Gneiss dome development & transcurrent tectonics in the Archean: Example of the Pukaskwa batholith and Hemlo shear zone, Superior Province, Canada.

摘要

Archean greenstone belts typically form narrow sheared basins separating bulbous tonalo-trondjhemo-granodioritic (TTG) batholiths. The role played by gravity in the development of such dome-and-keel structures is a key question in Archean tectonics. The Pukaskwa batholith - Hemlo shear zone (HSZ) is a representative example of the dome-and-keel structures that are common in Archean terrains. This region has received considerable attention because the HSZ hosts several major gold deposits that are currently being mined. Late dextral strike-slip kinematics of the HSZ are well recorded by abundant strain markers in greenstone rocks, whereas the quartzofeldspathic coarse-grained rocks of the Pukaskwa batholith bear no macroscopically visible fabric. The goal of this study is to understand the structural history of this greenstone belt-batholith system.;The Pukaskwa batholith is a heterogeneous assemblage of TTG gneisses bounded by the Hemlo greenstone belt to the north. The density of the Pukaskwa batholith rocks (density = 2700 kg/m3) is on average less than that of the Hemlo greenstone rocks (density = 3000 kg/m3). Since Archean geotherms were considered higher than modern equivalents, the effective viscosity of the TTG rocks might have been sufficiently low to allow their diapiric ascent through denser greenstone rocks. Alternatively, the emplacement of the TTG batholith might have been driven primarily by transpressive tectonics.;The anisotropy of magnetic susceptibility (AMS) provides valuable information on the internal fabric of the Pukaskwa batholith. This study provides the kinematic information needed to support either the diapiric or the transpressive tectonic model. AMS recorded east-west trending prolate and plano-linear fabrics across the northern section along the contact, suggesting that transpressional forces from the Hemlo shear zone affected the emplacement of the Pukaskwa batholith. Away from the contact, fabrics are generally flattened, indicative of doming through diapiric processes. Also, in order to fully evaluate the diapiric hypothesis, it is necessary to obtain reliable data on rock densities across the Pukaskwa batholith. The density of about 360 representative specimens from the Pukaskwa batholith has been measured and will constitute a valuable database for future gravimetric investigations by mining companies. The significant degree of correlation between high-field magnetic susceptibility and density in the Pukaskwa batholith should be taken into account in geophysical exploration in Archean terrains, only as a proxy for iron content.