Grenvillian U-Pb zircon ages of quartz porphyry and rhyolite clasts in a metaconglomerate at Vimsodden, southwestern Spitsbergen J U . A . BALASOV, A . M . TEBEN’KOV, Y. O H T A , A. N. LARIONOV, A . N. SIROTKIN, L. F. GANNIBAL and G . I. RYUNGENEN BalaSov, Ju. A , , Teben’kov, A. M . , Ohta, Y . , Larionov, A. N . , Sirotkin, A. N . , Gannibal, L. F. & Ryungenen, G . I. 1995: Grenvillian U-Pb zircon ages from quartz-porphyry and rhyolite clasts in a metaconglomerate at Vimsodden, southwestern Spitsbergen. Polar Research 14(3), 291-302 Proterozoic metasupracrustal rocks form a NNW-SSE trending basement zone along the western coast of Spitsbergen. The rocks show complex structures as a result of both Caledonian and Tertiary deformation, and most of the subordinate metaigneous rocks are not suitable for isotopic age determination. Some zircon-bearing rocks were found in the southwestern part of Spitsbergen and an attempt of U-Pb dating was performed. U-Pb dating was carried out on zircon fractions from quartz porphyry and rhyolite clasts in a meta- conglomerate unit of the Pyttholmen Formation northwest of Hornsund. southwestern Spitsbergen. The Pyttholmen Formation is considered to be a lateral equivalent of the upper part of the Gulliksenfjellet quartzite and in the same time as the upper part of the SkiUfjellet metavolcanites. Therefore, the obtained ages are applicable to the age of the Skilfjellet igneous activities. Some of the dated samples are strongly schistose and their magmatic origin is difficult to confirm; the interpretation of the isotopic results is not well constrained; however, some explanations are possible which refer to the known geological conditions; an igneous age of siliceous volcanic rocks of ca. 1200 Ma, inherited zircon ages of ca. 2500 Ma and a regional metamorphic age of ca. 930 Ma. The last age belongs to the Grenvillian period and is conformable with the Rb/Sr whole rock age obtained from the garnet-biotite schists of the I s b j ~ r n h a m n a Group underlying the Skilfjellet metavolcanites. Ju. A . Balafou, A . N . Larionou, L . F. Gannibal and G. I . Ryungenen. Geological Institute of Kola Science Centre, Russian Academy of Sciences, ul. Fersman 14, 184200 Apatity, Russia; A . M. Teben’kov and A . N . Sirotkin, Polar Marine Geological Expedition, Lomonosou, ul. Pobeda 24, 189510 Sr. Petersburg, Russia; Y . Ohta, Norsk Polarinstitun, P . 0. Box 5072 Majorstira, N-0301 Oslo, Norway. Introduction Pre-Old Red Sandstone basement occurs along the western coast of Spitsbergen. It is composed mainly of Precambrian rocks (Ohta 1994), except for limited occurrences of Cambro-Ordovician, low grade phyllites and subordinate amounts of carbonates and metabasic rocks, all of which were deformed and metamorphosed during the Cale- donian orogeny. The ages of these rocks are not well constrained, except for the high-pressure metamorphic rocks of Motalafjella, middle west- ern part of Spitsbergen (Dallmeyer et al. 1990; Bernard-Griffiths et al. 1993) and some Cale- donian ages from the south of Bellsund (Hauser 1982; Dallmann et al. 1990). Two unconformities were proposed within the Precambrian successions of northern Hornsund by Birkenmajer (1959, 1975, 1992), the Torellian and Werenskioldian. The upper, Torellian uncon- formity is defined by the base of the Slyngfjellet Conglomerate (Table 1, Fig. l ) , and its north- western extension was mapped in western Wedel- Jarlsberg Land by Bjornerud (1990). A Precambrian igneous complex, the Eimfjellet Group, has been known northwest of Hornsund (Hoe1 1918; Orvin 1940) below the lower, Wer- enskioldian unconformity (Table 1). It was stud- ied in detail by Birkenmajer (1959), Birkenmajer & Narebski (1960), Smulikowski (1965, 1968), KrasilSEikov & Kovaleva (1979), Teben’kov (1983) and summarised geologically by Bir- kenmajer (1981, 1992), Czerny et al. (1993) and Ohta & Dallmann (1994). A rhyolite conglomerate was described by Smulikowski (1968) at Vimsodden, NW of Not- tinghambukta (Figs. 1 and 2) and considered to be a part of the middle member of the Notting- hambukta Formation of the