Space-time variations in British Caledonian Granites: some geophysical correlations

Earth and Planetary Setence Letters, 45 (1979) 69-79 © Elsevier Sclentlhc Pubhshmg Company, Amsterdam - Printed in The Netherlands

69

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SPACE-TIME VARIATIONS IN BRITISH CALEDONIAN GRANITES SOME GEOPHYSICAL CORRELATIONS G C B R O W N and C A L O C K E Department of Earth Setenees, The Open Umverstty, Walton Hall, Mtlton Keynes MK 7 6AA (England}

Received April 5, 1979 Revised version received May 28, 1979

The characteristics of 30 "granites" from the postulated Caledonian (Iapetus Ocean) suture zone of mainland Britain are discussed Geophysical (gravity, aeromagnetlc), geochemical (U/Pb and initial 8"ZSr/a6Sr) and isotopic age data indicate that these British Caledonian mtruslons (390-600 Ma) can be divided Into two d~stmct groups temporally, each of which IS further subdivided spatially The temporal division applies throughout the British province and separates a pre-Sllurlan (group 1) suite of lowvolume, low-moblhty magmas, which were intruded under compressive conditions, from a Slluro-Devonlan (group 2) suite of large volume, mobile magmas intruded under tensional conditions The spatial subdivisions of groups 1 and 2 are made between Intrusions emplaced to either side of the postulated ENE-WSW lapetus Ocean suture which runs through the Solway Drth First, the group 1 gramtes northwest of the suture probably were produced by partial fusion involving Proterozolc continental crust (group 1N) whereas those to the southeast have isotopic characteristics simply Indicating a mantle or ocean crustal source (group 1S) Second, the Siluro-Devonian gramtes, which were all derived largely at the expense of Caledonian mantle, have different aeromagnetlc expressions depending on their position m the northwest (2N) or southeast (2S) Caledonldes These aeromagnetic characteristics are probably related to the differences In basement structure recently identified by seismic survey~ and they provide further evidence for the former existence and Slluro-Devoman closure of Iapetus Another slgmficant implication may be that Proterozolc basement is lacking from beneath the region ~mmedmtely adjacent to and southeast of the suture - southern Scotland and most of England

1 Introduction Lower Palaeozoic plate closure events, which f o r m e d the Caledonian-Appalachian m o u n t a i n belt, have b e c o m e widely recognased in Scandinavia, Greenland, Europe, Iberla, Canada and the U n i t e d States using a wide-variety o f geological techniques (summarlsed in references 1 - 7 ) The Iapetus ocean suture is an ENE-WSW line along the length o f the orogenic belt which passes through British Isles via the Solway Firth and Shannon estuary (Fig 1) Evidence for the position o f this suture comes from the structural f r a m e w o r k o f Western Europe [4] which reveals differences in m e t a m o r p h i c grade and intensity o f folding to the n o r t h w e s t abd southeast Locally, within Britain, similar spatial constrats are f o u n d In the petro-

chemistry o f Caledonian and older rocks [6,8] and In seismic [3] and other geophysical parameters [9] relating to vertical crustal structure [2] Intruding this crust, and o u t c r o p p i n g within Late Precambrlan and Lower Palaeozoic m e t a s e d l m e n t a r y hosts, are tonahte, granodlorlte and adamelhte bodies ( [ 6 , 1 0 13] and Fig 1) reminiscent o f those associated with compressive continental plate margins and continental collision zones Here we focus a t t e n t i o n on the gravity and aeromagnetac patterns locahsed over these "granItes" and use these features to amplify s o m e i m p o r t a n t aspects o f earlier geological studies - ( a ) A clear division into older and y o u n g e r granite types is defined, with lmphcatlons for a change in m a g m a genesis and m o b i l i t y within the Silurian (2) Deep crustal aeromagnetlc anomalies associated

70

--T

l

T

8W

6 O.

50

4

i

1,00m,les

~ &

[?

