Post-Sveconorwegian exhumation and cooling history of the Evje area, southern Setesdal, Central South Norway
KIRSTEN HANSEN, SVEND PEDERSEN, HENRIK FOUGT& PETER STOCKMARR
Hansen , K., Pedersen, S., Fougt, H. & Stockmarr, P. 1996: Post-Sveconorwegian exhumation and cooling history of th e Evje area, southern Setesdal, Centra l South Nor way. Nor.geol. unders. Bull. 43 1, 49-S8.
Fission track (Fl) dating of apatite and sphene from mon zoniti c dyke rock s in a M iddle to Late Proterozic intrusion from the Sete sdal region, southe rn Norway, ind icates coo ling fro m intru sion tem peratures to tem peratures of the surroundi ng s of ap proximate ly 250°C bef ore 800-600 Ma. Fluid incl usion stud ies suggest th e crusta l de pth to have bee n in th e or de r of 4-5 km at the time. Apati te FT result s indi cate th at during th e Palaeozoic th e rock s w hich now occ ur in the southern Sete sdal regi on were subjec ted to heating above th e closi ng temperature of apat it e. Apatite FT-length di stribution s and fi ssion track ages of nearly 300 Ma indicate that th e regi on had been buried t o a depth of more th an c. 4 km before th is tim e.
Kirsten Hansen and Svend Pedersen, Geologisk lnstitut, 0 sterVoldgade 10, DK-1350CopenhagenK, Denmark; Henrik Fougt, GEUS, Thoravej8, DK-2400Copenhagen NV, Denmark Peter Stockman, Carl BroAnlceg as, Granskoven8, DK-2600Glostrup, Denmark.
Introduction dominantly acidic but with basic components associated with plutonic rocks and interlayered immat ure sediments The Precambrian rocks of the Telemark-Agder-Rogaland (Pedersen & Konnerup-Madsen 1994a,b). area of southern Norway were formed during the Middle During the last part of the Sveconorwegian Orogeny, Proterozoic and the geological picture which appears rocks belonging to the Setesdal Igneous Province today is a result of deformation processes during the (Pedersen 1988) were emplaced into a heated crust. The Sveconorwegian/Grenvillian Orogeny (see, for instance, Setesdallgneous Province is divided into an older group Berthelsen 1980). The effects of Caledonian deformation consisting of a complex of granodioritic and dioritic have not been documented within Central South rocks, and a younger group which includes a series of Norway. The Precambrian evolution was followed by the complicated minor and generally bimodal (granit ic/mon formation of a sub-Cambrian peneplain which was cove zonitic) subvolcanic rock complexes (Pedersen & red by Lower Cambrian continental and shallow-marine Konnerup-Madsen 1994a). Minor bodies of granitic and sediments up to the Caledonian Front (e.g. Oftedahl monzonitic composition occur throughout the area sug 1980). The sedimentary record from both within and out gesting that additional bimodal bodies may be present side the Oslo Rift indicates that sedimentation continued below the present-day surface. Associated with the bimo into the Permian; and a sub-Permian peneplain was for dal rock bodies are the famous Iveland-Evje rare mineral med in the Oslo Rift representing a major phase of erosi bearing pegmatites (Bjerlykke 1934, Barth 1947, Fougt on of older sedimentary rocks (sandstones, limestones, 1993, Stockmarr 1994.). shales and phyll ites). Bjerlykke (1983) suggested on the The present study includes a number of fission track basis of sedimentation features and sea-level changes (FT) age determinations on apatite and sphene from that basinal subsidence occurred in the Oslo region in monzonites from one of the bimodal complexes, the Cambro -Silurian time. Hevrinqsvatn Complex (Fig. 1), w hich belongs to the In the southern Setesdal area, which is considered younger group of magmatic rocks within the Setesdal here, pre-Sveconorwegian rocks include supracrustals Igneous Province. The geology of the Hevrlnqsvatn and granitic to granodioritic intrusions with an assumed Complex has been stud ied by Pedersen (1975, 1980) who maximum age of 1350 Ma based on Rb/Sr whole-rock also carried out age determinations on the rocks. Rb/Sr analyses (Pedersen 1980). This age has recently been w hole-rock age studies yielded ages in the order of 900 to reconsidered by Pedersen & Konnerup-Madsen (1994a,b) 950 Ma (Pedersen 1980). and a more realistic age of 1290 Ma has been calculated. Reliable ages greater than 1290 Ma in the Telemark Agder-Rogaland area are rare although higher ages have The investigated area been obtained on zircons and baddeleyites from Telemark (5. Dahlgren, pers. comm. 1995). The major The Hevrlnqsvatn Complex (Fig. 1) includes rocks of gra crust-forming event apparently took place 1150-1100 Ma nitic and monzonitic compositions. The outcrop pattern ago and included the accumulation of volcanic rocks, indicates that the monzonite is situated below the grani- 50 Kirsten Hansen, SvendPedersen, HenrikFouqt & PeterStockmarr GU-BULL 43 , 1996
.' :::I
N
1km
H0VRINGSVATN COMPLE X ... PEGM ATI E
• MONZONITE
/ // j~~\ GR A ITE OLDER ROC KS ~ PORPHYRIC t:=j GRA OD IORITE ...... " (DEF ORMED ) t Fig. 1. Geological map of the Hovrngsvatn Complex. Sample localitiesare indicated (A= 1095, 8= 1092, C=91585, D= 91594, E=91602). te, and that the present level of erosion is close to the te/granite body. A younger element is that of a horizonta l roof of the int rusion.An important feature of th e system of monzonit ic dykes which is interpreted as for Hovringsvatn Complex is the development of cone sheet ming part of a bell-jar structure. Along with the se two systems inclu ding granitic as well as monzonitic sheets. dyke systems, irregul ar minor bodies of monzonite or These cone sheets clearly cross-cut the main monzoni- monzonite associated with granite are abundant . The NGU-BULL43 1,1996 Kirsten Hansen, Svend Pedersen, HenrikFougt& PeterStockmarr 51 youngest rocks belonging to the Hevrinqsvatn Complex tes, and this especially affected the monzonitic dykes and are peqrnatites. These occur in swarms especially in the resulted in very complicated fold patterns. The southern part of the complex. granite/monzonite body as well as the host gneisses, on Rb/Sr whole-rock age determinations by Pedersen the other hand, were defo rmed to only a minor degree. (1980) yielded ages of 945 ± 53 Ma (2a) for the Estimates ofthe PiT conditions from stud ies offluid inclu Hevrinqsvatn Comple x granite and 900 ± 53 Ma (Zo) for sions in the pegmatites indicate a minimum temperature the monzonitic cone sheet rocks, respectively. Initial Sr at emplacement of 430° C and a pressure of 1.4 kb. The isotope ratios are 0.7041 ± 0.0007 for the granite and same studies indicate that the lowest temperature in the 0.7040 ± 0.0002 for the monzonitic rocks. Rb/Sr ages on pegmatites during their formation was 280° C (Fougt minerals from two samples of the granite yielded mineral 1993). This also means that the temperature in the host isochron ages of 921 ± 34 Ma (zo) (whole rock - biotite: rock could not have been higher than 280° C at that time. 929 ± 20 Ma (2a )) and 853 ± 28 Ma (zo) (whole rock - bio A combination of the fluid inclusion study and KlAr tite: 874 ± 28 Ma (2a)), respectively. A KlAr age on biotite and Rb/Sr age determinations indicates that the crustal from the last sample yielded 856 ± 62 Ma (Pedersen cond itions 850 Ma ago corresponded to a temperature of 1973). This age is in accordance with earlier published 250-280° Cand a depth of around 4-5 km (equal to 1.4 kb) KlAr ages on micas from pegmatites immediately to the (Fougt 1993, Stockmarr 1994). south . A biotite from Haverstad yielded 871 Ma (Neumann 1960) while a muscovite from Iveland gave 847 Ma (Kulp & Neumann 1961). Outside the Fission track (FT) studies Hevrlnqsvatn Complex minor monzonitic and granitic bodies occur. These may belong to the complex or possi Previous fission track (FT) studies