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Home , Geology of Guernsey, Geology of Jersey

Geochronology of the 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 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 , 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 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 (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

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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 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 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 --early 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 ( 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).

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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. There are no major petrographic differences that might correspond to the double-isochron behaviour although some minor differences between samples do occur. Those specimens defining the older isochron all come from the margins of the gneiss and are usually finer- grained than the younger megacrystic varieties coming predominant~ from the

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central parts of the gneiss. These variations are of a minor nature and it has not proved possible to map the differences. The generally uniform character and absence of major xenolithic contamination makes postulate (b) unlikely also. Whilst extensive contamination of the margins by radiogenic strontium from older wall rocks would lead to some apparently high whole-rock ages it would seem unlikely to result in the clear-cut double isochron. Postulate (c) may provide the best explanation of the anomalies. If a younger 'rejuvenation' event has affected the gneisses then the older isochron may be interpreted as their true intrusive age whilst the younger at least approximates to the time of isotopic rejuvenation. Such a process implies isotopic rehomogenization during a phase of high grade metamorphism and/or remelting or some partial gb and Sr re- distribution during a metasomatic event. The geological evidence certainly cannot be reconciled with remelting but a younger metamorphic or metasomatic event affecting some of the gneisses and producing partial strontium isotope rehomogenization is possible. Two potassium ages from the 'younger' group of samples (I343 and I359) are concordant with their parent whole-rock isochron age indicating that some younger event has at least reset the potassium in these rocks. They have, however, remained unaffected by the much younger Cadomian metamorphism. Perhaps the simplest explanation is that those samples of the 'younger' group, by virtue of their very large feldspar megacrysts, are not whole-rock samples in the strict sense. Since much of the rubidium and strontium of these rocks resides in the constituent potassium feldspars their isotopic behaviour may dominate that of the total rock. In the

Icart & Perelle Gneisses, Guernsey

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0.78

0.74

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0.70 . , ~ "t , 0 1 2 87Rb/s"Sr 20.5 FIO. I. Rb-Sr isochron diagram, Pentcvrian gneisscs, Guernsey.

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marginal areas where the older samples are less coarsely crystalline and do not contain large feldspar megacrysts the isotopic pattern is more straightforward. In this case we can accept the older isochron age of 2620 4- 50 Ma as the time of emplacement of the Icart gneiss, whilst the younger isochron at least partially reflects the overprinting effect of a younger event. Tentatively, this younger event might be related to the second deformational phase and amphibolite- facies metamorphism which affected the Icart gneiss (Roach r966 ). In the case of the Icart gneiss the low initial strontium isotope ratio (o.7oi6 4- o-ooi5) is in keeping with the very old age of the gneiss and the Rb/Sr ratio of gneisses of this age and type.

Alderney Some preliminary Rb-Sr analyses tentatively indicate a Pentevrian age for the Western -dioritic gneiss. Two whole-rock samples (i278 , I282) a granite phase and normal give an intersection age of 222o 4- 12o Ma. By taking the granite sample alone and assuming the minimum possible initial 87Sr/86Sr ratio ofo'699, a maximum possible age for this sample is 23oo 4- x2o Ma. In either case the age is similar to that of the Guernsey gneisses and the correlation between the two islands is probably correct. These ages are comparable to those obtained from the Pentevrian rocks of Cap de la Hague about I6 km to the east (Leutwein et al. I973). Correlations with the French Pentevrian Leutwein and his co-workers have reported several Rb-Sr and K-At ages from the Pentevrian at St Brieuc, St Malo and Coutances (Leutwein i968 , Leutwein & Sonet i965, Leutwein et al. I968, I969) failing in the range 3oo- i25o Ma and clearly reflecting the overprinting of several younger events. However, the effect is less complete than in the Channel Islands and pre-Cadomian mineral ages (particularly Rb-Sr feldspar) in the range 9oo-I ioo Ma have been preserved. In view of their isotopic stability, some of the Rb-Sr potassium- feldspar ages from the St Malo gneisses may approximate to the true age of formation yet none approach those of the Sarnian area, i.e. Guernsey, Alderney and Cap de la Hague. At the present time the two areas of Pentevrian seem to be yielding different ages; those of the Pentevrian of the St Brieuc--St Malo area in the range 9oo-Iioo Ma and in the northern 'Sarnian' area, x96o-26oo Ma. Further work is needed in both areas to explore these differences and to deduce a chronology for the Pentevrian as a whole.

