FERNANDOCORFU,PAUL E.B.ARMITAGE,!<AREKULLERUD& STEFFENG.BERGH NGU-BULL 441,2003 - PAGE 61
Preliminary U-Pb geochronology in the West Troms Basement Complex, North Norway: Archaean and Palaeoproterozoic events and younger overprints
FERNANDOCORFU,PAULE.B.ARMITAGE, KARE KULLERUD&STEFFEN G.BERG H
Corfu,F.Arm itage,P.E.B., Kullerud,K.& Bergh, S.G.2003: Preliminary U-Pbgeochronolog yinthe WestTromsBasement Complex, NorthNorway: Archaean and Palaeopr oterozoic eventsandyoungeroverprints. Norgesgeo logiskeunder- sekelse Bulletin441,61-72.
Thispaperreportsnew U-Pbagesforzircon and titaniteinfour magm aticunitsof theWestTromsBasemen t Complex.A granite, presentas a megalenswit hin theNNW-t rendi ng SenjaShearBeltatthe sout hwestern tip of Kvaley a,yieldsazirconageof 2689±6Ma show ing that theunitis anintegralpartof theArchaeanbasement inthe regionrather than aPalaeop roterozoic intrusion.By contrast,aPalaeopr oterozoic origi nof theErsfjordgraniteis wellconfirmedby azirconage of1792±S Ma.Thisage shows thatthegraniteis coevalwith themaingenerat ionof mangerit es in Lof ot en and form edduringoneof thedominantplutoni c episodesacrosstheBaltic Shield.Zirco nin a smaller, late-synk in ematicgranit ic dyke at Otervika yields 1774±5Ma,indicating thatdefor mation alongmajo r NW-SE-striking,block-boundingshear zonesoftheSenja Shear Beltwasinthe waningsta gesatthistime.Tit anit e agesof about1770Mawerefound inthreedifferentsamples.Two ofthemalsocontaina secondary titanite gener- ationformedatabout 1755 Ma,indicatin gprolongedepisod es ofmeta mor phic resettin gandlow -temp eratu re new growt hand/orrecrystallization mostlikely during the waningstagesof Svecokarelianshearing.Onetitan ite popu-
lat ionwas st ro nglyaffecte d by anenigmat ic Neopr ot erozoicdisturb ance.Oneof the sam plesalso contai nsU-rich
accessorymineralswhich yield Carbon iferous ages,probablyreflect ing fluidactivity duringpost-Caledonianext en- sional faulting intheWest Trom sBasementComp lex
F.Corfu,Instituteof Geology,University of Oslo,PB 1047Blindern,N-03 16Oslo,tel:2285 6680, e-mail:fernando.[email protected]
P.E.B. Armitag e,NRI Department of Earth &EnvironmentalSciences,Universit y of Greenw ich at Medway,Chatham Mariti me,ME44TB,Englan d(formerlyUniversity oi Tromse),K.Kullerud&S.G.Bergh,Department ofGeology,University of
Iromse,N-9037Trom se,Nor way.
Fig.1.Sim plified geologicalmapof theBalt ic Shield.
Introduction
Precam b rianbasem en tis exposedalo ng the coastof nort h- ern Norw ayon a chainofislandsstretching from southern Lofot en to Vanna in WestTroms(Figs.1&2).This basement comprisescrustal blocks madeup of migmatites,granito id rocks and greensto nebelts,atleast inpart ofArchaean age, allmetamorp hosedand deformed to variab le deg rees and int rud ed by widespread granitoid plutons between 1900 and 1700 Ma.The various blocks are separated by major shear zones.In contrast to basem ent units exposed along the coastofcentral and sout hern Norw ay,this crustalseg- ment received onlya weak metamorph icoverpri nt during the Caledonian orogeny. It s role during the Caledonian orogeny remains,therefore,somethingof a puzzle. Although there are severallines of evidencesuggesting that it is sim- ply the autoc ht ho no us extension of the Fen noscandian Shieldemerging from beneath the Caledoniannappe stack (Griffin et al. 1978,Olesen et al. 1997). therehavealso been suggestions that it may be in a parautochtho no us to allo chthono us position (e.g., Brueckner 1971, Dallmeyer 1992, Mot uza 1998).The presentconfigurat ion was also sub- sta ntia llyinfluenced bypost-Caledonianextension and Late
1!..:"JlI!..' OsloRiB
I~
: , :::::
:JNeopro terozoi candPha nerozoic cover_ Caledonides
_ Sveconorwegion
~-+-+J lIB (Tronsscan dinavion
+-+.+ Ign eou s Belt)
~ Svecofennion
~ Karelian
. Archoea n
NGU-BULL 441, 20 0 3 - PAGE 62 FERNANDOCORFU, PAULE.B.ARM/TAGE,KAREKULLERUD
s
STEFFENG.BERGH- , - -
Palaeozoic to Mesozoic displacem ents between fault- boundedblocksrelatedto the openingof theNorth Atlanti c Ocean(Olesenet al. 1997).
Mapping and preliminary geochrono logy(seesum mary in Zwaan et al. 1998)have delineated the majorgeological componentsof the region, highlightingat the sametime a range of critical pro bl emsthat remain to be solved.These questio nsconcern (i)the developmen t and exte nt of the Archaean crust, (ii) the Palaeoproterozoic modifications of thebasem ent blocksby shearing,terrane accret ion , meta- morphismand extensive magmatism, (iii)the possiblecorre- lation of these blocks wit h Palaeoprot erozoic units and shear zones inthe Baltic Shield farther east, and finally (iv) the Caledonian and post-Caled onian evolution of the region. Inthis paper we makeacontributio n toward sthe resolut io nofsomeof thesequestions bypresentingnew U- Pb analyses for zirco n and titanite in variou sunits ofthe Precam brianbasement on Kvaleya(Figs.3&4).
Geological setting
The West Troms Basement Complex(WTBC;Fig. 2)comprises (i)a northeastern zone composeddominantly of intermedi - ateto mafic(tonalitic to anor th ositic)intrusive rocks under- lying Rinq vasseya and part s of Kvaleya, (ii) a dominantly granitic southw estern zoneconstit uting most of Senja and theremainder ofKvaleya(Zw aan 1995).and(iii)a networkof NNW-SSE-stri king ductileshear zones(e.g., theSenja Shear Belt) and narrow, medium- to high-grade metamorphic, partlyanatectic and mylon itic,metasup racrustal rocksthat divid ethe WTBCinto crustalbloc ks(Zw aan &Bergh 1994).