in southwestern bly to other, subsurface plutonic massifs. Scandinavia Field relationships suggest a rather elevated tempera ture and a moderate pressure in the region during the Zeck et al. (1988) reported FT ages from the Fenno intrusion of the monzonitic and granitic melts. Within the scandian Shield west of lake Vanern, southern Sweden. Hevrinqsvatn Complex irregular contacts between the Sphenes yielded pre-Caledonian FT ages and apatites intrusive rocks and their host are seen especially when post-Caledonian FTages and skewed FTlength distributi the host has an acidic or intermediate composition, whe ons. Their data suggest cooling below c. 250° C at c. 680 reas intrusions in rocks with a basic composition exhibit Ma ago and a post-Caledonian burial depth of 3-4 km. more or less rectilinear contacts . Internal contacts (1) bet Hansen (1995) recorded similar burial depths from the ween granite and monzonite and (2) between different island of Bornholm, which are due to a blanket of overly pulses of monzonite show very intricate structures, some ing sediments of c. 4 km thickness. of which may be primary, i.e. flow-related structures. The evolution of the Oslo Rift has been investigated by These contact relations and the presence of abundant Rohrman et al. (1993, 1994a) employing FT dating analy xenocrysts of alkali feldspars and quartz from the granite sis. The ages obtained for the rift flanks and floor yield a in some types of monzonite suggest that the granite was thermal and uplift history related to the rift evolution not quite consolidated when the monzonite intruded. In compared to the surroundings. Rohrman et al. (1994b, a single case, eutectic melting ofthe host - a gneiss with a 1995) investigated the morphotectonic evolution of sout granitic composition - during intrusion of a 5 m wide hern Norway and considered the post-Palaeozoic history monzonitic dyke can be demonstrated. Pedersen (1980) to be related to exhumation and basin extension follo suggested from studies of the granite system that this wed by Neogene domal uplift and erosion . Their results gneiss would melt eutectic at c. 600° C at a pressure showed a systematic increase in apatite FTages inland in below 5 kb. Norway. Indications of the pressure conditions are also given from the intrusive pattern of the bimodal complexes . Principles ofthe fission track method Evolution of cone sheets and horizontal dyke systems are usually limited to high or intermediate levels in the crust, The FT method is an age determination method which and these are generally considered to be subvolcanic. takes advantage of the time-dependent sensitivity of fis The morphology of the associated chambe r pegmatites sion tracks to temperature. Each track keeps a record of indicates emplacement under conditions of horizontal its thermal history experienced in the temperature-sensi stress in a brittle environment at depths of 4-6 km (Fougt tive interval (the annealing interval, closure or annealing 1993 and Stockmarr 1994, following the ideas of Brisbin temperature corresponds to 50% annealing for a simple 1986). cooling path (Naeser 1979)). In this interval, tracks can be The above indications taken together point towards a retained but are shortened in response to temperature temperature at the time of monzonite/granite e m p la ce and time; thus, the track length distribution c o n ta in s a ment in the order of 600° C and a pressure below 5 kb, record of maximum temperatures experienced during probably as low as 3-4 kb. The region was subjected to cooling and heating. For apatite the most sensitive inter 0 ductile deformation after the formation of the monzoni- val is c. 120-60 ( shifting both with composition and 52 Kirsten Hansen, SvendPedetsen. HenrikFougt&Peter Srockmarr GU-BUll 431,1996 towards higher or low er temperatures for shorter or long tively, the thermal