Strontium isotope composition of the Bindal Batholith , Central Norwegian Caledonides 0 YSTEIN NORDGULEN & BJ0 RN SUNDVOLL Nordgulen,0 & Sundvoll,B. 1992: Strontium isotope composition of the Bindal Batholith, Central Norwe gian Caledonides. Nor. geol. unders . Bull. 423, 19-39. A strontium isotope study of the Bindal Batholith has been carried out. The data generally do not permit precise and accurate age calculations; however, good estimates for initial Sr ratios are commonly obtained. Low initial ratios (c. 0.704-0.705) with little internal variation are pres ent in granitoids in the southeastern part of the oatnontn. Rocks with intermediate initial ratios (0.705­ 0.710 ) predominate and are present throughout most of the bathouth, whereas very high ratios (> 0.715) are found in tour maline granites and anatectic granitoids in the west. Considering a subduct ion related setting for the pluton ism , tne geographical distribution of Sr initial ratios would be consistent with a westward -dipping subduction zone. Contamination of the magmas at the level of emplacement is thought to be of minor importance, and disturba nce of the isotope system by secondary alterat ion appears to be relatively uncom­ mon. Cons iderable isotopic variation within plutons are prob ably a result of isotopic hetero geneity in the source materials which has not been obliterated by magmatic processes. The range in Sr initial ratios in the Bindal Batho lith reflec ts that the granitoids were derived in variable proportions from relatively non-r adiogen ic upper mantle to lower crust as well as isotopically heterogeneous crusta l rocks. 0 ystein Nordgulen, Norges geologisKeunderseketse. Boks 3006-Lade, N-7002 Tronarie im. Norway. Bjorn Sund voll, Mineralogisk-Geologisk Museum, Sarsgr. I, 0562 Oslo 5, Norway. Introduction Regional context The Caledonian Bindal Batholith, which is loca­ The Bindal Batholith (BB) occurs in the Helge­ ted in the Helgeland Nappe Complex in north­ land Nappe Complex (HNC), which belongs centra l Norway , consists of a variety of rock to the Uppermost Allochthon in the Scandinavi­ types ranging in compos ition from mafic clivi­ an Caledonides (Gee et al. 1985), As outlined ne gabbro to leucogranite. In this paper, we by Thorsnes (1987), Nordgulen & Schouen­ report the results of a Sr isotope investigation borg (1990) and Thor snes & t.eseth (1991), two of the batholith. At present, a total of c. 250 series of metasupracrustal rocks are intruded analyses are available, including 45 samples by the granitoids. One of these consists of analysed by Priem et al. (1 975), Nissen (1986, migmatitic gneisses, calc-silicate rocks and 1988) and Terudbakken & Mickelson (1986). marbles. Earlier work in the northern parts Partly as a consequence of the overall low of the HNC suggests that these rocks are Rb/Sr-ratios, acceptab le isochrons and age Precambrian in age (Riis & Ramberg 1981, determinations are rarely obtained. However, Terudbakken & Ramberg 1982, Brattli et al. the analyses allow reasonab ly precise estima­ 1982), The other rnetasupracrustal series, tes to be made for the initial I7Sr/"Sr ratios which comprises mafic and calcareous conglo­ (Sri) for most plutons and rock types, The merates, eale-silicate rocks, marbles, psamm i­ results are therefore of considerable interest tes and schists, is thought to represent a for classification and comparative studies, and cover sequence to ophiolite fragments in the are useful when trying to constrain poss ible HNC (Bang 1985, t.esetn 1985, Thorsnes 1985, source regions for the granitoids. Heldal 1987). Correlating the ophiolite frag­ Isotope analyses are not available from the ments in the HNC with the Early Ordov ician northwestern part of the batholith, i.e. the area Leka Ophiolite Complex, it is inferred that the between Vefsnfjord and Ranafjord, and from cover sequences must be Early Ordovician rocks along the eastern boundary of the Helge­ or younger in age (Nordgulen & Schouenborg land Nappe Complex (Fig. 1). 1990, Thorsnes & t.eseth 1991). The ophiolite 20 @ystein Nordgulen & Bjern Sundvoll NGU . BULL. 423. 1992 -, i '" 1';'- , \ ,-,:" 05601'• •~:~ : 5KlI N" ...... __ 'J ....:' "'" -,. ~ h' ~. .( ~ OS ' , A P " · . ;) A l S~" T T EI. PlUT AV Al $ TEt. ... ...5$1. 1=' 000£RE 0 .~ -JORDHOlv ",",[ ·U " UGl SU.W JEL LE"" . V " UST..,A, r- .E T GP G AWP[ '. ~ l U T • .. A ... • ... ....c >1 ElGn o •. ...PP E CO'"P\. E1 ...p " E "'H O~ · .E T PlUTQt. - 0 :. G5 '.' 0 [ ', _ A...." re CTlC GFl...·. ITOlOS "I M . e»;GSV Of: •• \f"SS ~ ...=TOU R... .. L I.. r G R AN ITE «vc o:. QL I ". A PP( COUPL E . 