Correlation of ophiolite-olistostrome units from Scotland
through Ireland to Newfoundland: two ophiolite belts
Henderson et al. (2009)
state that the field evidence seen on the 2008 HBC field excursion
was consistent with the possibility that at least some ultramafic
units in the HBC may have formed as fault gouges in an
ocean–continent transition basin, as proposed by Geoff Tanner at the
same meeting. The main advantage of this proposal is that the
ultramafic emplacement event would be syn-sedimentary and therefore
necessarily pre-Grampian, the Garron Point Group as an
ophiolitic cover sequence would become an unneccesary construct, and
the whole complex in the form of the Trossach's Group would be in
structural and stratigraphical continuity with the Southern Highland
Group. The rift scenario is theoretically credible, and has
already been proposed by David Chew (2001) for the ultramafic rocks
of the uppermost part of the c. 600 Ma Argyll Group on Achill Island
in Ireland (cf. p. 29 – ‘black graphitic muds have clearly
penetrated the serpentinites as soft sediments’)!
On the other hand the attempt by the authors to diminish the
ophiolite obduction hypothesis by emphasizing that the HBC
amphibolites are dissimilar to amphibolites of ultramafic and
eclogite-facies parentage found in the ‘soles’ of classical obducted
ophiolites is decidedly less persuasive, and is contrary to David
Chew's (2009) contention that the Bute amphibolite at least is most
likely associated with the early stages of Iapetus closure as a
supra-subduction zone ophiolite. It would be quite
legitimate to allow both scenarios, but the HBC then starts to look
like the proverbial camel that emerged from the committee charged
with designing a horse! The emplacement of the amphibolites as part
of a section of suprasubduction zone oceanic basin would still
require explanation even if it could be demonstrated that all the
serpentinites were emplaced during early Arenig (Ethington 2008) -
as distinct from Tayvallich - continental rifting. Early Arenig
rifting also runs counter to that fact that the Iapetus ocean was
already well established by this time. There is also the question of
how certain it is that the metagabbros participated in the Grampian
orogeny?
The map of Pickett et al. (2006) showing
the distribution of the Argyll and Southern Highland groups in
Scotland implies the existence in the region of the Mull of Kintyre
of a tight major fold culmination that in Ireland causes the Argyll
Group to appear adjacent to the Omagh thrust and the ‘paratectonic’
Tyrone ophiolite and its
associated ‘orthotectonic’ Dalradian core complexes. In Northern
Ireland the HBC does not appear along this contact, but according to
Chew (2003) the HBC reappears in Clew Bay - South Achill Beg in
Western Ireland. Here the ‘orthotectonic’ Clew Bay HBC lies
south of the much older ultramafic-bearing Argyll Group rocks (Chew,
2001) and are bordered to the south by the ‘paratectonic’ Deer Park
ophiolite (Chew et al. 2007) and the overlying successor South Mayo
trough. Explaining the absence of Southern Highland Group
rocks by equating the Clew Bay rocks with the ultramafic-free
Southern Highland Group would ignore the presence of serpentinites
on southern Achill Beg. Furthermore the Clew Bay rocks contain
pebbly psammites with Archean tDMs (Chew 2003), a feature that Chew
used to differentiate the Clew Bay rocks from the adjacent
Dalradian. In this respect it is interesting to note that the Lime
Craig conglomerate at Lime Craig quarry near Aberfoyle also contains
psammitic clasts with Archean tDMs (Dempster and Bluck 1989).
Is it possible that these rocks are slope and rise equivalents of
the Eriboll Cambrian containing an Archean - Lower Proterozoic (no
Mesoproterozoic) zircon population (Cawood et al 2007)? Or do the
zircon rims that have Hf470 values of c. –40 and Hf tDM model ages
of c. 3200 Ma from granitic pegmatites cutting the Tyrone Complex
Inliers have a bearing on this problem (Flowerdew et al. 2008). A
tDM study of the Cambrian Keltie Water Grits and the ‘early
Arenig?’Margie psammites would perhaps be enlightening, as would a
zircon search of the Loch Lomond and other Cr-rich sediments.
If the HBC and Clew Bay are indeed quasi-continuous, it might imply
that the HBC represents a tectono-stratigraphic unit that is
discordant relative to and separate from the Argyll/Southern
Highands succession rather than being in stratigraphic continuity
with the latter.
The serpentine body in the calcareous
unit on Achill Beg could indicate that the Clew Bay sediments were
originally underlain by obducted ophiolite, and it is possible that
the same could therefore be true of the sediments at Lime Craig
quarry in Aberfoyle. The Clew Bay and HBC rocks - and even the
chromiferous Loch Lomond sediments - may represent the leading
edge of an obduction system that involved both NW ophiolite
overthrusting and contiguous NW underthrusting of mantle material,
the latter triggering the development of the SW verging Tay nappe,
and presaging the flip in subduction sense (Church and Gayer 1973)
that led to the formation of the Midland Valley-Southern
Uplands-Longford Down-Notre Dame Bay arc. Alternatively, by
employing out-of sequence thrusting, it wouldn't be too difficult to
explain the ultramafic debris-bearing rocks as having been laid down
in a foreland basin in advance of the initial phase of ophiolite
obduction.