history could reflect change s in the er heating t imes, respectively (e.q. Gleadow et al. 1983, thermal regime such as changing geothermal gradients, 1986, Green et al. 1989), For sphene , the closure temp era ture is approximately 2S0-200°C (Gleadow & Brooks 1979, Hurford 1986). Samples studied and analytical techn ique FT analysis of apatite is especially suited to evaluating the low-tempe ratu re history, combining evidence from In t he present study, S samples of monzonitic dykes from FTage determinat ions and track-length distributions in a th e Hovring svatn Complex were studied by the FT met num erical model (e.q, Jensen et al. 1992) based on exper i hod . The samples were collected within the best mapped mental work (Green et al. 1989). Knowing the track densi int rusion in the Setesdal Igneous Province in order to ty, lengt h distribution and uranium concentr ation, a th er obtain a well defined geological cont rol of the sampling. mal history can be determined, Combined with other The samples were crushed and separated using con geological information the th ermal history can be inte r ventional magne tic and heavy liquid separation techn i preted in terms of tectonic development involving, fo r ques, All samples yielded abundant apatite and sphene, example, subsiden ce by bur ial or upl ift due to exhumati wh ile zircon occurre d in much lesser amounts. The apati on wh ich brings the rock to the surface by tectonic tes were mounted in Araldite, polished, etched in 1N and/ or erosional removal of the overburden. Alterna- HN03 for C. 30 seconds at room temperature to obtain ~ 60 10 9 5 ~ 60 1092 c: c: Ql Ql :J 40 :J 40 C" C" ....Ql ....Ql u, 20 u, 20 0 0 0 5 10 15 0 5 10 1 5 llm ~ m ~ 60 9 158 5 ~ 60 9 1594 ~ 60 9 16 02 c: c: c Ql Ql Ql :J 4 0 :J 4 0 :J 4 0 C" C" C" Ql Ql Ql "- ...... u, 20 u, 20 u, 20 0 0 0 0 5 10 15 0 5 10 1 5 0 5 10 1 5 llm ~ m ~ m Fig 2. Measured apatite FT len grh distributlons, Setes d at, Norway. Table 1. Fission track ages for apat ite s fro m Setesda l, Norway. Sample no. FTage (Ma) ± la Psx 10' P; x 10' Pd x 10' (no.) X' p% (number) (number) SRM612 CNI CN2 grains (no.) 91602 202.94± 16.52 31.06(398) 33.95(435) 13.8621 1(1844) 39.45161(1944) 37.59841(1853) 31% (20) 91594 223.72± 14.90 17.97(766) 17.83(760) 13.7416 1(1827) 40.26323 (1984) 37.42449(1844) 78%(20) 91585 24 1.13±23.53 5.127(271) 4.711(249) 13.7657 1(1831) 40.10091( 1976) 37.45927(1846) 86% (18) 1095 281.89±27.71 5.899(290) 4.618(227) 13.8139 1(1837) 39.77626(1960) 37.52884(1850) 41% (20) 1092 307.07± 31.78 4.596(269) 3.298(193) 13.7898 1(1834) 39.93858(1968) 37.49406 (1848) >99%(19) Rhos. rho, and rhod are track densities for spo ntaneous. ind uced and stan dard gla ss. respect ively. Zeta values used are for the glass stan dards SRM61 2 324.7(3.37%). CN1 112.70 (3.6 1%) and CN2 121.65(3.75%). Age uncertai ntie s are based on counti ng statisti cs and zeta value uncertainties (Galbraith 1981). NGU-BULL431, 1996 Kirsten Hansen, Svend Pedersen, HenrikFougt & Peter Stockmarr 53 Apatite Sphene 1095 1095 - 700 130 +2 500 +2 - - 110 +1 400 +' - . - : - 800 0 300 0 t- . ... ,; - .- 700 · 1 .. - I . .. · 2 . - 200 · 2 - 500 300 - 100 - 1092 1092 130 +2 +2 - 450 - '1 0 +' .- +1 .. 8 0 0 .... - 35 0 ... - 0 ,...... 0 t- 70 0 25 0 .- ., - · 1 .. . - 500 ·2 - 175 · 2 - 100 -300 50 91585 91585 - 700 '30 +2 50 0 +2 - - - '10 +' - 400 +1 .... - 800 0 t- . :. - 300 0 f- ...... - 700 ·1 .~ .. , ., · 2 - 200 · 2 - 500 -300 100 7 00 500 91594 91594 400 130 - - 110 +2 - 30 0 +2 - 800 +1 ... .. +' - 700 0 f- .. 0 I · 1 .. .- 200 ., - 50 0 · 2 · 2 - 300 ' 00 700 500 91602 91602 - 400 - '30 '10 +2 300 +2 - - 800 - t +1 . t •• . - 0 t- .' 0 t- .. . 70 0 .. - 200 .. . - · 1 . .. -1 · 2 . · 2 .