Vimsodden Subgroup of the Eimfjellet Group (Table 1; Birkenmajer 1992). The present paper reports some results of the U/Pb zircon dating from the clast of quartz- 292 J u . A . BalaSoa et al. Table 1 . Lithostratigraphy of the Precambrian successions i n southern Wedel-Jarisberg Land by Birkenmajer (1992) and our proposal which is similar lo Czerny et al. (1993). Correlation of the upper Virnsodden Subgroup is unknown since it is bounded by faults o n both sides. VKF: Vimrodden- Kosibapasset fault Birkenmajer (1992) Present paper Group Formation S of Werenskioldbr. N of Werenskioldbr. ---- GBshamna phy. Sofiebogen Hoferpynten dol. Sofiebogen (Torellian event) Slyngfjellet congl. Deilegga (Werenskioldian evenl) Deilegga Skalfjellet Vimsodden Eimfjellet (unexposed) Steinvikskardet fjellet qtz. Gulliksenflellet qtz. Feldspathic qtz. Revdalen Isbjornharnna Ariekammen lsbjarnhamna Skoddefjellet + 2 5 G a porphyry and rhyolite and matrix from the meta- conglomerate. Geological outline and sample occurrences New mapping (Ohta 6i Dallmann 1994; Czerny et al. 1993) indicates a major fault or fault zone from Vimsodden t o Kosibapasset (VKF, Fig. l ) , part o f which was observed along the southern coast of Vimsodden as a wide shear zone, includ- ing several faults with associated sulphide min- eralisation veins (Czerny e t al. 1992). T h e area north of V K F shows the Torrellian unconformity between the Deilegga and the Sofiebogen G r o u p s (Fig. 1 ) . To the south of VKF. the Eimfjellet and I s b j ~ r n h a m n a groups are distributed. T h e stratigraphic relation between these two areas is unknown. Birkenmajer (1975, 1981) defined the Werenskioldian event o n the southeastern side of Austre Torellbreen, north of V K F , where his Eimfjellet G r o u p is unconforrnably overlain by the Deilegga G r o u p . Birkenmajer's Deilegga G r o u p . however. has been reinterpreted as the Gisharnna phyllites of the Sofiebogen G r o u p a n d the Eimfjellet G r o u p has been shown t o be a different igneous complex, the Jens Erikfjellet volcanites (Fig. l), within the Slyngfjellet Con- glomerate of the Sofiebogen G r o u p , according t o our observations and Czerny et al. (1993). T h e Werenskioldian unconformity is therefore a mat- ter of debate. T h e lower part of the Vimsodden Subgroup of Birkenmajer (1992). most of his Nottingham- bukta Formation, occurs t o the south of V K F , shown as a part of t h e Gulliksenfjellet quartzite in Fig. 1, and the upper part, his Elveflya Forma- tion. t o t h e north, shown as the Vimsodden Sub- group in Fig. 1. A part of the Nottinghambukta Formation is considered here t o b e a lateral equiv- alent of t h e upper part of the Gulliksenfjellet quartzite and is termed the Pyttholmen Formation (Czerny et al. 1993, Table 1). T h e Vimsodden Subgroup (Birkenmajer's Elveflya Formation) around Vimsodden contains three or four beds of intensely deformed metadiamictites which have been considered tilloids. Birkenmajer (1959, 1991) and Harland (1978) considered them Pre- cambrian tillites; Harland et al. (1993) considered them t o be lower Vendian tillites. W e observed typical tillitic rocks in front of Werenskioldbreen on the strike direction of the metadiamictites, but the continuation with the latter is unknown d u e t o wide glacier a n d Quaternary covers. Czerny et al. (1993) considered these diamictites t o be Vendian tilloids. These new observations seem t o support t h e opinion of Harland e t al. (1993); however, since both sides of the Virnsodden Sub- group (Fig. 1) are bounded by faults (Fig. l), their stratigraphic position is unknown. T h e sampled meta-rhyolite conglomerate unit Grenoillian U-Pb zircon ages of quartz porphyry and rhyolite clasts 293 Legend: 1 Glacier and Quaternary cover 2-4 Sofiebogen Group. 2: G l s h a p n a phyllites 3 Hoferpynten carbonates 4 Slyngfjellet conglomerates 5 Vimsodden Subgroup 6 Jens Erikfjellet volcanites 7-10 Skilfjellet metavolcanites and Gulliksenfjellet quartzites 7 Metabasic rocks 8 Gulliksenfjellet 9 Metabasic rocks quartzites containing gabhro- diabase and granite blocks 10 Feldspathic quartzite at the base of the Skilfjellet volcanites 11 Isbj@mhamna Group 12 Deilegga Group 13 Faults The metaconglomerate unit is shown in solid black (exaggerated). VKF: Vimsodden- Kosibapasset Fault. ... ;.,:;_... - () .?,:.::;;..!!'