Using Oxygen Isotopes of Zircon to Evaluate Magmatic Evolution and Crustal Contamination in the Halifax Pluton, Nova Scotia

Oxygen isotope analysis of zircon (Zrc) is well suited for parsing out the magmatic history in granitoids. The Halifax pluton is the largest pluton (1060 km2) in the peraluminous South Mountain batholith. The Halifax pluton is mapped as a concentrically zoned body, with outer units comprising granodiorite, monzogranite and a mafic porphyry; these units are locally rich in metasedimentary xenoliths and magmatic enclaves. The exterior units surround a more felsic core of leucogranite [1]. Previous oxygen isotope studies of the pluton report high whole rock δ18O values that range from 10.7-11.7‰ [2], and indicate a significant supracrustal component in the source of the pluton. We report the first δ18O(Zrc) values from the Peggy's Cove monzogranite and an associated mafic porphyry. Samples were collected across 30 km of discontinuous exposures of the monzogranite. Values of δ18O(Zrc) vary from 7.71-8.26‰ (average = 8.15±±0.32‰(2 S.D.); n = 10). Small but systematic E-W regional variation in δ18O(Zrc) values suggests heterogeneous magmatic contamination within the monzogranite. Meter-scale magmatic enclaves, observed in close association with pods of diverse xenoliths and smaller enclaves at the western Cranberry Head locality, are slightly enriched in δ18O relative to the host monzogranite. These data combined support a model of magma mingling and heterogeneous mixing at the rim of the pluton, with contamination by high-δ18O rocks. Additional high-δ18O(Zrc) data from granodiorites on the northern margin of the Halifax pluton concur with these observations [3]. Typically, closed magmatic systems show increasing δ18O with SiO2 because more felsic magmas have a greater percentage of high-δ18O minerals such as quartz and feldspar. Thus, the Halifax pluton appears to exhibit an enrichment trend opposite of what would be expected of a closed evolving system. Emplacement mechanisms for the Halifax pluton proposed by previous workers suggest that the outer units intruded first, followed by the more felsic luecogranites at the core of the pluton [1]. Based on δ18O(Zrc) data, we propose a model in which early magmas were already enriched in δ18O from a metasedimentary source; during emplacement, the magmas mixed with and variably assimilated high δ18O(10-13‰; [2]) Meguma Group metasedimentary wallrocks and melts thereof, which increased the δ18O of the magma. The first magmas emplaced cleared the way for later, more evolved magmas to intrude without significant contact with country rock. Additionally, δ18O (Zrc) values are in disequilibrium with published whole rock and quartz δ18O values from the pluton, indicating that progressive contamination or subsolidus isotopic exchange elevated whole rock δ18O of the monzogranite after zircon crystallization. 1. M. A. MacDonald, R. J. Horne, Maritime Seds Atlantic Geol 24, 33 (1988). 2. F. J. Longstaffe, T. E. Smith, K. Muehlenbachs, Can J Earth Sci 17, 132 (1980). 3. R. M. Nowak, J. S. Lackey, J. W. Valley, GSA Abs (2007).