- 500 - 100 -300 Fig. 3. Radial plotsof apatite and sphene FTgrain agesfrom samplesfrom Setesdai, Norway. fully etched fission tracks, wrapped in alu-foil against a on in the apatite grains. Mounts and micas were mo un low-uranium mica dete ctor and irrad iated in the DR-3 ted on an object glass and similar areas in prismati c crys reactor at Research Center Rise, Roskilde, Denmark, with tal faces of apatites and mica mi rror images were coun a nominal fluence (t he time-integrated neutron flu x) of c. ted (dry) under a Zeiss Universal Microscope at a nom inal 9 x 1015 thermal neutrons/ern'. After irradiati on the micas enlargement of c. x1600 in tra nsmitted light. The calibre were etched in 40% HF for c. 38 minutes to obtain indu tion was carried out following the suggestions of Hurford ced tracks,giving a measure of the uranium concentrati- & Green (1983), using apatite from the Fish Canyon tuff 54 Kits ten Hansen, SvendPedetsen, HenrikFougt & Peter Stockman GU-BULL 431, 1996 Table 2. Apatite leng th distributions form Setesdal, Norw ay. t rack lengths of c. 13.5 urn and skewed length distributi ons (Fig. 2) domi nated by a single-stage exhumation pat Sample no. mean lengt h (urn) uncertainty no. of tracks (urn) 1 (J measured tern (Gleadow et al. 1986). The FT ages and length distri butions suggest that this pattern is of post-Caledo nian 91602 13.19±0 .19 2.38 151 age and that cooling below temperatures of 60-70°C (e.g. 91594 13.41±0.13 1.88 205 Gleadow et al. 1986) probably occurred rather late. 91585 13.51+0.18 2.23 149 1095 13.45±0. 17 2.07 149 Diffe rences in the cooling paths are suggested by the 1092 13.42±0.19 2.45 171 pattern of the apatite ages. Radial plots (Galbraith 1990) depict uncerta int y versus age for single grain ages (Fig. 3). The uncerta inty bar on the left hand side in each diagram app lies to all points in (Naeser et al. 1981) and the Mt. Dromedary banatite the diagram . Age uncertainties can be obtained by dra (Miller et al. 1990) as age standards. Fluence was monito wing a line through th e cent re of the uncerta inty bar on red using the glass standards NBS612, CN1 and CN2, the the y-axis and t he 10- or 20- ends of t he bar transferred to last two obta ined from Corning Glass Works, USA. the po int to the age scale. Precision increases to ward sthe Zircon and sphene were mounted in Teflon , etched in a right side of the diagram. From th e diagrams for apat ites eutectic melt of KOH-NaOH at c. 230°C and a mixture of it can be seen th at the statistical uncertainty may be the only parameter responsible for th e variation in age in HF, HN03, HCI and H20, respectively, wrapped in mica, irradiated and counted at c. x1600 (oil) enlargement in each sample, although compositiona l differences may transmitted light. Again, only prismatic faces were coun cause additional variation. ted . The calibration procedure was as for the apatites , but Sphene fission t rack ages (Table 3) vary between 590 using Fish Canyon zircon (Naeser et al. 1981) and Mt . and 790 Ma. These post-Sveconorwegian - pre Dromedary sphene (Miller et al. 1990) for a common cali Caledoni an ages represent points on the pre-Caledo nian bration of zircon and sphene. The Setesdal zircons were cooling path s which preceded general upl ift and pene not well suited for FT determination, crumbling during planation of sout hwestern Scandinavia (e.g. Oftedahl the etch ing of most grains, whereas the sphenes yielded 1980). Radial plots (Fig. 3) show that single-grain age vari well etched mounts. In this paper only apatite and sphe ations may be due to statist ical variatio n solely fo r sphe ne will be considered. nes. Track-length measurements for apatite were carried out measuring onl y the totally included horizontal tracks Model calculations parallel to prismatic faces, using a Kurta digital tablet at high resolution connected to a computer. The precision The modelled the rmal history (Fig. 4) is based on apatite of the measurements is believed to be better than 0.2 FTlength distribution