In the Baie Verte
(Burlington) Peninsula of Newfoundland, the ‘paratectonic’
Baie Verte ophiolite belt is bordered to the NW by the
‘orthotectonic’ Birchy Schist composed of a highly strained and
dismembered ophiolite assemblage (Church 1991). It in turn is
'underlain' by the Rattling Brook Group dominantly composed of
psammites with minor horizons of carbonate, graphite schist,
amphibolite and garnetiferous semipelite. However, the most
characteristic feature of the lower unit is the presence of
intercalated bodies of peridotite and peridotite / (actinolite)
pyroxenite / gabbro. The chrome-actinolite material after pegmatoid
clinopyroxene serves as a means of locating the boundary between the
Rattling Brook Group and the underlying eclogite/garnet-amphibolite-
bearing psammitic rocks, the latter possibly being equivalent to the
eclogite-bearing Slishwood Group in Sligo-Leitrim (Flowerdew and
Daly 2005). The pegmatoid clinopyroxenites (chrome actinolite)
also link the ophiolitic material in the Rattling Brook to that of
the upper plate ‘paratectonic’ ophiolites, in which they commonly
occur at the contact between ultramafic rocks and overlying
gabbros. The incorporation of the ophiolitic material into the
psammites could be achieved by a mechanism of reverse fault
'slicing' during subduction of the leading edge of the obducting
ophiolite, and normal fault 'dicing' during exhumation. There may
well be other mechanisms.
In summary, it may be useful to look at
the geology of the Highland Border region extending from Scotland to
the Southern Appalachians in terms of an upper 'paratectonic'
ophiolitic plate (distal to the subduction front), which includes
the Deer Park, Tyrone, Bay Verte-Betts Cove, Thetford-Magog
ophiolite belt (not present in Scotland), and perhaps even the
Maryland ophiolitic rocks of the Southern Appalachians, and a lower
'orthotectonic' ophiolitic subduction complex (proximal) which
includes the HBC, CBC , Birchy Schist-Rattling Brook, and perhaps
the New England Belvedere complex. The original ophiolitic leading
edge no longer exists, it having been 'frittered' away during the
subduction process. The HBC is part of the mixing zone, and the
serpentinites are remnants of the ophiolite.
References
CAWOOD, P. A., NEMCHIN, A. A., STRACHAN, R., PRAVE, T., and
KRABBENDAM, M. 2007. Sedimentary basin and detrital zircon record
along East Laurentia and Baltica during assembly and breakup of
Rodinia. Journal of the Geological Society, 164, 257–275.
CHEW, D. M. 2001. Basement protrusion origin of serpentinite in the
Dalradian. Irish Journal of Earth Sciences, 19, 23-35.
CHEW, D. M. 2003. Structural and stratigraphic relationships across
the continuation of the Highland Boundary Fault in western Ireland.
Geological Magazine, 140, 73-85.
CHEW, D. M., GRAHAM, J. R., and WHITEHOUSE, M. J. 2007. U-Pb zircon
geochronology of plagiogranites from the Lough Nafooey (=Midland
Valley) arc in western Ireland: constraints on the onset of the
Grampian orogeny. Journal of the Geological Society, 164, 747-750.
CHEW, D. M., PAGE, L. M., MAGNA, T., DALY, J. S., KIRKLAND, C.,
WHITEHOUSE, M. J. & SPIKINGS, R. 2008. Constraints on the timing
of deformation in the Highland Border Complex and correlative rocks
in western Ireland. (abstract). In Highland Workshop Meeting,
Edinburgh, April 2008, 35.
CHURCH, W. R. and GAYER, R. A. 1973. The Ballantrae ophiolite.
Geological Magazine, 110, 497-510.
CHURCH, W. R.1991. Discussion on a high precison U-Pb age for the
Ben Vuirich granite: implications for the evolution of the Scottish
Dalradian Supergroup. Journal of the Geological Society, 148,
205-206.
DEMPSTER, T. J . & BLUCK, B. J . 1989.The age and origin of
boulders in the Highland Border Complex: constraints on terrane
movements. Journal of the Geological Society, London, 146, 377-379
ETHINGTON, R. L., 2008. Conodonts from the Margie Limestone in the
Highland Border Complex, River North Esk. Scottish Journal of
Geology, 44, 75-81.
FLOWERDEW, M. J. and DALY, J. S. 2005. Sm–Nd mineral ages and
P–T constraints on high grade pre-Grampian metamorphism of the
Slishwood Division, Northwest Ireland. Irish Journal of Earth
Sciences, 23, 107–123.
FLOWERDEW, M. J., DALY, J. S., CHEW, D. M., MILLAR, I., HORSTWOOD,
M. 2008. In-situ Hf isotope measurements of complex zircons from
Irish granitoids reveal hidden Palaeoproterozoic and Archaean
sources at depth [abstract]. In: Highland Workshop, Murchison House,
Edinburgh, 2008.
HENDERSON, W.G., TANNER, P. W. G. & STRACHAN, R. A. 2009.
The Highland Border Ophiolite of Scotland: observations from the
Highland Workshop field excursion of April 2008. Scottish Journal
Geology, 45, 13-18.
PICKET, E. A., HYSLOP, E. K. & PETTERSON, M. G. 2006. The
Green Beds of the SW Highlands: deposition and origin of a basic
igneous-rich sedimentary sequence in the Dalradian Supergroup of
Scotland. Scottish Journal of Geology, 42, 43–57.
TANNER, P.W.G. 2008. An alternative model for the Grampian Orogeny
in Scotland (abstract). In High