The nort heastern part ofthe WTBC consists mainlyof gneisses sub divi ded into weakly banded to nebuliti c tonaliticto anorthosi ti cgneisseswith abundant cross-cut- tingdoleritedykes in the nort hand an assemblageof inter- mediate to maficbandedgneisses, inclu ding anort hosit e,to thesout hw est. On Rinqvasseya,the tonal it ic gneisses are tectonica lly overlain by a c.28 km-long and 10 km-wi de
Fig.3
o 10 20km
! d
c::::::::::J Neo proterozoic-Com brioncover endCaledonianna ppes
----- --- - - --- - ---- - ---- ------ West Trams Basement Complex(WTBC)
1:::::1 gabbro }
1: : :::1 diorite.quartzdiorite la rgely Palaeoproterozoic
~ granite.granod iorite
~ suprac rustalroc ks (Arc haeo n+/.Palaeoproterozoic (?)):
up toamp bibolitetocles/upto granulitetoc iesandonotexis
~ tonalifegneiss /infermediate-maficbanded gneiss/dom ainsric hin granitoids (Archaeon)
_' .northerr-;e nd ' of'Lofoten -Ve s ter 6Ie;;'-d omain -rAlcha oon-
+Proterozo icjFig.2.Geologi cal map of theWest Tram s Basement Complex (WTBC).Modified from Zwaan(1995).
FERNANDOCORFU,PAUL E.B.ARM/TA GE,KAREKULLERUD&STEFFENG.BERGH NGU-BULL 441, 2003 - PAG E63
West Trams Basement Complex:
~ Caledonianrocks
Mjelde-SkorelvvatnZone
metasupracrustal rocks Ersfjord Granite
intermediate-mafi cbandedgneiss
Gratind Migmatite:anorthositictotonaliticgneiss, minor amphiboliteandbiotiteschist Kattfjord Complex:tonaliticgneiss
~~
5km
N
J ~
o
Bakkejord Diorite:diorite,gabbro,tonalite
partly anatecticsupracrustal s withyounger granite and granodioriteIhighly shearedequivalent Brittle faults(Post-Caledonian)
Ductileshearzones(Proterozoic)
~
Antiform (Proterozoic)Fig.3.Geolo gic almapofKvaloya and northernmo st Senjashowing thelocat ionsofthe analysedsam ples.Modifiedfrom Zw aan etal.(1998).
NGU-BUL L441 ,2003 - PAGE 64 FERNANDO CORFU,PAUL E.B.ARM/TAGE,KAREKULLERUD&STEFFENG.BERGH
greenstone belt composed of mafic to felsic metavolcanic rocks of tholeiit ic and calc-alkalic affinity (Mot uza et al.
2001a.b),The greenstone belt had previouslybeen consid- ered to be of Palaeoproterozoicage(Zwaan1989,1995)but recent U-Pb age determinationson metavolcanic rocks have yielded Archaean ages of 2849 ± 4 and 2835 ± 14 Ma (Mot uza et al. 2001 a),which are similarto an age of 2842±3 Marepo rted byZwaan&Tucker(1996) for a tonalit ic sample in the underlying gneiss complex. On Kvaloya, meta- supracrustal rocks occur as steep,narrow belts,the most prominen tof whichisthe Mjelde-5korelvvat nZone (Binns 1983,Armitage 1999).The metasupracr ustal rocks are of mixed sedimentary and volcanicorigin,the volcanic rocks generally occurring as amph ibo litic units,also incl udi ng ultra maf ic bodies of komatiitic affinity (Mot uza 1998,Armit age, unpublished data).These inliers, and the main gneissic foliat ion,both have a NNW-SSE strike.The supracrustalrocks were metamo rphosed at greenschist-to amph ibolite-faciescondit ions(Zwaan 1995,Motu za1998).
The central and southwestern parts of the WTBCare dom inatedbygranit ic and migmatiticrocksof variouscom- position.One ofthe most prominent bodies is the Ersfj ord GraniteonKvaloya that intrudesbasementgneissessuch as the highl ydeformed,tonaliticto diorit icGratind Migma tite andthe massive Bakkej ordOiorit e(Figs.2&3).Undeform ed, metamorphosed mafi c dykesare prevalent in theBakkejor d Oiorit e,but they are scarcein the Gratind Migmatiteto the east of the Mjelde-Skorelvvatn Zone, where they occur as highl y deformed amphibolitic lenses parallel to the main foliation.Thiszonehas aprominent NNW-SSE-st riking folia- tion with steep westerly dips and consists of meta- supracrust al rocks whi ch haveformerlybeencorrelated wit h the RingvassoyaGreenstoneBelt(Binns 1983).Thekinemat- ics of the zone indicate that it developed during a Palaeop rot erozoicprogressivecrustalcontraction and trans- pressional event(Armitage 1999,Armitage& Bergh inprep.).
The Bakkej ord Oiorit e isflankedfart her to the west by the Kattfjord Complex, which comprises 'biot ite gneiss of tonalit ic composition'(Zwaan et al.1998)and is incontact wit hmetasupracrust alrocksoftheTorsnesShear Zone(Fig.
2).Part of the complex isexposed at Oterneset/5and nes- hamn, where a late-kinematic,foliation-cutt ing granite is itself cut by later,virt ually undefo rmed,granito idpegmat it e dykes.
The 30 km-w ide Senja Shear Belt is confin edbetween theSvanfjelletShearZone(Cumbest1986)in the southwest and the Torsnes Shear Zone (Nyheim et al. 1994) in the nort heast (Fig. 2).It is defined by narrower « 3 km-wide), approximately NW-SE-st riking,anast omosing,ductileshear zonesthat enclose mega-Iensesand blocks of gneissand granitoi drocks andnarrow beltsof highlydeform edmeta- supracrustal rocks, interpreted as intermediate to mafic metavolcanic rocksand mainlyquartziticterrigenoussedi- ment ary rocks (Zwaan&Bergh 1994)commo nly associated wit h ultramafites.The rockshaveundergoneamphibolite-
faciesmetamorph ism, locally reaching granulite facies with partial anatexis (Zwaan 1995). The deformation history included two main phases:early E-W oblique crustal con- traction possibl yassociated wit h accret ionary tectonics (01) followe dbyNW-directedsini stral strike-slip translation(02).
In the Mjelde-SkorelvvatnZone,Armit age(1999) mappeda subhori zon tal folding phase (his02) post-da tin g 01, but interp retedto bepartofthe sameprogressivedeformation event as 01.His 03 isthus equivalent to the regional02 event for sinistral shearing;thi s paperfollow s the regional nomen clat ure.Some of the granito id rocks are deformed wherethey extend into theSenja ShearBelt . The fact that pegmatites andaplites arefar lessdeformed than the main granit ic bodies suggests that intrusion of the latter was mostlikely cont emp oraneous with the shearing event.
Previou sage determ inat ions in the WTBC include the Archaean U-Pb ages of 2835-2850 Ma for tonaliti c and metavolcanic rockson Ring vassoya(Zwaan & Tucker 1996, Motuza etal.200 1a)and a U-Pb age ofc.2400 Ma for mafic dykescross-cutt ing the above units(Kullerud et al. in prep).
