Features of seafloor hydrothermal alteration in metabasalts of mid-ocean ridge origin from the Chrystalls Beach Complex

ABSTRACT

The Taieri Mouth locale of the Chrystalls Beach Complex (CBC) in the South Island of New Zealand includes well preserved to strongly deformed pillow lavas and flattened veins of epidote, quartz and chlorite intercalated with basalt flows and volcanoclastic breccias. The tectonic affinity for this rare igneous portion of the predominantly sedimentary CBC has not been well established in the context of its regional metamorphic geology. New field, petrographic, geochemical and isotopic evidence suggest a mid-ocean ridge origin for the Taieri metabasalts. Further, paleo-vertical networks of epidote-quartz-chlorite veins and cross-cutting faults provide a record of seafloor fracturing and fluid-flow. Altered pillows and epidote separates have δ¹⁸O isotope values ranging from 9.3 to 13.1‰. This indicates slightly enriched δ¹⁸O fractionation resulting from seafloor weathering and low-temperature (<250°C) exchange between seawater and hydrothermal fluids in basaltic fractures. Age-corrected ⁸⁷Sr/⁸⁶Sr ratios between 0.704135 and 0.70624 show low temperature fluid-rock interactions where the altered pillows and veins did not succumb to major mineralogic changes or isotopic re-equilibration after formation. In contrast, compressed s-fold epidote and coarse quartz veins near metasediments are suggestive of the elevated temperatures and pressures during accretion. We differentiate between episodic seafloor venting and accretional wedge-related alteration recorded within these metabasalts.

KEYWORDS:

• Otago region
• metabasalts
• seafloor hydrothermal alteration
• mid-ocean ridge processes
• veins
• epidote
• petrology
• geochemistry
• isotopic analysis
• ophiolite

Acknowledgements

We offer thanks to the following for their help: Erikka Olson in the field, Gemma Kerr with the XRD, Peter Crowley with quantitative SEM analyses, Nancy Piatczyc with the SEM, Matthew Cooper with the ⁸⁷Sr isotope analyses, and Isabelle Chambefort, Nick Mortimer, and an anonymous reviewer for their thoughtful feedback. The University of Otago Geology Department hosted L. Gilbert for sabbatical in 2018, during which this work was conducted. Samples are stored at Williams-Mystic; contact [email protected].

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are openly available in figshare at https://doi.org/10.6084/m9.figshare.12730352.

Additional information

Funding

This work was supported by a grant from the Northeast Section of the Geological Society of America.