Henderson Et Al. (2009) State That the Field Evidence Seen on the 2008 HBC Field Excursion

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 synsedimentary and therefore necessarily preGrampian, 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 eclogitefacies 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 suprasubduction 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 ultramaficbearing 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 quasicontinuous, it might imply that the HBC represents a tectonostratigraphic 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 outof sequence thrusting, it wouldn't be too difficult to explain the ultramafic debrisbearing 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 ofintercalated bodies of peridotite and peridotite / (actinolite) pyroxenite / gabbro. The chromeactinolite 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 eclogitebearing Slishwood Group in SligoLeitrim (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

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