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Role of fluids in the metamorphism of the Alpine Fault Zone, New Zealand.

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Title: Role of fluids in the metamorphism of the Alpine Fault Zone, New Zealand.
Authors: Vry, J. K.1, Storkey, A. C.2, Harris, C.3
Source: Journal of Metamorphic Geology. Jan2001, Vol. 19 Issue 1, p21-31. 11p.
Subjects: Fluids, Metamorphism (Geology), Fault zones, Stable isotopes
Geographic Terms: New Zealand
Abstract: Abstract Models of fluid/rock interaction in and adjacent to the Alpine Fault in the Hokitika area, South Island, New Zealand, were investigated using hydrogen and other stable isotope studies, together with field and petrographic observations. All analysed samples from the study area have similar whole-rock δD values (δDWR = -56 to -30‰, average = -45‰, n = 20), irrespective of rock type, degree of chloritization, location along the fault, or across-strike distance from the fault in the garnet zone. The green, chlorite-rich fault rocks, which probably formed from Australian Plate precursors, record nearly isothermal fluid/rock interaction with a schist-derived metamorphic fluid at high temperatures near 450–500°C (δD of water in equilibrium with the green fault rocks (δDH2O, green) ≈ -18‰; δD of water in equilibrium with the greyschists and greyschist-derived mylonites (δDH2O, grey) ≈ -19‰ at 500°C; δDH2O, green ≈ -17‰; δDH2O, grey ≈ -14‰ at 450°C). There is no indication of an influx of a meteoric or mantle-derived fluid in the Alpine Fault Zone in the study area. The Alpine Fault Zone at the surface shows little evidence of late-stage retrogression or veining, which might be attributed to down-temperature fluid flow. It is probable that prograde metamorphism in the root zone of the Southern Alps releases metamorphic fluids that at some region rise vertically rather than following the trace of the Alpine Fault up to the surface, owing to the combined effects of the fault, the disturbed isotherms under the Southern Alps, and the brittle–ductile transition. Such fluids could mix with meteoric fluids to deposit quartz-rich, possibly gold-bearing veins in the region c. 5–10 km back from the fault trace. These results and interpretations are consistent with interpretations of magnetotelluric data obtained in the South Island GeopHysical Transects (SIGHT) programme. [ABSTRACT FROM AUTHOR]
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Abstract:Abstract Models of fluid/rock interaction in and adjacent to the Alpine Fault in the Hokitika area, South Island, New Zealand, were investigated using hydrogen and other stable isotope studies, together with field and petrographic observations. All analysed samples from the study area have similar whole-rock δD values (δDWR = -56 to -30‰, average = -45‰, n = 20), irrespective of rock type, degree of chloritization, location along the fault, or across-strike distance from the fault in the garnet zone. The green, chlorite-rich fault rocks, which probably formed from Australian Plate precursors, record nearly isothermal fluid/rock interaction with a schist-derived metamorphic fluid at high temperatures near 450–500°C (δD of water in equilibrium with the green fault rocks (δDH2O, green) ≈ -18‰; δD of water in equilibrium with the greyschists and greyschist-derived mylonites (δDH2O, grey) ≈ -19‰ at 500°C; δDH2O, green ≈ -17‰; δDH2O, grey ≈ -14‰ at 450°C). There is no indication of an influx of a meteoric or mantle-derived fluid in the Alpine Fault Zone in the study area. The Alpine Fault Zone at the surface shows little evidence of late-stage retrogression or veining, which might be attributed to down-temperature fluid flow. It is probable that prograde metamorphism in the root zone of the Southern Alps releases metamorphic fluids that at some region rise vertically rather than following the trace of the Alpine Fault up to the surface, owing to the combined effects of the fault, the disturbed isotherms under the Southern Alps, and the brittle–ductile transition. Such fluids could mix with meteoric fluids to deposit quartz-rich, possibly gold-bearing veins in the region c. 5–10 km back from the fault trace. These results and interpretations are consistent with interpretations of magnetotelluric data obtained in the South Island GeopHysical Transects (SIGHT) programme. [ABSTRACT FROM AUTHOR]
ISSN:02634929
DOI:10.1046/j.0263-4929.2000.00299.x