Eclogitic rocks of Ireland and Newfoundland
View looking towards Ballyshannon from Bulbin Hill - rocks are 'Moinian' psammites of the Loch Derg Group
Eclogitic rocks, mostly retrograded to symplectic garnet granulites, occur in the cores of amphibolite units in psammitic rocks of the Lough Derg Group of western Ireland. There are several outcrops where the transition from igneous-textured metagabbro to foliated eclogite can be clearly observed. Similar eclogites are also preserved in the cores of amphibolite units within psammites of the Fleur de Lys Group of Western Newfoundland. In both cases, the psammites are bordered to the northwest by continental rocks of the western foreland of the Caledonides, and to the south west by melange and ophiolite units of the Rattling Brook/Baie Verte groups in Newfoundland and the melanges and ophiolites of Clew Bay in Ireland. The distribution of eclogites in this manner has been used as an argument in favour of correlating the Fleur de Lys Group of Newfoundland with the Moine -Dalradian of Ireland and Scotland.
Map showing eclogite localities in the Fleur de Lys Group (Church, 1969)
Map showing amphibolites, eclogites and metagabbroic bodies in the Lough Derg Psammites of Donegal, Ireland
Map showing relative locations of eclogites in the Fleur de Lys of Newfoundland and the Moine of Ireland
eclobally.jpg - folded body of mafic rock in white weathering psammites, composed of an internal core of eclogite/garnet-granulite and a dark outer rim of amphibolite, Ballyshannon, Ireland.
eclofleur.jpg - mafic body in metaconglomerates; road to Bear Cove, Burlington Peninsula; lighter coloured interior of the rock is eclogite, the darker margins of the outcrop are amphibolite; .
eclots.jpg - intergrowths of garnet and albite (kelyphitic texture); bright white crystal is quartz
Clinopyroxene - retrogressive clinopyroxene - albite symplectic exsolution textures in the Lough Derg eclogites
7aB23C2_1.jpg - symplectite texture - 1
7aB23C2_2.jpg - symplectite texture - 2
eclokely1.jpg - symplectite texture - 3
eclokely2.jpg - symplectite texture - 4
eclokely3.jpg - symplectite texture - 5
eclofleur2.jpg - eclogite boudin in possibly graded (east to west) Fleur de Lys psammites. First recorded outcrop of eclogite in the Newfoundland Appalachians, roadside outcrop near Eclogite Pond, on the road to Westport, Burlington Peninsula.
eclofleur3.jpg - contact of extremely tough eclogite (right) with psammitic gneiss (left); Bear Cove road near junction with Westport road.
eclofleur3ts.jpg - Atoll garnet; possibly retrogressed garnet that has undergone a second period of euhedral (straight boundaries) growth. Alternatively, the garnet may have been zoned and the survival of the outer atoll may have been due to the particular chemistry of the rim zone.
eclostaurofleur.jpg - psammitic unit within several hundred meters of the eclogite locality at Eclogite Pond; the psammites contain large crystals of what are likely to be staurolite.
Church, W.R., 1968. Eclogites, in Hess, H. and Poldervaart, A., Basalts - the Poldervaart Treatise on Rocks of Basaltic Composition, v. 2, p. 755-798.
Bally_Mag_F6.jpg - Fig 6 showing relationship between decreasing rock magnetic intensity and increasing garnet content.
p. 772 " In eclogites, garnet coexists with with pyroxene, and commonly with amphibole; in this case the porportion of pyrope in garnet is a function of the bulk composition of the rocks and the value of the iron-magnesium partition coefficient KD. Of the three minerals clinopyroxene, garnet and amphibole, garnet always has the higher Fe:Mg ratio (O'Hara, 1960., p. 150). At present there are no experimental data relating temperature and pressure to variations in KD.. However, it is to be noted that the value of KD. for clinopyroxene and garnet from the pyroxene-garnet-spinel rocks from Salt Lake Crater (Yoder and Tilley, 1962) and the French Pyrenees (Lacroix, 1917; KD. = 0.36) is much higher than that of any eclogite (glaucophane-eclogite = 0.03-0.07; eclogites from gneiss terraines = 0.08-0.19; diamond-bearing eclogite = 0.15-0.25"
p. 773" In coronite metagabbros, brown amphibole and biotite are often the first minerals to form, generally as coronas around magnetite or ilmenite, and pyroxene. Garnet may surround the amphibole, suggesting that crystallization has occurred in a water-deficient environment. Amphibole-eclogites may have formed under analogous conditions. "
Church, W.R. 1969. Metamorphic rocks of Burlington Peninsula and adjoing areas of Newfoundland, and their bearing on continental drift in the North Atlantic. p. 212-233 in North Atlantic - Geology and Continental Drift, Memoir 12, The American Association of Petroleum Geology.
