REE patterns of the metasedimentary rocks of the Land's End granite aureole (southwest England)

Chemwal Geology, 35 (1982) 265-280 Elsevier Scientd'te Pubhshing Company, Amsterdam - Printed in The Netherlands

265

REE P A T T E R N S O F T H E M E T A S E D h M E N T A R Y R O C K S OF T H E LAND's E N D G R A N I T E A U R E O L E { S O U T H W E S T E N G L A N D )

P MITR, OPOLI LOS

I.Inwersttv ol' A thens, Deparlment of Mineralogy and Petrology, TT621 Athens (Greece) (Received Aprd 22, 1981 ; revmed and accepted October ~~, 1981)

ABSTRACT

Mdropoulos, P., 1982 REE patterns or Lhe metasedlmentary rocks o1' the Land's End tffand.e aureole (southwest England). Chem. Geol., 35 265-280 Rock samples representing the vat,ous mel,asedimentary hornl'elsea of the Land's End granite aureole were collected and analysed for major and trace elements by X-ray Iluo re~ence analysts and for rare.earth elements (REE) by instrumental neutron activation analysis. These hornfelses show dmtmct,ive REE patterns highly enriched in light REE relative to the heavy. R EE Their R EE content has also been increased relatwe to the REE contenl o1' the unaJtered sed=ments due to the introduction of REE from the granite by

the act,on of hydrothermal solutions The R.EE patterns of the metasedimentary aureole rocks show vaxymg Eu anomalies, indicating unstable P - T conditions during the forms t=on ol' these rocks.

INTRODLICTION Det,aded geolo~cal, pet, roloL~cal and geochemical studies of the various met, a.rnorphie'-met,asomat, ic hornfelses, occurring in the Land's End ~anit,e aureole, soul,hwest, England, have already been undertaken by marly geologists. According to these studies the mineralogically, petrolot,dcally and chemtcally different, aureole rock types are regarded t,o have been derived by metamorphism, coupled with metasomat, tsm of IL~meous a.nd sedimentary rocks caused by Lhe i n t r o d u c t i o n of the wanit,e. However, controversies still exist, of the way Lhe various rock types of Lhe aureole have been formed. The compllcaLed structural geology of the area, the int, r o d u c t l o n of various ele ments from the ~anit,e Lo the aureole rocks by the action of hydrot,hermal solut, lons, the retrogressive acLton of the h y d r o t h e r m a l solutions under unstable P-"[' condlLions, are some of the main factors which have comphcated the problem of the genetw relat, mnshtp between the various aureole rock types. An a t t e m p t is made by this work to use the rate-earth element (REE) geochemmtry t,o trace possible genetic relationships between the different, rock types occurring in the Land's End wanit,e aureole.

(.)009 2541/82/(.)000--0000/$02 75 ,.') 1982 Elsevier Scientific Publishm8 Company

2~;I;

GE(.)LOGICA L SETTING 'T'he Land's End peninsula Ires in t,he suuthwest,ern part, of C,urnwall, England. II, is I'om~ed by a mass of I.~'anit,e surrounded by a narrow hell, of met,a.muq.'~hosed and metasomat, tsed basic tgneous and sedimentary c o u n t r y rocks sLdl exlst,mg at. various pomLs along tlhe co&~t. Ime ( F,g. 1 ).

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F'c,r lh l_edd~, rl 4 F'rl~.,:,t',:, l.i.~v~. ~ ,_q3rll_l"=., F-rlLJ ~ F'r~3nh %ind,-,

FtR I C,eolog,eaJ sket,eh map or t,he Land's End urea, sout, hwest, England, ~howmg t,he sample Ioeat,,ons

The Land's End 6n'a.rut,e ts one of t.he six great, bosses of ~antt,e uccurrmg in sout, hwesLem England. Bot, t, and Scot, t, (1964), usmg geophysical met,hods, have found t,hat, these exposed kn'antLes form cupolas on a large mt,erconnect mg granlt, e bat,holtt, h t,hat, exLends downwards t,o adept, h of at, least, l 0 km and has well defined roof and out, ward sloping wall reiGns. The age of t,he bat,holll,h has been round l o be ur 2 ' 7 0 - 2 9 0 Ma (Dodson and Rex, 19'71) 'T'h~ kn'antt,e has suffered mternal K met, a,somat, lsm as mdtcal, ed by the devel opment, of large K feldspar ~.'ry.st,a.ls in the ~antt,e as well as m t,he pelttlc xenoltt,hs found in t,he L~'a.ntt,e masses (Exley and St,one, 1964). Secondary alt,erat, tons of t,he ~anit,e, namely tourmalmizatton, ~eisenmg and kaoltnlza [,l(-)rl also o c c u r .