and FTage (Jensen et al. 1992). The urn. program uses the experimental results and the annealing model of Green et al. (1989). The age of the olde st track, which is also the shortest, in each sample is calculated in Results an inverse calculation proced ure which also gives ages and temperatures of individual histogram columns. This Apat ite fission track ages vary between c. 200 and 300 Ma inverse calculated therma l history is then adjusted to the (Table 1) and length measurements (Table 2) show mean measured histogram in a forward calculation avoiding Table 3. Fission tra ck ages for sph enes from Setesdal, Norway. Sample no. FTage (Ma) ± 1(J Psx 10' Pi x 10' Pd x 10' (no.) / P% (number) (number) SRM612 CNI CN2 gr ains (no.) 91602 675.23±52 .70(48.25) 270.5(1150) 55.52(236) 8.7759 1(1784) 26.36207 (1949) 25.1698 1(1861) 61% (15) 91594 590.16±45.64(42.05) 187.5(102 1) 44.80(244) 8.89250(1810) 26.45426(1956) 2S.42633(1880) 76% (l S) 91585 730.48±61.98(57.95) 139.0( 997) 26.35(189) 8.79777(1789) 26.37936 (1951) 25.21791(1866) >99%(16) 1095 786.88±67 .37(63.08) 146.2(1039) 25.75(183) 8.86700(1803) 26.43409(1954) 25.37021(1876) 92% (16) 1092 686.88±61.37(57.78) 146.7( 838) 29.76(170) 8.83421(1796) 26.40817 (1952) 25.29807(1871) 70% (16) As Table 1 but the zeta values used are for SRM612 329(1.74%), Cn1 111 (2.03%) and CN2 118 (1.99%). Figure s in paranth eses in column 2 rep resent co unting statist ic uncert ainty for individual grai ns and their mica im ages alone. NGU-BULL431, 1996 Kirsten Hansen, Svend Pedersen, HenrikFougt& Peter Stockmarr 55 250°C (closure temperatures of sphene c. 250-200°C, Calculated therm al histories for the Setesdal area Gleadow &Brooks 1979, Hurford 1986). Discussion The sphene FT ages reported in this study indicate that the sphene ages are associated with the general cooling 200 300 Age in Ma pattern after the Sveconorwegian Orogeny. The post Sveconorwegian uplift and thermal history of the area may then have been as follows: Fig. 4. Calculated thermal historiesofsamplesfrom Setesdai, Norway. At around 900 - 950 Ma ago the temperature and pres sure of the region, based on field evidence and investiga tion of the granite system, are estimated to have been in the order of 6000 e and 3-4 kb, respect ively. At the time of scatter assumed to relate to uncertainties in the measure pegmatite formation at c. 850 Ma it can be demonstrated ments. The obtained thermal history is believed to repre from studies of fluid inclusions that the temperature in sent a possible post-Caledonian geological thermal histo the host rocks was app roximately 2800 e and that the ry for the area, best in the low-temperature part due to pressure was in the order of 1.4 kb. the higher number of measured tracks in that part of the The fission track ages of the sphenes indicate that the histogram. Calculated histograms are shown in Fig. 5 and temperature of the investigated crustaI level cooled are seen to have similar shapes to the measured histo below c. 250 -200 0 e (closure temperature, e.g. Gleadow & grams. Brooks 1979, Hurford 1986) by at least 700 Ma BP. The Disregarding the rather uncertain evidence from the K/Ar and Rb/Sr systems of the micas gave similar or hig oldest tracks, the modelling suggests that there has been her ages than sphene FT ages at similar locations and a long period of constant cooling since 250 - 300 Ma BP. thus probably cooled slight ly earlier through their bloc This cooling was presumably related to uplift and erosion king temperatures of app roximately 3000 e (Purdy et al. following a period of higher temperatures, indicat ing a 1976, Wagner et al. 1977). The fast cooling (age) and the burial before 250-300 Ma of 3 - 6 km (geothermal gradi fluid inclu sion data may reflect a rapid exhumation of the ent 20 - 40°C/km and maximum temperatures for accu area. Between 700 and 600 Ma BP southwestern