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Journal of Petrology Advance Access originally published online on July 6, 2009
Journal of Petrology 2009 50(8):1477-1503; doi:10.1093/petrology/egp038
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© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Evaluating the Origin of Garnet, Cordierite, and Biotite in Granitic Rocks: a Case Study from the South Mountain Batholith, Nova Scotia

Saskia Erdmann1,*, Rebecca A. Jamieson1 and Michael A. MacDonald2

1Dalhousie University, Department of Earth Sciences, Halifax, ns b3h 3j5, Canada
2Nova Scotia Department of Natural Resources, Mineral Resources Branch, Halifax, ns b3j 3M8, Canada

RECEIVED JUNE 16, 2008; ACCEPTED MAY 25, 2009


  Abstract

Evaluating the origin of garnet, cordierite, and biotite in granites provides important insight into closed- and open-system magma evolution. We present field, textural, major- and trace-element mineral chemical, and Sr–Nd whole-rock isotopic data on garnet-, cordierite-, and biotite-rich zones from the peraluminous South Mountain Batholith. We infer that: (1) garnet-rich zones of decimeter to meter size with ≤30 vol. % large, subhedral garnet with abundant inclusions of detrital country-rock monazite represent partially assimilated metapelitic country rocks, where garnet is the incongruent product of biotite-dehydration melting; (2) cordierite-rich zones tens of meters to kilometers in dimension, with ≤5 vol. % large, subhedral to euhedral, zoned cordierite, formed by crystallization from relatively evolved magmas and subsequent crystal accumulation; (3) biotite-rich zones with large, subhedral to euhedral biotite with abundant euhedral apatite inclusions, making up ≤80 vol. % (centimeter-scale) or ≤25 vol. % (kilometer-scale) of the rocks, formed dominantly by fractional crystallization throughout the chemical evolution of the batholith. Our results suggest that for garnet and cordierite, a combination of textural and mineral chemical characterization is probably sufficient to determine their origin in granites. However, for biotite and other readily equilibrated minerals, evaluating both mineral and rock textures and major-element, trace-element, and isotopic compositions is essential.

KEY WORDS: assimilation; biotite; cordierite; fractional crystallization; garnet

*Corresponding author. Telephone: 902 494 3362. Fax: 902 494 6889. E-mail: serdmann{at}dal.ca


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