Polybaric magmatic evolution of the eocene penticton group alkaline rocks, south-central British Columbia

Volcanic rocks of the Challis-Kamloops belt erupted during the Eocene (55–45 Ma) throughout south-central British Columbia, Idaho, and Wyoming of the North American Cordillera. The Eocene volcanic rocks of southern British Columbia can be subdivided into three distinct groups (Kamloops, Princeton, Penticton) based on geography and composition. The southernmost Penticton Group is unique as it is composed of volcanic and intrusive alkaline rocks (e.g., Marron Formation, Terrace Mountain outlier, Allendale Lake pluton) as opposed to calc-alkaline rocks that are typical of the Kamloops and Princeton groups to the north. The rocks of this study are from the Penticton Group, are compositionally variably (SiO₂ = 51.2–64.7 wt%), and have highly fractionated chondrite-normalized REE patterns (La_N/Yb_N = 71–104) without noticeable negative Eu-anomalies (Eu/Eu* = 0.81–1.00). The rocks have similar Sr-Nd isotopic (⁸⁷Sr/⁸⁶Sr_i = 0.706422–0.706563; ε_Nd(t) = −4.3–6.1) ratios and Neoproterozoic depleted mantle model ages (T_DM = 794–873 Ma). Petrological modeling demonstrates that the primary process of chemical differentiation within the Penticton Group is fractional crystallization. However, it appears that the Marron, Allendale Lake, and Terrace rocks underwent fractional crystallization at different pressures (i.e., 4 GPa, 1.5 GPa, and 1 GPa). The polybaric differentiation of the Penticton Group indicates that the alkaline system evolved at different crustal depths in a similar manner as magmas related to the Yellowstone hotspot and markedly different from the Kamloops and Princeton groups. We suggest that the Penticton Group is underlain by ancient lithospheric mantle that was modified by Precambrian subduction processes and was isolated from the convecting mantle whereas the northern part of the Eocene volcanic belt is underlain by exotic accreted terranes (Intermontane Superterrane) and was derived from a different, younger lithospheric mantle.