GSA 2020 Connects Online

Paper No. 118-4
Presentation Time: 11:00 AM

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


BOGATU, Adina, University of Quebec at Montreal, BÉDARD, Jean H., Geological Survey of Canada, 490 rue de la Couronne, Quebec, QC G1K 9A9, Canada, LABROUSSE, Loic, iSTeP, UPMC Paris 6, 4, place Jussieu case 129, Paris, 75005, France, MARTIN, Celine, American Museum of Natural History, 200 Central Park West, New York, NY NY 10024-5102, TREMBLAY, Alain, Sciences de la Terre et de l'atmosphère, Université du Québec à Montréal, 201 President-Kennedy Av, PO Box 8888, Montreal, QC H3C 3P8, Canada and ZAGOREVSKI, Alexandre, 601 Booth St, Ottawa, ON K1A 0E8, Canada

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 (2nd 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.