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An evaluation of crustal assimilation within the Late Devonian South Mountain Batholith, SW Nova Scotia

Published online by Cambridge University Press:  20 February 2012

J. GREGORY SHELLNUTT  and
JAROSLAV DOSTAL
J. GREGORY SHELLNUTT*
Affiliation:
Academia Sinica, Department of Earth Sciences, National Taiwan Normal University, 88 Section 4 Tingzhou Road, Taipei 11677, Taiwan
JAROSLAV DOSTAL
Affiliation:
Saint Mary's University, Department of Geology, 923 Robie Street, Halifax, NS, B3H 3C3, Canada
*
†Author for correspondence: [email protected], [email protected]
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Abstract

The Late Devonian South Mountain Batholith (SMB) of southwestern Nova Scotia is the largest batholith in the Appalachian Orogen of Eastern North America and contains economic deposits of U and Sn. The SMB comprises at least 11 individual plutons, which range in composition from granodiorite to biotite monzogranite, leucomonzogranite and leucogranite. Previous studies have suggested that a combination of fractional crystallization, assimilation of Meguma Supergroup country rocks and an influx of magmatic fluids contributed to the chemical evolution of the SMB. The amount of crustal assimilation is estimated to be as high as 33%. MELTS modelling assuming a starting composition of granodiorite with H2O = 4 wt%, pressure = 4 kbar (~12 km) and fO2 = FMQ can reproduce the chemical evolution observed in the SMB. However, some leucogranites likely require an additional component (e.g. hydrothermal fluids) to explain their alkali metal enrichment (e.g. Na, K, Rb). Zircon saturation thermometry estimates indicate the Salmontail Lake and Scrag Lake granodiorite plutons had high minimum initial temperatures of 823 ± 6°C and 832 ± 2°C, respectively, which are similar to low zircon-inheritance granitoids. The TiO2/Al2O3 and alkali-lime ratios of the surrounding country rocks and the leucogranites indicate the amount of crustal assimilation is likely to be between 10% and 20%. Our findings suggest the granodiorites of the SMB were likely produced by partial melting of the sub-Meguma Supergroup (e.g. Avalon terrane) lower crust caused by the contemporaneous injection of high temperature mafic to ultramafic magmas.

Keywords

Late DevonianMeguma SupergroupSouth Mountain BatholithgraniteAppalachian Terraneperaluminous
Type
Original Articles
Information
Geological Magazine , Volume 149 , Issue 3 , May 2012 , pp. 353 - 365
Copyright
Copyright © Cambridge University Press 2012

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