25-2 Microstructural and Geochemical Changes Associated with Fluid Flow, Cataclasis and Subsequent Pseudotachylyte Formation in a Metapelitic and Foliated Garnet-Biotite Schist, Washington, USA

Session: Integrating metamorphism, mass transfer, and magmatism across the American Cordillera (Posters)

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Abstract:

The strongly foliated and generally pelitic to semipelitic Chiwaukum Schist (North Cascade Mountains) experienced peak metamorphism of approximately 600-650 °C and up to about 1 GPa in the northern part of the Nason Terrane. Foliation-parallel cataclasite (CCTE) zones, typically <10 cm thick, are common. These are thought to predate pseudotachylyte (PST) formation, which nucleated on the CCTE zones, either consuming them, or leaving some residual CCTE. Ar dating of the PST indicates formation at about 53.5-55.8 Ma, though possibly as old as 65 Ma, during exhumation.

The CCTE, mostly ultracataclasite, typically display two domains, a quartz-dominated (Q) domain, and ultrafine-grained, black, mica+graphite (M) domains. Extremely thin PST veins are locally present. Quartz rarely shows evidence of plastic deformation, including ribboning, BLG, and SGR recrystallization. Mostly it is brittlely deformed, granulated, highly fractured, showing undulose extinction and healed fluid-inclusion decorated microfractures. Micas are recrystallized, biotite is extensively replaced by chlorite, staurolite and kyanite altered to white mica, garnet is extremely fractured to ultra-small fragments (<5 microns) with some chlorite alteration and oxidation. Veining, with significant adularia, is common in the M domains.

In a thick (10 m-scale) cataclasite fault on Sloan Peak, strain is strongly partitioned between Q and M domains. Garnet is intensely crushed in the Q domains into strongly flattened oblate ellipsoids (pure shear), and thus the simple shear component is concentrated in the M domains, consistent with layer-parallel (usually adularia) veins.

Initial geochemistry on schist, CCTE, and PST shows the CCTE lost modest amounts of Ca, Na, and Mn, but gained K, LOI, C, Ba, Cs, and Rb. HFS elements mostly show slight increases in the CCTE, whereas the REES are little changed. Relative to the CCTE, PST is approximately isochemical except, remarkably, in the REEs. PST displays a negatively-sloping REE pattern, with negative Ce and Eu anomalies, enriched in the LREEs and MREEs but matching the CCTE in the HREEs. It is unclear how the PST became L/MREE enriched. Water-rich fluid penetrated the CCTE during strain, generally increasing LILs plus C and Ba, increasing the LOI and K by growth of white mica and chlorite, while removing Ca, Na, and Mn, largely by alteration of plagioclase. Mineralogy and microstructures suggest deformation at ~250 °C.   

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