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Lithospheric mantle xenoliths sampled by melts from upwelling asthenosphere: the Quaternary Tasse alkaline basalts of southeastern British Columbia, Canada



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The Canadian Cordillera, Mantle xenolith, Alkaline basalt, Mantle metasomatism, Continental lithospheric mantle


The Tasse basalts are exposed near the north shore of Quesnel Lake in southeastern British Columbia. They host a variety of mantle xenoliths consisting predominantly of spinel lherzolite with minor dunite and pyroxenite. Mineralogically, the xenoliths are composed of olivine, orthopyroxene, clinopyroxene and spinel characterized by forsterite (Fo87-93), enstatite (En90-92), diopside (En45-50-Wo40-45-Fs5), and Cr-spinel (6−11 wt. % Cr), respectively. All of the mantle xenoliths are coarse-grained and show granoblastic textures. Clinopyroxene and spinel display textural evidence for chemical reactions with percolating melts.

The mantle xenoliths are characterized by restricted Mg-numbers (89−92%) and low abundances of incompatible elements (Ba=2−11 ppm; Sr=3−31 ppm) and Yttrium (1−3 ppm). On the basis of REE patterns, the xenoliths are divided into three groups reflecting the various degrees of mantle metasomatism: (1) Group 1 consists of concave-up LREE patterns (La/Smcn=0.48−1.16; Gd/Ybcn=0.71−0.92); (2) Group 2 possesses flat to moderately LREE-enriched patterns (La/Smcn=1.14−1.92; Gd/Ybcn=0.87−1.09); and (3) Group 3 is characterized by strongly LREE-enriched patterns (La/Smcn=1.53−2.45; Gd/Ybcn=1.00−1.32). On MORB-normalized trace element diagrams, the majority of the xenolith samples share the enrichment of LILE (Rb, Ba, K), U, Th, Pb, Sr and the depletion of HFSE (Nb, Ta, Ti, Y) relative to REE. These geochemical characteristics are consistent with a compositionally heterogeneous sub-continental lithospheric mantle source that originated as sub-arc mantle wedge peridotite at a convergent plate margin.

The Tasse basalts have alkalic compositions characterized by low SiO2 (44−46 wt.%) and high alkali (Na2O+K2O=5.1−6.6 wt.%) contents. They are strongly enriched in incompatible elements (TiO2=2.4−3.1 wt.%; Ba=580−797 ppm; Sr=872−993 ppm) and, display OIB-like trace element patterns (La/Smn=3.15−3.85; Gd/Ybn=3.42−4.61). They have positive eNd (+3.8 to +5.5) values, with 338−426 Ma depleted mantle model ages, and display uniform OIB-like Sr (87Sr/86Sr=0.703346−0.703591) and Pb (206Pb/204Pb=19.40−19.58; 207Pb/204Pb=15.57−15.60; 208Pb/204Pb=38.99−39.14) isotopic compositions. The basalts erupted discontinuously along a >1000 km long SE-NW-trending linear belt with minimal compositional variation indicative of a homogenous mantle source. The Sr−Nd−Pb isotope and trace element systematics of the alkaline basalts suggests that they originated from partial melting of an upwelling asthenospheric mantle source. Melting of the asthenospheric mantle might have stemmed from extension of the overlying lithosphere in response to the early stages of back-arc basin opening in the Omineca and Intermontane belts. Ridge subduction beneath the Canadian Cordillera might have played an important role in the weakening of the lithospheric mantle prior to its extension. Alternatively, melting of the upwelling asthenosphere in response to the delamination of the lithospheric mantle beneath the Rocky Mountain Trench might have generated the alkaline lavas.

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NSERC, 254223

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