T118 Geochemical Signatures of Fluid-Rock Interaction: Earth Surface Weathering to Hydrothermal Systems II

Wednesday, 28 October 2020
10:00 AM - 12:00 PM
Meeting Rooms

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

Paper number: T118-5
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-354578

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