mulating fission tracks in apatite of 120°C, e.g. Gleadow Scandinavia was probably affected by a general uplift et al. 1986). The rate of cooling was perhaps slightly and peneplanation (as pointed out e.g. by Oftedahl enhanced during the last 50 - 100 Ma, but the data are 1980). inconclusive. The sphene FT ages were not reset after Pooled or bulk FT apatite (except 91602 and 1092) or peneplanation. This suggests that post-peneplanation sphene ages are similar within the 2 o uncertainty. Linear temperatures can be roughly estimated at less than regressions of distance versus sphene and apatite ages, ~ 60 1095 ~ 60 1092 c c :;,Cl> 40 ~ 40 C' C' ...Cl> ...Cl> u.. 20 u.. 20 0 0 0 5 10 15 0 5 10 15 Il m Il m ~ 60 9185 ~ 60 91594 ~ 60 91602 c c c ~ 40 ~ 40 ~ 40 C' C' C' ...Cl> ...Cl> ...Cl> u.. 20 u.. 20 u.. 20 0 0 0 0 5 10 15 0 5 10 15 0 5 10 15 Il m Il m Il m Fig. 5. Calculated length distributions, based on thermal histories. For comparison, the measured FThistogramsare shown as thin lines. 56 Kitsten Hansen, Svend Pedetseo,Henrik Fouqt & PeterStockmatr NGU-BULL 431 . 1996 c. 120°(, this overburden again having been remov ed. The presence of Low er Pa laeozoic phyllites in th e Hardangervidda area (Hardangervidda Group, Oftedahl 900 1980) suggests t hat t here were slight ly elevated tempera tu res during the post-peneplanation stage. The sedimen tation pattern in front of the Caledo nian nappe s may sug gest that the sub-Permian surface is likely to be close to the earlier (sub-Cambrian) peneplain (Bjorlykke 1983) '00 and probably part of the so-called Paleic surface (Gjessing 400 1967). The much lower FT ages wit hin th e Oslo Rift are 300 I att ributed to deposition and erosion of the lava pile , 200 (Rohrman et al. 1994a). The FT and geo logical evidence 0 2 6 7 8 ' m 1095 1092 91 58 5 91594 91602 taken together provide no conclusive evidenc e of t he depth to wh ich the rocks of the Setesdal area were buri ed before th e form atio n of the sub-Perrnian penep lain. Fig. 6. Measured dista nce versus age dia gram. Also shown are lin ear regres However , th e Setesdal rocks were at more tha n 120°C, sion lin es for sphene and apatite, respectively. The regression lines are not correspondin g to a depth of >4 km (geothermal grad ient altitude corrected, but such a correction only changes th eir posit ions slightly. of 30°Clkm ), befo re c. 300 Ma BP and to c. 3 km at c. 300 Ma BP . Modelling results for apat ite FT leng th distribut ions (Figs. 4 and 5) indicate a rapid exhumation (and coo ling) respectively, yield a regression line of y = -14.3x + 746 up to approximately 275 Ma BP. After this time th e coo (Ma) for sphene and y = -11.3x + 297 (Ma) for apatite (cor ling (exhumation) rate declined. Such a cooling path may relation coefficients 0.7 and 0.9, respectively). for FTages relate to the removal of overlying supracrustals (volcanic corrected to a similar elevation (440 m) (Fig. 6). The and sedimenta ry rocks) and/ or nappes, t he break occur regression lines may reflect chang ing mean exhumation ring at t he time of formation of the sub-Perm ian pene rates (m/Ma) along the line for apatite as follows: plain. Continued cooling correspondsto a furth er erosion 1/ [age/elevation above the closure ("1 00°(") isotherm = of up to two km of overlying mater ial. The data cannot age/ (1 000x(1 00/ geothermal gradient) + surface elevat i resolve the history in detail, but fu rther evidence from on (m))] giving 12.7 m/Ma (site 1095) and 18.0 m/Ma (site seismic profiles and maturity measurement s south of the 91602) at a di stance of 7.75 km between the end points. Norwegian coast suggest that sedimentary rocks may This suggests a probable post-Caledon ian tilting of the have covered t he coastal parts of southern Norway area of c. 5.3 rn/Ma or c. 1600 m in 300 Ma between the (Jensen &Schmidt 1992, Riis & Jensen 1992) in Mesozoic