Amphibole in a spinel lherzolite xenolith: evidence for volatiles and partial melting in the upper mantle beneath southern British Columbia

Pargasitic amphibole has been observed for the first time in an ultramafic xenolith from British Columbia. The xenolith is a chrome diopside-bearing spinel lherzolite trapped within an alkali basaltic lava flow at Lightning Peak, near Vernon, British Columbia. Amphibole (<5%) occurs within the xenolith as small grains, interstitial between other xenolith mineral phases, and always shows evidence of melting. Microprobe analyses of the amphibole reveal that it is a pargasite rich in MgO (MgO = 17.1–17.7 wt.%; Mg/(Mg + Fe2+) = 0.89) and CaO (10.4–10.7 wt.%). Textural and chemical evidence suggests that the pargasite is in equilibrium with the other phases in spinel lherzolite. The pargasite probably crystallized within the spinel stability field of the upper mantle from a volatile-rich metasomatic fluid that was produced by dehydration of subducted material. Melting in the amphibole may have been caused by one of three processes: superheating by the host alkali basalt, decompression as the magma ascended, or by in situ partial melting within the upper mantle. The partial melting of amphibole-bearing spinel lherzolite provides a possible mechanism for the generation of late Cenozoic alkalic magmas of the Intermontane Belt of British Columbia.

Meta-igneous granulite and ultramafic xenoliths from basalts of the Midland Valley of Scotland: petrology and mineralogy of the lower crust and upper mantle

ABSTRACTUltramafic xenolith lithologies representative of the mantle beneath the Midland Valley of Scotland comprise magnesian peridotite (predominantly spinel lherzolite) and cumulate wehrlites and clinopyroxenites. The lherzolites are typical of the worldwide type I (Cr-diopside) xenolith suite; their textures and mineral chemistry record a complex thermal and deformational history. The petrographical and mineralogical features of the wehrlite-clinopyroxenite suite can be interpreted within the context of a sequence of cpx + ol ± sp cumulates that have undergone a protracted period of subsoli dus re-equilibration. Mineral compositions are similar to those of type II (Al-augite) ultramafic xenolith suites.Although xenolith populations imply widespread lower crustal heterogeneity, meta-igneous basic granulites form a major component of this region beneath the Midland Valley. They are principally composed of pi + cpx + mt ± opx ± ap; a modal continuum exists from clinopyroxenite to anorthosite, skewed towards plagioclase-rich lithologies. Garnet is rare. Locally, evidence indicates re-equilibration from garnet granulite precursors. Rock densities range from c. 2·8 to c. 3·2 gm . cm−3, implying P-wave velocities in the range 6·5–7·5 km . s−1 consistent with the known seismic properties of the lower crustal layer beneath the Midland Valley. Phase relations are also consistent with equilibration of primary assemblages at depths of 20–35 km. Retrograde reactions indicate that portions of the crust may have had a complex pressure-temperature-time evolution. Major element compositions of the granulites are broadly basaltic, ranging from ne to hy normative with the mean corresponding to alkali olivine basalt. They are distinct, chemically, from basic granulites of the Lewisian complex of NW Scotland.