The geology of Ben Nevis, South-west Highlands, Scotland

摘要

The Ben Nevis volcanic-plutonic Complex was intruded into garnet gradeudPrecambrian metasediments in the Southwest Highlands of Scotland towards the end of theudCaledonian Orogeny. A variety of techniques have been applied to the Ben Nevis Complex inudorder to model its volcanic and plutonic evolution; these include: field mapping,udpetrography, mineral chemistry, whole rock major and trace element geochemistry and audcombined study of the isotopes of neodymium and strontium.ududRocks of the volcanic pile are subdivided into the following formations; the Allt a'udMhuillin formation, the Coire na Ciste formation, the Ledge Route formation and the Summitudformation, and these overlie a basement of Dalradian schist. Fine grained metasediments ofudthe Allt a' Mhuillin formation were deposited in a lacustrine basin into which entered audseries of non-volcanic mass flow units. Non-volcanic deposition was terminated with theudentry of the first volcaniclastic lahars of the Coire na Ciste formation into the Ben Nevisudbasin. Block and ash flows, lava flows and sills are also found in this formation. The LedgeudRoute formation testifies to a complex period of quiescence, airfall deposition and mass flowudmovement after which localised flows of lava dominate the overlying Summit formation. Audsingle felsite dyke is found intruding the volcanic pile and is correlated with early membersudof the Ben Nevis dyke swarm in the northern area of the Ben Nevis Complex.ududPlutonic rocks of the Ben Nevis Complex are subdivided into the Fine Quartz Diorite,udSgurr Finnisg-aig Quartz Diorite, Coarse Quartz Diorite, Porphyritic Outer Granite andudInner Granite; these were intruded to a high level in the crust. A dextral stress regimeudoperated during the intrusion of the plutonic rocks. Each of the above units was intruded as audseries of pulses.ududMass balance and qualitative trace element vector modelling (Rayleighudcrystallisation) is able to model the geochemical evolution of the Porphyritic Outer Graniteudusing plagioclase, amphibole, biotite and magnetite as the fractionating assemblage.udPyroxene is found as a phenocryst and as a groundmass phase in the quartz diorites withudcalcic amphibole found mainly as a replacement mineral after pyroxene; however, traceudelement modelling identifies amphibole as a fractionating phase.ududIsotopic modelling of the Ben Nevis Complex indicates the involvement of at leastudthree components; these being a mantle source, a lower crustal source with similar isotopicudcharacteristics to the Islay-Colonsay-Basement and, of lesser importance, Dalradianudmetasediments. No single parental magma can explain the geochemical and isotopic variationudof the Ben Nevis Complex.