(B) CADOMIAN IGNEOUS ROCKS The older Cadomian igneous complexes Perhaps the best example of the early Cadomian, pre-metamorphic plutonic phase is the Gneiss de Brest of western Finist~re (Bradshaw et al. I967, Bishop et al. 1969) which has yielded a Rb-Sr whole-rock isochron age of 690 ± 4 ° Ma. In the Channel Islands region the only definite equivalent of this phase is the L'ErEe adamellite of Guernsey. A Rb-Sr whole-rock/potassium feldspar isochron

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(Fig. 2"a) for this adamellite gives an age of 66o 4- 25 Ma thus setting an older age limit to the Cadomian metamorphism and post-metamorphic igneous com- plex and a younger age limit to the Brioverian sedimentation on Guernsey. On Alderney, the Cadomian metamorphism is conveniently bracketed by two phases of acid dykes intruded into the Western quartz- gneiss. The older, foliated granitic dykes yield a Rb-Sr whole-rock intersection age (I28I, I284) of 69o 4- 25 Ma whilst the younger, post-metamorphic quartz-porphyry dykes (I28O, I285) give a whole-rock intersection age of 63 ° 4- 3 ° Ma. The Rb-Sr ratios of the granodioritic orthogneisses of Sark are consistently low (about o'3) and accurate whole-rock dating of the gneisses is not possible. However, using a Rb-Sr whole-rock isochron approach one can at least distinguish between the age possibilities, namely, Pentevrian or Cadomian. Eight whole-rock samples from the Upper and Middle granite sheets and the Creux Harbour gneiss (Sutton & Watson 1957) are shown in Fig. 2b and despite their inherent imprecision, they cannot define a Pentevrian (about 26oo Ma) isochron. The approximate isochron age, 65o 4- 9 ° Ma, indicates an early Cadomian age for the orthogneisses of Sark. The question of the age of the migmatites and horn- blende and schists of Sark remains unresolved. Leucocratic granodiorite gneiss samples from the Ecr6hous, Paternosters and Minquiers reefs are grouped together on a single isochron diagram (Fig. 2c). The isochron age for these gneisses is 63o 4- 15 Ma indicating that these isolated but distinctive gneissic areas are of the same age. Also included in this group is a single analysis from the Gneiss de Tr6beurden, Tr~gorrois Peninsula (io59) which comprises a group of foliated leucocratic not unlike those of the Minquiers. A similar Cadomian age (c. 63o Ma) seems likely and the Pentevrian age postulated by Cogn6 (1962) can certainly be ruled out. In general, it is difficult to place these gneisses into the sequence of Cadomian orogenic events, but it appears that they were all emplaced during or possibly immediately prior to the main Cadomian metamorphism. A K-Ar hornblende age of 670 4- 17 Ma (io73 Hb) from the Perros granite (C6tes-du-Nord) is the only undoubted pre-metamorphic Cadomian mineral age obtained by the writer in the Channel Islands region but K-Ar and Rb-Sr biotite ages from the same sample (587 4- 14 Ma, 520 4- 17 Ma) indicate the usual Cadomian metamorphic overprint. The Rb-Sr whole-rock dating clearly indicates a widespread magmatic phase within the limits 630 4- I5 Ma and 690 4- 25 Ma extending from western Finistere, e.g. Gneiss de Brest, through the Tr~gorrois Peninsula (Perros granite and Gneiss de Trebeurden) and into the Channel Islands (Sark, Minquiers gneisses). It is important to note that most of these gneisses have in the past been correlated with the Pentevrian gneisses of St Brieuc and clearly it is unwise to make such correlations based on petrographic evidence alone. The younger Cadomian igneous complexes The younger Cadomian igneous complexes provide one of the best points of correlation within the Channel Islands (Table I). They lack the strong meta- morphic fabrics of the older Cadomian igneous rocks but occasionally show a