2E

q

o

d

for Brmsh Caledonian granites which further emphasise vanatmns in both time and space

2 Geophysical data

2E o

sources

58 N ~ L ' ~

--58 N '

0

\ 4 --

--54

,(

6W

~

2

0

Fig 1 The location of Caledonian gramtes m mainland Britain in relatmn to the lapetus Ocean suture hne Gramte ages are dlfferentmted into group 1 (black areas - pre-Sllurlan) and group 2 ( stippled - mmnly Sfluro-Devoman) Intrusions are numbered as follows 1 = Ardlethan, 2 = Ballachuhsh, 3 = Cam Chumneag, 4 = Foyers, 5 = Halladale, 6 = Helmsdale, 7 = La]rg-Rogart, 8 = Moy, 9 = Peterhead, 10 = Ratagaln, 11 = Stnchen, 12 = Ben Loyal, 13 = Calrngorm, 14 = Etwe, 15 = Lochnagar, 16 = Monadhhath, 17 = Mount Battock, 18 = Rannoch Moor, 19 = Strontlan, 20 = Cairnsmore of Fleet, 21 = Crfffell, 22 = Loch Doon, 23 = Mount Sorrel, 24 = Threlkeld, 25 = Chevxot, 26 = Eskdale, 27 = Market Welghton, 28 = Shap, 29 = Sklddaw, 30 = Weardale

with the younger intrusions serve to demonstrate the spatial contrast in crustal structure either side of the suture zone (3) The geophysical data are considered against a background of recent age and isotopic mfornratmn

The recent intensive LISPB (Llthospherlc Seismic Profile of Britain) Investigation of mainland Britain along a NW-SE traverse was highly successful in producing a broad picture of crustal layering ([3 ], Fig 2, see also section 4 3) In the absence of statable highresolution seismic data, first-order three-dimensional analyses of Individual igneous complexes have depended upon the interpretation of gravity data Such interpretations of Caledonian lntrusives are hmlted to the Lake District [14,15], north and east England [I 6,17] and the Scottish Southern Uplands granites [9,18] Gravity data for Scotland have become available recently in the form of IGS maps [19] and, although no Interpretation work is yet available, there are some interesting correlations of anomaly patterns with individual granite intrusions For the present purpose, all the available gravity data [ 1 4 - 1 9 ] were treated samllarly by subtracting an approMmated regional field from anomahes either associated with known granite outcrops or their postulated buried equivalents Residual gravity anomaly values for the Intrusive bodies discussed here (numbered in Fig 1) are given in Table 1 Where magnetic anomalies occur over granites, their analysis can provide additional Information on possible shape, for example, the Weardale and postulated Market Welghton Intrusions [ 16,17] The Interpretatlon of these magnetic data is more complex than for gravity because they usually reflect contributions from several sources For Instance, in addltmn to the remanent and induced magnetlsatlon of the intrusive rock, which for gramtes are usually small, there may be addxtlonal major components related to the presence of magnetic materials m both aureole and root zones which were produced by dehydratlve metamorphism during emplacement To gain m a x i m u m benefit from the magnetic data associated with Caledonian Intruswes, in Table 1 we have recorded (a) the amplitude of any aeromagnetlc anomaly, estimated to the nearest 50 nT from IGS maps [20], (b) the approximate width(s) of the anomaly patterns, bearing in mind that wider anomahes are due, m

71 general, to deeper sources, and (c) the amplitude of any anomaly on the digitally filtered aeromagnenc map [21] Only deep crustal magnetic sources remain after filtering because this map has an approximate 13-km wavelength cut-off

