Abstract

A quantitative comparison is made between fracture styles in two late Archean granite intrusions of the Superior Province—–the Lac du Bonnet Batholith (LDBB) and Eye–Dashwa Pluton (EDP). These intrusions have a similar geological setting, similar mineral and chemical composition, and similar physical properties but vary markedly in volume (LDBB = 9060 km3; EDP = 122 km3).The fracture style of the LDBB consists of mainly low-angle thrust faults within otherwise poorly fractured granite. Subvertical fractures are restricted to within 200 m of surface or zones encompassing the thrust faults. The mineral assemblage chlorite – iron oxide – carbonate is widespread in fractures. In contrast, fractures of the EDP are closely spaced, variably oriented, pervasive to depth, and dominated by subvertical transcurrent faults. Epidote is an abundant fracture-filling material.Most fractures formed in response to Early Proterozoic compression under low-greenschist conditions in the LDBB and upper-greenschist conditions in the EDP. Fractures in both intrusions were subsequently rejuvenated (clay – iron oxide filling materials) without appreciable modification to fracture styles. The presence of a strong planar fabric at one site, variation in the intensity of Early Proterozoic tectonism, and prolonged plastic deformation in the large LDBB are cited as possible causes for the observed variation in fracture styles.

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