TRACING GARNET ORIGINS IN GRANITOID ROCKS BY OXYGEN ISOTOPE ANALYSIS: EXAMPLES FROM THE SOUTH MOUNTAIN BATHOLITH, NOVA SCOTIA

Garnet in Nova Scotia's peraluminous South Mountain Batholith (SMB) displays diversity in its texture and composition that has challenged a comprehensive explanation of its origins. In this study, we have employed oxygen isotope analysis to fingerprint magmatic, peritectic, and xenocrystic SMB garnets to take advantage of contrasting delta O-18 of SMB and metasedimentary country-rocks and the slow rates of diffusion of oxygen in garnet. Among texturally well-characterized garnet, values of delta O-18 distinguish magmatic (8.21 +/- 0.19%; n = 10), metamorphic (9.38 +/- 0.13%; n = 6), and peritectic garnet (8.67 +/- 0.20%; n = 6). Values of delta O-18 of magmatic garnet are in equilibrium with coexisting zircon (delta O-18 = 8.14 +/- 0.23%; n = 21) in the SMB, confirming direct magmatic crystallization of garnet. Entrained metamorphic garnet porphyroblasts preserve high-delta O-18 values, confirming a slow rate of intracrystalline diffusion of oxygen in garnet. Averaging of metamorphic and magmatic contributions is evident from the intermediate delta O-18 of peritectic garnet, and corresponds to textural evidence that garnet crystallized dynamically, and that metamorphic wallrocks were partially melted and disaggregated by magmas. In the case of texturally ambiguous garnet found on the margin of the Halifax pluton, delta O-18 varies by 2.5 parts per thousand among closely spaced (separated by mm to cm) crystals, signaling heterogeneous populations of magmatic, peritectic, and xenocrystic garnet, and thorough mixing of the host magma. In total, delta O-18 analysis provides a powerful complement to existing methods of determining garnet provenance and a new means to deconvolute garnet assemblages in peraluminous magmas.