Geology and geochemistry of mafic-ultramafic rocks (Koli) in the Handol area, central Scandinavian Caledonides
STEFAN BERGMAN
Bergman, S.: Geology and geochemistry of mafic-ultramafic rocks (Koli) in the Handol area, central Scandinavian Caledonides. Norsk Geologisk Tidsskrift, Vol. 73, pp. 21-42. Oslo 1993. ISSN 0029-196X.
The ROdberget ultramafic-mafic Complex in the Handol area in Jiimtland, central Swedish Caledonides, consists of various ultramafic rocks, metacumulate rocks, metagabbros, mafic dykes, plagiogranites and a melange. It is interpreted as a fragmented ophiolite. The overlying Bunnran Fonnation is a metasedimentary-meta-igneous unit usually consisting of finely Jaminated cal careous psammitic and pelitic rocks with probable tuffitic components, intruded by and interlayered with mafic intrusive and extrusive rocks. Combined geological and geochemical evidence indicates a close spatia! and tempora! genetic relationship between the Rodberget ultramafic-mafic Complex and the Bunnran Fonnation. It is suggested that they originated in a basin where oceanic crust was fonned during deposition of material derived from continental and volcanic source regions, an environment comparable to the Andaman Sea in the Indian Ocean. As a general model, such basins with their high frequency of fracture zonesftransfonn faults could be the original settings of the solitary and detrital ultramafic rocks also in correlateable units in the central Scandina vian Caledonides.
Stefan Bergman, Department of Minera/ogy and Petrology, Uppsala University, PO Box 555, S-751 22 Uppsala, Sweden.
A wide variety of igneous and sedimentary settings are Complex and the Bunnran Formation and discusses represented in the allochthonous units in the Scandina models for their original setting and subsequent evolu vian Caledonides. Rocks formed in various early tion in terms of present and Caledonian environments. Palaeozoic oceanic environments are found in the eu geoclinal terranes (Koli Nappes) which structurally overlie the late Precambrian-early Palaeozoic conti Field relations and petrography nental margin of Baltica (Seve Nappes). In recent tec Previous work tonic models of the central Scandinavian Caledonides (Stephens & Gee 1985; Stephens 1988; Stephens & Gee Brief descriptions of the geology in the Handol area 1989), three eugeoclinal terranes have been distin (Fig. l) are included in publications based on early guished: the Virisen, Gjersvik and Hølonda terranes, regional work in the central Caledonides (Hogbom 1894; which correspond tectonostratigraphically to the earlier Tornebohm 1896; Frodin 1922). More recent contribu defined (Stephens 1980) Lower, Middle and Upper Koli tions are based on detailed studies of the Handol-Stor Nappes, respectively. The metasedimentary rocks in the lien (Sj6str6m 1983, 1986) and Tii.nnforsen areas Virisen terrane record evidence for proximity to the (Beckholmen 1984). A small number of chemical analy Baltoscandian margin from the Middle Ordovician; the ses of rocks from the study area have been presented in Gjersvik and Hølonda terranes have been inferred to regional studies of the ultramafic rocks in the Scandina have accreted to Laurentia during the Early Ordovician vian Caledonides (DuRietz 1935; Stigh 1979). In addi (Stephens & Gee 1989) prior to the closure of Iapetus. tion, some petrographic, textural and geochemical work The Koli unit of the present study area was initially in the area has previously been carried out (Hardenby included in the Virisen terrane (Stephens & Gee 1985), 1974; van der Kamp 1974; Westlund 1983). but recently (Stephens & Gee 1989) it was assigned to a The geology of the study area is shown in Fig. l. The separate iso1ated terrane (metamorphic complex of less Seve Nappes in the area have been subdivided into three certain affinity). parts (Sj6str6m 1983): the lowermost Blåhammarfjii.llet The brief presentations of the evidence for an ophiolite Nappe (greenschist-amphibolite facies) consisting of along the boundary between the Koli and Seve Nappes amphibolite, mica schist and calc-silicate rocks overlain at Handol, central Scandinavian Caledonides (Gee & by the paragneiss-dominated Snasahogarna Nappe Sj6str6m 1984; Sj6str6m 1986), promoted this study and (lower granulite facies), in turn overlain by the Tii.ljstens preliminary field results have been published previously valen Complex composed of imbricated thrust slices (Bergman 1987). This paper presents geological and geo dominated by mica schist and amphibolite metamor chemical data from the Rodberget ultramafic-mafic phosed in amphibolite facies. N N KOLl SEVE BUNNRAN FORMATJON �� TÅLJSTENSVALEN COMPLEX � Calcareous psammite, pelite l·:·:·: ·l Mica-schist, amphibolite * �mmmtlllllll Mafic rnetaigneous rocks � � r.:--:::� SNASAHOOARNA NAPPE RODBERGET ULTRAMAFIC " Paragneiss, amphibolite � MAFIC COMPLEX l" " "" l Metal?yroxenite, metag�bbro, �""F:"'"+,:,...+�] . BLÅHAMMARFJÅLLET NAPPE ...... amphibohte, metadolente 1 1 Mica-��hist, amphibolite, - Dunite, serpentinite, calc-slltcate rock talc-schist, melange
\ Younging direction
Major tectonic contact Y (thrust) V 1 æ > .
