Fig. 1:
Hand specimen. A sample of eclogite, number 317.17, a tough, massive rock composed largely of red garnet (pyrope- almandine) and
green clinopyroxene (omphacite).
Sample "6a" collected and presented (21 July 1978) by Andy Papworth, who was then a technician at the Department of Mineralogy & Petrology at the University of Cambridge. Thin section preparation was one of his principal duties. See also a
forsterite marble that he collected nearby on the same field trip.
Andy moved to Australia and became a senior technician for the Physics Department at the Australian National University in Canberra.
Andy passed away in 2022, but not before he made a remarkable gesture to posterity
in the form of a unique book (Papworth, 2021).
This book is mostly a guide to undergraduate lab experiments in physics,
but also manages to touch on some of the other passions of Andy's life,
including field hockey, science centres and museums, and rock music!
"Rock of the Month # 273, posted for March 2024" ---
The Glenelg eclogite
The regional geology of northwest Scotland is displayed on the map updated
by Stone and Jackson (2008). The oldest rocks in the British Isles are termed the Lewisian, a metamorphic and igneous baserment, often severely deformed, and divided into older (Scourian) and younger
(Laxfordian) elements. The Lewisian protolith predates the younger series of the Scottish
Precambrian (the Torridonian, Moine and Dalradian).
The local geology of the Glenelg region
is summarised by Janet Watson in her chapter on the Lewisian
(in Craig, 1965, pp.63-67).
The Lewisian appears in the Glenelg district in three main outcrops, inliers within younger Moine cover, and may have elements
of older Scourian and younger Laxfordian ages.
Gneiss, marble and amphibolite are typical bedrock lithologies, the latter probably deformed post-Scourie dykes, the
whole likely metamorphosed in the Caledonian orogeny.
The eastern belt of Lewisian in the Glenelg area displays metasediments,
notably extensive marble, and also eclogite.
The eclogites form lenticles or bands in acid gneiss, and are locally migmatized and converted into amphibolite
(the eclogites have a chemistry comparable to more familiar basic igneous rocks which
on metamorphism yield amphibolites).
The Glenelg inlier was described by Rawson et al. (2001).
The Eastern Glenelg Lewisian
contains some ultramafic rocks, such as olivine websterite (an igneous rock with essential
orthopyroxene and clinopyroxene, and often some olivine),
in the same small area as our sample, south of Loch Alsh and west
of Loch Duich. The ultramafic rock may represent a cumulate from a magma of basaltic
composition.
Rawson et al. concluded that
the websterite is an igneous rock crystallized from a melt emplaced in the Scourian,
which underwent
granulite regional metamorphism followed by cooling,
and then eclogite facies metamorphism during events of Grenville
age. A Grenville age (in the range circa 1100-1000 Ma) was derived
by Sanders et al. (1984) for eclogite near the Moine thrust.
Temperley and Windley (1997) found that eclogite-bearing
gneisses in the district were affected by pre-Caledonian extensional shear zones
The Eastern Glenelg Lewisian
contains a diversity of lithologies including forsterite marble, eulysite
and garnet biotite gneiss, migmatitic gneiss, and meta-igneous rocks such as
eclogite, amphibolite, pyroxenite and the garnet-bearing olivine websterite.
Fig. 2: Andy Papworth's labels for marble and (uphill, 1200 metres to the north) eclogite.
The busy ferry port of Kyle of Lochalsh lies a few km to the northwest, on the opposite shore of Loch Alsh.
Andy's detailed label indicates a mineral assemblage of garnet- clinopyroxene- quartz- rutile- amphibole- ilmenite- feldspar.
The locality is given as Cruachan Eilgeach, Glenelg, Invernesshire, grid reference NG842215 (Ordnance Survey, 1955
and later maps).
A polished thin section prepared by Precision Petrographics in 2024 gives us a closer look at this lithology.
A quick examination affirms that the rock is largely pale garnet and sheaves of the green amphibole
actinolite, presumably formed by retrograde alteration
of much of the original pyroxene.