'The basic igneous rocks of SW Cornwall constst, uf dolerit, tc m&,~ses In t,rudmg Irreg'ularly t,hrough t,he sedtmentary rocks, dolerit, k' sdls mt,erbedd~d wlt, h t,he sedtment,ary rocks and spiliLic pillow lava,.~ All t,hese basic bodies

267

are unquestionably of mag'matlc orl~n, having metamorphosed the sedimentary rocks (Flett, 1903). The basic rocks uccurring in the Land's End bq'anit,e aureole show metarnorphlc--meta.~omat, lc a.lt~=ration by the granite, so they are of pre-g]'antte age. Get)chemically, t,hey seem t,o be representatives oiº continental aika.lme bas',,dts ( Floyd, 19'72, 19'76). The sedimentary rocks o1" SW Cornwall are sandstones and slates o1' the Gra.mscaLho series and fine grained sdtstones and slates of the Mylor series. The Gra.mscatho beds are of Middle Devonian age (Hendriks, 193'7). Strut tura.l studies in the Porthleven area (Stone and Lambert,, 1956; Stone, 1966) rod=cared t,hat, the Mylor beds are younger. Therefore the Mylor sedLments seem t,o be of lat,e Middh-, Devonian to early bate Devon=an age. Dodson and Rex (1971), using K - A r dating, ilave determined the age of these two series to be 365-3,45 Ma. Bot,h the Mylor a.nd C,ramscatho aeries have undergone repeated lºoldmg and I'ault,tng which are associated with S W - N E structural trends due t,o late I')evoma.n and late Namurlan events and t,o the mtroductlon or the granite bat,hollth (Dearman, 1969). 'The rlne-gramed ar~qllaceous sediments assigned to the Mylor series occurrmg in the Land's End gramt,e aureole have been converted to various t,ypes of hornl'elses. K h = l (19,72) applied Q.mode cluster analysis and R mode I'act,or r:malysis on t,he basis of t,wenty-three ¢najor and trace elements to group the met,asedlment, ary. rocks o1" t,he aureole. He recot.,mlsed t,he I'ollowing four major zones ot' metasomatic alteration or deposition (Fig. 2): (a) Zone I, Ires within 8 m I'rom the grantt,e cont, act and includes highgrade metamorphosed sediments a,s well ,'~,~blot, it,to rocks and veins which were formed by the act4on of the late-stage hydroth~rm',d solut, iuns. The prmclpa.I mineraJs (.~1"thts zone are btot, lte -* and'aluslt,e -* K-feldspar. (b) Zone II, lies I'rom 8 t,o 21 m lºrom the gramt,e contact. It, includes moderately metamorphosed sediments and It,s predominant mineral is blot, ire i t ) Zone III, lies I'rom 21 to 36(.) m I'rom the 6~anit,e ,'ontact,. Its prmctpal minerals are blotlt,e and feldspar and, as in zone II, it, mcludes moderately met,amurphosed sedtment,s. DISTANCE Z O N E K-feldspar Biotite AndalUSlte No-feldspar Ca feldspar Quar'i z

Om

8rn I

21m II

.'t6Om lit

IV

L

IJ

Fig. 2. Miner',dOL,P/ of t,he four major zones of the mel, asedimenta.ry rocks of I,he aureole, arter Khan ({9'72).

26~

(d) Zone IV, lies more t,ha~ 36(J m from t,he grm~lt,e contact and It, it1 eludes silncnt'ned sediment,s The rocks of all t,he above four zones lying near hydroti~ern~a] veins are spot, ted and aJtered to kaolmnt,e and sent=re bearing rocks by hydrothermal acl,ivity. SAMPLING

A N D A N A L Y S I S (.IF T H E M E T A S E D I M E N T A R Y

R(')(."KS

The sampling scheme applned =n t,he area was formulat,ed t,o cover aJl t,he t,ypes of metasediment,ary rocks occurnng in the four metasomaLnc zones described by Khtm (1972). The collect, ion of Lhe samples was ext,ended t,o different, IocaJlt, leS where the met,a,.,~edmlent,ary aureole rocks occur (Fig. 1 ), in order to show a generaJ pal, tern ol' possrble variation. Samples of unmeta morphosed sednments collect,ed out,side Lhe aureole were also analysed t,o serve as a basis for comparison. Multnple samples were collected I'rom each of t,he rneta,.~ornat,ic zones, be<.'ause l,here IS a considerable varlat,ion even wit,htrt o n e z o n e al"id it, ts d i f f i c u l L t,o f'tr'lql t,h e m o s t , r e p r e s e n t , a L i v e sample by ma,'rq~ scc~pnc examinat, l~n alone. A Lhin sect, ion was prepared from each sanlple for mncroscopic mmlysis (Table II. The most represent, at,lye samples from each "['ABLE I Lot'ahl, ie=,.,and mineraJoglcal deseript.|un o f t.he an',dysed roc'k samples Sample No