Fart her south,the Ersfjo rd Granite had been investiga ted wit h the Rb-Srwhole-rock systemyieldi ng somew hat diffe r- entdates of 1706 ± 30 Ma(And resen1979)and 1779 ±17 Ma (Rom eretal. 1992).Andresen(1979) also reporteda highly disturbed Rb-Srarray for the Rodberghamn Granite crop- ping out on the mainland east of Senja.Two granitic to quart z-diorit ic bodies from Senj a have provided Rb-Sr isochr on ages of 1746 ± 93 Ma and 1768 ± 49 Ma(Krill&
Fareth 1984),and1822±5Ma(Lindstrom 1988).Zwaan et al.
(1998)alsolist severalunp ublished Palaeoprot erozoic U-Pb datesfor granit icto noriticunitson Senja.Oallmeyer(1992) reported Ar-Ar data for hornb lend e samplesfrom Kvaloya and Senjathat provid eacom plexrangeof datesbetw een 1950 and 900 Ma,thebest behaved of which define late Sveconorw egian(i.e.,c.900 Ma)ages.
U-Pb data
Sample characteristics and field relations
The paperreportsresultsforfive samples ofmagm aticrocks from Kvaloya (Fig. 3). Sample KV-2 represents coarse- grained foliated granit e from the southwestern tip of the island. The unitoccursas atectonizedmega-Iensformingan integ ral part oftheTorsnesShearZone along the northwest- ernedgeof theSenjaShearBelt(Fig.2).Two samp les,EG-1 and C35,repre sent theErsfjord Granit e,whichat the sam- pling localityisslightly heterogeneousinterm s ofgrainsize and abund anceof biot ite,andweaklystrained.As discussed above,thisunit played amajor rolein the intrusive history of the WTBCand itwasthereforeimp ort antto refinethe previ- ous age const raints, especially in view of the confl ict between thetwo existing Rb-Sr whole-rockdeterminations (Andresen 1979,Romeretal.1992).SampleO'I represents a foliation-cutting granite in the Kattfjord Complex at Oternes et.Thedyke wasinterpre tedby Pedersen(1997) as a syn- to late-kinemat ic (shear-ind uced) remn ant of the
Table 1.U-Pbdata,Kvaloya,WestTramsBasem entComplex
no. Mineral Weight U Th/U' Pbcom ""Pbl "'Pbl 2a ""Pbl 2a rho ""Pbl 2a ""Pbl "'Pbl "'Pbl 2a Disc. ."
characteristics' ""'Pb' mU6 2l8U6 ""'Pb' 2l11U6 mU6 ""Pb labs] rn:0
[~lgl' [ppml' [p p rn] [pg]' labs]' labs]' [absl6 [Mal' [0/0]' ~
z
KV-2:coarse grained foliated granite in Torsnes Shear Zone 0
1 zeusp--tips [34] 12 241 0.47 1.9 47379 12.771 0.027 0.50542 0.00098 0.97 0.18327 0.00010 263 7.0 2662.9 2682.6 0.9 2.1 0
2 z eusp [15] 8 229 0.48 1.5 38148 12.693 0.039 0.50323 0.00150 0.98 0.18294 0.00012 2627.6 2657.1 2679.7 1.1 2.4
8
:03 z tips[SOl 10 209 0.42 11.0 5963 12.541 0.034 0.49703 0.00127 0.97 0.18300 0.00011 2601.0 2645.8 2680.2 1.0 3.6 ,."c:
4 z tips[25] 12 328 0.41 2.1 59371 12.417 0.035 0.49414 0.00135 0.98 0.18226 0.00010 2588.6 2636.5 2673.5 0.9 3.9
:R
EG-l:Ersfjord granite c:r-
5 Z an-sbeq-s p 15 140 0.77 3.0 13761 4.966 0.010 0.31786 0.00059 0.95 0.11331 0.00007 1779.3 1813.5 1853.2 1.1 4.6 !"T1
6 Z eu-sbsp[18] 15 302 0.72 9.5 9220 4.618 0.010 0.30691 0.00059 0.96 0.10913 0.00006 1725.5 1752.5 1784.9 1.1 3.8 !'Jp
7 Zeu-sbIp 26 209 0.85 10.4 10036 4.608 0.011 0.30619 0.00066 0.97 0.10914 0.00006 1721.9 1750.6 1785.1 1.1 4.0 :0
s:
8 Zeu-sbtips[24] 19 273 0.70 8.6 11505 4.582 0.013 0.30489 0.00080 0.97 0.10900 0.00007 1715.5 1746.1 1782.8 1.2 4.3
iJ
9 Z eu tips[15] 10 226 0.74 4.4 9740 4.601 0.026 0.30486 0.00185 0.85 0.10945 0.00036 1715.3 1749.4 1790.3 5.9 4.8 !'T1Gl
