Abstract

Eleven U–Pb zircon dates from seven intrusives in the Nelson–Castlegar area provide new age constraints on the granitoid magmatism in southeastern British Columbia. Four samples from the Nelson Batholith date three distinct intrusive phases spanning ca. 10 Ma. One sample from the eastern porphyritic phase (Coffee Creek) yields the oldest age at 172.5 ± 5.0 Ma. Two samples from the central K-feldspar megacrystic phase (Crescent Bay) give precise ages at 162.0 ± 1.0 and 161.5 ± 1.5 Ma, and possibly date the youngest phase of the batholith. An intermediate age of emplacement at 166.0 ± 3.0 Ma is suggested for the southern tail of the batholith. The new and published data suggest that the Nelson Batholith was emplaced in roughly concentric zones. In contrast, the southern quartz diorite phase and the northwestern granodiorite phase of the Bonnington Pluton were emplaced coevally during the Middle Jurassic at 167.4 ± 2.0 and 165.0 ± 3.0 Ma, respectively. Middle Jurassic ages were also obtained for a sample of hornblende orthogneiss (166.0 ± 7.5 Ma) that intrudes the Trail Gneiss, and for a leucocratic gneiss sample (156.6 ± 6.0 Ma), a remnant from the Kinnaird Gneiss. A biotite granite sheet that intrudes the Kinnaird Gneiss yields a Middle Eocene age (40.5 ± 6.0 Ma), and possibly dates the youngest deformation event in the region. The Early Eocene ages (55.1 ± 3.7 and 50.6 ± 0.5 Ma) obtained from the Ladybird Granite and Coryell Syenite agree with previous estimates. Early Proterozoic ages of inherited zircon components in most of the samples agree with Nd–Sr isotopic evidence for incorporation of old crustal material in the intrusives empiaced in continental-arc settings.

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