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weak foliation indicating that tectonic forces were still operative. Within each complex only the youngest granitic members have proved suitable for Rb-Sr whole-rock dating but these at least provide a good minimum age for the com- plexes as a whole. On Guernsey, the most suitable member of this group for dating is the Cobo adamellite which yields a mineral/whole-rock isochron age of 57 ° 4-15 Ma (Fig. 2d). This age places an older age limit on the intrusion of the L'Ancresse granodiorite and the final metamorphism on Guernsey (Roach 1966). Similarly, the ages of the Cobo and L'Erde adamellites (57 ° and 660 Ma) bracket the intrusion of the gabbros, and of Guernsey, Herm and Jethou. Whilst being the most appropriate choice upon gelogical grounds, the 'older' Dicq and Longueville granites of south-east Jersey have low Rb/Sr ratios and are rather unsuitable for whole-rock Rb-Sr dating. Four whole-rock samples from these granites seem to define an imprecise isochron at about 580 Ma (Fig. 2f). However, the south-west granite of Jersey, previously regarded as one of the 'newer' granites has (unexpectedly) given a whole-rock isochron age of 565 :k 15 Ma (Fig. 2f), indicating that this granite must be grouped with the Cobo adamellite of Guernsey and the other older granites of Jersey. In the Cotentin, the younger Cadomian igneous phase is represented by the Mancellian granites of Lower Normandy, of which the Vire-Carolles granite is perhaps best known. This granite is of great importance in defining the age of the base of the Cambrian (Cahen & Dord 1964) and has been dated by several previous workers using K-Ar, Rb-Sr and U-Pb methods (Graindor & Wasser- burg 1962, Kaplan & Leutwein 1963, Pasteels 197o). In every case strongly discordant age patterns have been found (5o8-612 Ma). Similar discordant age patterns have been obtained for the Mancellian granites of the Chausey Islands (461-54o Ma) and Athis (489-505 Ma) (Graindor & Wasserburg I962 ) and are very similar to ages obtained from south-east Jersey (440-52o Ma). Pasteels (197o) dated zircons and monazite from two samples of the granite by the U-Pb method and obtained discordant ~07pb/93sU, 2°6Pb/~3sU and "°Tpbr°*Pb ages with a range from 497 Ma to 760 Ma. Confining his attention to the more reliable 2°Tpb/z°6Pb ages alone, Pasteels argued that those ages of the zircons, 760 4-35, 73 ° ~ 5 ° Ma were spurious and probably indicated xenocrystic origin, whilst the monazite ages, 554 4- 26, 579 q- 3 ° Ma were the most reliable. Using samples from the same localities as previous workers, a similar discordant mineral age pattern has been obtained by the writer, e.g. 1129 biotite, K-Ar -- 474 + I3 Ma, Rb-Sr -- 556 4- i i Ma, i i3o biotite, K-Ar = 519 4- 14 Ma, Rb-Sr -- 505 4- I I Ma. However, whole-rock samples and potassium feldspars yield an isochron age of 605 4- 15 Ma (Fig. 2e) in agreement with the maximum mineral age obtained by previous workers, 612 Ma, and which provides the most reliable age for the Vire-Carolles granite. Although a protracted cooling history could produce the above age discordances it is more likely that they are the result

FIo. 2. Rb-Sr isochron diagrams, Cadomiart igneous rocks, Channel Islands area. Symbols: solid circles = whole rock; open circles = potas- sium feldspar; open squares = biotite or muscovite; solid triangles = whole rock analyses not included in isochron ages.

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of some thermal overprint subsequent to intrusion. This would agree with the two- stage intrusion history (post-Cambrian and pre-Cambrian) suggested by Graindor (i957) and is supported by a single Rb-Sr whole-rock age of 508 =k io Ma from within the granite (Graindor & Wasserburg I962 ). A similar interpretation is presented for the discordant age patterns that are found on Jersey, 4 ° km to the NW, where 'older' granites, about 57 ° Ma old, are intruded by younger granites, 480-520 Ma. The newer granites of Jersey. Only those of the south-east complex are definitely 'young' since they intrude and remobilize gabbro, diorite and older granites. However, both north-west and south-east granites are distinctive alkali leuco- granites and to this group can also be added some of the supposed 'older' granites in the south-east complex--the and Gorey leucogranites. Rb-Sr whole-rock isochrons for the north-west and south-east granites yield ages of 490 4- 15 (lower Ordovician) and 520 4-4 Ma (upper Cambrian) respectively (Fig. 3), thus completely excluding the possibility of a Variscan age as previously suggested by Mourant (i95o). Mineral ages, especially Rb-Sr biotite and K-Ar hornblende ages, from these granites and the adjacent remobilized diorites are essentially concordant with the Rb-Sr whole-rock ages (Fig. 4, sections 2 and 3) but the K-Ar biotite ages are consistently younger (44°- 480 Ma), presumably the result of gradual cooling of the complexes as a whole. In retrospect it is important to note that Rb-Sr dating has revealed several

0.86 Newer Granites of Jersey STSr ss~rr G : Gorey L : E :Ft. Regent 0.82 • : S E Complex o : NW Granite

0.78

L Age:520+4my (SE)

0.74 490 +15 my (NW)

0.70 0 4 8 12 16 STRb/S6S r 20

FIG. 3. Rb-Sr isochron diagram, Newer granites of Jersey.