10 ZeuIpfr[5] 2 505 0.66 3.9 4775 4.338 0.093 0.29031 0.00622 1.00 0.10838 0.00010 1643.1 1700.6 1772.3 1.7 8.3 £,
11 Tfrbr 139 132 3.78 2.7 889 4.702 0.0 13 0.3153 1 0.00063 0.76 0.10816 0.000 19 1766.8 1767.6 1768.6 3.3 0.1 :0,."
12 Tfr br 98 131 3.77 2.2 1062 4.698 0.013 0.31494 0.00068 0.82 0.10819 0.00017 1765.0 1766.9 1769.2 2.9 0.3
"
TlensesIbr NA c:
13 124 120 3.45 1.8 1160 4.653 0.013 0.31413 0.00071 0.86 0.10744 0.00015 1761.0 1758.9 1756.4 2.6 -0.3 r-r-
C35:Ersfjordgranite (C-99-35 ) 9j
14 Zip fr[1] 0.83 3.0 3291 8.855 0.044 0.42262 0.00204 0.89 0.15197 0.00035 2272.4 2323.2 2368.2 4.0 4.8 c:
0
15 Zeu tip [1] 0.52 12.6 2404 4.779 0.Ql5 0.31684 0.00089 0.90 0.10940 0.00014 1774.3 178 1.2 1789.4 2.3 1.0 1(0
16 Zeutip[1] 0.22 2.5 8830 4.679 0.Ql2 0.31098 0.00072 0.92 0.10912 0.00011 1745.5 1763.5 1784.8 1.8 2.5 Vl
~
OT:Oternessetgranite,KattfjordComplex
17 Z eu-sb tips [1] 1 623 0.25 369 52.4 6.364 0.197 0.34725 0.00298 0.10 0.13292 0.00431 1921.4 2027.3 2137 56 11.6 ;::rn
18 Z eu-sb tips[12] 20 271 0.55 169 655 5.330 0.020 0.31916 0.00096 0.81 0.12111 0.00026 1785.6 1873.6 1972.7 3.9 10.9 0
19 Zrim-fr[1] 2 426 0.75 1.2 14039 4.648 0.045 0.31152 0.0030 3 0.99 0.10822 0.00011 1748.2 1758.0 1769.6 1.9 1.4 OJ:0rn
20 Z euIp[1] 1 248 0.34 0.6 7362 4.488 0.015 0.30076 0.00088 0.93 0.10822 0.00014 1695.1 1728.7 1769.7 2.3 4.8 Gl
21 Z concavetip [1] 9 72 0.25 15.0 785 4.177 0.018 0.28392 0.00095 0.79 0.10669 0.00029 1611.1 1669.4 1743.6 4.9 8.6 :t
22 Tfr br 269 111 3.43 1.5 1304 4.637 0.013 0.31190 0.00074 0.88 0.10783 0.00015 1750.0 1756.0 1763.1 2.5 0.8
23 Tlbr-clrims [20] 110 19 1.51 1.8 149 3.223 0.037 0.23441 0.00072 0.10 0.09972 0.00115 1357.6 1462.7 161 9 21 17.9
24 T frIbr ch i-inNA [6] 27 12 1.26 1.4 106 2.954 0.051 0.22413 0.00113 0.21 0.09559 0.00162 1303.6 1395.9 1540 32 16.9
25 T frlbr-cl 196 13 1.25 1.6 108 2.624 0.048 0.20160 0.00083 0.12 0.09441 0.00172 1183.9 1307.5 1516 34 24.0
26 Tfrcl-lbr 115 11 1.13 1.6 76.2 1.949 0.060 0.16525 0.0010 2 0.08 0.085 54 0.00262 985.9 1098.1 1328 58 27.8
27 Tfr clNA[1] 15 11 1.06 1.2 84.3 1.969 0.051 0.16123 0.00105 0.26 0.088 55 0.00221 963.6 1104.8 1395 47 33.2 z
CAT-l:syntectonic granitoiddyke,Mjelde-SkorelvvatnZone Gl
28 zip[1] 1 100 0.17 1.1 2638 12.000 0.045 0.47538 0.00 174 0.94 0.18308 0.00024 250 7.1 2604.4 2681 .0 2.1 7.8 c,
OJ
29 T fr br [50] 95 143 0.98 5.3 498 4.616 0.019 0.31078 0.0007 6 0.61 0.10772 0.00034 1744.5 1752.2 1761.3 5.8 1.1 c
30 TlensesIbr[29) 37 113 1.07 3.5 563 4.621 0.016 0.31265 0.00063 0.59 0.10719 0.00030 1753.7 1753.0 1752.2 5.0 -0.1 rr
31 Tfrbr[35] 55 169 0.91 6.0 509 4.525 0.016 0.30799 0.00058 0.52 0.10655 0.00033 1730.8 1735.5 174 1.2 5.6 0.7 .t>
.t>
32 Tlen sesIb r NA 164 127 1.04 5.3 437 4.451 0.018 0.30408 0.00054 0.42 0.10615 0.00038 1711.5 1721.8 1734.3 6.6 1.5
33 Uor NA [4] 1 315655 0.00 350 3150 0.408 0.00 1 0.05540 0.000 14 0.91 0.05337 0.00006 347 .6 347.2 344.7 2.7 -0.9 N
34 Ubr-rd[7] 5 241135 0.00 1113 344 7 0.368 0.006 0.0505 3 0.00080 0.99 0.05288 0.00013 317.8 318.5 323.7 5.7 1.9 00
w
1 Z
=
zirco n;T=
titan ite;U=
uranium-richmineral; eu=
euhed ral;sb=
sub hedra l; an=
anhe d ral;Ip=
long-p rismat ic;sp=
shor t-p risma t ic;eq=
eq uant;in=
inclusionsof otherminerals;fr=
frag ment;br=
brown;Ibr
=
pale brown;cl=
colou rless;or=
orange;rd=
red;chi-in=
inclusionsofchlorite;NA=
not abraded (alltheothersabraded .Krogh(1982); squarebracketsind icatethenum berofgrainsforfract ions of less»
"Uthan50 grains. Glm
1 weightand concentrationsofgrainsknown toabout+1-10%.except forsample sbelow3~lgwheretheuncertaintyisabout50% ()\
) V1
Th/U mod elrat ioinferredfrom208/ 206ratioandageofsample
.
Pbc=
totalcommon Pbinsam ple(init ial+blank)s rawdat a correcte d for fracti onati on and blank
, correctedfor fractionation. spike.blankand initialcom mo n Pb(us ingStacey and Kram er s(1975)model); errorcalculate d bypropagatin gthemainsources ofuncert aint y
t degre e ofdiscordance(in percent).
NGU-BULL 441,2003 -PAGE 66 FERNANDOCORFU,PAUL E.B. ARM/TAGE,KARE KULLERUD&STEFFENG.BERGH
Ersfjord Granite. SampleCAT-1 was collected from a fine- grained graniticdykethat int ruded syn-ki nematically with the later alshearingeventD2(=D3 of Armitage 1999)along the Mjelde-Skorelvvat nZone(Armitage1999).
Analytical procedure
Zircon and tit anit e were analysed by ID-TIMSusinga mixed losPb/2J5U spikeatthe Geological Museumin Oslo. Zirconwas dissolvedinteflon bombs in an oven at 184°Candtitanite in Savillexvialson a hot-plate.Pb and Uwere extracted from zircon with anion exchange resin in HCI medium (Krogh 1973), and titanite also wit h anion exchange resin, but experimentingwithdifferenttechniques and various combi- nation sofHCI,HBr, and HN03acid s(Corfu &Andersen 2002).
Zircon fract ions smallerthan 4 119 were measured without anychemical purification.GrainsofaU-richmineralinsam- pleCAT-lweredissolved inSavillex vialsandseparatedwit h the same HCI procedur e used for zircon.Themeasurement s were carried out on a MAT262mass spectrometer using multipleFaraday cupsin staticmode and/orby peak jump- ing using an ion-counting electron multip lier system.
Furt her detailsof the procedure are summarized in Corfu&
Evins (2002). Analytical uncertainties were calculated by propagatingthe main sources of error including terms of 1%
on thelO7Pb/""Pb and 2%onl06Pb/
'04
Pb ratios of initial com- mon Pb. Plott ing and regression calculatio ns were per- formed using the progr am of Ludw ig (1999).Decay con- stantsare those of Jaffey et al.(1971).Age uncerta intiesare reportedat20.Zircon data for the coarse-grained, foliated gran- ite (sample KV-2)
The four analysescarriedout on zircon from this sample rep- resent fractions of euhedralshort prisms orof crystal tips.
They provide data that are between 2 and 4% discordant and somewhatscattered (nos. 1-4, Fig.4a; Tablel). Because there isno good evidencefor the presenceof xenocryst ic cores, the scatte r is att ributed to multi ple Pb loss in the cour seofa complexgeolo gical histor y.Three of the analyses can befitted to aline with anupp er inte rcept of 2692±3 Ma,whereasanother set oftwofit s on a steeperlinewith an interceptat2686±3 Ma.By combining these two bounding ages and their uncertainties we obtain an age of 2689 ± 6 Ma forthe crystallizationof this body.