'-' Extensional shear 7nn<> l------\ ,. V V V
V V V V V V V V ,. .., Other fault V V V V V V V V V V V V V V V V V V Lithological boundary V V V V ...... V V V V V V V V V V V V� V V V V V V V / or structural marker vvvvvvv V V V V V V V V V
V V V V V � -- Road .. V V V V V V V Trondheim Nappe V V V V V V o Complex 000 Sample vv vvvv
• location vvvvvvv Koli Nappes z o � V El] (Tannforsen Synform) � ;><: V V V Cl V V V V V V SeveNappe 2l V V V V V V G Complex
V V V V V V §c;; Lower tectonic units :><: V V V V V [[[]] -1 S.BERGMAN � V V "' 1991 :><: "' 5 ...... O Fig. /. A. Location of map B in Scandinavia. T =Trondheim, (j = stersund. B. Map of the Tiinnforsen area and surroundings, simplified after Gee & Sturt (1985) and Beckholmen (1984). C. Geological map of the study :8 area with sample locations. Most of the boundaries from the western part of the area are from Sjostrom ( 1983). � NORSK GEOLOGISK TIDSSKRIFT 73 (I993) Ma.fic-u/tramafic rocks, Handol 23
nents in these two units. There are few exposures of the Forrnation Bunnran contact between the Rodberget ultramafic-mafic Com Calcareous psammitc,pelitc/ M M cooglomeratc/marole plex and the Bunnran Formation and its nature is not ' - � ' - Mafic extrusiveand obvious. However, in the Bunnerviken quarry (Fig. l) 1 • -' 1 • intrusive roclcs,meladoleritc i) the impression is a primary transition, from the cal Rodbe�etultramafic maficComplex careous melange in the former to the calcareous metasedi mentary rocks of the latter, rather than a tectonic Melange/lalc-schist contact. Mafic dykcs
Plagiogranitcif Fe-TI-amphibolitc Leucogabbro/ ROdberget u/tramafic-mafic Camp/ex layered melaeumulatcrocJcs Dunitc, serpentinitc The Rodberget ultramafic-mafic Complex consists of tectonic lenses of mainly ultramafic-mafic rocks dis � Major/minor tcctonic contact Mica-schist, amphibolitc persed over a distance of ca. 20 km along the tectonic paragneiss boundary between the Koli and Seve units. Many lenses have an entirely ultramafic composition but several (at least six) are composite, displaying two or more rock c. I SOm types. The exposed components are ultramafic rocks, various cumulate rocks, gabbros, mafic dykes and pla giogranites (Fig. 2), mostly containing metamorphic mineral assemblages. A lens containing all these compo nents is exposed at Mount Rodberget and River Jarpån (Fig. l). A melange (see definition in Raymond 1984) contains diverse fragments (see below) in a dominantly ultramafic matrix. The ultramafic rocks of the present area are found at two structural levels. They are sepa rated by a zone of strongly sheared amphibolite, mica schist and more or less calcareous Cr-rich quartzite (with Cr-mica and locally uvarovite and wollastonite). The latter is an excellent marker horizon throughout most of the area.