.::' ._.... ... ; ..... :......_. 13 . I . :...:. .:.: Place names: E: Eimfjellet G: Gangpasset J : Jens Erikfjellet K: Kosibapasset P: Pyttholmen S: Slyngfjellet SK: Skilfjellet V: Vimsodden. Square area: Fig. 2 Fig. 1 . Geological map of southwestern Wedel Jarlsberg Land, based on recent mapping (Ohta & Dallrnann 1994; Czerny et al. 1993). occurs at Pyttholmen, north of Kvislodden, and in the frontal flat of Werenskioldbreen (Fig. 2). The conglomerate is a member of the Pyttholmen Formation of Czerny et al. (1993) and has a total thickness of more than 150 m. One of the faults of VKF is inferred to occur along the southernmost coast of Vimsodden. To the south of the fault there is a 30 m thick layer of biotite-bearing, green-grey phyllites which con- tains large blocks of aphanitic basic rocks, gabbro- diabases and diorite. Each block is up to 5 m across and all are weakly metamorphosed. Dark green phyllites (55 m) occur on the bottom of the shallow water between the Pyttholmen island and the mainland. White-green quartzite (35 m) and a chlorite-sericite quartz schist (13 m) occur in the northern and middle parts of Pyttholmen island, respectively. The southernmost part of the island consists of the sampled metaconglomerate unit (20 m). The clasts consist entirely of rhyolic rocks in the northern part of the unit. Southwards, the content of quartz porphyry clasts increases, and some granite clasts can be observed in the sou- thern part. The size and frequency of the clasts 294 Ju. A . Balafov et al. Fig. 2 . Distribution of the Pvttholmen Formation decrease to the south; therefore, a southward younging structure is tentatively deduced. At ca. 700 m north of Kvislodden. an outcrop of green-grey phyllite occurs south of VKF. A white-grey, micaceous quartzitic schist (20 m), locally gritty. and a sandy phyllite with chlorite clots occur ca. 100 m south of the green-grey phyl- lite. To the south of these is a grey siliceous phyllite (ca. 50 m) containing a thin dark rhyolite layer, possibly a lava, and angular clasts of dark reddish rhyolite. A grey quartz schist ( 1 5 m ) , containing large feldspar fragments, occurs in the southern part of this locality and has scattered clasts of rhyolite u p t o 35 cm in size. This rock is well stratified and seven graded units were counted, all showing southward-fining. These sil- iceous phyllites and schists are located roughly in the strike direction of the metaconglomerate unit of Pyttholmen and are correlated with the latter. Some of the present authors consider that these phyllites and schists were originally siliceous lavas and pyloclastics, and not conglomerates. A phyllitic rhyolite (ca. 10 m thick), exposed isolately along a stream o n the southern frontal flat of Werenskioldbreen, is considered t o be correlatable with the metaconglomerate unit of Pyttholmen. Neighbouring scattered exposures are micaceous quartz schists. A white quartzite, showing brecciated structures, occurs ca. 150 m northwest of the phyllitic rhyolite. This quartzite is traceable discontinuously t o the southeast o n t o the western ridge of Angellfjellet, where i t turns back to the northwest along the lower part of Bratteggelva. and joins with the major part of the Gulliksenfjellet quartzite (Fig. 2). A thin meta- Legend: 1 Glaciers and Quaternary cover 2 Moraines 3 Green-grey phyllites, locally containing gabbro- diorite blocks schists quartzites rocks north of VKF. 4 Mica-chlorite-quartz 5 Gulliksenfjellet 6 Skilfjellet metabasic 7 Vimsodden Subgroup Black: metaconglomerate unit and its correlatable rhyolite. Italic nos: localities of the dated samples rhyolite lens occurs in the Gulliksenfjellet quartz- ite c a . 