Petrogenesis of Mantle Xenoliths in Alkaline Basalts in Southeastern British Columbia

English Abstract:

Tertiary alkaline basalts diatremes in Southeastern British Columbia are host to a variety of mantle xenoliths, predominantly spinel lherzolites with associated dunites and pyroxenites. Mineralogically, they are composed mainly of olivine, orthopyroxene, clinopyroxene and spinel characterized by forsterite (Fo_87-93), enstatite (En_90-92), diopside (En_45-50-Wo_40-45-Fs₅) and Cr-spinel (6-11 wt. % Cr), respectively. All mantle xenoliths are coarse-grained and show triple junctions. Clinopyroxene and spinel show evidence for chemical reactions with percolating melts.

The mantle xenoliths are characterized by 42.8-45.2 wt. % SiO₂, 37.5-42.7 wt. % MgO, 8.2-9.7 wt. % Fe₂O₃. Al₂O₃ (0.85-4.61 wt. %) and TiO₂ (0.02-0.16 wt. %) show moderate variations, resulting in super-chondriting Al₂O₃/TiO₂ (21-72) ratios. CaO values range from 0.61 to 3.8 wt. %, whereas the CaO/Al₂O₃ possess a narrow range (0.54-0.92). They have rather restricted Mg-numbers (89-92). They are strongly depleted in incompatible elements (Ba = 2-12 ppm; Sr = 3-31 ppm; Zr = 6-21 ppm, K₂O = 0.01-0.07 wt. %) and variably depleted in transitional metals (V = 41-94 ppm; Sc = 6-17 ppm). Yttrium (1-3 ppm) is also strongly depleted. On the basis of chondrite-normalized REE patterns, samples are divided into three groups: (1) LREE- depleted (La/Yb_n = 0.34-0.82) samples; (2) moderately LREE- enriched (La/Yb_n = 1.11-1.78) samples; (3) and LREE- enriched (La/Yb_n = 2.20-3.00) samples. N-MORB-normalized trace element patterns display negative Nb and Ti anomalies but exhibit positive Pb, Sr and Zr anomalies. These geochemical characteristics are consistent with a variably metasomatized sub-continental lithospheric mantle source, originated in a subarc mantle wedge.