There is also some interstitial quartz, and the rock is enriched in titanium,
as seen in abundant foxy-red rutile (TiO2).
Secondary minerals, besides actinolite, include traces of epidote (in late fractures cutting garnet),
pyrite and goethite.
Eclogites
Eclogite is a rarity in the British Isles, but occurs around the world in specific plate tectonic settings
(Miyashiro, 1973. pp.310-324).
The following is less a review than a sampler of the literature (older and newish: there are 669 records on eclogites
in the MINLIB bibliography, February 2024).
The chemistry of eclogite is essentially that of basalt, albeit the mineralogy is utterly transformed
in very high temperatures and pressures at depth.
Thus in the Himalayas, eclogites would have
originated as basaltic sills and dykes in passive margin sediments of the
Indian plate (Searle, 2017).
Eclogites are found around the world, with well-studied examples in Norway, South America
and elsewhere.
The essential mineralogy is omphacite plus garnet (Mg-Al-Fe-(Ca)), and rutile, and perhaps some quartz or kyanite.
Those in Glenelg were described relatively early, and compared with kindred rocks
in Italy and Bavaria (Harker, 1932).
Coarse-grained eclogites can be attractive rocks, and figure in
handbooks for rockhounds (e.g., Mottana et al., 1977) as well as
in introductory textbooks (Tarbuck et al., 2015).
Many examples - those found in outcrop - are the product of
high-temperature and pressure metamorphism in the deep crust (e.g., Fyfe et al., 1958;
Miyashiro, 1973; Vernon, 1976; Turner, 1981, pp.409-415).
Deep subduction and subsequent exhumation of metamorphosed crustal materials
generates eclogites in metamorphic belts, as in Hispaniola
(Haiti and the
Dominican Republic: Nagle, 1974).
Others examples are samples of the deep crust or upper mantle brought to the surface as
xenoliths in kimberlites. These deep-seated intrusions, a minority of which are the source of most diamonds,
entrain blocks of the rock layers they pass through on their ascent, and
these xenoliths can be found as curios in diamond mines.
Such mantle nodules can be very informative of conditions deep underground
(McClelland and Lapen, 2013; Wood et al., 2013).
It was realised, some five decades ago, in both the former Soviet Union and South Africa, that
the mineral chemistry of garnet and pyroxene, amongst other minerals,
could give explorers a better chance of finding kimberlites, and thus perhaps diamonds
(see, e.g., Sobolev and Lavrent'ev, 1971; Gurney and Switzer, 1973; Dawson and Stephens, 1975; Gurney, 1985).
Some eclogites are thought to have formed as igneous cumulates,
later modified by tectonism and metamorphism (Lappin and Dawson, 1975).
In detail, eclogites can be complex. Depending on their metamorphic history (a
tale of P-T-t-X: pressure, temperature, time and composition)
a range of additional minerals may be present, including kyanite, rutile, graphite, spinel, coesite and even diamond.
And reactions between minerals in eclogites and other rocks subject to high P,T conditions
(olivine and plagioclase, say) may form corona textures,
with haloes of a third mineral formed at the interface of two earlier minerals
(Griffin and Heier, 1973).
The mineral assemblages suggest that they formed in
collision zones such as the Dabie-Sulu orogen in China, where the continental crust
is subducted into the mantle, subjected to ultra-high-pressure metamorphism at depths >100 km,
and then rapidly exhumed: these rocks are mostly gneiss and eclogite, with
diamond and coesite inclusions in zircon and garnet. The exotic, reduced mineral assemblages
are found also in chromitites (in deeply-subducted ophiolites) that have undergone similar histories,
where rare minerals may surive "armoured" within chromite grains
(Robinson et al., 2015).
REFERENCES
Craig,GY (editor) (1965) The Geology of Scotland.
Oliver & Boy, Edinburgh and London / Archon Books, Hamden, CT, 556pp. plus map.
Dawson,JB and Stephens,WE (1975) Statistical classification of garnets from kimberlite and associated
xenoliths. J.Geol. 83, 589-607.
