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

Abstract

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 δ18O of SMB and metasedimentary country-rocks and the slow rates of diffusion of oxygen in garnet. Among texturally well-characterized garnet, values of δ18O 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 δ18O of magmatic garnet are in equilibrium with coexisting zircon (δ18O = 8.14 ± 0.23‰; n = 21) in the SMB, confirming direct magmatic crystallization of garnet. Entrained metamorphic garnet porphyroblasts preserve highδ18O values, confirming a slow rate of intracrystalline diffusion of oxygen in garnet. Averaging of metamorphic and magmatic contributions is evident from the intermediate δ18O 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, δ18O varies by 2.5‰ 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, δ18O analysis provides a powerful complement to existing methods of determining garnet provenance and a new means to deconvolute garnet assemblages in peraluminous magmas.