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mistaken geological correlations on Jersey. Firstly, some of the supposed 'older' granites of SE. Jersey, e.g. Gorey, Fort Regent granophyres, must now be grouped with the 'newer' group, e.g. Le Hocq granite; secondly, the supposed 'newer' granites of SW. Jersey must now be correlated with proven older granites, e.g. Dicq and Longueville adameUites; thirdly, the new Jersey granites as a whole are not related to Variscan igneous activity in any way, but must now be regarded as the final post-tectonic (alkalic) phase of Cadomian igneous activity in the Channel Islands region.

(C) THE CADOMIAN METAMORPHISM The Rb-Sr whole-rock ages discussed above place reasonable limits on the main Cadomian regional metamorphism, being younger than the L'Er6e adamellite of Guernsey (66o 4- 25 Ma) and the older dykes of Alderney (690 4- 25 Ma), probably synchronous with the igneous rocks of Sark (650 4- 9 ° Ma) and Minquiers (63o 4- 15 Ma), but older than the Cobo adamellite of Guernsey (57 ° 4- t5Ma), the older granites of Jersey (565 4-15 Ma) and the Vire- Carolles granite of Lower Normandy (6o 5 4- 12 Ma). The effective age limits, 6o 5 and 66o Ma can be further reduced by consideration of the mineral ages from the Channel Islands region. Over ioo K-Ar and Rb-Sr mica and hornblende ages have been obtained from the metamorphic basement and the younger Cadomian igneous rocks. Despite evidence of pre-Cadomian igneous activity the mineral age patterns consistently reflect the very strong thermal overprint associated with Cadomian orogenic events and with only one exception, the Perros granite, 670 Ma (IO73), the ages have a sharp older age limit at 620 Ma decreasing to a younger age limit at 44 ° Ma. The extent to which these ages approximate to the various orogenic events depends upon (i) the rate and degree of regional cooling aJ2er the main Cadomian metamorphism and (ii) the effect of the younger Cadomian igneous rocks which have interrupted and extended this cooling. To assess these two factors the mineral ages from the metamorphic basement and the younger igneous complexes are considered separately. The metamorphic basement. The K-Ar and Rb-Sr mineral age determinations considerably extend and amplify the pattern and interpretation previously published (Adams I967). All the K-Ar ages fall in the range 52o-5oo Ma with about 7o% of the hornblende ages in the range 57o--6oo Ma and about 7o% of the biotite ages in the range 53o-56o Ma (Fig. 4). The average discordance between hornblendes and is generally about 3 ° Ma but this is not always regular and occasionally may be reversed (e.g. I265, I26I). Such discrepancies usually arise from anomalously young hornblende ages which have either a complex crystallization history of hornblende-biotite-chlorite intergrowths, or compositional variations (and thus argon retention capacities) within the amphiboles (see Drysdall op. tit. I957). The Rb--Sr whole-rock/biotite intersection ages fall in the range 561-62o Ma (Fig. 4) generally about 3 ° Ma older than the corresponding K-At ages and similar to K-Ar/Rb-Sr age discordances observed by others (Hart I964).