Zircon data for the Ersfjord Granite (samples EG- 1 and(35)
Sampl eEG-1 conta inedanabundan t population of zircon but thoserecovered weredomi nantly of smal lgrainsize,a factor which had a negati veinfl uence onouratte mpts to ext ract conco rdant data.In addition, the pop ulation also contains traces of an inherited zircon component as reflectedby analysis no.5,which has alo'Pb/'06Pb age of 1853 Ma(Fig. 4c),significantly older than the age of the rest of the populat ion(Fig. 4b).Three fractions chosen to represent the
dominant, core-free zircon types in this sample yield a tightly spaced group of analyses,approximately 4%discor- dant (6-8).Two less precise analyses, repr esent ing zircon domains (frag ments of long and thin euhedral prismsand tips of such crystals)with avery low probability of having cores,overlap (9),or plot more discordantly (10)than this group.Because of the lack ofinternal spread,these fivedata pointsby themselves do not define aprope rdiscordia line, but they all fit (MSWD= 1.5)on aline constrained to pass through a lower intercept of 400 ± 100 Ma,the time of Caledon ian activit y known to have affected this region, yieldinganupper intercept age at about 1796 Ma.
To verify the valid it y of this proj ection ,zircons were extracted from a second sample from the same outcrop (C3S).Two analyses of single,pink euhedraltips (15-16)are moreconcordant than thoseinEG-1 and define a line pro- jectingto 1792±3 Ma.lnclusionof data points 6-10into the regression yields a similarupper intercept age of 1792 ± 5 Ma.The lower precision reflects some scatt er in the data (MSDW= 3.0)possib lydueto complex Pblossor residual amounts of inheritan ce in some of the fractio ns. A third analysis(14)on a squarefragment of a zircon prism from sampleC3S showsthat this was aninherite d grain with a primaryageof about2550 Ma, as defin edbytheproj ect ion from 1792Ma.
The zircon upper intercept age of1792±5Ma isconsid- ered to date the magmatic crystallization of the Ersfjord granite.The age is older than the Rb-Sr whole-rockage of 1706 ± 30 Ma reported for the same bod y by Andresen (1979),and alsosomewhatolder butwit hin erro rof theage of 1779±17Ma givenby Romeretal.(1992). Thisrelation- ship betwee n U-Pb in zircon and Rb-Sr whole -rock datais not uncommon ,and probab ly reflect sdistur bancesof the Rb-Sr system by metamo rphi c overp rints during the late stages of the Palaeoproterozoicevent s(seebelow)aswell as duringtheCaledonian orogeny.
Zircon data for the Oterneset granite (sample OT)
The zircon population of thissample isvery complexas it contains amajorproportion ofinherited material.This rela- tionship isindicated by thepresence of composite grains with cores and overgrowths,and was confir med by two analyses (nos.17 and 18),which yielded discordant data wit h pre-Svecofe nnianlO7Pb/106Pb agesof2140 and1973Ma (Fig. 4c).Three furt her analyses were carried out subse- quently on specific zircon domains deemed to be freeof inheritance.One single pieceof a crystalrim(no.19)yields a lessthan 2% - discordant analysis wit h alo'Pb/'06Pb ageof 1770 Ma whereasa single crystaltip,wit h a concave surface interpre ted to represent the interface of the tip to acore, yieldsa discordant analysis(no. 21;Fig.4b).These two data point s definea discordialinewit h a Caledo nian low erinter- cept age and an upperintercept ageof1774 ±5Ma,which is taken toindicate the timeofmagm ati ccrystallizati on ofthe
FERNANDO CORFU,PAUL E.B.ARM/TAGE,KARE KULLERUD&STEFFENG.BERGH
" - - - - -
NGU-BULL441,2003 - PAGE 67
0.31
@ magmatic zircon in KV-2
KV-2
2689 ± 6 Ma
c> ,'
-:
4
A7':::::
to ca.340 MaJ7 /
-: to ca.830 Ma
-/
0.51
33 350 4.7
4.7 4.3
4.5
to 950 ± 300 Ma
330 CAT- 1
350-320 Ma
310
CD U -ri ch mineral in CAT-1
@ (nearly) concordant titanite
1780 EG-1pale titanite
1756 ± 3 Ma
CAT-1titanite
~ 30 ./
1751 ± 8 M a---!?-5~..
22... EG-1
... ...29 brown titanite
IIIIIIIIIIIII( ••.•••./
1769 ± 3 Ma
./ 31 .,/
~" " "' /~~
titanite line to 667 Ma~ .,/ ..····32
...
0.31
/
0.30·~-~--1---~---+--~---I---~--+-
0.054
0.050 10
12.9
1300 6 12.5
(2550Ma)
\ 2500 ... ' ) / / ....././< 28( CAT-1j
2300 /...
Fig, 40
...14(C35)
F' 4b . . <\~e
I
l g190~"
,"" ""''':7(OT)
2100...\~,~\~000
'< _18(OT)
1
\.- 5(EG-1)o xenocrystic zircon
Fig.4dl---
_ ., MSWD=0.05
~
... :241100 notabraded
26
..
./...·/- 27 t0667+41 Ma
. . . ....
/0.34
0.18 0.50
0.26 0.42
00::J
M N _
JJe,
CDo
N
2.5 4.5 0.36 0.38
Fig.4.Concordiadiagr ams show ingtheresultsforzircon,titaniteandaU-richmineralinthefour samples investigated.The analytical dataforfig ures (a)to(e)arediscussedin thetext.
NG U -BULL 441,2 0 0 3 - PAGE68 FERNANDO CORFU,PAUL E.B.ARM/TAGE,KARE KULLERUD&STEFFENG.BERGH
granite.The thirdanalysisofa single euhedralprism(no.20) yieldsthe same2°'Pb/206Pb age as no. 19 but ismore discor- dant and plotsto the right of the discordia line through nos.
19 and 21.The positionof thisdata pointprobably indicates thatthe graincontained a smallinherit ed zirconcore inside the prism.
Zircon data for the syn-tectonic 03 granitic dyke in the Mjelde-Skorelvvatn Zone (sample CAT-1) Although very abundant,the zircon population of thissam- ple consistsalmost entirelyof highly metamict and altered, yellowi sh-grey,opaqueprisms.Becausethesecrystalswould likely yieldvery discordantU-Pbdata, no attempt was made to analysethem.Inaddition to thisdominant zircon type there arealsoverysubordinateamountsofclear transparent zircons,occurring assubroundedgrainsor asmo re or less euhedralprisms.Theanalysesofoneofthese prisms yieldsa discordantArchaeanage(no. 28;Fig.4c)st rong lysuggesting that all the clear zirconsare probable xenocrysts inherited from the country rock.No meaningful magmatic crystalliza- tion agecould therefore be extractedfrom this zirconpopu- lation.