DeseripLion

LoeaJit.v

M,neraJoRy

F!4,"p F!~9 Bq,'d

unaltered sednn,ents u n a l t e r e d sedlment,s at~dalusnte bearing ho~nl'el,s

(.]odretw Ponill, Praah Sands Porl.h L e d d e n

q u u r tz--s:erncn I.e - t ' h Iorn t.P a ndaJu~nte--bnot nte --plagnocla.se

B6 I

at=d',.gusit,P bearing hornl'els

Priest's Cove

andalusnt.e-l)uJt.nte--plagnucla..,~e

B64

el'~rdierll.e biot.d.e hornfels

Priesl's ('~Jv~

B,'p(/

cordterit,e biol,ite hornfels

Porth L e d d e n

B'2Y BL~

bnol, nl,e h o r n l e l s silicil'ied sedimer|l..~

The Crqpwn T h e Avarack

B4!J

serieih-, hornfpls

Purl.h L e d d e n

quartz-muscovtl.e-biot.nte

B6,'I

sericil.e horn felx

Priest,'~ C o w

--rhlornte~paque q uarl, z --m useovi t e - biol.n t.u -~.'hlord.e--opaque

A:?!-/

I'me granned g'rannl.e

Land's End

qua.rtz-feld..,,par--muscovnte

St. Ive,~

bnot il.e--t.q ~urm aline, f sch orl =t.e) hor rnblende ~Pvrc~xene (remains) - ' p l a g i o c l ~ u - i l m e n i t e (-~sphene)

quarl.'z.--ser,cnt.e--~'hlorlt.e "--'l'n UbC'( W I t,e

--~'or'u ndu m cordneriLe( ~ pin nl.e) - b n o t II.e - q u a r i.z (venniet.~) cord=erRe( ~ pnnnt.e )-boot.ate --'plagi(=elase b h J t i t e - - q u a r t . z - u p a q ue plagnc,'la.,~e--chlornl.e-bnotite -~quart.z ( vemleLs)-x~paque

-

AL!,'p

lea.st altered clLdernt.e

269 zone were selected, on the basis of the microscopic analysis and of the major-element ctlemistry, to be analysed by =nstrumental neutron activation analysis (INAA) for the REE. The samples were analysed for the major and tile trace elements (Table II) and for La and Ce from the R.EE (Table III) hy X-ray fluorescence (XR, P) analysm, using a Phdips'"' PWI450 automatic spectrometer wit,h a PW/466 sLxt,y-posd, mn sample ~'hanger. The method used here was essentraJly that, described by Leake et at. 11969). For the major elements samples were ex cited using a Cr.target X-ray tube. Per the trace elements Mo.t,arget and W target X-ray tubes were used. Corrections were made for the overlap of R,b.Klj on Y.K,~; for Ce Lt~ on Ba LI.~;for Y-K~ and Nb-K,~ on the Me h',~ Compton peak, for Sr-Klll,a on Zr Kc, and for the target c o n t a m m a t m n on Ni, Cr and Zn. Cr/V is quoted as it, is not possible to remove the effect of the overlapping V-Ltj peak. Cr values are therefore higher by - 6% of V ppm. W.L0 and Me K~, Compt, on peaks scattered t)y t,he sanlples were used to give correcttons for total mass absorptaon for emission lines shorter than the Pe absorption edge. Details on the instrumental conditions, the callbrabon and the precmion and accuracy or the method are given in Mitropoulos (1979). Per REE analysis INAA was used. The analyttcal procedure for INAA used m ties work is more or less analogous to that described by Gordon el, at. 1968), Hertogen and Gtjbels ( 1971 ), Plant, et at. 11976), Jacobs et at. i 19771. Sets of seven samples (unknowns and standards), were irradiated in the Herald reactor at, the Atomw Weapons R,esearch Est,ablmhment, Alder mast, on, at, a t h e r m a l n e u t r o n l l u x o f 2 . 5 . 1 0 n - 4 . 0 . 1 0 ~ n c m ~ s l f o r , - 18 h r The total integrated dose received by t,he samples was .-. 1.0"lO ~r' n c m " . The ganlma ray spectra were measured by the use of an Ortec °-') low-energy hthium-drifted germanium spectrometer ( G e - L i detecLor), mounted horizontally. The spectrometer is connected vm an Ort,ec *~cryogenic pre-amphher to an Ort,ec"-" amplifier and finally to a Northern Sclent, lhc" NSTO0 mult, ichannel analyser of 2048 channels. The multichannel analyser was set, to c o u n t between 75 and 220 keV. For the ident, lfication and integration of the analytical photopeaks a comput,er program was used, written by R.A. Gunnmk and coworkers at the Livermore Laboratory and modified for use on t,he ICL"-" 1906 compuLer of the tlniverstty of Birmingham by K. R andle. I~l Table III the analysed elements, the analytical photopeaks used, as well as potential interferences are Imted ( Leclerc and Cornu, 1974 ). The interna tional reference samples BCR,-I and NIM G and the departmental reference sample BOB were used as standards. BCR,-/ and BOB were used ['or the sam pies wit,h low La and Ce values, found by XRP whde NIM G was used for the samples with high La and Ce values. The accepted vaJues for BCR, 1 were taken from Flanagazl (1973) and Hooker eL at. (1975), and for NIM G from Jackson and Strelow 11976) and Abbey (1978). In general, motope d d u t m n values were preferred (Table IV ).

