New Zealand Journal of Geology and Geophysics abstracts

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Petrography and genesis of a wollastonite body and its associated rocks at Holyoake Valley, Nelson, New Zealand

W. A. WATTERS

Institute of Geological and Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand

Abstract  Samples from a wollastonite-rich lens, up to 55 m long and 10 m wide, within the contact aureole of the Canaan Granodiorite at Holyoake Valley, include a variety of calc-silicate rocks which locally contain abundant titanite. The mineralogy of the contact rocks indicates hornblende-hornfels facies conditions during metamorphism. Within the wollastonite body, an intrusive vein of monzonitic composition, up to 2 m across, probably resulted from crystallisation of granodioritic magma modified by partial desilication and contamination from adjacent calcium-rich rocks. Zoned calc-silicate rocks, apparently formed by reaction between wollastonite and basic hornfels that had been produced by recrystallisation of a dike of probable basaltic composition, are also present. Late intrusion of small amounts of quartzofeldspathic magma took place along discontinuities, including the dike boundaries, within and adjacent to the wollastonite body. Iron and titanium metasomatism was probably associated with intrusion of the quartzofeldspathic magma, while diffusion of calcium towards it led to the formation of heterogeneous contaminated rocks interpreted as endoskarns.

Available evidence is insufficient to allow a decision to be made between formation of the wollastonite by reaction between marble and enclosed quartz-rich layers or lenses, or by introduction of silica into marble from the adjacent granodiorite magma. Formation of the contact rocks probably took place with low values of XCO2 relative to XH2O in the fluid phase. Late changes accompanying cooling of the contact rocks in the presence of a fluid with high XH2O resulted in the crystallisation of small amounts of retrograde vesuvianite, prehnite, and apophyllite.

Keywords  Holyoake Valley; Nelson; Separation Point Batholith; marble; contact metamorphism; exoskarn; endoskarn; aplite; wollastonite; titanite; prehnite

New Zealand Journal of Geology and Geophysics, 1995, Vol. 38: 315-323

0028-8306/95/3803-0315 $2.50/0 (c) The Royal Society of New Zealand 1995

PDF file of entire paper: medium quality (905K); (scanned from paper original: notes about this process)

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