Caledonian and pre-Caladonian events in Moine rocks of the Cluanie area, Inverness-shire.
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.