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2'i 2 TABLE IV Acceptnd value.,, for t.he reference samples used in I.he present, work (ppm) BC,R I

NIM G

BOB

Sm Nd Eu

6.67 29 2 1.89

lb b 73 U 0.39

(']d

6.75

1(1.9

Tm Tb

06 I ()

2 1

35 I() '7 I 19 4 I) () b7 0 70

Yb

;I 47

13 3

2 h9

bu

() 5h

2 I

0 3~

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TABLE V

Abundances of the R EE (ppm) in the international reference samples M R G compar,,d with determinations by other workers Sample

Nd

Sm

Eu

MRG I

16 18 16 20 4

4.46 1.21 U 14 ().b3 -- (1'79~ 4.2 I 3 ()tlgb 0.46 . . . . 50 1.1 0 14 . . . . 6 ,-I ] 3 0.0b 0 h 0.3 I h I l) '1

JB /

296 b,-13 I.,19 32'1 b.2 I '7 24 8 4.8 1 52 I 5

Tm

-

. .

Th

Lu

Gd

Yb

I and JB I

References

p,esenl, work Jackson andStrelow(1976) Sen Gupta { 1977) Pal and Torrel (197H)

1 IN U27 ,4'73 2()9 ~ present work 1 3 (129 Sehillingetal (19'77) . . . . A n d o e t a l . l'1974) . . . 3.H 2 8 Pal arid Terrel (19'18)

* R.NAA values.

T h e accuracy of the m e t h o d wa~ t,ested by a~lalysmg two international reference samples, J B . i and MRG /. In Table V, the results o f these au)alyses are given along with the results o f published analyses from oLher laboratn nes, showing I'mr a g r e e m e n t The precision o b t a i n e d for all e l e m e n t s was f o u n d t,o range from 4.3 t,o 1().9%, expressed as relative standard d e v t a t m n . Details on the precis=on tests are given m M i t r o p o u J o s ( 1 9 7 9 ) . A d e t m l e d d e s c r i p t i o n o f the m e t h o d ul" I N A A used is also gwen In M i t r o p o u l o s ( 1 9 7 9 ) . REE PATTERNS

In T a b l e VI the R E E wdues are given f o r tim m e t a s e d t m e n t a r y rock sam pies anaJysed by I N A A a l o n g w i t h the R E E values for the u n a l t e r e d sedimen. tary rock samples F95 and F99, the u n a l t e r e d d o l e r l t l c rock sample A25, and the g r a m t e rock sample A29. T h e last f o u r samples were used as the basis for c o m p a r i s o n o f the R E E p a t t e r n s o f the m e t a s e d i m e n t a r y rocks