3 Granite ages, structural and petrochemical classification An important systematic classification of Caledonian intrusions by Read [28], on the basis of field petrographic and structural criteria, recogmsed a range of intrusive events across the whole of the British Caledoredes This classification divided the intrusions into two groups the older and the newer granites Newer granites were further sub&vlded into an early suite of forcefully emplaced bodies and a later group of "permitted" intrusions To judge from geochemical studies of granite samples, the evidence from geochemical maps and a prehmlnary review of granite ages with initial 87Sr/86Sr ratios it has been argued elsewhere [6,8,11,29] that the major change in granite naagmatlsm took place between the emplacement of the two types of newer granite, that is, between Read's [28] newer forceful and newer permitted mtruslons The timing of this change seems to correlate with the final closure of the Iapetus Ocean across Britain probably during mid-Silurian times [1,5,10], with the subsequent permitted granites being emplaced under conditions of crustal tension Accordingly, the intrusions listed in Table 1 and illustrated in Fig 1 have been subdivided into Group 1 (pre-SIlurlan) comprising Read's older and newer forceful granites In northern Scotland [8,29] and the Lake District [11 ] these intrusions are not associated with any geochemical anomalies when compared with their host metasedlments They have low concentrations of incompatible elements, high Sr contents and high K/Rb ratios compared with group 2 intrusions and are magmatitic dlorates and tonahtes Group 2 (Slluro-Devoman *) comprising Read's newer permitted granites These Intrusions are distinct * These age descriptions are based on the revised terminology introduced by Rundle [41] and are subject to continuing debate

from their host rocks in having variable and, in some cases, extreme incompatible element enrichments and low K/Rb ratios [6,11,29] They are generally granodlorltes and adamellltes - more acidic than group 1 intrusions Although these groupings have been derived using strutural, petrochemical and prehminary dating evidence, they should be compared with new absolute age determinations Wherever possible, ages are recorded in Table 1 using the compilation by Halhday et al [12] supplemented by three U-Pb ages [7] and independent Rb-Sr ages for Ardlethan [22], Cam Chumneag [23], Peterhead [24], Strlchen [25], Threlkeld [26], Weardale [27] and Eskdale [41] The new Rb-Sr ages for Loch Doon and Shap [12,42] are based on multlpoint whole rock lsochrons and, together with the Rb-Sr and U-Pb ages just mentioned, should be regarded as more reliable than the remaining seven K-Ar dates recorded in Table 1

3 1 Interpretatton ofgramte ages and isotope ranos K-Ar systems are known to provide erroneously young ages of emplacement in cases where Intrusions have undergone prolonged coohng This effect has been observed in the older, group 1 granites [30,31 ] emplaced during and just after the climax of Dalradlan metamorphism (ca 500 Ma ago [31,32]) Most of the ages quoted as K-At in Table 1 (derived from Pldgeon and Aftahon [7] and Halhday et al [12]), have been reviewed in the light of U-Pb data on zircons and, in some cases, have been corrected upwards to allow for a cooling interval [7] Since many of the U-Pb data comprise mixed populations of a little Inherited lead added to lead derived since emplacement, they form chords which intersect concordla at the age of emplacement (assuming no lead loss) The ages quoted as K-Ar are taken from Pldgeon and Aftalmon's [7] work and are based on their estimates using K-Ar dates and Caledonian U-Pb concor&a intersections In northern Scotland, Pidgeon and Aftahon [7] found that group 1 granites contain a more significant inherited zircon (lead) component than group 2 granites Consequently, group 1 granites have U-Pb systematics which are more discordant leading to less reliably placed concordxa intersections than for group 2 The group 1 granites are also syn- or just

TABL[ 1 Gravity [ 1 4 - 1 9 ] , aero magnetic [20] and filtered aeromagnetic anomaly [21 ] data for representative British Caledonian Intrustuns with age, Initial 87Sr/86Sr ratio and zircon data for reference [7,12,22- 27,41,42] Intrusion

Residual gravity anomaly approximate amplitude 1 (mgal)

Aeromagnetlc anomaly amplitude 2 (nT)

Width of aeromagnetlc anomaly 3 (km)

Faltered aeromagnetlc anomaly amplitude 2 (nT)

Age (Ma)

Average initial 87Sr/86Sr ratio 4

Presence of inherited zircon

Northern Scotland Group 1 1 2 3 4 5 6 7 8 9 10 1l

Ardlethan Ballachuhsh Cam Chulnneag Foyers Halladale Helmsdale Lairg-Rogart Moy Peterhead Ratagam Strachen

0 0 10

0 200 0

5, 1 -

-50 0 0

494(Rb-Sr) nd 557(Rb-Sr)