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Younger igneous complexes. The mineral ages for these rocks are similar to those of the metamorphic basement (Fig. 4) but only a few K-Ar hornblende ages fall in the range 57o-59 ° Ma and most are concordant with or younger than the co- existing biotites (53o-56o Ma). A particularly anomalous group of young K-Ar hornblende ages (495-515 Ma) comes from fine-grained diorites in NE. Guernsey, N. Herm and Alderney (Figs. 4, 5). It is possible that these young ages might reflect the effect of a hidden granite 47 ° Ma old in this area (cf. Jersey), but it is more probable that the ages reflect the fine-grained nature of the amphiboles and their complex parageneses. Drysdall (i957 0p. cit.) showed that in these rocks an original brown hornblende may be converted to bluish-green and then colourless amphiboles followed by the development of biotite, chlorite and iron/titanium oxides. It appears that the retentive brown hornblendes, where unaltered, yield reliable ages (e.g. I145, II4I ) but the other varieties (e.g. i276 , 1143 ) may have poor argon retention capacities which result in spuriously young ones. Geological interpretation The K-Ar and Rb-Sr mineral ages show no simple regional variations and the general uniformity of the age pattern reflects the regional nature of Cadomian orogenic events (Fig. 5). The oldest mineral ages, mostly K-Ar hornblende, from the metamorphic rocks provide a younger age limit for the main Cadomian metamorphism and this is especially true of areas such as Sark where the com- plication of the younger Cadomian igneous rocks is not present. With the Rb-Sr whole-rock data placing the main metamorphism at about 65 ° :k 3o Ma and the mineral ages (Fig. 4) providing a younger limit at 62o Ma, the best estimate for the main Cadomian metamorphism is 65o Ma. The age of the basic members of the younger Cadomian igneous rocks must be younger than the preceding metamorphism, 65o Ma, but greater than their maximum mineral age of 59 ° 4- IO Ma. The maximum mineral ages (e.g. i i53 Bi = 562 4- I4 Ma) of the granitic members agree well with the Rb-Sr whole- rock data at 57 ° 4- ~o Ma. Regional cooling of the metamorphic basement must have commenced at least 62o Ma ago and continued until 53 ° Ma, resulting in radiogenic 'closures' of various minerals at various times, namely, biotite with respect to Sr, 62o- 56o Ma; hornblende with respect to Ar, 6oo--53o Ma; biotite with respect to Ar, 58o-52o Ma. The sharp older age limit at 6oo-62o Ma must reflect the start of uplift, erosion and regional cooling of the orogen and it is significant that 4 ° km to the east, coarse arenaceous sedimentation in the basal Cambrian (c. 57 ° Ma ago) of the Cotentin Peninsula implies a source in this area undergoing vigorous uplift and erosion at the same time. Similarly, regional cooling of the basic members of the younger Cadomian igneous complexes must have started at least 59 ° Ma ago but locally this was interrupted by the younger granitic members of the sequence and late meta- morphic events. The last, retrogressive metamorphic phase on Guernsey post- dates the Cobo adamellite, 57 ° 4- 15 Ma, but is possibly synchronous with the emplacement of the L'Ancresse granodiorite for which biotite ages of 542 4- I4, 548 4- 14 Ma (K-At) and 551 4- 12 Ma (Rb-Sr) establish a minimum age.

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The thermal effect of these late igneous and metamorphic phases at 560 4- I O Ma has caused expulsion of argon from pre-existing rocks resulting in the con- centration of K-Ar ages, particularly of biotites, in the range 54o-55 o Ma, from both the metamorphic basement and the younger Cadomian igneous rocks (Fig. 4). Thus the final cooling of these rocks could not have started until about 55 ° Ma ago and continued until 52o Ma ago. On Jersey the emplacement of the 'newer'

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0 [3 Rb-Sr Bi 0 0 0 I 0 0 | i 430 470 5;0 550 59O 630 [] Rb-Sr In 0 Io." I 8 2 0 YOUNGER o° I CADOMIAN 8 ROCKS K-Ar 560- 600 my

• i t ''°° I 6 i | | i J 430 470 510 550 590 [] 3 0 Q I I Rb-Sr NEWER GRANITES & K-Ar JERSEY 0 I I 0 | i i 430 ,470 510 550

