Petrogenetic studies of the appinite suite of western Scotland
Mineral and rock chemistry together with petrography have been
used in association with previously determined field relationships to
elucidate magma genesis, conditions of crystallisation and geotectonic
environment of the appinite suite of western Scotland and northwestern
Ireland. The major part of the atudy has been related to the type area
of this suite, that of the Ballachulish district in the parish of Appin.
In addition, the major cluster of appinites in the Loch Lomond district
has been studied together with appinites from Colonsay and Ardara, Dongal.
Comparison has been made with the scyelite-pyroxenite-appinitic
type rock association of Central Sutherland and with the lamprophyres
of the Ballachulish district and lavas of Old Red Sandstone age (late
Silurian) in western Scotland. These comparative studies have been
used to establish which products of the late Caledonian igneous
activity were comagmatic and to determine whether the appinite suite
of the type area developed under unique conditions or whether comparable
maema genesis, emplacement and crystallisation conditions
were widespread in Caledonideso
The appinite suite of western Scotland and northwestern Ireland
was developed from a volatile-rich basaltic magma. For the :Ballachulish
cluster the magma. was of high-potassium calc-alkaline type
and evolved towards a relatively continental environment. For the
Ardara cluster it was transitional between calc-alkaline and
tholeiitic and evolved towards an island arc environment while for
the Loch Lomond cluster it was a normal calc-alkaline type evolved
in environments intermediate between the other two.
In the Ballachulish appinite suite, olivine appeared on the
liquidus at about 70-80 km depth. Subsequently emplacement into
high crustal level occurred. With magma uprise being impeded by structural traps of the folded Dalradian Supergroup, the rest of
crystallisation occurred under decreasing temperature but highly
variable PGas due to explosive activity in the subvolcanic pipes.
Following olivine, the successive fractionation of clinopyroxene,
amphibole, mica and plagioclase led to the progressive development
of rocks from ultramafic to acidic compositions. In addition,
accumulation of mafic phases occurred in the early st8.$'9sof
fractionation, under increasing PGas which led to explosive activity
that disrupted the early formed cumulates followed by their upward
movement as large "blocks", crystal mushes and individual crystals
with variable proportions of the residual liquid. Thus, appinites,
kentallenites and even diorites represent "mixed rocks" in which
particular phenocrysts are not in a matrix representing the particular
liquid with which they were in equilibrium. Explosive activity
associated with the emplacement of the marginal ultramafic rocks,
appinites and majority of the diorites occurred at higher PGas
(4-5 kb) than that associated with the emplacement of kenta1lenites
(1 kb). The late leucocratic diorites and granodioritea represent
crystallisation from a residual liquid which probably also contained
a small component of a hybrid secondary liquid produced by the
solution of silica from quartzite, which formed structural traps,
at high P-T. Gas-streaming continued after igneous emplacement of
various rock types.
Corresponding petrogenetic processes were also responsible for
the development of the Loch Lomond and Ardara clusters. However, due
to the variable strengths of the structural traps, in the Loch Lomond
cluster PGas reached before explosive breaching of the traps was
generally not so high as at Eallachulish while in the Ardara cluster
it was higher than both of the other two clusters.
The lamprophyres and lavas of the Old Red Sandstone age (late
Silurian) are genetically related to the appinite suite.
The scyelite-pyroxenite-appinitic type rock association of
Sutherland does not represent a cognate petrogenetic suite of rocks.
Scyelite and Si02-poor appinitic type rocks are the products of
crystallisation differentiation from a basaltic ma~ and are genetically
related to the appinite suite. Pyroxenites and Si02-rich
appinitic type rocks represent the hybrids of Ach'uain type.
The basaltic magma from which the various appinite clusters
crystallised were mantle-derived, at depths of ~o200 km and associated
with a northwestward dipping subduction zone in late Silurian time,
i.e. in the latter stages of the Caledonian orogeny .