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Fur the present, aLton uf the dtsLributton uf the REE tn t,he different, ruek Lypes various graph teal patterns have heen proposed. The corn m on cunven l,iun is t,o plot relat,tve abundances el' the REE against, t,hetr at,omlc numher. In gerler',,.d, normalization tu l,he abundance of the correspondlr'ig elemerlt in ~'hondrttes is used because no l'ractionatturl between the llghl,er and the heavier R,EE has been uhserved in these meLeurit,es (Schmii, L el, al., 1963; Hazktn el, aJ., 1966). For t,he s e d m l e n t a r y rocks norm'altzatton to an average North American ShaJe c o m p o s i t e (Haskm el, ',.d., 1966) has been used by some workers in parallel with Lhe n o r m a h z a t l o n to c h o n d r i t e s Norrna]iza. t,Jon uf the REE content, of the r'ut'ks of a rock suite tut, he ro~'k wtth the lowest REE cont, ent. has also been used (Sun and Hanson, 1975) in o r d e r to :.;how t,he variation of the REE ahundarwes wlt, hm the rock suite. For t,he 4'unst, ructton o f t,he REE patterns in I,he present, work nurmaJizuttun uf the REE a b u n d a n c e s tn t,he analysed rock samples t,o the REE abundarlces In ~'hc,ndrltes given by Frey el. al ( 1 9 6 8 ) was used in o r d e r that the REE pal. [,erns of the sediment, a~'y roq'ks carl he c o m p a r e d [u t,hose of the tt,meous rue'ks. It must, he nut,ed that Yh vaJues of the samples F99 and ["t49 are low, :~(.>t,hey have heen left, out, el' the REE pat, tenls (sue Fig. 4 B). It, ts possthle that t,hese Yb values are as low as the lower Itmtt, el' detect, ton of this element, for the met, hod ul" I N A A used (Mit, ropoulos, 19'79). The analysed rock samples uf sedimentary origin are classified according to the vartous met, a,sonmtle zones proposed hy Khan (1972) c~n the hasts uf theLr mlner,':,logy and meier-element chemtst, ry. According to t,hat, classtl'lca t,ion t,he REE pat, tenls of the analysed sediment, ary rock samples were kq'ou ped as follows: (a) Unaltered sedlrncnls The rock samples F9,5 and F99 ,~t' unalt,ered Mylar sediments ¢~olleet,ed from outside the granit,e aureule were an',.dysed fur t,he REE t,u serve as the basis for c o m p a r i s o n . TheLr c h o n d r i t e normalized REE pat, terns a p p e a r to have a templet,ely d i f f e r e n t shape than the REE pat, terns uf both the basle rocks and the t.,,rml~t,e (Fig. 3). 'They show a hiRh en r l c h m e n t In Itght FLEE lelat, Jve to t,he Ileavy R.EE and they d u n u t SllUW any Slt,mlt'tcaJlt e u r o p i u m anom.',ly. The e n r i c h m e n t In light, REE relative to the heavy R EE ts unaJogous Lo t,hat found ul the Aust, ralla.n post, Arehaean sedt rnents hy Nance and Tayh:)r (1976), but, these AustraJmn sedlment,s show rlegat, i,,~e europium al1(JmaJles. The ahserlce of' a.ny sikmil'icant europium anom ',.dy here may be uLt,rlbut,ed t,o the nat,ure o f L,he parent,~JI reeks from whtch tile Mylur s e d m m n t s were originated ( Nance and T a y l o r , 1976, 1977 ). (hi Neat" cot~tact, high grade nzelanzorphtc sedtments (zone I) The met,a selbment, ary rock,.; uf t,hts zone show a high tm:rease in AI whwh is largely due l,u the d e p l e t , ) n uf sdtca, because of the relative t m m o b t l l t y of AI tn peltt, tc rocks undergoing eat, ran e x c h a n g e react,,:ms (C,armlchael, 19691 'The mcrea~e in AI resulted m t,he f o r m a t i o n o f AI rich minerals such as andalus~t,e uJatl c o r u n d u m ("l"al')le I). Samples B48 and 1361 were analysed l'or t,he R EE (Tahle Vl). They show a Im'ge increase in REE eunt,ent, oI' abe)Lit, three t,tmes t,he REE content, ul' the unaltered s e d i m e n t s T h e t r R EE patt,erns (Fig. ,4 A)

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are analogous wlt, il t,he R,EE patt,erns of t,he unalLered sediments showing high enrtchmenL in light, R,EE (L,R, EE) relabve to t,he heavy EEE (HR, EE), The R,EE pat, t,em o1" sample B48 (which shows lower REE c o n t e n t and =ts apparent, distance from the granite was larger} shows a small negative Eu anomaly (c) Moderately metamorphosed sedmJents (zones I I and Ill). The sedimen t.ary rocks of t,his zone show t,o a lesser degree a d e p l e b o n in St resultmg in t,he enrichment, in AI. Theu" main mineralogical constiLuents are biotite and cordterite. T w o c o r d m r i t e - b m t i t , e - b e a r m g rock samples, 1364 and B50 and one of banded btobt, ic sediments, B22, were analysed for REE. Their REE eont,ent is higher t,han the R.EE c o n t e n t s of the unaltered sediment,s (Table VI) but, it, Is lower than the REE c o n t e n t of the rock samples from zone [. The R,EE pat, t,erns of samples B50, B64 nad B22 (Fig. 4A) show an enrich m e n t in LR, EE relative t,o the HR, EE similar to t h a t shown m the R,EE pat,t,erns of t,he unalLered sediments. The R,EE pat, terns or t,he samples B50, B64 and B22 show a negat, lve Eu a n o m a l y . (d) Slhei[wd sediments (zone IV). These sediments are characterised by an increase m t,he SI(:), cont,ent, which m assocmLed with t,he f o r m a b o n of a large n u m b e r of small veinlet~ t h r o u g h o u t t,he roe.ks. They are also enriched in Zr,