Lower ultramafic leve/. - This level is dominated by foliated, more or less serpentinized dunite in well exposed tectonic lenses (maximum recorded thickness is ca. 250 m) bounded by strongly deformed serpentinite. It is underlain by mylonitized mica schist, amphibolite and psammite of the Tåljstensvalen Complex (Seve). Primary features within the hetter preserved dunitic
Fig. 2. Schematic composite sections showing relationships between lithological bodies have been recorded locally (Fig. 3A, B). These units based on field data from (i) Ri vers Handiilan and Bunnran and the include compositional- and grain-size layering with al Bunnerviken quarry and (ii) Mount Riidberget and River Jårpån. The line linking ternating orthopyroxene-bearing porphyritic dunite and the two columns correlates these and also marks the base of the Bunnerviken lens as defined by Beckholmen ( 1984). fine-grained dunite and rare mm-thick chromite trails. Irregular 0.3-1 m thick pods of fine-grained dunite have gradational contacts to a more coarse-grained variety. A The Koli units (upper greenschist-lower amphibolite grain-shape foliation defined by coarse olivines or py facies) tectonically overlying the Seve in the southern roxenes is developed in both rock types. Gabbroic rocks part of the Tannforsen region (Tånnforsfåltet; Torne of the cumulate section have been found in tectonic bohm 1896) have previously been described as the Bun contact with serpentinized dunite in several places indi nerviken lens ( Beckholmen 1984) and its western part as cating structural breaks in the lithological column. the Handol Formation (Sjostrom 1983). The subdivision In thin section, large olivines with undulose extinction presented here differs slightly regarding the Bunnerviken have recrystallized to small strain-free crystals with lens as defined by Beckholmen ( 1984, see Fig. 2). Based straight grain boundaries meeting in triple points. Sev on field relationships, two units are distinguished: the eral generations of serpentine and late tale veins replace Rodberget ultramafic-mafic Complex overlain by the olivine. Reddish brown chrome spinel is partly altered Bunnran Formation. A schematic composite section to magnetite and is associated with secondary Al-rich (Fig. 2) shows the relations between the different compo- chlorite. Relict orthopyroxene is found in some samples. :!4 S. Bergman NORSK GEOLOGISK TIDSSKRIFT 73 ( 1993)
Fig. 3. A. Layering in slightly serpentinized dunite. The hammer is 54 cm long. 400 m southeast of the Bunnerviken quarry. B. Fine-grained dunite pod (upper part) with gradational contact to porphyritic orthopyroxene-bearing dunite. The foliation defined by oriented grains and aggregates is interpreted as primary ( see text). The pen is 16 cm long. 400 m southeast of the Bunnerviken quarry. C. Grain-size layering in meta-cumulate rock with actinolite pseudomorphs after pyroxene. The pen is 14 cm long. Mount Rodberget. D. Layered metagabbro with mafic dyke transecting the layering. The hammer is 54 cm long. Mount Rod berget. E. Multiple sheets of metadolerite (shaded) and plagiogranite (crosses). The contacts and fine-grained margins ( dotted) are clarified on the line drawing. Loose debris is white. The hammer is 54 cm long. River Jårpån. F. The melange: fragments of ultramafic to felsic igneous rocks, jasper, quartzite, quartz, schist, marble and manganiferous rock in a dominantly ultramafic matrix. The hammer head is 15 cm long. Bunnerviken quarry. G. Psammite beds and laminae with sharp bases grading upwards into pelite. Note incipient formation of composi tional layering transecting bedding due to metamorphic differentiation along crenulation cleavage. The pen is 14 cm long. River Handolan. H. Amygdaloidal metabasalt overlain by graded (right-way up) metasedimentary sequence. The pen is 14 cm long. River Handolan. l. Psammite with ripple-drift cross-lamination. The pen is 14 cm long. River Handolan. J. Metaconglomerate with clasts of psammite, pelite, marble, granite and Cr-bearing mica in a quartz-white mica-cal cite matrix. The pen is 14 cm long. River Handolan. K. lntrusions of metagabbro and metadolerite, now concordant and lens-shaped, in laminated pelite and marble to the teft and psammite and pelite in the centre. Note thin metadolerite intrusion within metagabbro in upper right corner (see arrows). The thickest part of the dark metagabbro lens is ca. 3 m. Tjuvfloarna quarry. NORSK GEOLOGISK TIDSSKRIFT 73 (1993) Mafic-ultramafic rocks, Handol 25 26 S. Bergman NORSK GEOLOGISK TIDSSKRIFT 73 (1993)
