BORON ISOTOPIC CHARACTERISATION OF SERPENTINITES FROM THE ATLIN TERRANE, CANADIAN CORDILLERA: EVIDENCE FOR PRESERVED OCEANIC CORE COMPLEXES?

Ultramafic massifs from the Atlin (formerly Cache Creek) terrane, northern Canadian Cordillera, display an incomplete ophiolitic sequence as upper crustal rocks generally lie directly on mantle peridotites. Low-angle normal faults characterized by foliated serpentinites, rodingites and cataclasites mark the mantle-crust contact. Serpentinites from this contact at Union Mt. and Squanga Lake were investigated by microscopic petrography and Raman spectroscopy. Raman methods allow phase identification of different serpentine minerals: lizardite-crysotile (0-300°C; <1.0GPa) and antigorite (300-460°C; higher-P). Boron isotopic compositions of serpentine minerals are representative of the fluids that altered the ultramafic rocks. Previous work showed that serpentine hydrated by seawater has δ¹¹B ranging from +40 to +10‰, while serpentine hydrated by slab-derived metamorphic fluids has δ¹¹B ranging from +10 to -6‰ (shallow fluids) or -6 to -20‰ (deep fluids). At least three generations of serpentine minerals from the Atlin terrane were identified and analysed for δ¹¹B: 1) early lizardite-chrysotile with δ¹¹B values of +13 to -1‰; 2) a second generation of antigorite-chrysotile with δ¹¹B values of -2 to - 13‰; and 3) late antigorite flakes with δ¹¹B values of +4‰ to -2‰. Identification of several isotopically distinct serpentine generations support the hypothesis of: (1) early alteration of peridotites (early serpentine with +δ¹¹B) by seawater-derived fluids, possibly associated with exhumation during oceanic core complex formation; (2) possible infiltration of aqueous fluids (2^nd generation of serpentine with strongly negative δ¹¹B) during obduction and imbrication of ophiolite nappes; (3) re-exhumation (late antigorite flakes with δ¹¹B straddling 0‰), possibly reflecting late stages of obduction and thrust-stacking.