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Silicate Mineralogy of the Middle Zone Cumulates and Associated Gabbroic Rocks from 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

The Middle Zone (MZ) of the Insch intrusion lies in a geographically intermediate position between the Lower Zone (LZ) and Upper Zone (UZ) cumulate sequences, but is more complicated than either in comprising two intricately associated, but texturally distinct, components, the MZ cumulates and the fine-grained granular gabbros (FGG). In addition, there is a minor group of porphyritic granular gabbros (PGG), containing abundant plagioclase phenocrysts. A further variety of gabbroic rock, namely quartz-biotite norite (QBN) occupies a similarly intermediate position in the Boganclogh extension of the main Insch mass. These principal components (MZ cumulates, FGG, and QBN) show considerable mineralogical overlap with each other and with the lower part of the UZ succession (UZa). Unlike the UZ rocks, they are essentially olivine-free (apart from the most basic granular gabbros) and consist mainly of plagioclase, orthopyroxene, and Ca-rich clinopyroxene, with accessory Fe-Tioxides and minor interstitial biotite and/or hornblende. Mineral compositions are in the range An70–55 (plagioclase), En71–44 (opx), and Ca46Mg42Fe12 to Ca45Mg30Fe25 (cpx), but in detail each rock group can be characterized mineralogically as well as texturally. It is concluded that the MZ cumulates and FGG (including PGG) are essentially complementary, formed in different locations, and under slightly different conditions, but in the same magma chamber. The complex relationships between them, and the apparently haphazard geographical variations in mineral compositions, may be the combined results of the magmatic events, possibly including the movement of large xenolithic fragments, and later block faulting. The substantial compositional overlap of the more evolved MZ cumulates by UZa is explained in terms of replenishment by magma of slightly more primitive (and potentially olivine-bearing) composition after the deposition of the MZ. The Boganclogh QBN is believed to represent a more hydrous fraction of the Insch MZ magma.

Type
Petrology
Information
Mineralogical Magazine , Volume 52 , Issue 366 , June 1988 , pp. 309 - 322
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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References

Ashcroft, W.A., and Munro, M. (1978) The structure of the eastern part of the Insch marie intrusion, Aberdeenshire. Scott. J. Geol. 14, 557-9Google Scholar
Ashcroft, W.A., and Munro, M. Kneller, B.C., Leslie, A.G., and Munro, M. (1984) Major shear zones and autochthonous Dalradian in the north-east Scottish Caledonides. Nature, 310, 760-2.Google Scholar
Busrewil, M.T., Pankhurst, R.J., and Wadsworth, W.J. (1973) The igneous rocks of the Boganclogh area, N.E. Scotland. Scott. J. Geol. 9, 165-76.Google Scholar
Clarke, P.D., and Wadsworth, W.J. (1970) The Insch layered intrusion. Ibid. 6, 7-25.Google Scholar
Gribble, C.D. (1967) The basic intrusives of Caledonian age of the Haddo House and Arnage districts, Aberdeenshire. Ibid. 3, 125-36Google Scholar
Irvine, T.N. (1982) Terminology for layered intrusions. J. Petrol. 23, 127-62.Google Scholar
Parsons, I. (1979) The Klokken Gabbro-Syenite Complex, South Greenland: cryptic variation and origin of inversely graded layering. Ibid. 20, 653-94.Google 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.Google Scholar
Read, H.H., Sadashivaiah, M.S., and Haq, B.T. (1965) The hypersthene-gabbro of the Insch Complex, Aberdeenshire. Ibid. 76, 111Google Scholar
Thy, P. and Esbensen, K.H. (1982) Origin of fine-grained granular rocks in layered intrusions. Geol. May. 119, 405-12Google Scholar
Von Gruenewaldt, G. (1973) The Main and Upper Zones of the Bushveld Complex in the Roossenekal area, Eastern Transvaal. Trans. Geol. Soc. S. Africa, 76, 207-27.Google Scholar
Wadsworth, W.J. (1982) The basic plutons. In Igneous Rock of the British Isles (Sutherland, D., ed.) John Wiley, 135-48.Google Scholar
Wadsworth, W.J. (1986) Silicate mineralogy in the later fractionation stages of the Insch intrusion, N.E. Scotland. Mineral. Mag. 50, 583-95.Google Scholar
Wadsworth, W.J. Dunham, A.C. and Almohandis, A.A. (1982) Cryptic variation in the Kapalagulu layered intrusion, Western Tanzania. Ibid. 45, 227-36Google Scholar
Whittle, G. (1936) The eastern end of the Insch igneous mass, Aberdeenshire. Proc. Lpool. Geol. Soc. 17, 64-95.Google Scholar
Wilson, J.R., and Larsen, S.B. (1985) Two-dimensional study of a layered intrusion-—the Hyllingen Series, Norway. Geol. Mag. 122, 971-24Google Scholar