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277 Rb, W and Ba (Table II) elements which are considered t,o have been inLroduced int, o the aureole rocks from the granlt, e. Sample B2 from Lhe stlicified sediments of t,he Avarack was ana.lysed for Lhe REE (Table VI). Its R,ER cont,ent, in sptt,e of tt,s large apparent, disLance from the granite contact,, is surprisingly high, analogous wit, h that, of t,he near contact, andalustte bearing metased=ment,s. Its REE pattern shows a slmtlar shape to that of t,he REE pat, terns o1" the unalt,ered sediments but, it, has a high negahve Eu anomaly ( Fig. 4 B).

(e ) Late-stage hydrothermally altered sediments. All t,he metasedimenLary rocks o1' the aureole have been afrected by lat,e-st,age iwdrothermal solut, ions. They show hydrot,hermal alteration such as sermit, izaLion and argilhzaLion. Two rock samples (B49, 1363) of sericlte.bearing hornfelses have been analysed for the R,EE. They show the same level of R,EE content, (i.e. two times the REE cont, ent, of the unaltered sedim'ents) as t,he rock samples from the zone of t,he moderately met,amorpilosed sediment,s along with which they have been found. They also show similar R,RE patterns with a negative Eu anomaly. It, seems likely that t,he R,EE charact,eru,~tics of these serictt,e bearing homfelses have not, been affected by the act,ion o1" the secondary hydrot.hermal solut, lons (Ftg. 4 B). That, probably meaJls that, the secondary hydrothermal solutions had low REE content, t,o affect, the R,EE cont,ent of t,hese hornfeL~es at, any slgmficant level. That suggest,ion ts supported by t,he very low La, Ce and Y values found by XR, F analysis for t,he aplit, lc veins (Mit, ropoulos, 1979) occurnng at vartous places wlt,htn the aureole. DISCLISSION AND CONCLLISIONS The analysed rock samples of sedimentary origin from the Land's End granite aureole show distinctive characterlst, ics in terms of REE geochemlst .ry which can be used to distinguish the aureole rocks of sedimentary origin from those of il~meous origin and from the granite (Fig. 3). These characteriStlCS are: (a) They have similar chondrite.normalized R,EE patterns (Fig. 4A and B), showing the same degree o1" enrichment In 'all the R,EE which is shown by the sufficiently stable La/Sm and La/Tm rat,ios (Ftg. 5). This probably means t,hat t.he met,amorphism tt,self has not, modified at any s~gn=r~cantlevel the shape of" t,he tntt, ial REE dist, ribut, ion patterns of the sedimentary, rocks of the aure,.)le. (b) They show lligh enrichment i.n R,EE content relative Lo the REE content of the unalt,ered sediments (up t,o Lhree Ltmes), giving evidence t,hat, R EE have been mtroduced from the granit,e by the action of t,he hydroFig 4 A. Chondrlle normalized R ER pal.terns or Lhe met,asedimenLs of zone I (samples B,./,~ and B61 ) and or zones II and III (samples B,50, B64 and B22) o1' the gra.niLe aureole. B Chondrit, e normalized R.EE paLLerns of the meLasedlments of zone IV (sample B2) and of the hydrothermally altered sedimenL~ (samples B49 and B63) or Lhe gramt,e aureole

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t,hermaJ solut, lons. T h e p a r ~ l e l e u r l t ' h m e n t , trl 'all t,he R BE p r o b a b l y metals t,hat, t,he LR, P,E/H R, BB rat, lo o f t,he hydrot, h e r m a l solut, tons was d i f f e r e n t t,o

t,hat, o r t,he g'rm~tt,e. T h e enrnchment, in R.P,E is htgher for t,he r o c k s o f g o n e I near t,he bq'anlt,e cont,a~'t, t,han ror t,he r o c k s o f t,he z o n e s I1 and III. T h e high ~'oncent, rat, lon o f R EE m t,he sdtclfled sedlment,s ts p r o b a b l y d u e t,o the c h a n g e s m t,he p h y s t c o c h e m t c a l condlt, tons trl Lhe h y d r o t , h e r m ~ soluLtons, which have also gtven rise t,o t,he preclpit,at, lon o f t,he silica.

