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Use of Expert Systems and GIS in Land Evaluation 总被引：1，自引：0，他引：1
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The Zoned Clinopyroxenes of the Shiant Isles Sill, Scotland 总被引：1，自引：0，他引：1
The zoned clinopyroxenes in samples from a near-vertical sectionthrough the main Shiant Isles sill have been studied using theelectron microprobe. In terms of Ca: Mg: Fe the over-all fractionationtrend is much greater than had been believed from bulk pyroxeneanalyses and is of a distinctive calcic augite-hedenbergitetype. In this respect it differs from both the augite-ferroaugiteand salite-aegirine trends generally regarded as typical oftholeiitic and alkaline basic magmas respectively, and it isproposed that this trend represents a third major type typicalof mildly alkaline basic magmas. The Al, Ti, and Na contentsof the pyroxenes are dependent on the degree of fractionationas represented by Mg/(Mg+Fe) and it is suggested that the Aland Ti contents are controlled by the Ti content of the liquidand the crystallization of titaniferous magnetite whereas theentry of Na into the pyroxene as aegirine is dependent on theactivity of Fe3+ in the liquid. To provide a basis for discussion of the crystallization ofthe zoned clinopyroxenes the existing petrogenetic hypothesesfor the Shiant Isles sill are reviewed and a new model proposedfor its emplacement and differentiation. It is concluded thatthe differences between the pyroxenes of the mildly alkalineShiant Isles sill and those of more strongly alkaline basicmagmas result from fundamental chemical differences in the magma.The lower Ca contents of the early pyroxenes are due to theslightly higher Si activity of the magma and the subsequenttrend towards hedenbergite rather than aegirine is due to crystallizationunder conditions of relatively lower oxygen fugacity, probablyas a result of lower initial H2O content of the magma. 相似文献
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Chemistry of the Shiant Isles Main Sill, NW Scotland, and Wider Implications for the Petrogenesis of Mafic Sills 总被引：3，自引：0，他引：3
Major and trace element data for the Tertiary, Shiant IslesMain Sill, NW Scotland, are used to discuss its complex internaldifferentiation. Vertical sections through the sill exhibitsharp breaks in chemistry that coincide with changes in texture,grain size and mineralogy. These breaks are paired, top andbottom, and correspond to the boundaries of intrusive units,confirming a four-phase multiple-intrusion model based on fieldrelations, petrography, mineralogy and isotopes. Whole-rockchemistry is consistent with this model and necessitates onlyminor revisions to the intrusive and differentiation mechanismspreviously proposed. The rocks contain strongly zoned minerals(e.g. olivine Fo70–5, clinopyroxene Mg# = 75–5,plagioclase An75–5) indicating almost perfect fractionalcrystallization, but whole-rock compositions do not show suchextreme variations. Thus, while residual liquids became highlyevolved in situ, they mainly became trapped within the crystalnetwork and did not undergo wholesale inward migration. Someinward (mainly upward) concentration of residual liquids didoccur to form a ‘sandwich horizon’, but the morevolatile-rich, late-stage liquids that did not crystallize insitu appear to have migrated to higher levels in the sill toform pegmatitic horizons. Parental liquid compositions are modelledfor the intrusive units and it is concluded that the originalparent magma formed by partial melting of upper mantle thatwas more depleted in LREE than the sources of most ScottishTertiary basaltic rocks. Incompatible trace elements in thepicrodolerite–crinanite intrusive unit support isotopeevidence that its parent magma was contaminated by crustal material.Attempts to reconcile the chemical characteristics of the sillwith a recently proposed petrogenetic model based on a singleintrusion of magma differentiated by novel, but controversial,processes fail comprehensively. It is predicted that the complexpetrogenetic history of the Shiant Isles sill is not unusualand could become the model for other large (>50 m thick)sills. KEY WORDS: alkali basalt; differentiation; geochemistry; multiple intrusion; Shiant Isles; sill 相似文献
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The xenolithic Tertiary ultrabasic dykes of south-west Skyeare composed principally of forsteritic olivine, plagioclase,and clinopyroxene with accessory chrome spinel and magnetite.The plagioclase: pyroxene ratios are relatively constant withineach dyke but the absolute amounts of these minerals are dependenton the olivine content which varies, often considerably, acrossthe dykes. Five main types of olivine distribution are recordedfrom the dykes and in all five there is a concentration of olivinecrystals towards the centre of the dyke. The average size ofthe olivine crystals also varies across the dykes with increasesin size generally corresponding to increases in the olivinecontent. The petrogenetic hypothesis of composite intrusion previouslyproposed for these dykes by Bowen (1928) is re-examined andfound to be inadequate. It is suggested that the dykes, whichwere intruded as magmatic suspensions of olivine crystals androck fragments, are unlikely to have been differentiated intheir present positions and the possibility that the differentiationoccurred during their emplacement is examined. It is shown thatthe mineral distributions and crystal size variations occurringin the dykes are, on the whole, analogous to those expectedto arise during laminar flow in vertical conduits, and it isconcluded that flow differentiation occurred during the intrusionof the ultrabasic dykes of south-west Skye.
1Present address:Department of Geology,University of Toronto,Toronto5,Ontario,Canada. 相似文献
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