Rare earth element and high field strength element characteristics of whole rocks and mineral separates of ultramafic nodules in Cenozoic volcanic vents of southeastern British Columbia, Canada

Abstract

Alkali basalts in southeastern British Columbia of Cenozoic age, having within-plate characteristics, contain a compositionally diverse suite of Cr-diopside ultramafic nodules. These nodules are either not metasomatse, or record different styles of lithospheric metasomatism, revealing that the continental lithospheric mantle under southeastern B. C. possesses a heterogeneous nature developed during a complex history. Three, distinctive trace element patterns are exhibited by the nodules. Nodules with trace element pattern 1 are severely LREE depleted, where ( La Yb)n = 0.20-0.37, and ΣREE = 3.2-6.0 ppm, with pronounced positive Th and Nb but generally negative Zr, Hf, and Ti anomalies on primitive mantle-normalized diagrams. Relative to primitive mantle, these nodules have low La Th (1.8-6.7 vs. 8) and La Nb (0.4-1.0 vs. 0.96) ratios, indicative of enrichment of Nb and Th relative to LREE. Nodules with trace element pattern 2 are LREE enriched (LREE 10 times pattern 1), where ( La Yb)n = 2.0-3.4 and ΣREE = 1.4-10.2 ppm, with generally smooth coenrichment of Th and Nb with LREE. They are characterized by positive Zr and Hf but negative Ti anomalies. Nodules with trace element pattern 3 exhibit greater enrichment of LREE, where ( La Yb)n = 7.0-21 and FREE = 15-17 ppm, combined with negative Nb, Zr, Hf, and Ti anomalies. Orthopyroxene and olivine from the LREE-depleted nodules have complementary Zr, Hf, Ti, and Nb anomalies to clinopyroxene which exhibits negative HFSE anomalies. Moreover, while REE, Zr, Hf, and Ti reside principally in clinopyroxene, the orthopyroxene and olivine separates contain Th and Nb at a similar level to clinopyroxene. Therefore, LREE-depleted xenoliths have variably positive Th and Nb and negative Ti, Zr, and Hf anomalies, depending upon modal mineral proportions. Accordingly, clinopyroxene analyses are not necessarily representative of whole nodule compositions; specifically in terms of positive Nb anomalies in nodules vs. negative anomalies in clinopyroxene. The LREE-depleted continental lithospheric mantle under southeastern British Columbia has a distinctive trace element pattern, differing from the oceanic lithospheric mantle. Two different metasomatic processes have been invoked to explain the LREE enriched nodules: plume-related melt for positive HFSE anomalies (pattern 2) and hydrous or C02-rich fluid for negative HFSE anomalies (pattern 3). © 1995.