Titanite data
The U-Pb data for titanite from the Ersfjord (EG-') and Oternessetgranites(OT)and the Mjelde-Skorelvvatn Zone granit icdyke(CAT-t) reveal a comple x multistage history (Fig. 4d,e).
TheErsfj ordgranite(EG-')conta insdark brown titan ite that provides twooverlappingconcordantanalyseswit h an age of 1769±3 Ma(nos.11-12;Fig.4dl.and a distincttyp eof light-brown titan ite fragmentsand lenses, one analysisof which(no.13)is concordant at an age of 1756±3 Ma.
Titanite from sample CAT-' has a somewhat similar appearance asthat in the Ersfj ord granite,consist ing ofa mixtureofbrownandpalefragmentsand ovoidlenses.One ofthe analysesof brow ntitanit e (no. 29)is1% discordant and inthe Con cordiadiagram itdisplaysanaffinitywit h the brown titanite in EG-'.By cont rast,another select ion of brow n titanite fragment s from this sample (31) has a younger lO'Pb/206Pb age and fitson a line through two data pointsfor cohabitingpale-browntitanitelenses,oneconcor- dant (no. 30,the abraded grains)and the otherdiscordant (no.32,theunabraded ones).A regression through the three analyses yieldsan upperintercept age of 1751±8 Ma anda lower intercept ageofc.950±300Ma.Theupper intercept is identicalwit hinerrorwit h the age of the palebro w ntitanite lenses insampleEG-', suggesti ng that they bothrepresent a coeval generati on grown during asecondaryevent. The distincti on inage betweenthe two brown titan it egenera- tions in sampleCAT-' is somewhat puzzling becausethe two select ions were made using roughlythe same criter ia, thoughat differenttimes.
TheOterneset(OT)titanitepopulation provid esa differ- ent type of data pattern.In terms of morphologyand colour,
thepopulat ionresembles very much that inthe othe r two samples as it comprises deeply coloured brown grains toget herwit h a very pale-brown to colourlesstitan itevari- ety.One analysis of brown titanite(no.22,Fig. 4d)isless than 1%discordant, indicating an age sim ilarto that of theolder browngeneration in samplesEG-' andCAT-t.Bycontrast, the pale-brow n varietiesexhibit a much higher degree of discordance between17 and 33%(Fig.4 e).The abraded frac- tions(nos.23,25,26)arecolinearwit h the nearlyconcordant analysisyieldingan upper intercept age of 1768±4Ma and alower intercep t ageof 667 ± 41 Ma.The two unabraded tita nite fract ions(nos.24 and27)deviate,respectively,to the left and to therightfromthis line.
Thehigh degree of discordance of the paletitanite in the Oterneset (OT)sampl e is dueeith erto mixingbetwee n two distinct growt hphases or to some secondary process that rewor kedtheorigin altitanitecausing Pb loss. Someof the analyses were carried out specifically to evaluate this question.Fract ionno.23,whichrepresent s pale-bro wn rims separated from brown tita nite interiors,provide sthe least discord ant datapoint of the group(Fig. 4el.thus indicati ng thatthe rims do not representa new lygrown phase.A sec- ond attempt (no. 24) was made using colourlesstitan ite fragme ntsthat hadinclusions of chlorite,assumingthat the chlorite may represent new growth during a low-grade overpr int.The resultof this test, howeve r,leadsto thesame conclusion as for the clear rims.The thirdtestwas carriedout on a singlecolourless fragment(no.27),to evaluatewhet her the multigrain fractionsmay represent mixtures of oldand youn g pale titanite.Although thissing le grain yields the most discordant analysis,its posit ion is not much differen t fromthat ofthe multigrainfractionofsim ilarfragments(no.
26).Hence,onemustconcludethat the discordanceismost likely the result of some recrystalliza tion process that affected the titan it e rat her than ofnewgrowth .Thisrecrys- tallizationprocess wasaccompaniedby considerable Pbloss and probably also byloss ofUand especiallyofTh, because the U concentr ati on wasreduced to the 10-20 ppm level from over 100 ppm in brow ntitanite,andTh/U decreased from 3.4 tovaluesof1.5-1.0.
An interestin g aspectof theseOTdataconcernsthe 667
±41 Malower inter ceptagedefined bythe titaniteline(Fig.
4e).lsthis Neoprote rozoicage geologicallysign ificantorjust an artefact of complex Pb loss? A related question is whet herwe mightbe biasing the age byusingonlythe data pointsfor abraded fractionsto calculatetheline show n in Fig. 4e. Because analysis no.27deviates significantlyto the righ t of the line,a different regression using all the pale tita nit e data(but not no.22)decreasesthevalu e of the lower int erceptage to 580±230 Ma,but thisline has alow proba- bilit yof fit (MSWD = 4.6)leadi ng to a highly infl ated erroron the age.Onthe otherhand,a regression of analyses23-26 alone, leavingout the analysis of the single unabraded frag- ment no.27,has an MSWD= 2.7 andyields intercepts that are less precise but indistinguishablefrom those shown in
FERNANDO CORFU,PAUL E.B.ARM/TAG E,KAREKULLERUD&STEFFENG.BERGH NG U-BU LL441,2003 - PAG E 69
Fig.4e.Hence,the majority of thetitaniteresultsfor this sam- pleseem to be pullingconsistentlytowardsthe 667±41Ma low er-in tercept age.The exception is analysis27,but this may simplyreflectthe presenceof titanitedomainsaffected by younger disturbances.Thus,the possibility appears to exist that this regio n was affect ed by a Neoproterozoic tectonometamorph ic event.
U-rich mineral in syn-tectonic granitic dyke (CAT-1)
In addition to zircon and titanite,samp le CAT-l also con- tains brown to orange andred, paramagnetic fragm entsof an unidentifiedheavy mineral. Twofractions (nos.33 and 34;
Fig.4f)haveU contents of 20 to 30%and provide nearlycon- cordant Carboniferou sages;the unabr aded grainsyield a slightlyreversely discordant age of about 347 Ma, whereas theabraded grainsare youngerat about 318 Ma(Fig.4e).
The differenceinage betweenthetw o analysesmeansthat there are systematiccomplicationsthat need to be resolved by more detailed work beforethese agescan be properly interpreted.Keeping this in mind,however, the data can be taken as an indication that the Mjelde-SkorelvvatnZone was probably faulted duringoneor a sequenceof Carboniferous events leadi ng to thedeposi tion of such high -U minerals.
Hydr othermal act ivity mayhave been relatedtothe devel- opme nt of brittle faultsthat cut across the Precambrian grain of the Mjelde-SkorelvvatnZone andare common near theCAT-lsam ple site (Armit age 1999).
