Redox state of the South Mountain Batholith: A reconnaissance study using zircon geochemistry

Abstract

The late Devonian South Mountain Batholith (SMB) of Southwestern Nova Scotia is the largest plutonic igneous body emplaced during the Appalachian orogeny, with a current surface expression of 73,000 km2. The batholith is composed of 13 distinct plutons that are broadly peraluminous in composition, ranging from tonalite to syenogranite. A parameter that has been particularly difficult to quantify for the SMB is the redox state, as measured by the oxygen fugacity (fO2), which exerts a profound control on magmatic phase stability, element partitioning, and importantly, the potential for economic mineral deposits. We are attempting a redox state survey of mineralised and unmineralised phases of the SMB using the newly-calibrated Ce-in-zircon oxygen barometer. This method combines bulk rock and zircon compositions to calculate apparent zircon/melt partition coefficients for Ce, a parameter which varies with the Ce4+/Ce3+ in the melt, and hence oxygen fugacity. A total of 23 samples were collected, of which 13 were selected for zircon separation lithogeochemical analysis based on spatial distribution, mineralogy and preliminary geochemical data acquired with XRF. Zircons from the unmineralised Halifax and Sandy Lake plutons, and mineralised New Ross plutons were imaged using Cathodoluminescence to determine textural domains, which were a guide to subsequent analysis by electron microprobe and LA-ICPMS. Preliminary examination of REE patterns revealed two distinct groups of zircons in the SMB. The first pattern depicts a continuous increase in REE concentration with increase in ionic radius (La to Lu), large positive Ce anomaly and negative Eu anomaly; these characteristics are typically associated with magmatic zircons. The second pattern also shows a negative Eu anomaly, but a subtler increase in REE concentration from La to Lu and a lack of Ce anomaly; these characteristics are typically associated with hydrothermal zircons. Oxygen fugacity calculations using zircons that exhibit magmatic REE patterns yield four main observations. First, the fO2 recorded by the SMB shows an apparent internal variation of 4 orders of magnitude. Second, the margin of the Halifax pluton is more reduced than the interior. Third, the mineralised New Ross pluton appears to be the most reduced phase of the sampled phases. Finally, for the SMB, the concentration of dissolved water in the melt imparts a stronger control on redox state than temperature.

Keywords: oxygen fugacity, redox state, zircon, Ce-in-zircon oxygen barometer, cerium, mineralised granites, South Mountain Batholith, S-type granite.