Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Abstract

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzburgites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al 2 O 3 contents more akin to those found cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al 2 O 3 and TiO 2 ) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source.Petrographic evidence suggests that the mantle beneath the Wyoming craton was reenriched in three ways: (i) by silicate melts that formed mica websterite and clinopyroxenite veins; (ii) by growth of phlogopite from K-rich hydrous fluids; and (iii) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly LREE-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalised diagrams. Lack of enrichment in high field strength elements (e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatised by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE-enrichment. They have 143 Nd/ 144 Nd values that range from 0.5121 (close to the host minette values) to 0.5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143 Nd/ 144 Nd values (0.5113) and extremely high 87 Sr/ 86 Sr ratios in their constituent phlogopite, indicating an ancient (probably mid-Proterozoic) enrichment. This enriched mantle lithosphere later contributed to the formation of the high-K Eocene host magmas.The cumulate group ranges from clinopyroxene-rich mica peridotites (including abundant mica wehrlites) to mica clinopyroxenites. Most contain >30% phlogopite. Their mineral compositions are similar to those of phenocrysts in the host minettes. Their whole-rock compositions are generally poorer in MgO but richer in incompatible trace elements than those of the tectonite peridotites. Whole-rock trace element patterns are enriched in LILE (Rb, Cs, U and Pb) and depleted in HFSE (Nb, Ta Zr and Hf) as in the host minettes, and their Sr-Nd isotopic compositions are also identical to those of the minettes. Their clinopyroxenes are LREE-enriched and formed in equilibrium with a LREE-enriched melt closely resembling the minettes. The cumulates therefore represent a much younger magmatic event, related to crystallisation at man...

New Occurrence of Pyroxenites in the Veria-Naousa Ophiolite (North Greece): Implications on Their Origin and Petrogenetic Evolution

“…This behaviour of Ca and Cr is not in agreement with other natural examples [61,70,71] and may imply percolation of a melt that was richer in Ca and Cr than the pyroxenite's precursors. Relic crystals with lobate boundaries and newly-formed, euhedral spinel crystals in Ol-orthopyroxenite show a range of Cr# resembling spinels from both marginal basins and boninitic basalts (Figure 4b-d).…”

“…Natural and experimental data reveal that Si-rich melts [56,57], or aqueous [58] or supercritical fluids [59] are released from the underlying subducted oceanic slab into the sub-arc region driving the dissolution of mantle olivine and the neoformation of pyroxenes and Al-phases in a range of pressures between 0.8 and 3.4 GPa [60][61][62][63][64][65][66][67][68]. Experimental results of Grant et al [69] at 800 MPa suggest that neoblasts of orthopyroxene and clinopyroxene form at a relatively narrow temperature range of around 800 to 900 • C (with few exceptions up to 1100 • C).…”

Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes

“…However, the closely associated mela-aillikites fingerprint more reduced longterm isotopically enriched material within the Torngat UML magma source region and we have demonstrated that this component most likely resided in an old phlogopite-, K-richterite-, Ti-oxide-, and apatite-bearing assemblage. Such hydrous dominated, Ti-rich metasomatic assemblages are known from many cratons worldwide including the Wyoming and Churchill Provinces of Laurentia (Peterson and LeCheminant, 1993;Carlson and Irving, 1994;Downes et al, 2004). They form part of mantle-derived xenolith suites entrained by volatile-rich mafic/ultramafic magmas and appear to represent reaction products between hydrous alkaline melts of lamproitic composition and cratonic peridotite (Waters, 1987;Sweeney et al, 1993;Konzett et al, 1997;Grégoire et al, 2002;Downes et al, 2004).…”

“…Such hydrous dominated, Ti-rich metasomatic assemblages are known from many cratons worldwide including the Wyoming and Churchill Provinces of Laurentia (Peterson and LeCheminant, 1993;Carlson and Irving, 1994;Downes et al, 2004). They form part of mantle-derived xenolith suites entrained by volatile-rich mafic/ultramafic magmas and appear to represent reaction products between hydrous alkaline melts of lamproitic composition and cratonic peridotite (Waters, 1987;Sweeney et al, 1993;Konzett et al, 1997;Grégoire et al, 2002;Downes et al, 2004). Rarely observed contact relationships indicate these assemblages form veins and striations within the deeper parts of cratonic lithosphere (Kramers et al, 1983;Waters and Erlank, 1988;Waters et al, 1989;Carlson and Irving, 1994;Rudnick et al, 1999).…”

Consistent patterns of rare earth element distribution in accessory minerals from rocks of mafic-ultramafic complexes

“…than Yb contents (3.6-7.2 t.ch.) [125]. The REE patterns of these phlogopites have the shape of lines with a gentle negative slope and are described by the values of (La/Yb) parameter in the range of 2.3-7.5 ( Fig.…”