((')They show an m('reaslng negat, tve europium anomaly front t,he sedl ment,ary, aureole rocks o1' zone I (near t,he Isq'antt,e conLact,) l,owar(ts t,he s,-,dl menLary aureole rocks of zone IV (ouL,er p',.u't,s or t,he aureole), I~qwng eVl dence or probable declining oxygen I'ugm'lLy ACK NOWLBD(.3 BM BN'T'S

T h e aut, h o r IS g'rat,eful t,o Mr E.D. Lacy for t,he cont, m u ~ g u i d a n c e ml<:l help d u r i n g t,he <'ourse ur t,hts w o r k , t,o Dr. G . L . H e n d r y for v a l u a b l e d a y t,o d a y a d v w e and d t s c u s s l o n on t,he a.nalyl, lcal p r o c e d u r e , 'mid t,o t,he SLate S c h o l a r s h i p s Foundat, ton o f G r e e c e ror t,he fmancl',d support,. T h e S.R C grant, for use o f trradmt, lou facdtt, t e s t s also at'knowledged.

279 REFERENCES Abbey, S., 19'78 Calibrat.tonstandards. X-ray Spe(,trom , 7. 9 9 - 1 2 1 . Ando, A., Kurasawa, H., Ornori, T. and Takeda, E., 1974. 1974 cornpilat~on of data on Lhe OSJ geochemical rel'erence samples JG I granodiortte and JB I basalt,. Geochern J , 8 : 175-192. BoLt, M.H and Scott,, P., 1964 Recent geophyst(,al studies .n S.W England In Present. Views of Some Aspects of the Oeology of Cornwall and Devon. R Oeol Soc C,orn wall, 150th Annu. Vol., pp. 2 5 - 4 4 . Carmichael, D M , 1969. C)n Lhe mechanism of prograde rnetamorphle react.tons in quartz bearing pehtl(, rocks. Contrlh. Mineral. Petrol , 20. 2 4 4 - 2 5 2 . Dearman, W R , 1969 An ouUine of the st ruct.u,al geolok,W,of Cornwall Pro('. Geol Soc London, No 16b,4,pp 3 3 - 3 9 . De Soot e, D , Oilbels, R. and Hoste. J , 1975. Neutron Act.ivat..on Analys,s. Wilev-lnt.er science, London, ,~36 pp Dodson, M.H. and Rex, D.C., 1971 K - A r ages for Porthleven and Mylorslates. Q.J. Geol. Soc. London, 126. 4 6 5 - 4 9 9 . Exley, C S. and Stone, M , 1964 The t~'q'anil.,crocks of S W England. R. Geol Soc Corn walI, C'omrnern Vol.,pp. 1 3 1 - 1 8 4 . Flanagan, FJ.. 197:] 1975values for,nternat, lonalgeochemical reference samples. Oeo chlm Cosmochlrn. Act,a , 3 7 . 1 1 8 9 - 1 2 0 0 F l e t t , J . S , 190',]. Pet.rography of West Cornwall. G.B Geol S u r v , S u m P r o g , p p . 1 b0-162, Floyd, P.A., 1975. Oeochem,stry, origin and te(`ton,c env.ronment ol" the basic and acidic rocks of Cornubm, England. Proc. Oeol. Assoc., 83 3 8 5 - 4 0 4 . Fh.byd, P A , 19'76 Geochemical var,at,,on in the greenstones of S.W. England J. Pel,rol , 1'7 b 2 2 - b 4 5 . Prey, F.A., Haskm, M.A., Poetz, J.A. and Hi, kin, L.A . 1968. Rare earth abundances in som~ ba.,~lCrock.,, J. Geophys. R es., 7:] 6(185--6()97. Gnrdon, G E , Randle, K., Goles, G.O., Corllss, J.B., Beckon, M.H. and Oslpy, S S., 1968. Instrumental activation analysB of standard rocks wmLh high resolution ), rays detec lors. Geochim. Cosmochim. Act,a, :.12 3 6 9 - 3 9 6 . Haskln, L.A , Wildeman, T R ,Frey, P.A., Collins, K.A., Keedy, R A. and H~,~kin, M.A , 196~ Rare earths ,n sedirnenl.s. J. Geophys. lies., 71: 6 0 9 1 - 6 1 0 6 . Hendrlks, EM L , 1937. Rocks, s u c c e s m o n a n d s t r u c L u l e i n S . Cornwall Q J . GeoI. Soc. London, 33 3 2 2 - 3 6 7 . Hert ogen,J andG,.ibels, R , 1971.1nstrumenLalneutronactlval.ionanalyslsofrocksw.th slow.energy phot, ondeLecLor'. AnaI. Chlrn Acts, 5 6 : 6 1 - 8 2 Hooker, P.J,(')'Nions, R K and Pankhurst, RJ., 197b. Determmat.onof R E B m LISOS standard rocks by mixed solvent ton ex(,hange and rnass.speetrometri(, isotope dilu t ton. Chore O e o l , IF~. 1 8 9 - 1 9 6 . Jack~on, P F S andStrelow, F.W E . 19'76 In S Abbey(Bdltor),SY.2, S Y 3 , M R O I Report on the collaborative analysis of three Canadian rock samples for use as tort.fled reference matermL'~,. C A N M E T Rep 76 36, 5'7 pp Jacobs, J W , Korot,ev, R L , Blanchard, D.P., H~,~kln, L.A., 19'/7 A well tested procedure for instrument',xl neutron activation analysis of s,licate rocks and minerals. J Radio anal. C h e m , 4 0 9:3-11,4 Khan. I H , 1972 (']eochpmlstry of t,he aurecde of Lhe Land's End gramte. Ph.D. Thesis, I.In.ver,,,ity , f B,rrningham, Birmingham (unpubhshed). Leake, B.H , Hendry, G.L., Kemp, A., Plant, A.G , Harvey, P.K., Wdson, J R , CoaLs, J S., Aucott, J W., Lunel, T. and Howarth, R.J., 1969. The chemical analysLs of rock powders by automatic X ray fluorescence. Chem. O e o l , 5 7 - 8 6 Leclere. J C a n d C o r n u , A , 1974. Neutron AcL,vat,on Analys.sTables Heyden, London.

