Abstract

Laboratory measurements of compressional and shear wave velocity to confining pressures of 600 MPa for a suite of representative samples collected from the Pikwitonei granulite belt and God's Lake domain, an Archean crustal cross section in the northwestern Superior Province, provide the basis of comparison of these terranes with the seismic characteristics of Archean lower crust. We found that felsic rocks in the Pikwitonei granulite belt and God's Lake domain, which make up the bulk of these terranes, have a similar average compressional wave velocity of 6.5 km/s at 600 MPa, indicating that felsic rocks show little velocity change across the amphibolite–granulite facies transition. Compressional wave velocities for mafic rocks from each terrane are between 7.1 and 7.3 km/s. Apparent Poisson's ratio ranges from 0.24 to 0.26 and 0.26 to 0.28 for felsic and mafic rocks, respectively. These velocity data compare favorably with data for similar lithologies from the Kapuskasing uplift. Using the relative abundances of the constituent lithologies, the weighted average compressional wave velocities of the God's Lake domain and Pikwitonei granulite belt at 600 MPa are 6.56 and 6.63 km/s, respectively. These values, coupled with velocity distribution functions based on the population statistics and relative abundance for each lithology, show that there is no correspondence between the seismic characteristics of the Pikwitonei granulite belt and typical Archean and Proterozoic lower crust. The average properties of the Pikwitonei granulite belt and God's Lake domain, however, correspond well with typical Archean and Proterozoic middle crust. This suggests that either the Pikwitonei granulite belt represents an extreme felsic end member of Archean lower crust or that the deepest levels of the Superior Province crust are not exposed in the Pikwitonei granulite belt. Similar distribution function diagrams for acoustic impedance show that the Pikwitonei granulite belt is characterized by high acoustic impedance contrasts, but the high-impedance component is low in abundance. If the strong reflections observed under the Pikwitonei granulite belt in recent Lithoprobe surveys are not due to other causes, such as favorably oriented bodies of metamorphosed banded iron formation, diabase, or rock units not exposed in this region but present at depth, then they are caused by surprisingly small volumes of mafic metavolcanic rocks.

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