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Discussion |
Department of Geology, National Museums & Galleries of Wales, Cardiff CF10 3NP, UK Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia School of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK (e-mail: aewright@onetel.net. uk) School for Professional and Continuing Education, University of Birmingham, Birmingham B15 2TT, UK
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Jana Horák writes: |
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 TOP Jana Horak writes: W. Compston, A. E....References |
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Firstly, the conclusion that the new age data for the Arfon Group enables a redefining of the Sarn Complex terrane affinity is questioned. In particular the statement
it also now seems unlikely that the Sarn Complex should be compared with the Padarn area as the Parwyd Gneiss has yielded a Rb–Sr date of 542 ± 17 Ma (Beckinsale et al. 1984). This is not now regarded as the age of the amphibolite facies metamorphism (Horák et al. 1996) but must indicate a considerable retrogression event in the gneissose basement in Late Precambrian–Cambrian time
does not appear to have any relevance to any of the data presented. The Parwyd Gneiss is represented by a single exposure of retrogressed garnet amphibolite and felsic gneiss, which lies entirely within the Ll
n Shear Zone. Although previously included within the Sarn Complex, recent work has defined this as a separate unit of uncertain affinity (e.g. Gibbons & Horák 1990; Gibbons & McCarroll 1993). In contrast to the gneisses, the Sarn Complex is a poorly exposed heterogeneous plutonic complex, of which the most northwestern margin has been involved in ductile and brittle deformation. The Rb–Sr isotopic data age for the Parwyd gneisses has no bearing on the affinity or age of the Sarn Complex and furthermore the published U–Pb age of 615 ± 2 Ma for a gabbro (not granite, as stated by Compston et al. 2002) from the Complex (Horák et al., 1996) falls well within error for the age of the basal part of the Padarn Tuff Formation (614 ± 2 Ma, Tucker & Pharaoh 1991). In addition to this, although the affinity of the Parwyd Gneiss is uncertain, a Sm–Nd model age for the felsic gneiss of 1.5 Ga (Davies et al. 1985) shows greater affinity to the model ages of plutonic rocks in Southern Britain (Davies et al. 1985; Noble et al. 1993), than data for the Monian Gneisses (Sm–Nd model ages, paragneiss 1.8 Ga, amphibolitic gneiss 1.1 Ga, Horák 1993).
A second point relates to the four undated outliers on Anglesey that Greenly (1919) correlated with the Arfon Group. If the Bwlch Gwyn tuff, correlated with the Padarn Tuff, dated at 614 ± 2 Ma (Tucker & Pharaoh 1991), lies unconformably on the blueschist-bearing shear zone (dated at 550 Ma, Dallmeyer & Gibbons 1987) then the age data clearly question this correlation. However, Horák & Gibbons (2000) clearly indicate that the Bwlch Gwyn Tuff (and Baron Hill Formation) forms ‘isolated exposures within the Berw Shear Zone, which represent fragments of the Cymru Terrane that were tectonically interleaved with the Monian Composite Terrane following cessation and dismemberment of the Avalonian arc’. As Compston et al. (2002) do not include any age constraint data for any of the four outliers, the data presented does not appear to have any bearing on, or progress the discussion related to, the affinity of these units.
The statement that ‘modern terrane analysos has made little advance on the correlation problems of the first attempt at plate tectonics interpretation of the area’ belies an appreciation of the value of terrane analysis, not as an end in itself, but as a means of identifying similarities or differences between adjacent blocks and establishing the common elements (if any) between them. In particular the identification of the Menai Strait Fault System as containing a major ductile, sinistral, shear zone has revealed the relative motion the Monian Composite Terrane and the Avalonian terranes of Southern Britain. (e.g. Gibbons 1987, 1990a, b; Gibbons & Horák 1996). Furthermore terrane analysis has provided a meaningful context in which to place recent radiometric ages (e.g. Tucker & Pharaoh 1991).
