Geochronology of the Channel Islands and adjacent French mainland C. J. D. ADAMS SUMMARY Rb-Sr whole-rock dating of igneous and metamorphic event 62o-650 Ma ago, and 7) a metamorphic rocks of the Channel Islands post-metamorphic igneous phase 56o-61o Ma region has delineated the following major ago. 8) On Jersey a further post-tectonic events: I) Formation of the Pentevrian base- granite phase, 48o-52o Ma, can be recognized ment gneisses ¢. 26oo Ma ago in the northern Within this framework, nearly 13° K-At and Guernsey, Alderney and Cap de la Hague Rb-Sr mineral ages from the area are inter- region. 2) Possible isotopic rejuvenation of preted as reflecting initiation of cooling and these gneisses during a second metamorphic uplift of the Cadomian orogen as a whole in episode ¢. I95oMa ago. 3) Formation of late Precambrian or Cambrian times. Pentevrian gneisses in the southern, St Malo- Only a very few, young minor intrusions are St Brieuc region, Iooo-IxooMa ago. 4) connected with Variscan orogenic events. Deposition of the Brioverian sedimentary series Some preliminary comparisons can be made in the interval 65o-Iooo Ma. 5) Emplacement with late Precambrian igneous and meta- of igneous rocks 65o--69o Ma ago during initial morphic rocks elsewhere in the Massif Armor- phases of the Cadomian Orogeny (late Pre- icain, southern Britain, eastern Canada and cambrian) followed by 6) a main regional U.S.A. GEOLOGICALLY, the Channel Islands region (which in this work includes parts of the Cotentin and the Trdgor Peninsula) lies in the northernmost part of the Massif Armoricain, a large area of Precambrian and Palaeozoic rocks extending over the entire region of Brittany. Since fossiliferous rocks are absent from the Channel Islands, dating of the plutonic and metamorphic rocks has inevitably been made by comparison with similar rocks on the adjacent French mainland where a fairly complete Precambrian and Palaeozoic succession is present. i. Geological setting (A) REGIONAL OUTLINE The oldest rocks of the region are the Pentevrian gneisses, first recognized by Cogn6 (1959) from the St Brieuc region (C6tes-du-Nord), which underlie a thick sequence of late Precambrian or Brioverian sediments (Graindor I957, Cogn~ I962 ). Other gneisses of the Channel Islands region have been correlated with this area (Chauris I967, Verdier I968 , Brown et al. I97I , Roach et al. i973, lZyan I973) and it appears that the Pentevrian lies in two areas (i) a southern area including the Tr~gorrois, St Brieuc, St Malo and Coutances complexes and (ii) a northern or 'Sarnian' area including Guernsey, Alderney and Cap de la Hague. The Brioverian supracrustal sequence overlying the Pentevrian consists of argillites, arenites, spilites and more acid volcanic rocks, dl geol. Soc. Lond. vol. z32, I976, pp. 233-25o, 5 figs. x Table. Printed in Northern Ireland Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/3/233/4896958/gsjgs.132.3.0233.pdf by guest on 24 September 2021 234 C. J. D. Adams divided into lower, middle and upper groups (Barrois i895 , Graindor i957 Cogn 6 1962). The Cadomian (late Precambrian) orogeny affected both Brioverian and Pentevrian rocks and resulted in the Brioverian/Cambrian unconformity seen in Lower Normandy. The main phase of Cadomian deformation resulted in ENE.-WSW.-trending folds but in some areas, e.g. along the western side of the Bale de St Brieuc, there is evidence for an earlier phase of folding along N.-S. axes. The principal Cadomian metamorphism appears either to accompany or post-date the main deformation and this is usually accompanied by syn- or post-kinematic plutonism (Bradshaw et al. I967, Bishop et al. I969, Cogn6 i97o , Roach et al. I972 ). Uplift during the Cadomian orogeny produced a land mass in northern Brittany extending into the Channel Islands. As a result of uplift marking the close of the Cadomian orogeny several molasse basins were formed in Lower Palaeozoic times in the northern part of the MassifArmoricain and by the beginning of the Ordovician the region was largely stabilized. A major marine transgression, represented by the Gr~s Armoricain, marks the start of a relatively thin sequence of sediments of stable platform type. In late Devonian--early Carboniferous times Variscan (Bretonic) earth movements resulted in basinal type sedimentation in the Carboniferous. (B) CHANNEL ISLANDS In the Channel Islands, gneissic complexes occur on Guernsey (Bonney & Hill i884, Farquharson i924, Plymen I933, Roach I966 and pers. comm.), Alderney (Morgan, W. R. I957, Univ. Nottingham thesis (unpubl.), Sark (Hill & Bonney I912, Wooldridge 1925, Sutton & Watson i957) and the Ecrrhous, Paternosters and Minquiers Reefs (Mourant i933a,b , i938, Graindor & Roblot I957). On Guernsey, there is evidence that the gneisses predate possible