7 0 0 m southeast from the mouth of Bratteggelva. T h e quartzite contains sandy schist layers in its upper part on the eastern coast of Nottinghambukta ( t h e Nottinghambukta For- mation of Birkenmajer, 1992). These observations from Vimsodden to Not- tinghambukta show that t h e Pyttholmen For- mation consists of quartzite, locally with greenish tint, a n d quartz schists similar t o t h e Gul- liksenfjellet quartzite and green-grey phyllites which locally contain blocks of coarse-grained igneous rocks, similar t o those commonly >den (Czerny e t al. 1993) in the Skdlfjellet Subgroup of Birkenmajer (1992). T h e SkAlfjellet and the Vimsodden subgroups were thought t o be lateral equivalents by Bir- kenmajer (1959, 1992) and the Gulliksenfjellet quartzite and the' Skdlfjellet Subgroup of Bir- kenmajer a r e laterally transitional (Czerny e t al. 1993). W e consider that the Pyttholmen For- mation is a lateral facies variation of t h e upper Gulliksenfjellet quartzite, and a t the same time, it is the western facies variety of the upper Skdlfjellet Subgroup (Fig. 3). Accordingly, the rhyolite a n d quartz porphyry clasts in t h e meta- conglomerate unit of the Pyttholmen Formation a r e considered t o b e derived from t h e Skdlfjellet Subgroup, forming the westernmost distribution of the felsic pyroclastic-eruptive rocks of the sub- group. It is difficult t o judge whether the clast- bearing, strongly schistose rocks of the unit con- sist totally of volcanic materials or contain certain amount of sedimentary components. Solid clasts were chosen for t h e dating, but some schistose Grenuillian U-Pb zircon ages of quartz porphyry and rhyolite clasts 295 NW Legend: 1 Metaconglomerate unit of the Pyttholmen 2000 Formation quartzites (sandy) schists 2 Gulliksenfjellet 3 Mica-chlorite-quartz 4 Metarhyolites 5 Metabasic rocks with 1000. gabbro-diabase and granite blocks 6 Green and green-grey phyllites 7 Feldspathic quartzites 8 Unexposed conformable with the The Isbj0rnhamna Group is 0 SE Angellfiellet Fig. 3. Lateral changes of litho-facies between the Pyttholmen Formation, the Gulliksenf~ellet quartzites and the Skslfjellet metavolcanic rocks, modified from Czerny et al. (1993). Table 2. Modal composition of the dated rocks. Symbols of minerals are after Kretz (1983), except for feld (feldspars) and Op. (opaque minerals). The remaining content of samples 3 , 5 , 6 and 7 are secondary and accessory minerals which are difficult to count due to fine grain size; +: recognised. -: not recognised. matrices were also included. The sedimentary materials, which may be mixed in the schistose samples, are considered to be derived from the same source as those in the upper Gulliksenfjellet quartzite. Samples Seven samples zircon dating, each 5- 0 % were collected from the metaconglomerate unit; the localities are given in Fig. 2 , and their modal compositions are shown in Table 2. Samples 2 (Fig. 4, left, clast) and 4 (Fig. 4, right, clast) are brownish grey and reddish quartz porphyry clasts, respectively, ca. 50 cm in length, showing relict-porphyritic and -glomeroporphy- ritic textures with 0.5-1 .O cm phenocrysts of quartz and K-feldspar. Round outlines of clasts Fig. 4. Metaconglomerates. Left: schistose rhyolite conglomerate, white rhyolite clasts in a grey schistose matrix (Pyttholmen). Middle: micaceous quartz schist, either the matrix of conglomerate or strongly schistose rhyolite (north of Kvislodden). Right: schistose rhyolite conglomerate; the clasts are dark in colour (north of Kvislodden). 296 J u . A . BalaSov et al. are well preserved. but cataclastic textures are developed within the clasts. Samples 3 (Fig. 4, middle) and 5 (Fig. 4, left. matrix) are schistose parts of the metaconglom- erate and have lepido- and granoblastic textures. Relict-porphyritic and - glomeroporphyritic tex- tures, consisting of up to 1 .O cm size quartz and K- feldspar grains, are locally preserved. suggesting that these samples are strongly sheared rhyolite in rhyolitic pyroclastic. Some amounts of the matrix of conglomerate may be incorporated in the samples, but they are impossible to evaluate petrographically. A significant amount of musco- vite and a small amount of chlorite occur along the cleavages. The layers around these rocks