LOWER CRUSTAL AND POSSIBLE SHALLOW MANTLE SAMPLES FROM BENEATH THE HEBRIDES - EVIDENCE FROM A XENOLITHIC DYKE AT GRIBUN, WESTERN MULL

A Permian (268 +/- 2 Ma) olivine-nephelinite dyke cutting Proterozoic (Moinian) psammites on the west coast of Mull, contains an abundance o f xenoliths inferred to be from the lower crust and possibly uppermost mantle. The majority are pargasite pyroxenites grading to hornblendit es. Next most abundant are pyroxene granulite orthogneisses. High-grad e meta-arenites are also relatively common. Scarce xenoliths and relat ed megacrysts are composed of anorthoclase, ferrosalite, apatite, magn etite and ilmenite (af-cpx-ap-mt-il suite). Scarce kaersutite megacrys ts are thought to be derivative from pegmatitic hornblendites. The par gasite pyroxenites are derived from ultramafic protoliths that have ex perienced recrystallization, modal metasomatism (with introduction of amphibole) and deformation prior to entrainment. Olivines are wholly p seudomorphed but pyroxenes are diopsides (Fs(7.8-9.2)) with up to 12.5 % Al2O3. Plagioclase (An(45.6)) is a rare accessory. Whether the pyrox enite protoliths were upper mantle peridotites or lower crustal ultram afic cumulates is indeterminate. The mesocratic xenoliths are granulit e-facies orthogneisses comprising plagioclase (An(38-22)), augite (Fs( 16-22)), pseudomorphed pigeonite (?), magnetite, ilmenite and apatite. Pb/U SHRIMP dating of zircons indicates a crystallization age of 1850 +/- 50 Ma for the orthogneisses, suggesting that the Archaean/ Palaeo proterozoic Lewisian gneisses were magmatically underplated by younger Proterozoic (Rhinnian or Ketilidian age) rocks. The pyroxenite and gr anulite gneiss xenoliths may be coeval fragments of a lower crustal an d possibly uppermost mantle sequence. The most evolved of the orthogne isses are two-feldspar quartz diorites containing subordinate sanidine (Or(64-74)). The magmas parental to the postulated underplating intru sion are thought to have been mildly alkaline (transitional) basalts. The metasomatic introduction of amphibole that affected the ultramafic rocks is attributed to pervasive influx of Fe, Ti, K, LREE (etc.)-ric h small fraction partial melts from the asthenosphere. The af-px-ap-mt -il megacryst suite, however, appears to represent a distinct younger event involving intrusion of a geochemically evolved vein system withi n the amphibole pyroxenites. Temperature estimates based on coexisting (a) feldspar and (b) oxide pairs indicate establishment of equilibriu m at approximately 800 degrees C, suggesting a geothermal gradient of c. 27 degrees C km(-1).