Upper ultramafic leve/.- This consists of up to 50 m thick Mafic dykes. - Mafic dykes have intruded the layered strongly deformed tale schist with antigorite, chlorite, cumulate rocks (Fig. 3D), often at a high angle to the carbonate, tremolite and magnetite in addition to tale as layering, and the Fe- Ti-amphibolites and the pla principal constituents. Small ( dm-sized) retrogressed mafic giogranites. Fine-grained margins are common and dyke pods interpreted as disrupted dykes and more coherent but intruding-dyke relationships (Fig. 3E) have been found isoclinically folded retrogressed amphibolite dykes have at several places, although never in sufficient numbers to been recorded. In dose association with the tale schist there be recognized as a sheeted dyke complex. Dykes are is a ca. lO m thick melange (see below). A tectonic lens of present in all of the composite ultramafic-mafic lenses. massive or slightly fo liated serpentinite is situated within Porphyritic, sub-ophitic and massive fine-grained vari the cumulate rocks. The serpentinite has an entirely eties have been recorded and their thicknesses vary be metamorphic mineral assemblage consisting of antigorite, tween O. l and l m. Mineralogically the mafic dykes are magnetite, tale and carbonate but pseudomorphs after amphibolites consisting of hornblende, plagioclase olivine are outlined by a network offine-grainedmagnetite. ( oligoclase) and epidote with minor amounts of opaque minerals, biotite, chlorite and Ti-oxides. Apatite and Cumulate rocks and amphibolites. - The cumulate section actinolite are rare. Secondary caleite has been observed is best exposed in the easternmost part of the area (Mount in some samples. Variably altered plagioclase phe Rodberget and River Jårpån, Fig. 2). It is at least 400 m nocrysts are the only primary magmatic phases present. thick (the upper contact is not exposed) and comprises layered metapyroxenite, metagabbro and meta-anor Melange. - The melange (Raymond 1984) contains frag thosite, massive leucogabbro and Fe-Ti-amphibolite. ments of various rock types in a dominantly ultramafic In the layered cumulate rocks, layers range in thickness matrix (Fig. 3F). At the type locality in the Bunnerviken fr om less than a centimetre to several decimetres and can quarry its thickness is ca. lOm. It is matrix-supported be followed along strike for a few tens of metres. The and the strongly foliated matrix is dominated by antigor layering in the lower parts is defined by size variations of ite and tale but chlorite- and carbonate-rich zones are crystal aggregates that consist mainly of actinolite and also present in the upper part. Local concentration of minor antigorite and carbonate (Fig. 3C). Such aggre carbonate in patches and irregular veins provide evidence gates often have rectangular outlines up to 5 x 3 cm in for secondary carbonate introduction. The fragments size. They are interpreted as pseudomorphs after pyroxene range in size from a few millimetres to several decimetres
(uralite). A few crystals of relict clinopyroxene have also (the !argest observed is 0.5 x 0.3 m). Their aspect ra ti os been found locally. vary from l : l to less than l : l O, in part depending on The massive leucogabbro and the layered metagabbro their competence relative to the matrix. Some are angular (Fig. 3D) consist of varying proportions of actinolite and but most are rounded and oblate in shape with both long more or less altered plagioclase (labradorite). The degree and intermediate axes subparallel to the matrix foliation. of alteration is extremely variable over short distances. At one locality (north of Lake Bunnersjoarna), the folia Some meta-anorthosite layers consist almost entirely of tion in an angular serpentinite fragment is at a high angle plagioclase, while other leucocratic layers are almost to the matrix foliation, indicating the possibility of defor completely saussuritized. Pockets of pegmatitic gabbro mation prior to formation of the melange. A strongly have been recorded within the leucogabbro, which is most retrogressed and boudinaged mafic dyke has been found abundant in the upper part of the cumulate section. The in the melange. upper contact of this leucogabbro is intrusive, as demon Many rock types are represented by fragments: serpen strated by its fine-grained amphibolite xenoliths. tinite, white and green tale schist, metagabbro, amphibo The amphibolites overlying the layered cumulates are lite, felsic plutonic rock, biotite schist, quartzite, vein inconspicuous in the field but display a distinctive chem quartz, marble and manganiferous rock. Rocks of ultra istry. They are depleted in most incompatible elements mafic composition dominate the fragment assemblage. and show signs of accumulation of hornblende/ Fe-Ti-ox Serpentinite fragments are dark green or black and fine ides ( see below). These rocks are mineralogically similar to medium-grained. Apart from some chromites rimmed to the mafic dykes although their abundance of opaque by magnetite, no relict igneous phases have been found. phases is higher. Sheets of leucogabbro are present within Metagabbroic fragments often have a well-preserved this unit. igneous texture although the primary mineralogy is to tally replaced. Many of the quartzitic fragments are red Plagiogranite. - The felsic rocks (plagiogranite) are found and fine-grained, suggesting that they originated as within the layered cumulates and most commonly in the jaspers. Grey and white varieties have also been mafic dykes and the Fe-Ti-amphibolites as cross-cutting recorded. The marble fragments consist of either white dykes (Figs. 2 and 3E) (O.l -0.4 m wide) or lenses or calcite or, more rarely, pink rhodochrosite. This Mn-rich sheets with unclear contact relations. Their fine-grained rock is found as massive ellipsoidal aggregates, disrupted groundmass consists of quartz, plagioclase, hornblende, layers or as rims around white carbonate fragments. An chlorite and opaque minerals. Well-preserved zoned euhe important exotic fe lsic plutonic fragment consists of 1- dral plagioclase phenocrysts are common. 2 mm large rounded bluish quartz, altered feldspar, white NORSK GEOLOGISK TIDSSKRIFf 73 (1993) Mafic-ultramafic rocks, Handol 27
