The significance of Mn‐rich ilmenite and the determination of P–T paths from zoned garnet in metasedimentary rocks from the western Cape Breton Highlands, Nova Scotia

Abstract

The Jumping Brook Metamorphic Suite in the western Cape Breton Highlands of Nova Scotia is part of an inverted Barrovian sequence that formed during a Late Silurian–Early Devonian promontory–promontory collision in the Canadian Appalachians. In this study, systematic discrepancies between geochemical observations and thermodynamic model predictions led to the discovery of a systematic relationship linking the style of garnet core isopleth intersection (GCII) to the pyrophanite (MnTiO3) component of co‐existing ilmenite. Samples that yielded tight GCIIs at or near the garnet‐in curve were found to contain ilmenite with negligible pyrophanite components, whereas samples yielding GCIIs far removed (up to 105°C) from the garnet‐in curve were found to contain ilmenite with significant pyrophanite and/or ecandrewsite (ZnTiO3) components. Based on petrographic and geochemical observations, Mn(±Zn)‐rich ilmenite are interpreted to have sequestered Mn throughout prograde metamorphism due to sluggish intracrystalline diffusion. The amount of reactive Mn input into the thermodynamic models from whole‐rock analyses were, in some cases, overestimated, resulting in garnet‐in curve topologies that extend to erroneously low P–T conditions. Modifications to the whole‐rock chemistry that account for Mn sequestration into ilmenite, however, yielded robust model results. Our results show that, in addition to uncertainties in thermodynamic data sets and phenomenon related to reaction kinetics, Mn‐rich ilmenite may superimpose additional complexities related to the interpretation of predicted equilibria involving garnet. Numerical simulations of garnet crystallization were used to infer P–T paths of metamorphism for one sample from the garnet zone (Mn corrected) and two samples from the staurolite zone (Mn uncorrected) of the inverted sequence. Model results are remarkably similar among the three samples and indicate that garnet crystallization occurred along relatively steep (31–37°C/km) clockwise P–T paths. The peak conditions of garnet crystallization and metamorphism (560–590°C, 7.4–8.0 kbar) are interpreted to have been attained approximately simultaneously, such that the paths are characterized by tight prograde‐to‐retrograde transitions. The hairpin nature of the P–T paths is interpreted to represent the onset of thrust‐related exhumation and isograd inversion along ductile shear zones, consistent with available field and geochronological constraints.

The (chemical) potential for understanding overstepped garnet nucleation and growth

“…This is typically implemented by modifying measured rock compositions to remove unreactive phases (e.g. Spear 1988;Marmo et al 2002;Evans 2004;Konrad-Schmolke et al 2005;Zuluaga et al 2005;Caddick et al 2007;McCarron et al 2019), by progressively modifying assumed length-scales of chemical equilibrium and calculating 'local compositions' accordingly (e.g. Guevara and Caddick 2016;Lanari and Engi 2017), or by This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.…”

“…Therefore, the uncertainties on the equilibria for Mn-bearing phases are likely greater than the uncertainties on end members with more robust experimental constraints. The strong dependence of garnet-in on the MnO bulk rock content may partially be a function of poorly constrained thermodynamic data or may be impacted by the sequestration of MnO by accessory phases (McCarron et al 2019). Simply recalculating phase equilibria in a MnO-free system is not an appropriate way to mitigate this phenomenon, with the calculated initial stability of garnet then moving to higher P-T than suggested by independent constraints (e.g.…”

Simultaneous operation of opposing reaction mechanisms: The influence of matrix heterogeneity on post‐kinematic garnet crystallization in an inverted metamorphic sequence

“…Given their homogeneous distributions of major phases at the hand sample scale and the approximately spherical garnet morphologies, this approach is considered appropriate and has been successfully applied in the past (e.g. George & Gaidies, 2017;McCarron, McFarlane, & Gaidies, 2019).…”

The Early Jurassic Metamorphic History of the Yukon-Tanana Terrane of Northwestern British Columbia: Insights from a New Inverse Garnet Fractionation Modelling Technique

“…Many metamorphic processes can affect the composition of garnet such that models are unable to accurately predict observed compositional zoning (e.g. Pattison and Tinkham, 2009;Pattison et al, 2011;Spear and Wolfe, 2019;McCarron et al, 2019). In this study we sidestep possible issues such as metamorphic overstepping, open system behavior, inaccurate thermodynamic databases, and chemical sequestration by other phases by taking an inverse modelling approach.…”