Wainui shear zone geology : investigations into a Cretaceous shear zone on the margin of the Separation Point batholith, northwest Nelson

Borcovsky, Damien Anatol.

The Wainui shear zone represents a 400-1400 m thick, 20 km long belt of steeply dipping, wellfoliated granitic and schistose rocks that form the western boundary of the Cretaceous Separation Point Batholith in northwest Nelson. The Separation Point Batholith is granitic in composition and makes up the northern-most portion of the Separation Point Suite, which forms the western edge of the greater Median Batholith. Geological units along the western margin of the Wainui shear zone include the Devonian Rameka Gabbro, Paleozoic Mount Arthur Marble and Pikikiruna Schist, and small granitic plutons of the Separation Point Suite. The shear zone preserves evidence for progressive solid-state deformation and fabric development that generally transcends from the lesser-foliated margins into the centre of the shear zone. Microstructural and mineralogical evidence indicates that the ductile fabrics developed during cooling from a minimum of middle amphibolite facies to greenschist facies conditions. Sense of shear was east-side up and the shear zone is interpreted to have initiated at least soon after crystallisation of the granite batholith as a response to convergence and rapid uplift along the western edge of the Median Batholith. Brittle deformation in the centre of the shear zone and constraints on the maximum amount of uplift that could have occurred during ductile deformation indicate that deformation is likely to have continued to temperatures below the brittle-ductile transition. Several faults and subparallel joint sets through out the Separation Point Batholith at a high angle to the shear zone are suggested to have formed during late Cretaceous extension. Retrogressive metamorphism and fluid-rock interaction coupled with intense leaching of base metal cations during deformation of central shear zone rocks implies a pervasive and mobile fluid phase. Quartz-rich foliated rocks exhibiting the distinctive chromium-nickel trace element signature indicate large syntectonic additions of quartz to the rock, which is also consistent with large volumes of fluid moving through the shear zone. These rocks are interpreted to represent later-stage, lower temperature ductile deformation. Extensive schists and many quartz-rich foliated rocks in the centre of the shear zone exhibit a distinctive trace element signature characterised by elevated nickel and chromium, which cannot have been derived from rocks presently exposed along the shear zone margins. A component to the shear zone is thus required to have been derived from depth and strong evidence suggests that it may be mafic to ultramafic rocks of the Riwaka Complex, which is presently exposed ~25 km south of the main study area.

iv, 104, vii leaves : col. ill., col. maps ; 30 cm. + 1 folded map in pocket.

2004Borcovsky

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