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:%, ¢30"W

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granites has locally interrupted and further extended this cooling history (see section 2B). (D) VARISCAN OROO NY It is now certain that no major Variscan plutonic rocks occur in the Channel Islands although Variscan granites are present nearby on the French mainland. For example, a Rb-Sr biotite age of 297 4- 6 Ma from the Barfleur granite at the north-east tip of the Cherbourg Peninsula has been reported by Graindor & Wasserburg (I962). Also, the writer has obtained K-At and Rb-Sr biotite ages of 305 4- 6 Ma and 316 4- 7 Ma respectively (I 131 ) for the FlamanviUe granite in the NW. Cherbourg Pensinula. In both cases these granites are high level rapidly-cooled plutons and therefore the mineral ages are probably close to the time of intrusion. The third example, outside the Channel Islands area, is the Trrgastel-Ploumanac'h granite, C6tes-du-Nord which yields a Rb-Sr whole-rock isochron age of 3oo 4- 5 Ma. The above granites and many other Variscan granites in this region are cut by dyke swarms similar to those invading the Cadomian igneous complexes and younger sediments of the Channel Islands. A dolerite dyke (1296) of a type known to post-date the Alderney (Cambrian ?) sediments gives a K-Ar whole-rock age of 317 4- 9 Ma, thus placing a younger, pre-Stephanian age limit on the sediments. On Guernsey, the age of the last igneous event, the intrusion of mica lampro- phyres, is given by a K-Ar biotite age of 296 4- 8 Ma (I 163). A younger age limit for the plutonic rocks and the Rozel of NE. Jersey is provided by a K-Ar hornblende age of 427 4- 13 Ma (lower ) from a hornblende (I 21 o) cutting the sediments. 3" Correlations (A) PENTEVRIAN AND BRIOVERIAN The Pentevrian rocks of the Channel Islands region probably extend westwards into the Trrgor Peninsula (Verdier 1968, Ryan I973) and possibly to NW. Brittany (Cogn6 1962, Cogn6 & Shelley I966 , Chauris 1967). However, in southern Brittany and the Massif Central a pre-Brioverian basement has not been seen. With only an imprecise picture of metamorphic events in the Pentevrian at 2600, 195o and I IOO Ma it is nonetheless important to recognize that these events correlate with major orogenic episodes of the Archaean and Proterozoic shield areas of Canada, Scotland and Scandinavia. Similar to other late Precambrian terrains in NW. Europe, Brioverian sedi- mentation occurred in the interval 69O-lOOO Ma ago, in good agreement with recent trace fossil evidence of a late Riphean or Vendian age (Squire i973).

(B) CADOMIAN The main episodes of the Cadomian orogeny can be readily traced across the northern part of the Massif Armoricain. For example, the Older Cadomian rocks are definitely represented in Finist~re by the Gneiss de Brest (Bishop et al. 1969,

Fxo. 5- Geology and distribution of all mineral age determinations.

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Adams & Taylor, unpublished work), in the Tr6gorrois (C6tes-du-Nord) by the Perros granite and probably in the Cotentin (G. Power pets. comm.), whilst the younger Cadomian magmatism occurs in Lower Normandy as the Mancellian granites (Graindor & Wasserburg 1962, Kaplan & Leutwein 1963, Leutwein 1968, Leutwein & Sonet 1965) and in northern Britanny as the Renards, Guimiliau and Plougenven granites (Leuwein et al. 1968, 1969, Adams & Taylor, unpub- lished work). Cadomian ages similar to those found in northern Brittany have been reported from southern Brittany (Barri~re et al. 1971, Vidal 1972, Cogn~ & Vidal I972), the Massif Central and north-west Spain (Priem et al. 1966, Priem & Verdurmen 197o ) although much of the detailed orogenic history has been obscured by later Variscan events. To the north, late Precambrian mineral ages in the range 55o-65 o Ma (mostly K-Ar) have also been found for Precambrian igneous and metamorphic rocks of Anglesey (Moorbath & Shackleton I966), the Malvern Hills (Lambert & Rex 1966), Charnwood Forest (Meneisy & Miller 1963) and Pembrokeshire and the Welsh Borderland (Adams, unpublished data). In most cases the ages have been interpreted as 'cooling' ages only, reflecting uplift of the complexes after formation but some Rb-Sr whole-rock data for igneous rocks on Anglesey (Moorbath & Shackleton 1966) and in the Malvern Hills igneous rocks (Lambert I969) confirm a late Precambrian age. Finally, significant comparisons can be made across the Atlantic with late Precambrian rocks in Newfoundland (McCartney et al. i966), Cape Breton Island (Cormier 1972) and New England (Fairbairn et al. 1967) where both K-At mineral ages and Rb-Sr whole-rock isochron ages follow a very similar pattern to that of the Cadomian rocks in north-west France and the Channel Islands. Clearly these results lend support to continental reconstructions of the North Atlantic region (e.g. Bullard et al. 1965) and suggest the probable juxtaposition of the two areas in late Precambrian times.

ACKNOWLEDOE~mZ~rS.I wish to thank Dr S. Moorbath for his help and encouragement during the tenure of an N.E.R.C. Studentship at the Geological Age & Isotope Research Laboratory, Oxford University; and Drs A. C. Bishop and R. A. Roach for discussion, and for criticism of this paper.

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