contain dark reddish, elongated rhyolite clasts strongly penetrated by cleavages. Sample 6 is a light grey-brown. phyllitic rhyolite in hand-specimen and is petrographically a micaceous quartz schist, similar to sample 3 and 5 . except that no chlorite has been observed. Sample 1 has a greenish colour and is a finer grained siliceous rock compared to the others, lacking clasts of phenocrysts. Sericite-chlorite aggregates are localised along cleavages in the granoblastic texture of quartz and feldspars. This seems t o be an aphanitic part of the rhyolite or sedimentary matrix. Sample 7 is a dyke, 30 cm wide and 1.5 m long, that cuts the metaconglomerate. The dyke is restricted to the inside of the metaconglomerate unit and is considered to have formed soon after deposition bv segregation in a hot pyroclastic flow or segregated during a later metamorphism. The rock has a coarse-grained pegmatitic texture of quartz and feldspars and shows cataclastic frac- tures. Table 3 Chemical composition of the dated rocks Two analyses of Smulikowslu (1968). 338M and 338C. are included as samples 8 and 9 , respectively The analyses were made at the SEVZAP- GEOLOGIJA Laboratory, St Petersburg, Russia, by the X-ray fluorescence method o n the K R F - I 8 device Chemical analyses of the dated rocks, together with two from Smulikowski (1968) are shown in Table 3. Due to intense development of cleavages accompanied by distinctive amount of sericite, and locally chlorite, primary chemical com- positions were strongly modified. especially the K 2 0 contents. Strict considerations of an igneous rock group can not be made for the present rocks, since some of them may contain certain amount of sedimentary materials. Apparently the rocks have very high K2O (aver- age of 9 samples = 7.5%) and very low Na,O (average of 9 samples = 0.47) contents (Table 3). Most of these rocks plotted in the rhyolite composition field on the total alkalis vs SiOz dia- gram (Fig. 5a), except for sample l , a subalkaline dacite and samples 8 and 9 , trachytes. Sample 8 has normative Ans2 and is considered to be a glassy andesitic rock modified by later addition of K 2 0 . although Smulikowski (1968) described it as a rhyolitic clast. Sample 9 lacks specific petro- graphic description in Smulikowski (1968). The samples are scattered across the alkaline and sub- alkaline field (Fig. 5a, Irvine & Baragar 1971), and all subalkaline rocks have a calcalkaline nature (Fig. 6, Miyashiro 1974). TiOz vs FeO*/ MgO ratios (figure not shown) show a typical calcalkalic trend (Miyashiro & Shido 1975). Samples 2 and 4 are clasts relatively free from penetrative cleavages, and a variation trend of total alkalis passing these two gives an alkali-lime index of ca. 52 5 -3 (Figs. 5a and 5b). Since a large addition of K 2 0 during later metamorphism is expected, the index will be higher in the primary rocks. i.e., alkali-calc and calc-alkali rock series. High K 2 0 and low N a 2 0 contents apparently show that the rocks are of the S-type (Chappel & White 1974). The boundary between the S- and I-type on the K 2 0 - N a 2 0 diagram is more than 2 wt % N a 2 0 at 2 wt % K 2 0 (Chappel & White 1974). The maximum NazO content of the present samples is 1.32 wt 70, therefore, even if some amounts of K 2 0 were considered to be secondary additions, the present samples remain within the S-type field in the diagram. Although some samples may contain the matrix of conglomerate, some, at least samples 2 and 4, are solid igneous rock clasts and they are in the S-type field. This is supported by the trace element vari- ations (Table 4 and Fig. 7 ) . Since only four elements for standard spider diagram are avail- able, Fig. 7 is an unusual spider diagram and the analyses were normalised to the average I-type Grenvillian U-Pb zircon ages of quartz porphyry and rhyolite clmts 297 I I I 1 I . 1 8 a- 1 ~ - - - 55 60 65 70 7 5 Si02 wt % Fig. 5. Total alkalis and C a O vs SiO, diagrams, two samples from Smulikowski (1968), nos. 8 (Smulikowski, no. 19, table 2) and 9 (Smulikowski, no. 18). are included. 