Vein assemblages and fluid evolution in 18 O-depleted Neoproterozoic igneous rocks of the Mira terrane, Cape Breton Island, Nova Scotia 1 This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology. 2 Laboratory for Stable Isotope Science (LSIS) Contribution 251.

• Published in: Canadian Journal of Earth Sciences
• Main Authors: Potter, Joanna, Longstaffe, Frederick J., Barr, Sandra M.
• Other Authors: Murphy, Brendan J.B.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2012
• Subjects: General Earth and Planetary Sciences; Breton Island; Mira
• Online Access: http://dx.doi.org/10.1139/e11-074; http://www.nrcresearchpress.com/doi/full-xml/10.1139/e11-074; http://www.nrcresearchpress.com/doi/pdf/10.1139/e11-074

Fluids responsible for regional 18 O-depletion of Neoproterozoic igneous rocks in Avalonia are investigated here through a petrographic, microthermometric, and stable isotopic examination of fluid inclusions and minerals from the abundant vein networks of the Mira terrane, Cape Breton Island. Six categories of vein assemblages — from oldest to youngest — are present: (i) quartz–albite, (ii) quartz–epidote, (iii) quartz, (iv) quartz–chlorite–calcite, (v) quartz–calcite, and (vi) calcite. Vein system temperatures were initially as high as ∼300 °C and gradually decreased to ∼200 °C. Moderate salinities (<8 equivalent wt.% NaCl) characterize entrapped fluids in the early quartz–albite veins but decrease in later quartz–epidote and quartz–calcite veins to <1 equiv. wt.% NaCl. The limited range of fluid δ 18 O values (–1.9‰ to +1.4‰) calculated for most of the vein assemblages is suggestive of a seawater-dominated system, as are the δD H2O values (–12‰ to –3‰) obtained for epidote. Decreasing fluid salinities, however, suggest that meteoric water became dominant during later stages of vein formation. The carbon isotopic compositions of trace CO 2 and CH 4 from the fluid inclusions (δ 13 C CO2 = –22‰ to –4‰; δ 13 C CH4 = –52‰ to –37‰) are indicative of externally derived (i.e., non-magmatic) fluids of organic origin.