ends of t he profile (11.6°). The age variation may th us times. The sediments may infer change s in uplift/coo ling suggest a post-peneplain tilt ing and a pre-peneplain rates in southern Norway. Furthermore, Neogene uplift is thermal str atification in the area, as alt itude dependence inferred by the tilt ing of these sedim entary rocks and by may have occurred before the penep lanat ion. accum ulat ion of a thi ck succession of Tert iary sedimen ts The apat ite FT data of Rohrman et al. (1995) for the in the Central Trough. Hunnedalen profile, southern Norway, are interpreted as Rohrman et al. (1995) documented cooling into the ind icating that the rocks cooled slowly through the anne apatite annealing interval in Jurassic times from the FT aling interval in Jurassic time s. Their ages are slight ly data of coast-near samples, south western Norway, a slow younger than those found fo r our samples, which show a cooling through the Jurassic and Cretaceou s, and finally similar age/elevat ion dependence and mean trac k furt her FTdata indi cated fast cooling to surface tempera length. The apatite ages of the Setesdal profile fit well tures since Tertiary times.Such a coo ling path is compati int o the g en eral ap atit e age p attern g iv en in Rohrman et ble w it h the genera l cooling sch eme o f the Sete sd al sam al. (1995). The suggested differential exhumation of c. ples, for which, however, coo ling into the apatite annea 1600 m obta ined from linear regression, however, diffe rs ling interval occurred already c. 300-250 Ma ago and per sign ificantl y from t he regional exhu mation pattern sug haps at a higher crustal level. Also, the modelled cooling gested by Rohrman et al. (1995) and may relate to local paths for the Setesdal apatites are very similar to t he coo evolution and uncertainty in age determination. The coo ling paths found fo r the Bamble sector south of the Oslo ling ages for sphene and the modelled cooling pat hs fo r Rift (Rohrman et al. 1994a). The apatite FT data from the apatites suggest that temperatures in the period betwe Oslo Rift yield sim ilar (on t he rift flanks) or young er (on en 600-800 Ma and 325-275 Ma ago did not exceed c. the rift floor) ages for the apatites but wit h mo re variable 200-250°C in the study area and since then have not mean track lengths (Rohrman et a1. 1993, 1994a). sugges exceeded c. 120°C. If the present surface is close to or t ing that the thermal history outside the rift is cont inuous below the sub-Cambrian peneplain (e.g. Oftedahl 1980) with tha t of the Setesdal province. but above th e sub-Cambrian 120°C isotherm, the area was later buried to depths with temperatures in excessof NGU-BULL 431, 1996 Kirsten Hansen, Svend Pedersen,Henrik Fougt &Peter Stockmctr 57 exhumation rate through the Late Palaeozoic and the QC/ Km Mesozoic. A possible increased Neogene exhumation is 90 not revealed from the data. The simi larity of the apatite 80 and sphene FT data for southwestern Sweden and sout 70 ~ 60 hern Norway may point to a common post-Caledonian SO evolution for Fennoscandia. 40 30 A 900 800 700 600 M. Acknowledgements Thi s study was carried out using g lass st andard s supplied by Dr. Schreuers, Corning Glassworks and age st andards supplied by Drs. c.w. Naeser, U.5. Geological Survey, and A.J'w Gleadow, La Trobe Uni versity, Me lb ourne, Australia. Dr. C,K, Brooks and Dr. J. Konnerup-Madsen (both ~ from th e University of Cope nhagen) corrected and discu ssed t he 350000 r man uscrip t. One of the referees in particular is tha nked for valuable sug 100 B! ?! ~ > ge stions for improving the manuscript. Dr. D. Roberts kind ly improved 900 800 700 600 500 400 300 200 100 M. the English. P(Kb) D(Km) 3 9 References 2 6 Barth , T.F'w. 1947: The nickeliferous Iveland -Evje amphibolite and it s relation. 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