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Silicate stages mineralogy in the later fractionation of the Insch intrusion, NE Scotland

Published online by Cambridge University Press:  05 July 2018

W. J. Wadsworth
W. J. Wadsworth*
Affiliation:
Department of Geology, The University,Manchester M13 9PL
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Abstract

Despite poor exposures and lack of ‘stratigraphic’ control, there is consistent mineralogical and textural evidence that the Insch Upper Zone (UZ) represents a cumulate sequence developed from a single episode of progressive fractionation. This sequence has been subdivided on the basis of the phase layering, with cumulus plagioclase, pyroxenes and Fe-Ti oxides providing continuity from the Middle Zone (although the pyroxenes appear to be temporarily restricted to inter-cumulus status in the lower part of the UZ succession). The reappearance of cumulus olivine after its absence throughout the Middle Zone, marks the base of the UZ, and it is later joined by cumulus apatite, and then by alkali feldspar and zircon. Higher in the succession cumulus orthopyroxene, followed by olivine and Fe Ti oxide, eventually disappear. There is pronounced cryptic variation of the principal cumulus minerals (olivine Fo47−6, orthopyroxene En58−24, clinopyroxene Mg#63−6, plagio-clase An60−39 and the cumulus alkali feldspar is distinctly Ba-rich when it first appears. Comparison with the later fractionation stages of other layered intrusions shows broad similarities to the Bushveld and Skaergaard intrusions and closer similarities to the Fongen-Hyllingen body, which, like Insch, is a Caledonian synorogenic intrusion. The behaviour of the Insch pyroxenes, which show smoothly progressive Fe-enrichment despite their temporary absence as cumulus phases early in the UZ, together with the apparent range in cumulus plagioclase compositions in each sample, are taken to indicate that the cumulus mineral assemblages have been to some extent modified by re-equilibration with trapped inter-cumulus magma.

Keywords

Silicate mineralscumulate sequenceInsch intrusionScotland
Type
Silicate mineralogy
Information
Mineralogical Magazine , Volume 50 , Issue 358 , December 1986 , pp. 583 - 595
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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References

Bevan, J.C. (1982) Reaction rims of orthopyroxene and plagioclase around chrome-spinels in olivine from Skye and Rhum (NW Scotland). Contrib. Mineral Petrol, 79, 124-9.CrossRefGoogle Scholar
Butcher, A.R. (1985) Channelled metasomatism in Rhum layered cumulates—evidence from late-stage veins. Geol. Mag, 122, 503-18.CrossRefGoogle Scholar
Cawthorn, R.G. (1976) Calcium-poor pyroxene reaction relations in calc-alkaline magmas. Am. Mineral, 61, 907-12.Google Scholar
Clarke, P.D., and Wadsworth, W.J. (1970) The Insch layered intrusion. Scott. J. Geol, 6, 7-25.CrossRefGoogle Scholar
Dunham, A.C. and Wadsworth, W.J. (1978) Cryptic variation in the Rhum layered intrusion. Mineral Mag, 42, 347-56.CrossRefGoogle Scholar
Faithfull, J.W. (1985) The Lower Eastern Layered Series of Rhum. Geol. Mag, 122, 459-68.CrossRefGoogle Scholar
Henderson, P., and Wood, R.J. (1981) Reaction relationships of chrome-spinels in igneous rocks—further evidence from the layered intrusions of Rhum and Mull, Inner Hebrides, Scotland. Contrib. Mineral. Petrol, 78, 225-9.CrossRefGoogle Scholar
Irvine, T.N. (1980) Magmatic infiltration metasomatism, double-diffusive fractional crystallization and adcumulus growth in the Muskox intrusion and other layered intrusions. In Physics of Magmatic Processes (R. B. Hargraves, ed.). Princeton University Press, 326-83.Google Scholar
Irvine, T.N. (1982) Terminology for layered intrusions. J. Petro, 23, 127-62.CrossRefGoogle Scholar
Read, H.H., Sadashivaiah, M.S., and Haq, B.T. (1961) Differentiation in the olivine-gabbro of the Insch mass, Aberdeenshire. Proc. Geol. Assoc, 72, 391-413.CrossRefGoogle Scholar
Read, H.H., Sadashivaiah, M.S., and Haq, B.T. (1965) The hypersthene-gabbro of the Insch Complex, Aberdeenshire. Ibid. 76, 1-11.CrossRefGoogle Scholar
Sparks, R.S.J., Huppert, H.E., Kerr, R.C. McKenzie, D.P., and Tait, S.R. (1985) Postcumulus processes in layered intrusions. Geol. Mag, 122, 555-68.CrossRefGoogle Scholar
Tait, S.R. (1985) Fluid dynamics and geochemical evolution of cyclic unit 10, Rhum, Eastern layered series. Ibid. 122, 469-84.CrossRefGoogle Scholar
Von Gruenewaldt, G. (1973) The Main and Upper Zones of the Bushveld Complex in the Roossenekal area, Eastern Transvaal. Trans. Geol. Soc. S. Afric, 76, 207-27.Google Scholar
Wadsworth, W.J. (1982) The basic plutons. In Igneous Rocks of the British Isles (D. Sutherland, ed.). John Wiley, 135-48.Google Scholar
Wadsworth, W.J. (1985) Terminology of postcumulus processes and products in the Rhum layered intrusion. Geol. Mag, 122, 549-54.CrossRefGoogle Scholar
Dunham, A.C. and Almohandis, A.A. (1982) Cryptic variation in the Kapalagulu layered intrusion, Western Tanzania. Mineral. Mag, 45, 227-36.Google Scholar
Wager, L.R., and Brown, G.M. (1968) Layered Igneous Rocks. Oliver and Boyd, Edinburgh.Google Scholar
Wilson, J.R., and Larsen, S.B. (1985) Two-dimensional study of a layered intrusion—the Hyllingen Series, Norway. Geol. Mag, 122, 97-124.CrossRefGoogle Scholar
Wilson, J.R. Esbensen, K.H., and Thy, P. (1981) Igneous petrology of the synorogenic Fongen-Hyllingen layered basic complex, south-central Scandinavian Caledonides. J. Petrol, 22, 584-627.CrossRefGoogle Scholar