167-2 Late Cretaceous to Eocene petro-tectonic setting of the northern Coast Mountains batholith, British Columbia and southeast Alaska

Session: Integrating Geochronology and Geochemistry to Decipher the Tectonic Evolution of Orogenic Belts

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Abstract:

The Coast Mountains batholith (CMB) of British Columbia and southeastern Alaska was emplaced from Jurassic to Eocene time and is the largest magmatic arc in the North American Cordillera. Understanding changes in its petrologic development and tectonic setting through both time and space informs models of the evolution of Cordilleran margins globally. New zircon U-Pb and Lu-Hf data from 26 intrusions comprising a transect across the northern CMB are used to constrain the tempo and sources of magmatism in its northern segment. Plutons in the western part of the transect have relatively uniform ages clustering between 93 and 88 Ma and are mostly diorites that make up part of the Admiralty-Revillagigedo belt. Intrusions directly east of this belt form steeply dipping, foliated tonalitic sills of Paleocene age (65 – 57 Ma) that are associated with the Coast shear zone (CSZ), a Paleocene-Eocene intrabatholithic ductile shear zone. Similar tonalitic sills delineate the CSZ along much of the batholith. East of the tonalite sills, the batholith is Late Cretaceous to Eocene in age (85 – 54 Ma) and dominated by undeformed Paleocene granodiorites and granites. Zircon Lu-Hf results indicate that Admiralty-Revillagigedo diorites have a uniform and juvenile source, with sample averages between +7 and +9. Tonalite sills of the CSZ are also uniform, but more evolved, with Lu-Hf scattering to sub-chondritic values and sample averages between -0.5 and +1.5. Plutons to the east of the sills are isotopically variable, but most have sample averages between +5 and +9 and are similar to western intrusions. We interpret the dominantly primitive Lu-Hf character of the western and eastern domain plutons as reflecting a mantle-dominated ‘baseline’ source for the northern batholith. The tonalite sills in the axial part of the batholith require the addition of an older crustal component, likely Yukon-Tanana terrane, in the melt source at depth or as a contaminant. Restriction of this crustal signal to sills within the CSZ suggests a tectonic sliver of Yukon Tanana terrane lies within the shear zone and was likely transported northward by dextral slip on the CSZ.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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