mica and opaque minerals. Discontinuous layers, ca. the finely laminated metasedimentary rocks and a calcite l cm thick, composed of biotite and chlorite may repre marble unit dose to the base of the Bunnran Formation sent pelitic fragments. A fragment of uncertain origin at River Handolan is several metres thick. Although the consists of abundant carbonate and opaque phases with marble is intense1y deformed, recognizable but poorly minor tale and serpentinite and well-shaped plagioclase preserved Pelmatozoan stem segments have been ob porphyroblasts. served here. These were discovered by A. G. Hogbom and mentioned in Strand & Kulling ( 1972) but have escaped attention elsewhere in the literature, since they Bunnran Fo rmation are of limited use as age or environmental indicators. In the stream bed of Handolan, there is a matrix-sup Overlying the ROdberget ultramafic-mafic Complex is the ported polymict metaconglomerate (Fig. 3J, cf. Fig. 2). It Bunnran Formation, a heterogeneous metasedimentary is ca. 6 m thick, laterally impersistent and is in contact and meta-igneous unit. Most common are calcareous with pelitic and mafic igneous rocks. The matrix consists psammitic and pelitic rocks with some marble and inter of quartz, white mica and calcite. The more or less calations of mafic metavolcanic rocks. Intrusions of strongly flattened clasts are composed of psammites metagabbro and metadolerite are abundant. Multiple pelites, brown weathering marble, granite and abundant deformation and metamorphic mineral growth in upper aggregates ( �0.5 cm) of green Cr-bearing mica (0.5% greenschist-lower amphibolite facies have obscured most Cr203). The clast population is critical for the interpreta primary structures. However, the heterogeneous nature tion of the provenance of the metasedimentary rocks in of the deformation has led to local preservation of the Bunnran Formation and is further discussed below. primary sedimentary and igneous structures and textures. At a few localities, knots ( � 1.5 cm) of quartz, calcite Y ounging directions, mostly recorded from graded beds, or quartz-cored calcite are present in a quartz-white are indicated in Fig. l. It has not been possible to mica-dominated matrix. Hexagonal outlines of quartz establish a meaningful lithostratigraphy because of inter knots have been observed. The origin of this rock is nal shear zones and isoclinal folds, the Jack of persistent unknown. marker horizons and limited exposure. Similar lithologi Euhedral plagioclase crystals embedded in a fine cal units are commonly repeated throughout the entire grained quartz-mica matrix are not uncommon and are exposed thickness of the Bunnran Formation (Fig. 2), regarded as igneous in origin. Altemating thin layers of which is at least 1000 m. The relative importance of quartzo-feldspathic metasedimentary material and am tectonic repetition versus cyclicity related to the deposi phibolite have been interpreted as tuffite (Sjostrom tional environment is unknown, but both are probably 1983). significant. Meta -igneous rocks. - Mafic igneous rocks (metagab Metasedimentary rocks. - A characteristic fe ature of the bros, metadolerites and metabasalts) make up a signifi metasedimentary rocks is the compositional layering cant proportion (ca. 30-50%) of the rocks in the (Fig. 3G-I, K) with individual layers of widely varying Bunnran Formation. In the field, they stand as topo thickness, but commonly 0.5-5 cm. The layering can in graphic highs, white the surrounding metasedimentary places be shown to represent sedimentary bedding or rocks are mostly covered with Quatemary deposits. The lamination by associated sedimentary structures (Fig. mafic rocks are generally concordant with the layering in 3G-I), but elsewhere an origin by metamorphic differen the metasedimentary rocks and have tectonic lensoidal tiation during formation of crenulation cleavage can be shapes (Fig. 3K) up to 130 m thick. They are composed demonstrated (Fig. 3G). The rocks consist of varying of blue-green homblende, plagioclase, oxides, ± epidote, proportions of quartz, mica, plagioclase and calcite, with ±bi o tite, ±ch1 orite, ±carbonate. Some mafic rocks the addition of homblende, gamet, chlorite or clino carry gamet. Plagioclase is the only primary magmatic zoisite in particular layers. Scattered grains of green phase recorded, and is locally very abundant as euhe Cr-bearing mica in quartz-rich layers have been found in dral-subhedral phenocrysts. Relict clinopyroxene was several places. These are probably reaction products after reported by DuRietz ( 1935). Selective growth of meta detrital chromite. morphic homblende and gamet in pelitic metasedimen Compositionally graded beds, generally 2-5 cm thick, tary rocks along the contact to the mafic rocks is not are