0 714 n d 0 710

nd n d yes

13 0 0 0 -9 8 0 4

200 100 250 0 100 250 200 0

8 2 10, 4 3 6, 2 5 -

100 0 0 0 0 0 0 0

400(K-Ar) n d 420(K-Ar) n d uncertain 417(Rb-Sr) 400(K-At) 462(Rb-Sr)

0 706 nd 0 706 nd 0 718 0 717 0 705 0 717

yes n d yes n d yes nd yes yes

-13 -30 -32 -22 -19 -28 nd -5

250 200 650 200 250 300 200 200

5 20, 2 10, 3 3 20 5 10,3 10, 2

0 100 200 0 150 150 50 150

n d n d 410(K-Ar) 415(K-Ar) nd n d nd 435(K-Ar)

nd nd 0 705 0 706 nd nd nd 0 706

n d nd trace s trace s nd nd nd trace s

Group 2 12 13 14 15 16 17 18 19

B~n Loyal Calrngorm gtlve Lochnagar Monadhhath Mount Battock Rannoch Moor Strontlan

Southern Uplands 20 21 22

Caarnsmore of Fleet Cnffell Loch Doon

-31

0

-

0

390(U-Pb)

0 707

no

-22 -18

200 0

5 -

0 0

406(U-Pb) 408(Rb-Sr)

0 706 0 705

no minor

-1 0

50 0

2 -

0 0

454(U-Pb) 459(Rb-Sr)

nd 0 706

no nd

-20 -19 -18

250 0 100

100 0 0

390(K-At) 429(Rb-Sr) nd

0 706 0 708 nd

nd n d nd

-15 -16 -28

200 0 -100

0 0 0

394(Rb-Sr) nd 410(Rb-Sr)

0 707 nd 0 706

no nd nd

England Group 1 23 24

Mount Sorrel Threlkeld

24 26 27

Cheviot Eskdale Market Welghton Shap Sklddaw Weardale

Group 2

28 29 30

10, 3 8 5 5

n d = no data available 1 2 3 4

Amphtude given relative to a regional approximated from the surrounding area, sign indicates the centred anomaly termination Amplitude given relative to a background value approximated from the surrounding area Approximate figures - where anomaly is composed of at least two components two figures are given The most reliable Initial ratios are hkely to be those for which Rb-Sr dates are quoted Inmal ratios appearing after K-Ar ages were calculated by constructing isochrons through whole rock sample analyses for 87Rb/86Sr whose slopes were determaned from the quoted K-Ar ages [12] s Data for Lochnagar and Etive are hardly distinguishable from those where no inherited zircon Is present Recent detailed studles of Strontlan [ 12] indicate a zircon memory in the granite core only with none in the surrounding tonahte and granodlorite

73 post-tectonic They were emplaced into warmer crust, and thus had more prolonged coohng histories and greater post-emplacement argon loss than for group 2 Therefore, the agreement between K-Ar and U-Pb dates for Foyers, Helsdale and Ratagaln is less good than for the group 2 granites at Etlve, Lochnagar and Strontlan Bearing in mind that the Southern Uplands granites belong to group 2, it follows that, to a first approximation, there is an age difference between group 1 and group 2 (Table 1) The rather scattered ages of group 1 granites reflect a genuine age variation among syn- and post-tectonic forcefully emplaced granites whereas the younger group 2 granites cluster in the lower Devonian at about 400 Ma (390-415 Ma) except for late Ordovlclan/early Sdurlan intrusions at Eskdale and StronUan which had more complex origins discussed elsewhere [12,41] At this point we note that these conclusions, based on age data from different sources and of different qualmes, are tentative and should be continuously reviewed as more evidence is obtained However, they prowde a basis for the geophysical observations an section 4 One significant conclusion from this section is that the ISotopic characteristics of group 1 gramtes differ northwest and southwest of the suture zone (Fag 1) and thereby define two subdivisions The granites of northern Scotland have high initial a 7Sr/86Sr ratios and inherited zircons (group 1N) whereas those of England (see Table 1, also Pldgeon and Aftallon [7] and Thorpe [341 for other early Palaeozoic intrusions) have low initial 87sr/aasr ratios and concordant zircons (group 1S)