Finally, Compston et al. (2002) use Monian Composite Terrane, Monian Superterrane and Mona Complex interchangeably throughout the text, to refer to the Monian rocks of Anglesey and NW Lln, with little regard to the difference in meaning of these three terms. This is both confusing and misleading. Although Monian Composite Terrane (Carney et al. 2000) is preferred to Monian Superterrane, as current thinking suggests that the component Monian terranes had amalgamated prior to juxtaposition with mainland Wales along the blueschist belt shear zone (Gibbons & Horák 1996), it is acknowledged that usage of composite terrane versus superterrane is a valid topic for discussion. The use of the term ‘Mona Complex’ however cannot be condoned. As explained by Gibbons & Horák (1990) the Monian rocks cannot be considered as having formed without any significant displacement between them as originally envisaged by Greenly (1919). This is clearly demonstrated by the contrasting metamorphic pathways recorded by the three terranes. ‘Mona Complex’ is therefore an anachronistic term and it is more helpful to refer to the three individual terranes (Monian Supergroup, Coedana Complex and Blueschist Terrane) which collectively comprise the Monian Composite Terrane.
29 May 2002
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W. Compston, A. E. Wright & P. Toghill reply: |
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TOPJana Horak writes:W. Compston, A. E....References |
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The relevance of our Arfon data to terrane correlation with the Sarn Complex and the Parwyd Gneiss depends entirely on the interpretation placed on the retrogressive metamorphism of the Parwyd Gneiss. We were assuming that this event was indicative of the age of some of the major tectonism suffered both by it and the Sarn Complex (as seems to have been the case for the Ercall Granophyre in the Welsh Borderland). As the Arfon Group seems to have been in a stable tectonic environment and unmetamorphosed between about 572 Ma and the top of Lower Cambrian times it still seems most likely that the Parwyd Gneiss, at least, was remote from Arfon until the Menai Strait Fault System had finally amalgamated these terranes. We recognize that the Sarn Complex is of identical age to the Padarn Tuff, but whether it belongs to the same terrane depends upon the date of any events subsequent to its emplacement, and these have not yet been determined.
A similar argument applies also to the four outliers, previously regarded as either Cambrian or correlatives of the post-Padarn Tuff Arfon Group, that occur on Anglesey. The reasonable assumption from Dallmeyer & Gibbon's (1987) dating of the blueschists, is that the sedimentary, metamorphic and tectonic activity affecting the Monian Supergroup and the Blueschist Terrane spanned the period of deposition of the Arfon Group. We have no personal experience of the four inliers, but based purely on the earlier published evidence, they seem to be later than, or at least unaffected by, the tectonism and metamorphism affecting the Mona Complex and cannot therefore be correlatives of the Fachwen. We would agree, of course, that if any or all of them are simply fault bounded slivers within the Monian Composite Terrane, as suggested byHorák & Gibbons (2000), then it becomes possible to correlate them with almost any other lithostratigraphic group.
We use Mona Complex as rock stratigraphic term, admittedly old fashioned but much shorter than alternative phrases, for the rocks of the Monian Supergroup, Coedana Complex and the blueschists, as the terms Monian Composite Terrane and Monian Superterrane are not rock-stratigraphic terms. Horák uses ‘Monian rocks’ in the same way, but this is open to confusion as more logically referring only to rocks of the Monian Supergroup.
Lastly, we defend our assertion that modern terrane analysis has made little advance on the correlation problems of the first attempt at a plate tectonic interpretation of the area. We do not question the value of terrane analysis, being some of its earliest disciples, nor the advances it has made in our understandingof the Avalonian or any other orogenic belt, but those of us working at the time of the plate tectonic revolution essentially used terrane analysis (‘identifying similarities or differences between adjacent blocks and establishing common elements (if any) between them’) to achieve correlations (Wright 1969), which are still being proved essentially correct by the much more accurate dating methods now being used.
24 July 2002
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References |
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TOPJana Horak writes:W. Compston, A. E....References |
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Carney, J.N., Horák, J.M., Pharaoh, T.C., Gibbons, W., Wilson, D., Barclay, W.J., Bevins, R.E., Cope, J.C.W. & Ford, T.D. 2000. Precambrian Rocks of England and Wales. Geological Conservation Review Series, Joint Nature Conservation Committee, Peterborough, 20.
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