Petrology and geochemistry of the Tasse mantle xenoliths of the Canadian Cordillera: A record of Archean to Quaternary mantle growth, metasomatism, removal, and melting
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| Authors: | Polat, A; Frei, R; Longstaffe, FJ; Thorkelson, DJ; Friedman, E |
| Year: | 2018 |
| Journal: | Tectonophysics 737: 1-26 Article Link (DOI) |
| Title: | Petrology and geochemistry of the Tasse mantle xenoliths of the Canadian Cordillera: A record of Archean to Quaternary mantle growth, metasomatism, removal, and melting |
| Abstract: | Mantle xenoliths hosted by the Quaternary Tasse alkaline basalts in the Canadian Cordillera, southeastern British Columbia, are mostly spinel Iherzolite originating from subcontinental lithospheric mantle. The xenoliths contain abundant feldspar veins, melt pockets and spongy clinopyroxene, recording extensive alkaline metasomatism and partial melting. Feldspar occurs as veins and interstitial crystal in melt pockets. Melt pockets occur mainly at triple junctions, along grain boundaries, and consist mainly of olivine, cpx, opx and spinel surrounded by interstitial feldspar. The Nd, Sr and Pb isotopic compositions of the xenoliths indicate that their sources are characterized by variable mixtures of depleted MORB mantle and EM1 and EM2 mantle components. Large variations in epsilon Nd values (-8.2 to +9.6) and Nd depleted mantle model ages (T-DM = 66 to 3380 Ma) are consistent with multiple sources and melt extraction events, and long-term ( > 3300 Ma) isolation of some source regions from the convecting mantle. Samples with Archean and Paleoproterozoic Nd model ages are interpreted as either have been derived from relict Laurentian mantle pieces beneath the Cordillera or have been eroded from the root of the Laurentian craton to the east and transported to the base of the Cordilleran lithosphere by edge-driven convection currents. The oxygen isotope compositions of the xenoliths (average delta O-18 = +5.1 +/- 0.5 parts per thousand) are similar to those of depleted mantle. The average delta O-18 values of olivine (+5.0 +/- 0.2 parts per thousand), opx (+5.9 +/- 0.6 parts per thousand), cpx (+6.0 +/- 0.6 parts per thousand) and spinel (+4.5 +/- 0.2 parts per thousand) are similar to mantle values. Large fractionations for olivine-opx, olivine-cpx and opx-cpx pairs, however, reflect disequilibrium stemming from metasomatism and partial melting. Whole-rock trace element, Nd, Sr, Pb and O isotope compositions of the xenoliths and host alkaline basalts indicate different mantle sources for these two suites of rocks. The xenoliths were derived from shallow litho spheric sources, whereas the alkaline basalts originated from a deeper asthenospheric mantle source. |
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