4 Geophysical correlatmns First we consider granites both north and south of the Iapetus suture according to the time periods now estabhshed and finally focus attention on the suture zone itself, taking Into account geophysical data for the Southern Uplands granites 4 1 Pre-Sdunan gramtes, group 1

Some members of this suite are responsible for small gravity anomahes [19] from - 9 to +13 regal (Table 1) but many have no associated residual

anomaly This indicates that the volumetric extent of the Intrusions may be rather small, although their highly variable compositions and diffuse contacts [8,11,28,29] makes the prediction of density contrast for such estimates highly suspect Some of the group 1 granites, notably Ballachullsh, Helmsdale and Peterhead contain prominent low-density, relatively acid components at outcrop, yet their gravity anomalies are no more marked than the remainder The largest positive anomaly in this group occurs over the Foyers granite and has been successfully modelled as a thin granitic layer overlying a basic body [33], a conclusion also supported by magnetic anomaly data which are exceptional for group 1 (Table 1) The majority o f the group 1 Intrusions are expressed on the aeromagnetlc map [20], sometimes as complex anomahes with at least two wavelength components Anomaly widths are small ( 1 - 6 km with two exceptions, Table 1) which indicates shallow sources, a conclusion endorsed by the absence of a deeply buried magnetic expression on the smoothed map [21 ] Likely magnetic sources lie within shallow metasedlmentary host rocks and the Intrusive rocks themselves, which sometimes contain a prominent magnetic mineral component and there is no Impression of deep magnetic disturbances To summarlse, the geophysical evidence from group 1 Intrusions indicates little density-magnetic contrast with their host rocks and we interpret this evidence as indicating small intrusive volumes Together with the ISOtOpic data for group 1N Intrusions, these data are consistent with magma genesis involving crustal melting or crustal equlhbratlon of mantle-derived melts during a period of steepened thermal gradients [10,32] Given sufficiently elevated temperatures, Proterozolc zircons are available in Dalradlan and Molnlan cover [12] and these may be an equally viable source of partial melt during climatic metamorphism to fusible Lewislan basement ([3,7], and Fig 2) The southern group 1S granites have stmllar geophysical expressions but lack the ISotopic Indicators of crustal melting Either a deeper, mantle melt source is implied or a source within juvenile continental crust It is instructive to note the similar characterlSttCS of the late Cadomlan, early Caledonian intrusive

74 suite of southern Britain [34] which forms minor outcrops (e g the Malvern and Johnston complexes) that are fault-bounded fragments of much larger intrusions Despite the associated high-grade metamorpluc rocks (ca 560 Ma) of Anglesey and southern England ([4], and R S Thorpe, personal communication), which indicate elevated crustal temperatures, a convincing case can be made for subduction related magma genesis [34] Later sediments m this region are lowgrade shales and greywackes (e g the Welsh Ordovlclan and Silurian) which have been nowhere near melting condmons There IS no indication of a widespread thernral event at the time of emplacement (ca 450 Ma) of the group 1S granites considered here and we propose a deep subduction zone or mantle origin for these intrusions Such an origin is consistent with the gravity data in Table 1 because (1) the small Threlkeld microgranite is petrologically similar to its host rocks andesltiC Borrowdale volcanlcs [26] - and therefore has a similar density, and (2) the Mountsorrel granophyrlc dmrite has a rather high density (2800 kg/m 3 [43]) and does not contrast significantly with its host rocks (Precambrlan slates) 4 2 Stluro-Devoman gramtes, group 2