Geological implications
Thenew results,ingeneral,support thepresent geological unde rstand ing of the region and constra in the tim ing of major tectonom agm atic events.The data (i) confirm the presence of Archaean basement, (ii)document the occur- rence of 1800-1790 Ma intrusive units correlative with ext ensive late-orogenic (Iat e-Svecofennian) suit es present throughout the region,and (iii) pick up a well defined late- tectonic overprint at 1770-1750 Ma, reflecting the late Svecokarelian tectonothermal act ivity that has affected a vast region of the BalticShieldfromthe Atlant iccoastinthe west to the Belomorian belt in the east (e.g., Gaal &
Gorbatchev 1987).Thereare alsoso meindications concern - ing thetimingof Neoprote rozoic and Palaeozoic events,but in this respect the data are not yet clearly defined and remain ratherspeculative.Below,we discussour resultsand thei r possiblerelationship to these eventsin chro no logica l order.
Archaean
The presence of Archaeancrustin the WTBChasbeenprevi - ously documented by Zwaan &Tucker (1996)and Motuza et al.(2001a)on Rinqvasseya, an observationconfirmed by the 2.4 Ga age of a mafic dykesw arm inthesame area(Kullerud et al. in prep).The present work documents a furt her Archaean age for a coarse-grainedgraniteat thesouthwest-
erntip of Kvaloy.This graniteisnowpartof a mega-Iens sur- rounded by highly deformed metasupracrust al and mylo nitic rockswit hin the Palaeoproterozoic Senja Shear Belt (Zw aan 1995,Pedersen 1997).We interpretthegranite as a remnant of a formerlymore exten siveArchaean base- ment suite in theregion.
Indirect ind icat ions forthepresence ofoldcrust arealso given by the abundanceof Archaean zircon s as inherited xenocryst ic grain sin other units.
Outs ide the WTBC, the presenceof Archaean crustis well known in the Vesteralen area to the south (Heier &
Compston 1969,Taylor1974,Jacobsen &Wasserburg 1978, Corfu,unpublisheddata)where itappears totaperout(Skar 2002).TheseArchaean crustalrem nants correlate with the sout hern part s of the Archaeancraton exposedin the Balt ic Shield east of the Caledonide s(Romer et al. 1992,Gaal &
Gorbatchev 1987).
Palaeoproterozoic: 1800-1790 Ma magmatism The age of1792± 5 Ma forthe Ersfj ord granit eplacesthis intrusionin thecontextofan intensive andvery widespread magmaticepisodethatisrecord ed across theBalt icShield.
One example is the anorthosite-m angerite-ch arnockite- granite (AMCG) suite in the Lofoten - Vesteralen regio n (Griffi n et al. 1978).Theserockswere probably derived by melt ing of underthrusted maficand felsic lower crust, and the melts were subseq uent ly processed throu gh compl ex fractionation and contamination processes prior to their emplacement in the middle crust (Markl 2001,Corfu in press).ln many other parts of theBaltic Shiel dthe 1800-1790 Ma magmatic suiteincludesabundant granitic rocks of pre- dominant lycrustaI derivat ion(e.g.,Ohlander&Skj old 1994, Ahii ll&Larson 2000, Skar 2002, Weihedet al. 2002)but there isalso a suite wit h shoshonitic affinit y interpretedto have been produced by melting of subcont inental Iithospheric mantle(Eklund etal. 1998).These eventsalso correlatewit h late-Sveco karelian deformation along major NW-SE to N-S- trending crustaI-scaleshear systems (Hi:ig dahl et al.2001, Weihedet al. 2002).Elsewhere in the Shie ld theintenseflare- up of mag mati c act ivity and deformation has been dis- cussedin terms ofa shiftfrom N-NEto E-SE in thedirection of plate convergence (Romeret al. 1992,Ahiill &Larson2000, Weihed et al. 2002, Corfu in press).The Senja Shear Belt records a polyphase tectonotherm al evoluti on including a main early phase of E-W oblique crustaI contracti on and possibly accretionary tectonics(01) follow ed bya superim- posed phaseof st rike-slip translat ion (0 2; Zwaan & Bergh 1994,Nyheim et al. 1994, Pedersen 1997).Ped ersen(1997) related thedevelopmentofsome of these str uct uresalong the Astridal ShearZone on Senja(Figs.2 &3)toobli q uecon- vergence and contraction .By analogy wit h thelarge-scale regional relat ionships,especially theevidencefor E-W con- tract ion(e.g.,Weihed et al.2002),it could be speculated that the 01 defor matio nwas related tothe1800-1790 Ma event and to the emplacement ofthe mainmassof the Ersfjord
NGU-B ULL441,2003 - PAG E70 FERNANDOCORFU,PAUL E.B.ARM/TAGE,KAREKULLERUD&STEFFENG.BERGH
Granit e. NoD1 str uct ureshave been observed in the granite, howev er, and thus thelat ter would havebeen,at best,very late-ki nematicwith respecttoD1.
Palaeoproterozoic: 1775 -1750 Ma magmatism, metamorphism and deformation
Ourst udy prov ide sevidence foratleast twoepisodesthat caused new growth and/or reset ting of titanite in the Ersfj ord Granitesome30 to 50 m.y.after itsint rusion. The first oftheseepisodes correspond s to the intru sion (dated by zircon and titan ite) of the granit ic dyke (a T) in the Kat t fj ord Complex at 1774± 5Ma.This event is also indi- cated by the 1769± 3 Ma age of brown titaniteinsam ple EG-1and bytitanitein thesyn-te ctonic granit e dyke of the Mjelde-Skorelvvat n Zone (CAT-l). Thus, this 1770 Ma episode is marked by magmatism that for me d the OternesetGranite and alsoby metamorphismthatresetor form ed titanitein the Ersfj ord Granite.Because it wasnot possibl e to date magm at ic zircon in samp le CAT-1, it remains uncertain whet herthe 1768Ma age of the oldest titanitegenerationin this sample representsthe act ual time of emplacement(as for O'I)orjust asuperimposedeventas observed in EG-1.Thetit anite ageprovides, inanycase,a low erlimi tfor the timeofintr usion ofthedyke.
Ayounger generati on of titanite is present inboththe Ersfjord Granit e and the syn-tectonic dykein the Mjelde- SkorelvvatnZone wheretheyyield overlappingagesof 1756
±3Ma and 1751± 8 Ma, respect ively.This generationcon- sistspredom inantlyof pale-b rownto colourlesstit anite that canbedifferentiated relati vely easilyfrom theolder brown varieties.This is avery commonsituation(e.g.,Corfu&Stone 1998), whereby the secon dary titanit e generat ion grows locallyduring retrogres siveevents fro m Tiliberated during breakdown of Ti-bearing phases such asbiot it e orilme no- magnet it e. Normally,onealsoobservesadistinc tdecrease in Uand Th/U betweenthe first and the second generat ion, but this isnot the case for the EG-1 and CAT-1 titanite s (Table1).It is,nevert heless, assumed that the second gener- ationof titaniterecord sgrowthduringsomelat e-metamor- phic and possiblydeform ationaloverp rint.
