This study addresses the problems of identifying and dating the Precambrian to Caledonian events which affect the Proterozoic Moine metasediments of the central Northem Highlands of Scotland. Particular emphasis is placed on determining the timing and environment of emplacent of early amphibolite suites, and their relevance to the nature of the Grenville event in northern Scotland. In the area to the east of Loch Cluanie, two distinct suites of pre-tectonic amphibolites intrude Moine metasediments and granite gneiss. The fine grained, non-garnetiferous metadolerite suite are chemically and isotopically similarto modern N-type MORB. The coarse grained, garnetiferous metagabbro suite show a more evolved chemistry, consistent with derivation from a metadolerite-like magma by a process involving assimilation of Moine wall rocks and fractional crystallisation. While the LlL element chemistry of both suites has been markedly altered during metamorphism, the REE and HFS elements appear to have remained stable. The metadolerite suite yield an Rb-Sr whole-rock age of 1004 ± 47 Ma, which dates hydration of the suite during the first, 01' deformation event. The absence of major compressive structures associated with the 01 event, coupled with the MORB chemistry of the metadolerites, suggest that the Grenville event in this area may have been extensional. The West Highland granite gneiss may have been formed by crustal anatexis during metamorphism associated with this extensional phase, accompanied by extension-related basic magmatism. Rb-Sr and Sm-Nd ages from pegmatites, metagabbros, and metasediments suggest that the major recumbent isoclinal folding of the D2 event is late- Precambrian to early Cambrian in age, and is not related to the Grenville event, as has often been assumed. 02 resetting of the Rb-Sr systems of the metagabbros is dated at 562 ± 24 Ma, an age which overlaps within error with the time of emplacement ofthe Carn Chuinneag granite. Rb-Srdating of a memberofthe syn- D3 pegmatite suite provides an age of 442 ± 5 Ma for Caledonian upright reworking associated with the formation of major internal ductile thrusts (e.g. the Sgurr Beag slide) and the Loch Quoich Line. No evidence was found in the Loch Cluanie area for metamorphism ordeformation during a Morarian event.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:361421 |
| Date | January 1990 |
| Creators | Millar, Ian Lawson |
| Publisher | Royal Holloway, University of London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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