This younger group of granites IS well represented within a rahter narrow time period across the Caledoredes and in Britain most intrusions are of SiluroDevonian age Reference to Table 1 shows that they are distinguished clearly from the earlier, group 1 suite on geophysical grounds Except for StrontIan, in some ways an atypical member of group 2, all have large negative residual gravity anomahes which exceed those for group 1 granites Amphtudes range up to - 3 2 regal for Etlve [19], north of the suture, and to - 2 8 mgal for Weardale [16], to the south, implying that significant density contrasts are preserved toconsiderable depth Although these intrusions are richer In quartz and feldspars at outcrop, and so are of lower density than group 1 Intrusions, detailed modelling indicates that they occupy large volumes of the upper crust For example the Calrngorm and Lochnagar Intrusions, which have a density contrast o f - 1 0 0 kg/m 3 with country rocks, extend to some 12 km depth with a steep intrusive contact to the north and a more shallow contact to the south and west (C A Locke, in preparation) Similarly the pro-

posed bathohth at Market Weighton has been shown to extend to considerable depth ( 9 - 1 2 kin) if modelled with density contrasts of - 1 2 0 to -1 O0 kg/m 3 [17] The smaller Ship granite in the Lake District is a steep sided body extending to 8 kin depth [14, 15] with a density contrast of - 1 2 0 kg/m 3 from the country rock Gravity over the Strontlan granite is exceptional and may reflect the complex nature of this intrusion (see footnote to Table 1 and Halhday et al [12] and Munro [35]) However, its geochemical [6,11], isotopic [7,12] and magnetic signature (Table 1) are of group 2 type The aeromagnetlc anomalies over group 2 granites are also significant, particularly in the northern Caledonldes where they rise to 650 nT over Etlve and 250 nT over Monadhhath [20] As with group 1, some anomalies appear to comprise at least two components and the larger component is 10-20 km wide for several group 2 intrusions Both this evidence and that from the filtered map ([2l] and Table 1) indicate a deep magnetic source associated with the six largest group 2 granites of northern Scotland (group 2N) Such large anomalies could not be produced by the weakly magnetic intrusive rocks themselves, the Cairngorm granite, for example, has too small a remanent magnetasatlon (2 × 10 - 7 emu/g) and induced magnetlsatlon (2 5 × 10 - s emu/g) Yet the locahsed nature of the anomalies defines a close association with these Intrusions Possible sources are (I) basic crystal cumulates at depth, and (2) metamorphic effects in the basement around the granite root zones On balance, we favour the latter first, because there is no gravity evidence for high density crystal cumulates and second, because the analogous group 2 of the southern British Caledonides lack these prominent anomalies and have penetrated a different type of lower crust (Fig 2) Only the Cheviot massif has a significant anomaly of presumed deep source and elsewhere the magnetism of group 2S granites is due to shallow metamorphic rocks, often in contact aureoles [15,16] To summarlse, the geophysical evidence from group 2 intrusions indicates strong magnetic and density contrasts with their host rocks which we interpret as indicating large intrusive volumes (all of group 2) overlying deep magnetic source regions (group 2N only) This evidence defines group 2 in a more effective way even than present isotopic data

75

?

I--I

U n c e r t a i n structure

Younger rocks

Caledonlanbeltmetamorphlcsandgranltes(61 ~] ~

62kin/s)

LowerPalaeozoicrandgranltes (5 8-60km/s) re-Caledoman basement (>6 4 km/s) Uncertalnbasement (<63km/s) Lower Crust ( ~7 km/s) Upper mantle ( ~8 km/s)

~_

6

0

O~

J Distance (km) 700

E o.

20

E3

ZONE OF UNCERTAIN RELATIONSHIPS BETWEEN LOWER CRUST AND UPPERMOST MANTLE

40 Fig 2 Cross-section through the crust of northern Britain based on the LISPB profile and the interpretation of Bamford et al [3] Inset shows schematically the upper crustal structure of the Southern Uplands where alternate cross-hatched and black bands are Lower Palaeozoic sedimentary wedges - the direction of imbrication is indicated - g n d the area with plus signs is a gramte intrusion For further details and discussion see section 4 3 of the text (The key to this figure has been modified from that of Bamford et al [3] to be consistent with the fact that pre-Caledonlan basement has not been posltwely identified from the southern Caledomdes )

and all the evidence combines to place magma genesis in the upper mantle or crustal underplate [6] (see also section 3 1) Group 2N granites rose through Proterozolc (Lewlslan) basement and developed magnetlsed root zones whereas group 2S granites rose through crust devoid of such basement (see also sections 4 3 and 5) The contrast in filtered magnetic anomalies for group 2 granites north and south of the Iapetus suture zone of mainland Britain is a strlkang feature of Table l which underlines the contrast in crustal structure and composition deduced independently [2,3,8,9] These observations throw into sharp focus the group 2 intrusions of the Southern Uplands which are considered in relation to the trends now established either side of this region