oH() Md.ropoulos, P, 1979 A contrtbul,.ion to tile R E E a n d trace elementgeochem,stry o l ' t h e rock.,, ol the Land's End grand.e aureole, S W England. Ph D. Thes=t,, University or B=rm,ngham, B=rmmgham (unpubl,shed). Nanee, W B and T a y l o r , S R , 1976 Rare earth element patterns and crustalevoluLion, I Australtanpost Arehaeansedtmentary rocks Geoehim (3osmoehim. Act a , 4 0 ' 1!t:t9-1 !tl-~t Nance, W B and T a y l o r , S . R , 19'7'7 Rare earthelernenl, patterns a n d e r u s l a l e v o l u t t o n , II A.rehaean sedimentary roek.,~ from Kalgoorhe, Austral,a Geoehtm Cosmochim. Act.a, 4 1 22~:t-2;~1 Pal, ~q. and Terrel, D.J., 1978 I N A A o1' 29 tnternatmortal geochemtcal reference samples (3eostand Newslett.,'2 18'7-192 Plant,J.,(3oode,(3 (3 and H e r r m g t o n , J . , 19'76 An mst.rumental neul,.ron ael,,ivatton method for mult=elementgeochem=caJ mapping. J Oeoehem. Explor , 6 2 9 9 - 3 1 9 . Schdhng, J(3 , 197'7 Mesozoic sequencesort.hecenLral North At.lantw In P Aumento, W (3 Melson et al (Edd.ors), Inittal R eporLs ol' the D~qDP, Vol 3'7, LI ~q (3overnmenl, Printmg OfFice, Washingt, on, D (3 , pp b91 -,r'~97 Schm=tt., R A , Smdh, R H , Lasch, J E , Mosen, A W , (.)lehv, D A. and Vat~thevsk=s, J , 191,;;]. Abundances of the I'ourl,.een REE, scandium and ytl.rtum m meteoritic and ter restrtal matter (3eochtm Cosmoehtm Aet a, 27 I:P"/7-6~,2 Sen 1,3upta, J (3 , 1977 Determmatton ol' traces o1' R.RE, Y and q31 tn several mternal..tonM lel'erence samples and comparison of data with other pubhshed values (3eosta.nd Newslett., I 1 4 9 - 1 5 b . Slonv, M , 1966 Pold structures ,n I..heM.vhJr beds, neat PorLhleven,Cornwall Geol MaR , I(.)2~ 440-.-161,). Sl,one, M. and Lambert, J.L M , 1966. Lqhear I'old,ng ,n the Mvlor slates, near Porthleven, Cornwall (3col M a g , 9 3 ; t 3 1 - 3 3 b Suwt, S S and Hanson, O N , 197b (.')rtgm o1' Ross Island ba.,~an,totds and Itmtl,.attons upon the heterogvnetlv of mantle sources for alkali ba.salts and nephelmltes. Contr,b Mmeral Petrol., b~' 77-1(16