The magmatic and met amorph ic events in the period between 1775 and 1750 Ma probab ly rep resente d thenat- uralcontinu ationand gradual declining in inte nsity of the principal 1800-1790 Mamagmat icepisode, remain ingactive nonet heless acrossthe Shield (e.g.,Romer & Smeds 1997, Ahiill &Larson2000,Bibi kovaet al.2001,Hbgdahletal. 2001, Rehn strb m 2003).The NNW-SSE-stri ki ng Senja Shear Belt andrelatedhig h-strainzon esof the WTBC(Fig.2)represent struct uresof shield-wid e proportions linki ng up wit h the Svecokarelian deformation zones (e.g., Balt ic-Bot hnian megashear)ontheBalticShieldto theeast(Fig.1;Berthelsen
& Marker 1986, Gaal & Gorbatchev 1987, Henkel 1991, Olesenet al. 1997).ltis,thus, likely thatthe timingofthe late movementsrecorded in theWTBCalsoap pliesto the struc- tureasawhole.The D2 eventiswell verifie dstructurallyin
sheared rock s of the Torsnes, Astridal and Mjelde- Skore lvvatn Shear Zon es.The age of the lat e-kine matic granit ic dyke represented by samp le O'I indicates that region al deformat io n was still active by 1774 ± 5 Ma.The CAT-1 granitic dyke in the Mjelde- Skorelvvatn Zone is clearly syn-t ectonic with respect to the D3 deformation event of Arm it age (1999),equivalent to the D2 event of Pedersen (1997),thus const raining itstim ing at or before 1768Ma,the ageof titani te.Becausewe have been unable to datezircon,the age ofintrusioncouldbe older thanthat ofthe titan it e.
Neoproterozoic to Palaeozoic resett ing oftitanite
Thetit aniteresultsin sampleO'I,and more weaklythose in sam pleCAT-1,are suggestiveof a Neoprot erozo ic distur- bance.As arguedabove,the availableevidencefavour spar- tial resettingbysome mechanismof recrysta llizat ionor per- hapsleachin g rath erthan newgrowt h.Because wedidnot succeedin finding titanite thatwoul d be entirelyreset,the significanceof the 667±41 Ma lowe r intercept age remains somew hatenigma tic,and therefore itis notyet possibleto affix a defi nit e interpretation to that age.There could be somecausallink with the ap parent Sveconor w egian,Ar-Ar hornbl end e ages reported by Dallmeyer (1992) from Kvaleyaand Senj a,butbecauseof the unsystema tic nature of the Neop rote rozoic datesin this region , it isdiffi cult to reach a conclusive inter pret ation concerning their signifi- cance.Amuchbetter constrained late-Neoproterozoictitan- ite age(637 Ma)hasbeen found in a granitic gneiss of the Skarja napp e in the Seve Nap pe Com plex farther east (Rehnst rb m et al. 2002), but there the titanite actually recordsa phase ofcrystallizat ionduring deformationof the host rock.Thepre-t ecton icprovenance ofthe unitremains speculative.
Thepresentdata set allow susto concurwit h theconclu- sion of Dallmeyer (1992) concerning the weaknessof the therm al overp rint that affe cte d the WTBC during the Caledonian orogeny. This is evidenced mainl y by the absence or only limited amount of U-Pb discordance observed for mostof thetitanit es.Onaverage,thedegr ee of tit anit e discordance isconsiderablylow er thanthat seen,for examp le, inl.ofo ten -Vesteralenfart herto thesouth(Corfu, in press,and unpub lished data),and clearly much less than that reported by Skar (2002)in the basem ent wind ow sin Nordland.
The ap paren t Carboniferous ages provided by the unid entified U-rich mineralsin sample CAT-1 may reflect britt lefault ing in the Mjelde-Skorelwatn Zon e causingfluid circulationand deposition of U-richminerals.Thiscan likely belin ked tothe numero us brittlefaults thatoverprintedthe shear zone inthevicinityof the CAT-1int rusion(Armitage 1999).Palaeomagn et ic st udi esofother britt le fault systems inthe WTBeand Lofote n-Vesteralen have indicated atleast tw o compon ent s offault ing in the Perm ian andTertiary that
FERNANOO CORFU,PAUL E.B.ARMITAGE,KAREKULLERUO&STEFFENG.BERGH NGU-B ULL441, 2003- PAGE 71
are relatedto specificstages oftheopening of the North Atlantic(Olesenet al.1997).Thepresentdatamaybe record- ing an earlierphasein this evolution.
Conclusions
The new U-Pbresults from the WTBC leadto thefollowing conclusions:
(1) Archaean granite (2689 ± 6 Ma) is present in the southwestern part of Kvaleya forming megablocksfloating wit hinsheared rocksof the SenjaShearBelt.
(2)TheErsfj ordGranit e yields azircon age of 1792± 5 Ma,showing that it was correlat ivewit h the exte nsiveand compositionally diverse plutonic suite sthatwere emplac ed across the Balti cShield ina major event at 1800-1790Ma.
Alt hou gh not direct lyconstrainedby our dataand lacking evidence of 01 deformation, it is possiblethat thisevent coincided broadly wit h the first E-W contractional phase (0 1)recordedin theregionalshearsyste ms,by analogywith the relationships seen elsewher e in the Baltic Shield (e.g., Weihed et al. 2002).
(3)Ayounger, late-syn kinematic granitic dyke cutting the Kattfjord Com plexwas emplaced at1774±5 Ma.A sec- ond, late-kin emat ic syn-02 dyke inthe Mjelde-Skorelvvatn Zonecouldnot be dated by zircon, but thetitanit eindicat es anage of 1768±4 Ma, whichis thus a minimum age for 02 deform at ion,causedbySE-d irect ed strike-s liptranslation in thelargeregional shearzones.
(4)TheErsfjordGranite and the granitic dykefrom the Mje lde-Skorelvvat nZone both contain two titanitegenera- tions,anearlyone formed ataround 1769Ma anda younger one formed at around 1755 Ma.Titanit e in the Kattfjord Complexdyke yields an age of 1768±4 Ma, but also evi- dencefor a verystro ng Neopro terozo ic distur bance, possi- bly indicativeof aregional eventat thattime.
(5) The granitic dyke cutting the Mjelde -Skorelvvatn Zone contai nstraces ofa very U-rich mineral, which yields Carboniferous ages,probably indicat ingcrystal lization from hydrothermal fluids during brittle faulting of the shear zones.By cont rast,noneofthe mineralsystemswere sub- stantiallyaffect edbytheCaledoni an event, confi rming the results of previousstudiesin the region .
Ackno wledgemen ts
ConstructivecommentsbyG.Motuzaandhelpfulrevi ewsbyK.B.Zwaan and L.P.Grom etare grat efullyackn ow led ged.
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