4 3 Gramtes and structure tn the Southern Uplands The three major granite bodies in this region intruded greatly tilted and thickened sedimentary sequences which have been Interpreted [36,39] as obducted prismatic oceanic sedimentary wedges overlying basaltic ocean crust (see schematic insert to Fig 2) The intrusions are of the younger, group 2 variety with Sduro-Devonlan ages and large negative residual gravity anomalies [9,18] However, they lack the prominent magnetic signature of their Northern Scottish counterparts and are of distinct southern character (Table 1) This suggests that the major break in Caledonian crustal structure occurs at about the Southern Uplands Fault and it is tempting also to

76 place the Iapetus suture in this location (cf [6,10,11]) To the north, it IS known that Lewlsian-type basement probably extends beneath the Midland Valley [3,37] and perhaps even further south in central Ireland [44], but the precise position of the ENE-WSW-trendIng suture has been debated The LISPB seismic evidence ([3], Fag 2) confirms a major break in crustal structure near the Southern Uplands Fault but stratlgraphlc evidence now favours a suture some 70 km to the southeast through the Solway Firth [1,36,38] The seismic cross section indicates that Lower Palaeozoic volcamclastlc materials thicken at the expense of basement from about 5 to 15 km moving south across the fault "Basement" seismic velocity also falls to the south and, more important, the sharp Moho reflections of northern Scotland disappear under both the Southern Uplands and northern England causing considerable uncertainty in the nature and thickness of the lower crust Regional gravity and magnetic analyses [9] also support this structural break position but the Imbricated Lower Palaeozoic sequence seen at the surface in the Southern Uplands comprises pelagic shales overlain by strongly dlachronous greywacke fans which prograded to the south [36] These sediments were successively obducted as prismatic wedges (Fig 2), analogous to those of active subductlon zones, along the northern Iapetus margin as subductaon activity intensified to the southwest [38,39] It follows that although the prominent structural break In the British Caledonldes occurs along the northern margin of the Southern Uplands suture zone, final welding probably occurred along the Solway hne Also by analogy with modern subductlon zones, we anticipate that the 5-km layer of uncertain basement material underlying Lower Palaeozolcs on the seismic cross section (Fig 2) is metabasalt of ocean crustal affinities [36,38,39] It should be emphaslsed that this is no more than a logical suggestion consistent with the termination of recognlsed lower continental crust at the Southern Uplands Fault and the notion of southward sedamentary progradatlon onto former ocean crust, now trapped in the suture zone We conclude that the basement to the north of the Southern Uplands Fault comprises Lewislan and MorarIan rocks [2,8,29] and that the suture zone comprises entirely Palaeozoic material (as outlined above) To the south, from the former Irlsh Sea landmass to

the south coast of Britain, most of the basement comprises late Precambrlan to early Palaeozoic Cadolnlan rocks formed in the period 700-550 Ma [4,34,45] Few relics of older continental crust are recorded in southern Britain (except possibly for the Rosslare complex [45,46]) and convincing evidence for earlier Proterozolc rocks to the south of the Iapetus suture is confined to continental Europe [4] The entire Caledonian continental crust of southern Scotland and northern England, between these Proterozolc continental forelands, may have accumulated by underplatJng, sedimentation, volcanism and granite intrusion of an ocean crust foundation during the period 700-400 Ma in back-arc and marginal basins No basement older than Cadomlan and Caledonian is required in this region and no clear evidence for such basement is available As in northern England (section 4 2), the Southern Uplands granites intruded Lower Palaeozoic sediments after closure and their strontium and lead isotopic characteristics record a derivation in "lower crust or upper mantle" with "some involvement of Lower Palaeozoic sediments" [12] In terms of the model developed here, the primary melt source, like that of the group 2N granites, was either mantle or crustal material whlch, at the tame of emplacement, was of recent mantle derivations Similar modern granites, Intruding immature "geosynchnal" sediments with no ancient crustal foundation have been recorded behind the Pacific subductmg margin of southwest Japan [40] and it seems that thickening of sedimentary cover over ocean crust (Fig 2) may have provided the necessary therlnal insulation for melting to be Initiated [10,11]