5a: Total alkalis vs SiO,. Solid circles with numbers: present samples. A , variation trend of N a 2 0 + K,O, including the least modified rocks (samples 2 and 4); B, alkaline-subalkaline division after Irvine & Baragar (1971). Broken lines, classification of felsic rocks; lined area, area of the felsic igneous rocks from the Skilfjellet metavolcanic rocks (Ohta, Teben’kov and Czerny, unpublished data). 5b: CaO vs Si02. Same symbols as in 5a. Dashed line, variation trend of CaO . A A -M Fig. 6 . AFM diagram with two samples from Smulikowski (1968) included. Solid curve: tholeiite-calcalkalic boundary (Ivrine & Baragar 1971), area inside dashed-dot curves: field of common calcalkalic rocks (Ringwood 1974), area inside dotted curve: Scandinavian leptites (Magnusson 1970; Parak 1975; Lundberg & SmeUie 1979), solid circles and lined area: same as Fig. 5a. granite (Taylor & McLennan 1985). The present rocks reveal distinct differences from the I-type granite, but show general similarity to the S- type, except for T i 0 2 V and Ni, and rather good correlation with the pattern of the average shales (Taylor & McLennan 1985), except for V, Cr, Ni Table 4. Trace element analyses of the dated rocks. The analyses were carried out in the VNII OCEANGEOLOGIJA Spectra Laboratory of St. Petersburg, Russia. The values with an aster- isk (*) are determined by mass-spectrometric analysis; nd = not determined; - = not detected. av-I = average of the I-type; av-S = average of the S-type granites; av-sh = average of post- Archean shales. All three averages from Taylor & McLennan (1985) are shown for reference. and Co. The reasons of these dissimilarities can not be explained as in the case of definite igneous rocks, since they may contain some sedimentary materials. High values of Cr can be explained by a possible supply of chromite from the gabbro- anorthosite rocks of the Skftlfjellet Subgroup, which contain cumulate facies of chromite (Czerny et al. 1993). The trace element charac- teristics suggest an addition of areno-argillaceous materials into the present rocks, during both igneous and sedimentary processes. The high K 2 0 298 Ju. A . BalaSorl et al. 101 A O ' t V 0.05 5 Rb Sr Ba Sc Y Ti Zr V Cr Co Ni Cu Ga F i g . 7. Analysed trace elements normalised to the average 1- type granite, (Taylor & McLennan 1985). Vertical lines. present rocks, broken line. average S-type granites; dotted line. average post-Archean shales (both from Taylor & McLennan 1985). nature infers a crustal origin, if this nature is primary. It is difficult to state that all present samples belong to a comagmatic series from their chemical data. However, general similarities of chemical nature between the present rocks and the felsic rocks of the Skilfjellet metavolcanites indicate that they belong t o the same igneous rock group, and this is supported by the observed lateral tran- sition between this metaconglomerate unit and upper Skiilfjellet Subgroup. Uncertainty of a co- magmatic nature for the samples makes the inter- pretation of isotopic results ambiguous; however, some considerations can b e attempted within the limit of data quality. T o b k 5. Percentages of the two morphotypes of separated zircons M - l = morphotype 1 : M-2 = morphotype 2. Sample no M-1 M-2 I 70 30 100 0 3 100 0 4 98 2 5 99 5 6 95 10 7 90 7 Zircons Separated zircon grains were grouped into two morphotypes (Table 5 and Fig. 8): (1) Intact crystals with (111) and (110) facets and their fragments, 0.07-0.1 m m in size, with length/width ratios of 1.2-1.5, a r e by far the most prevalent type. They a r e unzoned. transparent with smooth surface t o half-transparent with rough surface, mainly colourless. though some are pale pink in colour. (2) Grains having complex natures are grouped together in this type. Some have half-transparent inner and turbid (possible d u e t o rough surface) outer parts and show lilac colour, and are frag- ments of idiomorphic grains. Z o n e d grains rep- resent u p to 2-3% of the total zircon population. Some are rounded, and oval grains have scale- like surfaces and tiny ridge-trough patterns o n the pyramidal facets, similar t o corroded grains (Krasnobaev 1985). T h e fractions of this morpho- type certainly contain some inherited o r detrital zircon grains. Fig. 8 . Examples of the morphotypes of the dated zircon grains. Above: morphotype 1, fragment of idiomorphic grain. Blow: morphotype 2, rounded and zoned grain. Numbers in the figures are sample number and morphotype number. 