'I 'of'; « ...l V" " ..... I.' O' .Z O"<ITE -=....:...:~ G R ..... ITE G R AN OOIORI H '. ' "I'P ...R A Ol,F ~ E L l [ T PlUT :.:-:::: QU ARTZ MQN ZON ITE. S'f'E:NITE "I V ..;QlSV .. ~ .... POAP HV RITIC GR ANITE L'" LIF"JElL V"5 51. V G V OSJOE '. GASBRO o 0 PQ RP'"l v RIT1C GRANO OlORIT[ ... ~. EV ER · .E S ~ : .>:.: D.ORlTE TONALI TF GRA NOOIOR IT[ HI C~,j D H )P" I H .' o « C".SCAl ',' ASSlr • 0" • v G AAe RO DIOR.TF "" , ZOOIORITE ~ './C RQOi-' m S F"JA U [ T '.A PP[ CO....' PLE . VARIOUS V E TASUPRA CRUSTAL RQC oo;S R .., RE INFJEl L M ASSIF ~ GREENSTO r-.E GAe e RO ULTRA..i AFIC ROC " S 5 soP. JOROE", 5 " Soo.:A LV,l£R ~ ~ LE..,A OP H I,)LlTE CO V PLE , o 10 2'!i. " r " ORQHAT H.~ ~ : . RQC"S e ELOW THE H"'O C & RNC TV l OVE"> ••• ...SSIF TP ~EPR A . FJEllE· "lUTe>:'. ... ... r.l RUST Il", Ff RRlO THRUS T VV 'lISTW .O,A', [ "' ASS r Fig. 1, Geological map of the Bindal Batholith. NGU - BULL. 423. 1992 Stront ium isotope compos ition 21 fragments as well as their cover rocks were ppm) Rb or Sr, however, were subjected to deformed and metamorphosed prior to being isotope dilution (ID) determination. Sr isotope cut by Late Ordov ician granitoids. Juxtapositi­ ratios and ID-determinations by mass-spec­ on of the ophiolite related rocks with the ol­ troscopy, and Rb/Sr ratio determ ination by der metasupracrustals must also have occur­ XRF-spectroscopy were carried out at the red in the Ordovician. Laboratory for geochronology and isotope The Bindal Batholith consists of more than geology at the Mineralogical-Geological Muse­ 50 plutons and occup ies a substa ntial part um, University of Oslo. The analytical procedu­ of the HNC (Fig. 1). Descript ions of parts of res used have been published elsewhere (Ja­ the batholith were provided by Kollung (1967), cobsen & Heier 1978). For some samples , Myrland (1972), Nordgulen (1 984), Theis sen Rb/Sr ratio!' were determ ined by XRF-spec­ (1986), Gustavson (1988), Nordgulen & Mit­ troscopy at the NGU-Iaboratories (Table 1). chell (1988) and Nordgulen & Schouenborg The mass-spectrometer, a VG 354 five-collec­ (1990). tor instrument, yielded a value for the NBS The rocks are generally equigranular or 987 Sr standard of 0.71025 ± 3 during the porphy ritic, medium- or medium- to coarse­ period of analysis. The erro r of the XRF­ grained and predom inantly granodioritic to determinations were estimated to ;;;; 1% , and granitic in composition. A few plutons are tona­ that of the ID-method ;;;; .5%. litic, and gabbros, diorites and monzonitic All isochron calculations and age data quo­ rocks are present in some areas. Tourm aline ted have been performed or recalculated using granites and anatectic granites are fairly abun­ the decay constants recommended by Steiger dant in the weste rn part of the batholith (Fig. & Jaqer (1 977). All errors are quoted at the 1). Petrograph ic and chemical data show that 2a level. Elemental and isotopic data are listed the major ity of the rocks are I-type according in Table 1 together with (" Sr/" Sr)o ratios cal­ to the class ification of Chappell & White (1974). culated with respect to an age of 440 Ma. For However , some plutons show transitional be­ samples with " Rb/'·Sr ratios less than 1, a haviour towa rds A-type granites, and the ana­ 20 Ma shift in the assumed age will cause a tectic rocks in the west may be regarded as very small change « 0.0003) in the calculated S-type granites (Nordgulen et al. 1988). initial ratio, whereas samples with " Rb/'·Sr Only a limited number of age determ inations ratios of c. 10 will have a change of c. 0.003 are available from the BB. Rb-Sr whole-rock in the initial ratio. and mineral data indicate a fairly wide age span ranging from the Late Cambrian to the Middle Silurian (Priem et al. 1975, Gustavson & Prestvik 1979, Nissen 1986, 1988, Terudbak­ ken & Mickelson 1986). Recently, a number of U-Pb age determ inations on zircons have yielded Late Ordovician to Early to Middl e Silurian ages for different rock types in the Strontium isotope data batholith (Nordgulen & Schouenborg 1990, Nordgulen et at., in prep). The U-Pb data thus The southwestern partof the batholith suggests a relatively narrow age range for the plutonism and indicates that the dates obta i­ Introduction ned by the Rb-Sr method must be conf irmed Several large to intermediate size plutons by more precise dating techn iques. spanning a wide compositional range are pre­ sent in this area. Analyses are available for the Krakfjellet, Terrakfj ellet and Heilhornet Plu­ tons in Bindal, and the porphyritic Sklinna Plu­ ton, which is located southwest of Leka (Fig. Analytical methods 1). From the Holm peninsula, north of the Heilhornet Pluton, data are presented from All sample prepa rations and chemical prepa ra­ tourm aline granite, anatectic granite and a tions were performed at the NGU-Iaborator ies.