Late Archean and Paleoproterozoic evolution of the subcontinental mantle is recorded by large-scale, widespread magmatic and structural features. These may include a cryptic tectonic suture, lithospheric folds, a crustal-scale fault (STZ), at least two granite "blooms" (Snow Island and Nueltin), several diabase dyke swarms, and a giant ultrapotassic magma field. They can only be effectively investigated by incorporating integrated broad-reach techniques, such as petrological and isotopic research (Peterson, Cousens and Davis), long wavelength gravity data (Roest et al.), and teleseismic and magnetotelluric experiments (White, Jones, et al. and Eaton). As in other Archean cratons world-wide, the WCP is thought to be underlain by a refractory, buoyant mantle root, extending down to ca. 400 km depth. Although the root, as established from seismic data, is apparently remote from crustal features targeted by the bedrock mapping projects, its existence places significant constraints on crust-mantle interaction and crustal melting in models derived for other components of the WCP NATMAP program. Because lithospheric roots are classically interpreted to result from massive basalt and komatiite extraction from the upper mantle, the assumed late Archean age for the WCP mantle root may have implications for the tectonic setting of the penecontemporaneous greenstone belts. It may also present a hurdle to the application of delamination models, formulated in younger settings, to explain the widespread nature of crustal and mantle-derived magmatic suites (ca. 2.6 Ga Snow Island and ca. 1.8-1.7 Ga Nueltin granitoids). Furthermore, a huge volume of ultrapotassic material (Christopher Island Formation) was erupted at ca. 1.85-1.80 Ga, derived by melting of strongly metasomatized upper mantle. Although Province-wide upper mantle enrichment could be related to shallowly dipping Paleoproterozoic subduction beneath the WCP (ca. 2.0-1.8 Ga), it is not obvious how a late Archean mantle root would be preserved. Nevertheless, given the presence of diamonds in ultrapotassic dykes, the timing and distribution of upper mantle metasomatism has implications for mineral exploration.

The age and nature of the root are tentatively predicated on sparse seismic data for the upper mantle. Spatial variation of seismic shear wave velocity for the WCP grossly outlines a deep seated boundary between faster lithosphere to the SW and slower lithosphere to the NE. This suggestion derives some support from the distribution of a limited number of Sm-Nd model and U-Pb magmatic ages in both the Rae and Hearne "provinces" (Middle to Late Archean in the southwest vs Late Archean in the northeast). This raises the possibility that the major tectonic boundary in the WCP is cryptic and lies at a high angle to the STZ, separating a middle Archean crust to the SW from younger late Archean crust to the NE. Such a boundary might also be reflected in variations in crustal thickness, presence/absence of a high velocity zone at the base of the crust, distribution of electrical resistivity within the crust and upper mantle, as well as differences in seismic and electrical anisotropy, and degree of mantle metasomatism. Similar characteristics could enable the STZ at to be imaged at depth to determine its gross orientation, and whether it is an intra-crustal fault or a fundamental lithospheric-scale structure.

The proposed teleseismic - magnetotelluric experiment (1998-1999) would deploy 8 geophysical instruments along profiles parallel and perpendicular to the STZ. Complete analysis of the data would include anisotropy measurements, as well as mapping of sub-horizontal lithospheric boundaries and the seismic and conductivity structure of the lithosphere. According to existing criteria, the test of the age of the lithosphere is as follows: if it is indeed late Archean, then mantle structure should reflect the penetrative nature and orientation of the late Archean crustal structure. Otherwise it may reflect the discontinuous nature of Paleoproterozoic crustal deformation, or show no geometrical correlation with crustal structure at all.

The lithogeochemistry of granitoids is a powerful, broad-reach, petrotectonic tool. Strategically upgrading the distribution of well constrained Sm-Nd model ages across the WCP, and obtaining new Pb isotope data, will contribute to testing the possible existence of an older cratonic nucleus to the SW, and the postulated differences between Rae and Hearne "provinces" (Peterson and Cousens; 1997 and ongoing). The principal gap in the existing data set lies in the NE Hearne, where NATMAP bedrock mapping will be focused. The widespread distribution of ca. 2.6 Ga Snow Island suite granitoids across the WCP does not correspond to the belt-like configuration expected for classical subduction-related magmatic arcs, and their genesis remains a fundamental problem in the crustal evolution of the WCP. By exploiting them as crustal and mantle probes, one can test for contemporaneous basaltic underplating, with direct repercussions for the age of the inferred mantle root. Mafic and ultrapotassic dyke swarms intruded from ca. 2.45 Ga to ca. 1.8 Ga, represent further probes into the evolving lithospheric mantle through the Paleoproterozoic.

The salient point here is that these broad-scale crustal and lithospheric features reflect the boundary conditions determining the late Archean and Paleoproterozoic tectonic evolution of the WCP. Without constraints, interpretations of the NATMAP geological mapping will remain inherently equivocal.

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Alan G Jones / 24 April 2004 / [email protected]