400 square kilometres of eastern Aberdeenshire have been surveyed, using magnetic surveying and shallow level drilling, to supplement outcrop mapping. This has shown the existence of the following metasedimentary series in this area: (a) Migmatitic series - sillimanite/fibrolite and garnet bearing metasediments, often migmatitic and with abundant small granite intrusions. (b) Ellon series - cordierite, garnet, andalusite and sillimanite/ fibrolite metasediments. (c) Collieston series - andalusite and cordierite metasediments. Detailed chemical, mineralogical and petrographical studies of these metasediments have shown that these rocks may have originated in the following manner. An initial episode of regional metamorphism led to the development of cordierite, garnet and andalusite in the metasediments. Compositions considered suitable for the development of cordierite are shown to be present within the metasediments of the Migmatitic Series and it is proposed that the absence of cordierite in these rocks is explicable in terms of the higher pressures which prevailed in the Migmatitic Series during the regional metamorphism. The climax of the regional metamorphism was followed by a local rise in temperature, the sillimanite overprint of Chinner (1966), which brought the rocks in the Ellon and Migmatitic Series into the sillimanite/fibrolite stability field, and in addition produced anatexis in the Mignatitic Series. The rocks of the Collieston Series were unaffected by this later, essentially thermal event. The boundaries between the different series are considered to be metamorphic, there being no evidence of a structural break. Following the climax of metamorphism but, before the final episode of metamorphism and folding, basic magma was emplaced to produce a large complex intrusion, previously considered to consist of two separate masses, at Arnage and Haddo House. The magnetic and drilling surveys have greatly modified the pre-existing boundaries of this Arnage/Haddo basic intrusion. In particular, this mass is found to extend as a one kilometre wide zone, seven kilometres south from the previously known limits. The basic rocks consist of cumulates, norites and diorites. The intrusion is bounded by an aureole with cordierite and sillimanite in the metasediments in the inner aureole and cordierite and andalusite in the outer aureole. Chemical and structural evidence indicates considerable differences between the Arnage/Haddo and Belhelvie intrusions, and the disposition of all the "younger" basic intrusions of north east Scotland in an antiformal structure is questioned. The amphibolites of this area', with one exception, possess chemical compositions and exhibit trends of chemical variation closely akin to basic igneous rocks thus, they are considered to be of igneous origin. The one amphibolite of possible sedimentary origin is diopside bearing and very calcareous. On the basis of field relations and textures an "older" and "younger" series of amphibolitised rocks is considered to be present but the age of many of these amphibolites is indeterminate. In addition it has not been found possible to distinguish between the two series on the basis of chemical compositions. Some of the amphibolites along the margins of the Arnage/Haddo intrusion are considered to represent sheared off portions of this mass. Pyroxene occurs in amphibolites found within the inner aureole of Arnage/Haddo whilst cummingtonite occurs in the outer aureole. Diorite veins are also occasionally found within some of the amphibolites in the aureole and are also shown to have developed in response to the physical conditions within the aureole. The absence of garnet in the amphibolites within the Ellon and Collieston series is thought to be due to 1) the low pressures which prevailed during the regional metamorphism and 2) the lack of amphibolites of appropriate composition. The restricted occurrence of garnet in the amphibolites within the Migmatitic Series is thought to be due solely to the unfavourable compositions of most amphibolites. The Al6+ content of the calciferous amphiboles can be related to the pressure of crystallisation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:592366 |
| Date | January 1975 |
| Creators | Duncan, Ian G. |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk/R?func=search-advanced-go&find_code1=WSN&request1=AAIU409354 |
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