characterized by a quartz-rich base and a continu uncommon. The microstructure of these porphyroblasts ously increasing amount of mica towards the top of the is similar to undoubted regional metamorphic porphy bed (Fig. 3G, H). Rare cross-lamination (ripple drift) is roblasts further away from mafic bodies. This implies shown by asymmetric lenses of quartz-rich metasedimen that the contact phenomena are mainly due to material tary material enveloped by mm-thick mica-rich laminae transfer by magma-sediment interaction rather than con (Fig. 3I). Laminations probably disturbed by soft sedi tact metamorphism. ment deformation have also been noted. The concordant relationships and locally preserved Calcite is a common matrix constituent, especially in chilled margins suggest that the medium-grained psammitic rocks. Thin carbonate layers are frequent in metagabbroic bodies intruded as sills in the metasedi- 28 S. Bergman NORSK GEOLOGISK TIDSSKRIFT 73 (1993) mentary rocks. Primary ophitic textures and less com is considered less likely since the dunitic rocks have mon pegmatitic gabbro pockets and mafic xenoliths are characteristics (foliation, layering, fine-grained dunite preserved in the interior of some of the weakly deformed pods) which resemble those in the transition zone be metagabbros. tween the metamorphic peridotite and the overlying cu Fine-grained and locally ophitic metadolerite dykes mulates in ophiolites (Coleman 1977). Although this with thicknesses from a few centimetres to several metres transition zone has commonly been interpreted as cu intrude all other meta-igneous rocks and the metasedi mulate in origin (e.g. Coleman 1977), evidence fo r a mentary rocks (Fig. 3K). Chilled margins are locally dominantly residual origin by partial melting of or preserved but the contacts are often sheared. The angle thopyroxene out of harzburgite has been presented between the dykes and the layering in the metasedimen (Nicolas & Prinzhofer 1983). tary rocks is usually less than 20°, but may have been The fo liation in the hetter preserved dunitic rocks may larger prior to deformation. Angular metagabbroic xeno have originated in the mantle. Support for this is given liths have been fo und. by van der Kamp (1974), who reported slip along Metabasaltic rocks (Fig. 3H) are intercalated with the {Ok l}[ 100] in the older large olivines at Handol indicating metasedimentary rocks. It is not always possible to de upper mantle, high-temperature deformation (Rayleigh eide in the field whether a fine-grained mafic rock is 1968; Carter & Ave'Lallement 1970). This also argues intrusive or extrusive, but extrusive varieties have been against an origin as a layered intrusion for the Rodberget identified by their amygdaloidal top surfaces. The ultramafic-mafic Complex. quartz- or calcite-filled amygdules are more or less ellip The layered cumulate rocks may have fo rmed by accu soidal in shape due to deformation and commonly mulation of Fe-Mg-silicates and plagioclase. Repeated � 5 mm long, with a maximum recorded length of supp1y of magma is indicated by the dykes transecting 20 mm. the layering. The cumulate rocks seem to be in a normal upward facing position in view of the upwards increasing leucocratic component and typical high-level massive Discussion of fie/d relations and petrography gabbro, pegmatitic gabbro and plagiogranite in the upper parts. The transition from calcareous melange in the Rodberget Melanges are widespread in the modem and ancient ultramafic-mafic Complex to the calcareous metasedi geological record and may be formed in a variety of mentary rocks of the Bunnran Formation and the pres environments by tectonic or sedimentary processes (e.g. ence of metadolerite dykes in both units suggest that they olistostromes, serpentinite diapirism, tectonic disruption) have a close spatial genetic relationship. The presence of or they may be polygenetic (Raymond 1984). In general, fe lsic plutonic and pelitic detrital material together with distinctive criteria indicating their origin are difficult to the Cr-rich material in both units supports such an establish and later tectonic overprinting, such as in the interpretation. Handol area, further complicates interpretation. Appar There is no general agreement in previous work con ently the igneous fragments in the melange come from cerning the exact location of the boundary between the various levels of the ophiolite. The seemingly transitional Seve and the Koli rocks in the area. In this study, the nature of the contact between the melange and the spatial association of dunitic ultramafic rock and cumu Bunnran Formation may indicate a sedimentary origin late metagabbro (Fig. 2), and the presence of cumulate for the melange. metagabbro at the same structural level as the talc-schist Mn-rich rocks similar to those fo und as clasts in the (Koli, Fig. 2), are considered to be good reasons for melange may be formed in a variety of settings. Mn-rich including all ultramafic rocks in the Koli Nappes. This is nodules and coatings are well known from marine and also consistent with textural and metamorphic data from lacustrine environments where sedimentation rates are associated