Church, W.R. 1970. Metamorphic eclogites from the Burlington Peninsula, Newfoundland, Canada. Prog. Abst., Geol. Assoc. Canada Ann. Meet., Winnipeg, p. 13.
Church, W.R. 1978. Eclogite-bearing amphibolites from the Appalachian Mobile Belt, NorthWest Newfoundland: dry versus wet metamorphism: a discussion. Jour. Geology, 86, p. 655-659. "the presence of eclogites in gneissic terrains depends not only on their environment of formation being "dry" but also on their being subsequently protected against "retrogressive" amphibolization during infiltration of water rising from deeper levels, either following the onset of higher temperatures dehydration reactions, or as a result of the crystallization of granite rocks during uplift and cooling in the later stages of metamorphism." " The Kd values for amphibole-garnet compared with those of clinopyroxene-garnet for the same rock (figs 3, 4) indicate that the primary carinthine amphiboles of the eclogite formed at temperatures as high as 550 degrees C, whereas the amphiboles of the amphibolites could have formed at temperatures as low as 400 degrees C."
Recent relevant papers regarding eclogite exhumation:
Martinez, F., Goodlife, A.M., and Taylor, B., 2001. Metamorphic core complex formation by density inversion and lower-crust extrusion. Nature, 411, p.930 - 934.
papuaeclogite2.jpg - illustration of the concept of core complex formation by density inversion
papuaeclogite3.jpg - explanation of papuaeclogite2
Cuthbert S. J. and Ballèvre M., 2002. Exhumation of metamorphic terranes: Introduction. Mineralogical Magazine, v. 66, no. 1, p. 1-3
Baldwin, S.L., Monteleone, B.D., Webb, L.E., Fitzgerald, P.G., , Grove, M., & Hill, E.J., 2004, Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea. Nature, Sept, v. 431, p. 263-267
papuaeclogite.jpg - rapidly exhumed 4.3 Ma-old eclogite-bearing core terraine separated from ultramafic rocks by a highly sheared (D2) carapace.
CONTRIBUTION OF THE
DEPARTMENT OF GEOLOGY
UNIVERSITY OF WESTERN ONTARIO
LONDON, CANADA NO. 159-b
Prog. Abst.G.A.C. Ann. Meet., Winnipeg, 1970 P. 13.
METAMORPHIC ECLOGITES FROM THE BURLINGTON PENINSULA,
W. R. Church
Eclogites occur as boudinaged mafic masses within garnetiferous psammitic metasediments of the Fleur de Lys Supergroup of the Appalachians of Newfoundland. They are composed of omphacite, garnet, amphibole, zoisite, muscovite, quartz,rutile and ilmenite. The eclogites are (at the moment) the only known occurrences in the Appalachians, and only the second known example of classic type (within a gneissic terrain) in North America.
The eclogites have quartz basalt-normative compositions with low K20 values (0.01 wt. percent) and 100 Mg/Mg+Fe (mol) of 51 to 56. One retrograded sample of biotite-bearing eclogite contains 0.73 (wt. percent) of K20 and 6 percent normative quartz (mol. norm). It has a 100 Mg/Mg+Fe (mol) value of 42.and a Poldervaart and Parker Crystallisation Index of 38 which is an uncommonly low value for eclogite.
The omphacite has the composition: Si02-53.45; Ti02-0.07; Al203-9.49; Fe203-3.48; FeO-2.25; MgO-10.94; CaO-14.53; Na20-5.61 (microprobe analysis). The iron-magnesium partition coefficient determined by microprobe analysis for the pair garnet-amphibole in the eclogite, compared with determinations for garnet-amphible from various metamorphic terrains of different grade, suggests that the eclogites formed under moderate metamorphic conditions of the staurolite-kyanite grade of metamorphism.
The metasedimentary sequence in which the eclogites occur is comparable to that of the Moine series of Scotland and Ireland, the More suite of Norway, and the Hecla Hoek succession of Spitsbergen, all of which contain eclogite. The Fleur de Lys and Moine successions, and probably the More suite, represent thick early marginal accumulations of clastic material derived from the craton and deposited in the Appalachian-Caledonide geosyncline during the late Proterozoic and Cambrian, and metamorphosed prior to early Ordovician time. The eclogites were formed by metamorphism of early mafic igneous rocks intruded into a thick sequence of psammitic rocks which had already been compacted and de-watered prior to the onset of metamorphism.