OXYGEN ISOTOPE EVIDENCE FOR CONTAMINATION OF GRANODIORITES OF THE HALIFAX PLUTON, NOVA SCOTIA

Strongly peraluminous (A/CNK = 1.1–1.2) granodiorites of the Devonian South Mountain batholith (SMB) contain abundant metamorphic xenoliths and xenocrysts. Such granodiorites occur as the outermost units of the SMB, in contact with metamorphic wallrocks of the high-δ¹⁸O (10-13‰, Longstaffe et al. 1979, CJES) Meguma Group. Thus the granodiorites appear to record increasing contamination toward the margin of the batholith. To address how contamination varies spatially in the SMB, we have analyzed δ¹⁸O of co-existing zircon (Zrc) and garnet (Grt). The utility of δ¹⁸O analysis of these two minerals lies in the fact that slow oxygen diffusion in both minerals preserves magmatic δ¹⁸O and garnet typically appears late as a liquidus phase. Thus, comparing δ¹⁸O of the two minerals can be used to deconvolute late-stage contamination from magma source.

This study focuses on a granodiorite unit exposed on the northern margin of the Halifax pluton. Cathodoluminescence imaging of zircon reveals normal growth zonation, with some inherited cores that are relatively small (< 5 vol.%). Values of δ¹⁸O(Zrc) in the granodiorite range from 8.13–8.64‰ (avg. = 8.32 ± 0.22‰, 1SD; n=4). In contrast, δ¹⁸O(Zrc) in the Harrietsfield monzogranite, the innermost unit of the Halifax pluton, is lower on average (7.86 ± 0.20‰, n=3) despite the monzogranite being more evolved. Average δ¹⁸O of magmatic garnet in the granodiorite is 8.31 ± 0.05‰ (n=2) and is indistinguishable δ¹⁸O(Zrc), which indicates minimal contamination after zircon crystallized. A metamorphic garnet xenocryst yields δ¹⁸O of 9.03‰ that matches the δ¹⁸O of metamorphic garnet of the Meguma Group (Lackey et al. 2006, GSA Abst). Preservation of original δ¹⁸O in the xenocryst indicates that it did not undergo significant δ¹⁸O exchange with the host magma.

Overall, δ¹⁸O data confirm greater contamination at the margin of the Halifax pluton, which corroborates textural evidence for contamination. In addition, the xenoliths likely represent the refractory parts of upper crustal contaminants whereas the labile contaminant contribution is resolved by δ¹⁸O analysis.