pelitic rocks (Bergman 1992). very low (0.5-5 mm/year). Mn-mineralizations have also It is suggested that the association of ultramafic been recognized in aragonite veins deposited from cold rocks, layered cumulate rocks, gabbros, plagiogranite, sub-seafloor solutions, in ultramafic rocks from the Ro mafic dykes and melange in the Handol area represents manche Fracture Zone in the Atlantic Ocean (Bonatti et a dismembered ophiolite, in agreement with Gee & al. 1980) and Zabargad Island in the Red Sea (Bonatti et Sj ostrom ( 1984). The two basal layers of an ophiolite al. 1983). Manganese carbonate hands (Sugisaki et al. as defined by the Penrose Conference ( Conference Par 1991) concentrated at specific horizons associated with ticipants 1972) appear to be represented in the area. chert or shale have been suggested as indicative of a An extensive mafic sheeted dyke complex (100% hemipelagic-neritic environment in isolated basins such dykes) and mafic volcanic rocks in contact with other as fore-are and back-are basins or rifted continental ophiolitic components have not been found, however. margins with narrow basins. Since any of these modes of This may be due to erosion, poor exposure andjor formation may be applicable to the Mn-rich fragments high strain obliterating primary features (see also be observed in the Handol area, their depositional environ low). An alternative model for the origin of the Rod ment and mode of incorporation into the melange cannot berget ultramafic-mafic Complex as a layered intrusion be inferred. NORSK GEOLOGISK TIDSSKRIFT 73 (1993) Ma.fic-ultramafic rocks, Handa/ 29
In the Bunnran Formation, the intercalation of mafic During sampling, an effort was made to obtain large, extrusive rocks with water-lain metasedimentary rocks fresh, non-porphyritic and non-amygdaloidal samples demonstrate eruption in a subaquatic environment, al that were not strongly sheared, weathered or affected by though unequivocal pillow lavas have not been fourtd. other secondary alterations. In order to maximize their The water depth during eruption is constrained to less representativity, samples weighing several kilo grams were than ca. 2000 m by the presence of amygdales in basaltic taken in most cases. Largest possible geographical rocks (Cas & Wright 1988; cf. Moore 1965). spreading of sample sites was aimed at but a concentra The clasts in the local conglomerate ( channel deposit?) tion in some areas was inevitable due to the distribution in the Bunnran Formation indicate possible sediment of appropriate outcrops. sources for this unit. The presence of granite and quartz Samples fo r analysis were crushed in steel jaws and ite clasts strongly suggests a clastic input from a conti rock chips were powdered ih a tungsten carbide shatter nental source. The green Cr-mica aggregates, probably box. W and Co analyses are not presented due to the derived from detrital chromite, also indicate ultramafic possibility of contamination from the shatterbox. Whole or mafic rocks in the source region. rock chemical analyses of 54 samples were made by ICP-AES (Inductively Coupled Plasma�Atomic Emission Spectroscopy) at the Swedish Geological Co., Luleå. All Geochemistry calibrations were based on certified international· stan dards. The stated precision is hetter than 1.5% (relative The chemical composition of an igneous rock represent standard deviation) fo r major elements and for trace ing a liquid is a function of the composition of the elements at concentrations ten times greater than the source, amount of and conditions during partial melting detection limit (Table 1), 10% at trace elements at con and differentiation, interaction with wallrock and other centrations five times greater than the detection limit and magma, etc. Assignment of one or more of these pro 50% at the detection limit. cesses to a given suite of rocks, and quantification of the The precision was tested by duplicating the analysis of variables involved, requires extensive trace element and two samples (numbers 298 and 126). The results (Table isotope studies of fresh rocks with preserved ph�nocryst l) are generally within the precision stated by the labora assemblages. The ambition here is to obtain geochemical tory for sample 298 but deviations in Se, V, Nb and Yb information from the ultramafic to felsic igneous rocks in are clearly higher for number 126. In order to evaluate the area, emphasizing those mafic rocks which represent the accuracy of the analyses, the international standard liquid compositions, to complement field and petro BCR-1 was included twice in the sample series. The graphic data so as to create a basis for further discussion results of this test are also presented in Table l. With of the environment of fo rmation of the Koli rocks in the respect to the detection limits and reliability of published Handol area. standard values (Flanagan 1973), the results obtained
Table l. Comparison between duplicate analysesof samples 126, 298 and the international standard BCR-1, together with recommended values for BCR-1 (Fianagan 1973), and the detection limits for ICP-AES during the analytical prooedure, for evaluation of precision and accuracy.
