Lower crustal and possible shallow mantle samples from beneath the Hebrides: Evidence from a xenolithic dyke at Gribun, western Mull

Abstract: A Permian (268 2 Ma) olivine-nephelinite dyke cutting Proterozoic (Moinian) psammites on the west coast of Mull, contains an abundance of xenoliths inferred to be from the lower crust and possibly uppermost mantle. The majority are pargasite pyroxenites grading to hornblendites. Next most abundant are pyroxene granulite orthogneisses. High-grade meta-arenites are also relatively common. Scarce xenoliths and related megacrysts are composed of anorthoclase, ferrosalite, apatite, magnetite and ilmenite (af-cpx-ap-mt-il suite). Scarce kaersulite megacrysts are thought to be derivative from pegmatitic hornblendites. The pargasite pyroxenites are derived from ultramafic protoliths that have experienced recrystallization, modal metasomatism (with introduction of amphibole) and deformation prior to entrainment. Olivines are wholly pseudomorphed but pyroxenes are diopsides (Fs^sub 7.8- 9.2^) with up to 12.5% Al^sub 2^O^sub 3^. Plagioclase (An^sub 45.6^) is a rare accessory. Whether the pyroxenite protoliths were upper mantle peridotites or lower crustal ultramafic cumulates is indeterminate.

The mesocratic xenoliths are granulite-facies orthogneisses comprising plagioclase (An38_22), augite (Fs 22), pseudomorphed pigeonite (?), magnetite, ilmenite and apatite. Pb/U SHRIMP dating of zircons indicates a crystallization age of 1850i50 Ma for the orthogneisses, suggesting that the Archaean/ Palaeoproterozoic Lewisian gneisses were magmatically underplated by younger Proterozoic (Rhinnian or Ketilidian age) rocks. The pyroxenite and granulite gneiss xenoliths may be coeval fragments of a lower crustal and possibly uppermost mantle sequence. The most evolved of the orthogneisses are two-feldspar quartz diorites containing subordinate sanidine (Or^sub 64-74^). The magmas parental to the postulated underplating intrusion 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.)-rich 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 within the amphibole pyroxenites. Temperature estimates based on coexisting (a) feldspar and (b) oxide pairs indicate establishment of equilibrium at approximately 800C, suggesting a geothermal gradient of c. 27'C km - '

Keywords: xenoliths, pyroxenite, granulite-facies, lower crust, upper mantle.

Many localities are now known in the British Isles where upper mantle and/or lower crustal xenoliths occur (Hunter & Upton 1987). These predominantly late Palaeozoic occurrences involve minor intrusions, diatremes or, rarely lava flows, related to silica-undersaturated basic magmas (Upton et al. 1983; Halliday et al. 1993). A hitherto unknown locality was recently discovered near Gribun, on the western coast of the Isle of Mull [National grid reference: 444 334], where a thin dyke (c. 1 m width), trending at 153`, cuts the Proterozoic psammites of the Morar Formation within the Moine Supergroup (Fig. 1). The dyke is part of the Permian NW-SE swarm across the Hebrides (cf. Speight & Mitchell 1979; Morrison et al. 1980) and is crowded with angular xenoliths up to 20 cm across. The majority are melanocratic amphibole pyroxenites whereas the second commonest category comprises mesocratic to leucocratic granulite-facies gneisses. Additionally the dyke contains scarce discrete megacrysts of anorthoclase, kaersutite, magnetite, ilmenite and apatite. Some composite megacrysts of apatite-magnetite and apatite-magnetite-ferrosalite, together with rare xenoliths of anorthoclasite containing ferrosalite, apatite and FeTi oxides, are also present.

The occurrence provides an opportunity to study some of the lower crustal, (and possibly uppermost mantle) samples from the late Palaeozoic lithosphere of NW Scotland. The locality lies some 18 km NW of the Great Glen Fault, where the present-day crust-mantle boundary is thought to be between c. 27 and 30 km depth (Rollin 1994). The crustal thickness in Permian times, however, is in some doubt in view of the subsequent extensional processes in the Mesozoic and early Tertiary but is unlikely to have been substantially thicker.

The host dyke

The dyke containing the xenoliths is an olivine nephelinite composed of microphenocrysts of (serpentinized) olivine and titanaugite in a matrix dominated by clinopyroxene, magnetite and apatite. The dyke magma was distinctly primitive, with MgO c. 10 wt%, and Ni and Cr contents of 287 and 424 ppm respectively, in conjunction with high contents of incompatible elements and a Na2O/K20 (wt%) value of 0.66 (Table 1). This magma is inferred to have been generated by small fraction decompression melting of the asthenosphere as a result of extensional stress in the late Palaeozoic (cf. Upton et al. 1992).

A sample of the dyke groundmass was analysed for argon isotopes. The 4Ar39Ar analysis followed the method described by Rex et al. (1993), with the following modifications: samples were irradiated at the Ford Reactor, Ann Arbor, Michigan. Interference correction factors were ^sup 40^K/^sup 39^K=0.03, 36Ca/39Ca=1000 and 37Ca/39Ca=0.24. Standards used were Tinto biotite (Rex & Guise 1986) and hornblende Fy12b (q.v. Fy12a in Rex et al. 1993). Isotopic analyses were performed with a modified MS10 mass spectrometer; measured atmospheric 4Ar/39Ar was 288.80.2 and sensitivity 1.35 x 10-7cm3 V-'. The isotope correlation plot was produced using data from 'Isoplot' (Ludwig 1990). Best-fit line and intercepts were made using Yorkfit 1 and errors were calculated from 95% confidence level values. The spectrum obtained after step-heating the samples (Fig. 2, Table 2) showed that the gas from the low-temperature steps was rich in atmospheric argon and was probably evolved from clays and other alteration products. The temperature steps from 800-1320deg"C gave an age range of 261281 Ma with an integrated age of 268+/- 2 Ma (1 a) confirming a Permian age for the dyke (although we cannot claim that this is a plateau age).