RARE-EARTH ELEMENT AND HIGH-FIELD STRENGTH ELEMENT CHARACTERISTICS OFWHOLE ROCKS AND MINERAL SEPARATES OF ULTRAMAFIC NODULES IN CENOZOIC VOLCANIC VENTS OF SOUTHEASTERN BRITISH-COLUMBIA, CANADA

Alkali basalts in southeastern British Columbia of Cenozoic age, havin g within-plate characteristics, contain a compositionally diverse suit e of Cr-diopside ultramafic nodules. These nodules are either not meta somatised, or record different styles of lithospheric metasomatism, re vealing that the continental lithospheric mantle under southeastern B. C. possesses a heterogeneous nature developed during a complex histor y. Three distinctive trace element patterns are exhibited by the nodul es. Nodules with trace element pattern 1 are severely LREE depleted, w here (La/Yb)(n) = 0.20-0.37, and Sigma REE = 3.2-6.0 ppm, with pronoun ced 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 (0.4-1.8 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 CREE = 1.4-10.2 ppm, with g enerally smooth coenrichment of Th and Nh with LREE. They are characte rized by positive Zr and Hf but negative Ti anomalies. Nodules with tr ace element pattern 3 exhibit greater enrichment of LREE, where (La/Yb )(n), = 7.0-21 and Sigma REE = 15-17 ppm, combined with negative Nb, Z r, Hf, and Ti anomalies. Orthopyroxene and olivine from the LREE-deple ted nodules have complementary Zr, Hf, Ti, and Nb anomalies to clinopy roxene which exhibits negative HFSE anomalies. Moreover, while REE, Zr , Hf, and Ti reside principally in clinopyroxene, the orthopyroxene an d olivine separates contain Th and Nb at a similar level to clinopyrox ene. Therefore, LREE-depleted xenoliths have variably positive Th and Nh 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 clinopyroxe ne. The LREE-depleted continental lithospheric mantle under southeaste rn British Columbia has a distinctive trace element pattern, differing from the oceanic lithospheric mantle. Two different metasomatic proce sses have been invoked to explain the LREE enriched nodules: plume-rel ated melt for positive HFSE anomalies (pattern 2) and hydrous or CO2-r ich fluid for negative HFSE anomalies (pattern 3).