Metamorphism and Plutonism in the Waipuna Valley, North Westland

Ritchie, Tom

This report describes the first field descriptions, mineralogy, quantitative P-T-t conditions of the Greenland Group in the Waipuna Valley, Westland. The Greenland Group in this area has been subdivided into four groups (I-IV) based on textural and mineralogical features. Group I rocks contain detrital textures and chlorite zone greenschist facies mineralogies. The rocks increase in grade to Group IV amphibolite facies gneisses and migmatites. Quantitative estimates on the conditions of deformation range from 420 C in Group I, through to 670 C in the Group IV gneisses. Group III rocks, however, contain the most informative mineral assemblages. Group III rocks contain an assemblage comprising andalusite-staurolite-biotite-quartz-oligoclase-Fe-oxide (M2) overprinted by biotite-sillimanite-muscovite-quartz and plagioclase (M3). These assemblages suggest that they formed under a relatively high geothermal gradient. The timing of the latest phase of metamorphism has been constrained by the U-Th-Pb dating of monazite, which is texturally controlled and grew at 108 1 Ma. Monazite growth may have coincided with the imposition of deformation fabrics, such as recrystallisation of quartz. The Waipuna Valley Greenland Group has been intruded by a suite of granitic dykes, stocks and plutons, here named Upper Grey Granites. The granite geochemistries indicate correlation with the nearby Rahu Suite magmatism. This interpretation is corroborated by U-Pb zircon ages which reveal mid Cretaceous emplacement between 111 and 105 Ma. Early granites (111 Ma, intruding Group III and IV Greenland Group) have deformation textures continuous with the their host rock. The youngest granite (105 Ma) intrudes Group I Greenland Group and there is a biotite hornfels assemblage developed in the aureole. The field, metamorphic and geochronological characteristics of theWaipuna Valley Greenland Group and Upper Grey Granites are interpreted to represent a rocks on either side of Cretaceous ductile detachment fault, here named the Waipuna Shear Zone. Group I and II Greenland Group represent upper plate portion, whereas the Group III gneisses and Group IV migmatites represent the lower plate. Although mylonitic rocks are exposed, the detachment surface is not. Nevertheless, the overall relationships described for the Waipuna Shear Zone are strikingly similar to those of the nearby Paparoa Metamorphic Core Complex, which has been interpreted as having formed during continental extension immediately prior to opening of the Tasman Sea. The rocks of the Waipuna Valley extend the known extent of Cretaceous shearing. In addition, the evidence for Cretaceous amphibolite facies metamorphism and dehydration of the mid crust, directly south of the Reefton Goldfield, may have significant implications for the timing of mineralisation.

viii, 137 pages A4, map in back pocket

2013Ritchie

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