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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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