Fyfe,WS, Turner,FJ and Verhoogen,J (1958) Metamorphic Reactions and Metamorphic Facies.
Geol.Soc.Amer. Memoir 73, 259pp.
Griffin,WL and Heier,KS (1973) Petrological implications of some corona structures.
Lithos 6, 315-335.
Gurney,JJ (1985) A correlation between garnets and diamonds in kimberlites.
In "Kimberlite Occurrence and Origin: a Basis for Conceptual
Models in Exploration" (Glover,JE and Harris,PG editors), Geology Department
and University Extension, University of Western Australia, Publication No.8,
revised edition, 298pp., 143-166.
Gurney,JJ and Switzer,GS (1973) The discovery of garnets closely related to diamonds in the Finsch
pipe, South Africa. Contrib.Mineral.Petrol. 39, 103-116.
Harker,A (1932) Metamorphism, a Study of the Transformations of Rock-Masses.
Methuen & Co. Ltd., London, 1st edition, 360pp.
Lappin,MA and Dawson,JB (1975) Two Roberts Victor cumulate eclogites and their re-equilibration.
Physics and Chemistry of the Earth 9, 351-365.
McClelland,WC and Lapen,TJ (2013) Linking time to the pressure-temperature path for ultrahigh-pressure
rocks. Elements 9 no.4, 273-279.
Miyashiro,A (1973) Metamorphism and Metamorphic Belts.
George Allen and Unwin (originally published in Japanese in
1965), English translation, 492pp.
Mottana,A, Crespi,R and Liborio,G (1977) Guide to Rocks & Minerals.
Simon and Schuster, Inc, Fireside Books, translated from Italian
original (Prinz,M, Harlow,G and Peters,J, AMNH, editors), 608pp.
Nagle,F (1974) Blueschist, eclogite, paired metamorphic belts, and the early
tectonic history of Hispaniola. Bull.Geol.Soc.Amer. 85, 1461-1466.
Papworth,A (2021) The Art of Experimental Physics.
Australian National University Printing Service (ANU Print),
296pp.
Rawson,JR, Carswell,DA and Smallwood,D (2001) Garnet-bearing olivine-websterite within the eastern Glenelg Lewisian
of the Glenelg inlier, NW highlands. Scot.J.Geol. 37, 27-34.
Robinson,PT, Trumball,RB, Schmitt,A, Yang,J-S, Li,J-W, Zhou,M-F,
Erzinzger,J, Dare,S and Xiong,FH (2015) The origin and significance of crustal minerals in ophiolitic
chromitites and peridotites. Gondwana Research 27, 486-506.
Sanders,IS, van Calsteren,PWC and Hawkesworth,CJ (1984) A Grenville Sm-Nd age for the Glenelg eclogite in north-west
Scotland. Nature 312, 439-440.
Searle,M (2017) Colliding Continents. A Geological Exploration of the Himalaya,
Karakoram, & Tibet. Oxford University Press, xxvi+444pp.
Sobolev,NV and Lavrent'ev,JG (1971) Isomorphic sodium admixture in garnets formed at high pressures.
Contrib.Mineral.Petrol. 31, 1-12.
Stone,P and Jackson,AA (2008) Bedrock Geology UK North. An explanation of the Bedrock Geology Map
of Scotland, northern England, Isle of Man and Northern Ireland. British Geological Survey,
1:625,000 scale map and booklet, 88pp., in one folder, 5th edition.
Tarbuck,EJ, Lutgens,FK, Tsujita,CJ and Hicock,SR (2015) Earth: an Introduction to Physical Geology.
Pearson Canada, Inc., 4th Canadian edition, 533pp.
Temperley,S and Windley,BF (1997) Grenvillian extensional tectonics in northwest Scotland.
Geology 25, 53-56.
![eclogite hand specimen [396 kb]](http://www.turnstone.ca/317-17.jpg)
![A.Papworth specimen labels [400 kb]](http://www.turnstone.ca/APlabels.jpg)