THE SHIANT ISLES MAIN SILL - STRUCTURE AND MINERAL FRACTIONATION TRENDS

The Shiant Isles Main Sill, of Tertiary age, is a classic example of a composite, differentiated alkaline basic sill. The first unit to be i ntruded was a 2 m thick olivine teschenite which was emplaced with phe nocrysts of olivine (mg > 83) [mg = Mg#] and, perhaps, plagioclase. Th is was intruded by a 24 m thick picrite sill consisting of a mush of m elt and suspended olivine phenocrysts (mg > 83) with a D-shaped modal profile. The 140 m thick picrodolerite-crinanite unit was formed by a magma carrying similar to 10% olivine (mg > 83) as the main phenocryst phase, together with some calcic plagioclase phenocrysts, being empla ced into the top of the picrite unit before the host rock was complete ly solidified. The olivine phenocrysts settled towards the bottom to f orm the picrodolerites. In-situ differentiation processes occurred und er conditions of almost perfect fractional crystallization, during whi ch very strongly zoned ophitic crystals of olivine (fayalitic rims) an d clinopyroxene (hedenbergitic rims), and zoned laths of plagioclase ( anorthoclase rims), formed. The last unit consists of similar to 2 m o f granular olivine picrodolerite which was intruded into the upper cri nanites, again before the host rock was fully solid. The mineral zonin g patterns are interpreted using published cation diffusion coefficien t data, and used to show that the picrite unit might have cooled to th e blocking temperatures for Mg and Fe diffusion in < 5 years, and that even the relatively thick crinanite unit cooled very fast, so preserv ing the zoned Fe-Mg olivine and pyroxene compositions. The composition s of coexisting ilmenites and spinels define a redox trend which initi ally lies close to fayalite-magnetite-quartz buffer conditions, but la ter becomes more reducing and approaches magnetite-wustite buffer cond itions. The final stages of development occurred during sub-solidus de uteric processes and involved formation of analcime and zeolites, as w ell as localized sulphide mineralization.