1- 1 1- 2 3- 1 4- 1 5- 1 6- 1 6- 2 7- 1 7- 2 I R a d lo g . l so lo p rd lo s A g e m ea su re d ls o to p ra ll o L o ad in g R a d lo g . C om m on U 20 6P b 20 6P b 20 6P b 20 7P b 20 7P b 20 6P b 20 7P b 20 7P b 20 6P b (m g ) P b @ p m ) P b@ pm ) (p pm ) 20 4P b 20 7P b 20 8P b 2 M P b er ro r% 23 5U er ro ff i 23 81 1 er ro r% rh o 20 6P b 23 5U 2 ~ 8 u 5. 00 34 .4 9 0. 41 17 5. 2 28 55 10 .8 1 4. 68 0. 08 8 0. 14 2. 15 0. 28 0. 18 0. 25 0. 87 13 80 11 64 10 51 1. 70 75 .1 3 0. 89 25 9. 1 29 68 7. 65 6. 14 0. 12 6 0. 33 4. 61 0. 40 0. 26 0. 22 0. 56 20 19 17 50 15 12 13 .0 0 33 .7 2 0. 54 13 9. 2 27 68 11 .8 5 2. 96 0. 07 9 0. 17 2. 19 0. 43 0. 20 0. 39 0. 91 11 83 11 78 11 75 4. 90 32 .1 0 0. 77 13 6. 9 16 29 11 .4 3 3. 20 0. 07 9 1. 41 2. 16 1. 43 0. 20 0. 23 0. 16 11 72 11 68 11 67 3 8. 20 15 3. 10 0. 87 67 0. 6 84 78 12 .2 6 4. 37 0. 08 0 0. 25 2. 25 0. 36 0. 20 0. 25 0. 71 11 98 11 97 11 97 3 s. 0. 30 21 7. 29 10 .7 0 98 2. 4 85 1 8. 51 5. 83 0. 10 2 0. 59 2. 92 0. 69 0. 21 0. 31 0. 52 16 59 13 88 12 19 a - z a 4. 50 28 .3 4 0. 77 11 7. 0 15 55 11 .4 1 2. 26 0. 78 7 0. 26 2. 03 0. 44 0. 19 0. 35 0. 81 11 25 11 04 11 64 1. 50 31 .1 4 8. 29 16 3. 3 18 6 6. 39 2. 54 0. 08 3 1. 84 1. 95 1. 92 0. 17 0. 35 0. 31 12 70 IO W I0 1 5 2. 50 87 .6 5 3. 24 28 3. 6 12 35 6. 87 6. 26 0. 13 5 0. 31 5. 28 0. 45 0. 28 0. 32 0. 72 21 67 18 65 16 06 9 6 3 '"0 d 2 G a 3 R t 4 W W 300 J u . A . BalaSori et al. analyse at the geochemical laboratory of Kola Science Center. Apatity, Russia. Samples 2, 3 . 4 . and 5 d o not contain enough amount of morpho- type 2 zircon for analyses and sample 2-1 was unsuccessful in analysis. The treatment of zircons for dating following the procedure of Krogh (1973). Uranium and lead concentrations and lead isotopic composition were determined using an MI-1201-T mass-spec- trometer with a single collector. employing a specially cleaned silica gel emitter and H 3 P 0 4 on the evaporating rhenium ribbon filament. The lead blank during the analysis was less than 0.5 ng and that o f uranium 0.05 ng. The analytical error of the isotope measurements of lead was 0 . 2 % . The error of the isotopic dilution techniques. using 2'JxPh and z35U spikes is estimated to be 1 % . T h e analytical procedures were calibrated by running the All-Union IGFM-87 and NBS-981 standards. T h e age calculation has been done using the program of Ludwig (1991a. b) Results Results o f the isotope analyses are shown in Table 6 and Figs. 9 and 10. Samples 3, 4 and 5 plot closely to each o t h e r ; therefore. they are shown separately in Fig. 9. Specimen 4-1, which is a solid quartz porphyry clast with little penetrative cleavage, lies o n the concordia (Fig. 9) and gives an age of 1198 ? -5 M a . Samples 3-1 and 5-1 are slightly off from the concordia and a chord through them and 4-1 gives an upper intercept age of 1201 t -57 Ma. T h e lower intercept of the chord. 924 ? -765 Ma. is with a large uncertainty. since the cord is subparallel to the concordia, but a small partial resetting is suggested by the lower intercept. Alternatively. samples 3-1 and 5-1 show an age a few million years younger than that of sample 4-1. as does sample 6-1 (Fig. 10) which is almost on the concordia and shows ca. 1110 Ma. The 1-1 and 1-2 zircon fractions define a chord with a lower intercept age of 919 f -16 Ma and an upper intercept age of 2456 ? -31 M a (Fig. 10). The two morphotype fractions from sample 7 yield a lower intercept age of 933 ? -9 Ma and an upper intercept age of 2538 * - 2 4 M a . T h e 6-2 fraction plots near the chord of the fractions from samples 1 and 7. T h e composite chord of the fractions 1-1, 1-2, 7-1, 7-2 and 6-2 gives a lower intercept age of 931 ? -54 Ma and an upper intercept age