5 Conclusmns Caledonian orogenic events had widespread lmphcanons for the geological evolution of many continental regions bordering the modern North Atlantic Although the present study of associated igneous events is confined to a small, but typical part of the orogen, which contains the ancient plate suture, the spacetime sequence of plutonlc events gives an insight into both magma genesis and the development of crustal structure It also provides a basis for comparisons with other regions In the orogen which may receive

77 similar attention in the future For example, there are indications that the major group of 440- to 360-Ma granites of northeastern Newfoundland [47] and those of Atlantic Canada and New England [48,49] may yield similar time-space variations, though they were emplaced in regions of variable crustal deformation [48] and with decreasing ages further southwest in accord with the hinged closure of Iapetus proposed by Phillips et al [1] Specific conclusions based on the geophysical and isotopic data for the 30 granites discussed here from mainland Britain are as follows (1) An older pre-Sxlurlan, largely syn-tectonlc, granite suite is defined in northern Scotland (group IN) with small gravity and magnetic anomahes which reflect gross similarities to the host metasedlments from which they were probably derived by anatexls These granites rehct Proterozolc zircons and have strontium isotope ratios consistent with a crustal source although deeper melt initiation cannot be excluded Granites of similar age in the southern Caledomdes are volumetrically small at outcrop (group 1 S) and reflect a mantle derivation either directly, or Indirectly, via a subductlon system during Caledonian times (2) A younger, Slluro-Devonlan suite (group 2) IS recognlsed by an association with large negative residual gravity anomalies caused by large intrusive volumes and, in many cases, slgmficant aeromagnetlc anomalies These granites have concordant U/Pb ratios, low initial 87Sr/S6Sr ratms and were generated by p a m a l fusion within a subductlon zone environment, or within Caledoman continental underplate, followed by some contamination during ascent by older continental crust or its erosion products - the Lower Palaeozoic sediments Most of these granites were intruded in a tensional post-tectonic regime following Iapetus Ocean suturing (3) Granites of the northern and southern Caledonldes have differing characteristics Group 1N Intrusions bear the imprint of crustal isotope ratios in most cases whereas group 1S Intrusions have near mantle ratios (see 2 above) More important, the group 2N granites have associated deep aeromagnetlc anomahes which do not characterlse group 2S The group 2N anomahes may have resulted from metamorphism of lower crustal ProteroZOlC basement during ascent but, whatever the cause, their absence from the southern region testifies to a

fundamental difference in crustal properties either side of the suture (4) The Southern Uplands intrusions are of group 2S character Although this evidence corroberates the regional geophysical and seismic breaks m crustal continuity at the Southern Uplands Fault, a model is favoured whereby the Iapetus ocean sutured along the Solway-Shannon line leaving thickened Lower Palaeozoic sediments resting on an ocean crustal foundation In southern Scotland and northern England All the group 2 granites were intruded after the closure event when fusible ocean crust and/or continental crustal underplate became thermally insulated by the thickened sedimentary cover (5) These observations support a view of crustal structure [3,4,34,45] which involves Proterozolc basement north of the Southern Uplands Fault with Cadomlan-Caledonlan granmc and volcanlclastlc materials resting on former ocean crust to the south and extending beneath much of England (6) The Foyers and Strontlan granites both have exceptional geophysical characteristics but these are different in the two cases

Acknowledgements We thank R S Thorpe, I G Gass and J A Plant for constructive criticism of an earlier manuscript draft C A L acknowledges the receipt of an NERC studentship

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