Brioverian sediments (Plymen I922) whilst on Jersey, the Jersey sedimentary and volcanic series (Mourant I933a , Graindor i957) have been correlated with the Brioverian of the Cotentin (Transon I85I , Graindor I957) and are intruded by Cadomian plutonic rocks. Throughout the Channel Islands the Pentevrian and Brioverian rocks have been affected by the Cadomian tectonism and regional metamorphism (Roach 1966) and invaded by several generations of Cadomian plutonic rocks (Bonney & Hill I884, Parkinson I9O7, Wells & Wooldridge i93i , Plymen 1933, Mourant I95 o, Henson I956, DrysdaU i957 (Southampton Ph.D. thesis (unpubl.), Morgan I957 op. tit., Roach I966 ). On Jersey, the situation is complicated by younger episodes of granite intrusion and dyke emplacement (Wells & Wooldridge x93 I, Bishop, A. C. 1954, Univ. London Ph.D. thesis (unpubl.)). On Alderney and Jersey, unfossiliferous sediments post-date the Cadomian igneous activity but are cut by minor intrusions of probable Variscan age (Bigot i888, Plymen i92x ,Cornes i933, Mourant I95O, Sutton & Watson I97O). Details of the geology of the individual Channel Islands and correlations between them are presented in Table I and further discussion of the geology of the region may be found in Bishop et al. (i975). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/3/233/4896958/gsjgs.132.3.0233.pdf by guest on 24 September 2021 Geochronology of the Channel Islands ~35 ¢) ==. m m m la, h., tm ,.-4.,4 ® "~ @ -= gem ,,g lli = == H .o ~.r.t ~ h m o to ,.~ .~ e.- h E e-. k ¢, ~.,4 o o o ~ o ol F4 , . 1 .~ ~ .,, ® z = , ~, • ~ ~ ~,~ ~'~o ~B~ ~ ~o ° o,,4 ~ .. i,.,.l XSV'HcI XSV'H,:I 1:=:1 S~O~NOI ~IXO~OX S~O~NOI ~[GqO r..) . , . , I i I I '-' =l ~l °l ~'~o o i o z o a a ,i, o ~ d II-IO~ I i i i 0 ~ w~ o 0 0 o o 0 0 0 0 01 ............. 1"..-. ~ . ,. ,r-- Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/3/233/4896958/gsjgs.132.3.0233.pdf by guest on 24 September 2021 236 C. J. D. Adams 2. Geochronology Rb-Sr dating of the igneous rocks of the Channel Islands has provided a valuable framework within which the significance and interpretation of K-Ar and Rb-Sr mineral age patterns on these and other metamorphic rocks can be discussed. For this reason, the Rb-Sr whole-rock isochron ages on Pentevrian and Cadomian rocks are presented before consideration of the mineral age determinations. A tabulation of I63 samples with s'Rb/SeSr; s'Sr/SeSr ratios; Rb-Sr and K-Ar ages, *°Ar radiogenic content, and 40Ar atmospheric/4eAr total percentage is given in Supplementary Publication No. I8OI 3 (5PP.), deposited with the British Library at Boston Spa, Yorkshire UK and with The Geological Society Library. Decay constants used are: STRb:2fl = 1.39 x IO -ix year-l; 4°K:2fl ---- 4"72 x xo -x° year -x, )le = o'584 x IO -x° year -1. A full discussion of the initial strontium isotope values obtained for the igneous rocks of the Channel Islands will be given elsewhere. (A) THE PENTEVRIAN BASEMENT Guernsey. Dating of the pre-Cadomian rocks is particularly difficult for two reasons. Firstly, the great majority of the Rb-Sr and K-Ar mineral ages indicate a very strong, widespread, Cadomian thermal 'overprint' and no reliable pre- Cadomian ages have been obtained in this way (Adams i967). Secondly, Rb-Sr whole-rock dating, although unaffected by such an overprint, is difficult to apply to the predominantly diorific Pentevrian gneisses which have uniformly low Rb/Sr ratios. Although in general the granitic gneisses of Guernsey are more suitable in their Rb/Sr ratios they include heterogeneous paragneisses which probably do not satisfy some fundamental requirements of the Rb-Sr method. For this reason selection of samples was confined to gneisses of more igneous character, principally the Icart granitic gneiss which is more uniform petro- graphically and shows sharp contacts with adjacent metasediments. The 17 specimens of Icart gneiss do not, as might have been expected, define a single isochron (Fig. I) and an average isochron age of 2300 ± 2oo Ma must be meaningless since none of the data points would fall upon such an isochron line. It is clear that two significantly different populations are present and two isochrons are drawn giving ages of 262o 4- 5 ° Ma (initial s~Sr/s6Srratio -- o.7oi6 4- o.ooi5) and I96o 4- I4O Ma (initial 87Sr/86Sr ratio --o'o7o28 4- o'0o44). Such a dual isochron indicates a failure of at least some of the samples to satisfy certain fundamental assumptions of the dating method, implying that the samples (a) are of various ages, or (b) had variable initial 87Sr/8eSr ratios, or (c) have not remained closed systems with respect to Rb and Sr subsequent to formation. Postulate (a) may bc excluded, since the geological evidence clearly indicates a single phase of emplacement for the Icart gneiss.