of 2508 t -125 M a with M S W D = 9.5. T h e samples incorporated in this composite chord a r e phyllitic rocks and a peg- matitic rock and the three fractions of morpho- type 2 among them certainly contain inherited and/or detrital zircon grains. Interpretation and conclusion D u e mainly to metamorphic modifications. the dated samples can not be proved by their co- magmatic origin by petrographic and chemical examinations. T h e discords showing high upper intercept ages suggest the presence of detrital and/or inherited zircon grains in the samples, therefore some samples a r e not pure igneous rocks, but the matrix of conglomerate. It is dif- ficult to give any conclusive interpretation of t h e isotopic data. However, since all dated samples are from o n e monomictic conglomerate unit, the clasts are supposed t o b e derived from the same igneous source and t h e matrix is the same material as the laterally equivalent Gulliksenfjellet quart- zies. S o m e explanations will be attempted based o n the assumption above, though alternative interpretations a r e possible. T h e concordia age of fraction 4-1 and the upper intercept age of the chord through fractions 3-1, 4-1 and 5-1 ca. 1200 Ma, a r e considered t o b e t h e magmatic age of the rhyolite and quartz porphyry, since the zircon grains in these fractions are euhedral and homogeneous crystals. Fractions 3-1. 5-1 and 6-1 may give younger ages of the igneous activity which prolonged for some million years. T h e lower intercept ages obtained by various chords a r e in a range of 919-933Ma and are considered t o imply partial resetting by later regional metamorphism (Fig. 10, I in inserted figure). This age is conformable with a pre- liminary Rb-Sr whole rock age of 936 t -15 M a obtained from garnet-biotite schists of the I s b j ~ i r n h a m n a G r o u p by Gavrilenko e t al. (1993). We tried isotopic analyses of some rocks from the group, but t h e results show a large scatter and unsuccessful. T h e Isbj0rnhamna G r o u p is con- formably underlying the Skilfjellet Subgroup (Czerny et al. 1993; O h t a & Dallmann 1994) at all observed localities of their contact. Accord- ingly, t h e regional metamorphism of a n inter- mediate-pressure series, greenschist-middle amphibolite facies grade, which affected t h e Grenuillian U-Pb zircon ages of quartz porphyry and rhyolite clasts 301 Skilfjellet igneous rocks and the sediments of the Isbj@rnhamna Group, is considered to be of Qenvillian age. The upper intercept ages, ca. 2500 Ma, ob- tained from the fractions of samples 1, 6 and 7, suggest the presence of a Late Archean or Early Proterozoic crystalline basement that was either partly incorporated at depth or supplied detrital zircons at the surface to the present rocks (Fig. 10, I1 in inserted figure). Samples 1 and 7 have no sign of ca. 1100-1200 Ma. This suggests that sample 1 is a sedimentary matrix of the met- aconglomerate. Sample 7, a pegmatitic rock, can be explained as a segregated dyke containing detrital zircon grains derived from the wall rocks, mainly matrix of the conglomerate. This inter- pretation confirms that the lower intercept age of Fig. 10 indicates a regional metamorphism, but not an igneous age. A similar, but somewhat young upper intercept age of ca. 2100Ma, has been obtained as the protolith age of the eclogite in Motalafjella about 150 km north along the western coast of Spits- bergen (Bernard-Griffiths et al. 1993). A meta- morphic zircon age of ca. 2400 Ma has been reported from the gneisses of the Eskolabreen Formation, structurally lowest lithological unit in southern Ny Friesland, northeastern Spitsbergen (BalaSov et al. 1993). Acknowledgemen&. - T h e authors are grateful to the leadership of Polar Marine Geological Expedition for providing the oppor- tunity of this geochronological work in Svalbard. M. G. Bjorn- erud, Miami University, Ohio, U.S.A. and J. Czerny and M. Manecki, Stanislaw Staszic University of Mining and Metallurgy, Krakow, Poland, contributed very much during the field works. We thank A. 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