1261 2981 BCR-1
Recommended Detection A B A B A B value limit
Si02 51.0 50.2 47.0 46.9 55.1 53.4 54.5 O.ol Ti02 1.07 1.12 1.38 1.40 2.37 2.37 2.20 O.ol Al203 15.1 15.4 16.4 16.5 13.4 13.8 13.6 0.01 Fe203 11.6 11.8 9.12 9.24 13.4 13.9 13.4 0.01 MnO 0.18 0.19 0.15 0.15 0.19 0.19 0.18 O.ol MgO 6.38 6.46 8.33 8.51 3.62 3.56 3.46 0.01 Ca O 9.02 9.30 8.97 9.18 7.02 7.37 6.92 0.01 Na20 4.38 4.32 4.81 4.85 3.49 3.46 3.27 0.01 K20 0.10 0.24 0.25 0.26 1.69 1.80 1.70 0.01 P20s 0.11 0.12 0.20 0.21 0.31 0.40 0.36 0.01 LOI 0.7 0.6 2.8 3.3 0.4 O.l Total 99.6 99.7 99.4 100.5 100.6 100.2 100.3
Se 37 42 33 33 25 32 33 2 V 304 336 213 216 364 406 399 5 Cr 27 26 249 258 36 17 17.6 5 Ni 26 23 120 124 21 14 15.8 5 Sr 175 182 215 217 313 332 330 l Ba 13 20 32 32 581 716 675 2 y 21 20 25 26 33 34 37.1 2 Zr 41 49 102 103 155 180 190 5 Nb 6 18 8 8 23 39 13.5 5 Yb 13 3 3 3 18 4 3 l 30 S. Bergman NORSK GEOLOGISK TIDSSKRIFT 73 (I993)
Tahle 2A. Whole-rock chemical analyses of metacumulate rocks and ultramafic rocks from the Rooberget ultramafic-mafic Complex. (CG - metagabbro, P - metapyroxenite, D - serpentinized dunite, S - serpentinite, T - talcschist). The oxides are given in weight-percent and elements in parts per million. The oxides are recalculated on an LOI-free basis. FeO, represents total iron recalculated as ferrous iron.
162 174 189 201 285 199 169 194 195 163 190 178 187 CG CG CG CG CG p D D D s s T T
Si02 48.1 49.2 46.8 45.3 49.7 47.0 40.4 43.2 43.0 40.2 44.6 51.4 46.9 Ti02 0.08 0.16 0.09 0.05 0.16 0.10 <0.01 0.01 0.02 0.02 0.06 0.21 0.38 Al203 18.1 18.4 16.3 24.1 20.4 11.2 O. l 1.8 1.7 0.7 0.6 2.3 7.2 Fe203 4.23 5.11 5.08 3.41 5.54 6.7 8.4 8.7 8.8 16.6 9.5 10.9 13.5 MnO 0.09 0.10 0.10 0.06 0.11 0.12 0.12 0.13 0.13 0.13 0.08 0.15 0.11 MgO 11.6 10.6 13.9 10.0 8.78 19.4 50.7 44.4 43.5 41.0 43.5 29.0 26.5 CaO 16.9 15.4 16.8 16.2 13.6 13.8 0.2 1.8 1.5 0.4 0.8 4.4 3.5 Na20 0.71 1.36 1.14 0.60 2.70 0.74 0.03 0.10 O.o? 0.05 0.03 O.Q2 0.61 K20 <0.01 0.04 <0.01 <0.01 0.11 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 P,o, Se 40 42 37 17 41 47 <2 9 9 4 7 7 17 V 84 98 89 35 115 98 <5 21 20 6 6 31 106 Cr 1031 897 551 458 658 1418 4067 2712 2172 2415 2663 3620 5243 Ni 208 140 170 155 104 375 2557 2023 1789 1094 2154 1904 1659 Sr 101 157 118 118 207 15 2 5 2 6 45 30 Ba <2 7 5 21 2 <2 <2 <2 <2 18 13 4 y <2 4 2 <2 3 2 <2 <2 <2 <2 <2 4 4 Zr <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 15 were less accurate for Ba, Cr and Yb ( one analysis) and grained mafic dykes (Table 2C), which are spatially for Nb (both analyses). The conclusion is that Yb and related to cumulate rocks and ultramafic rocks, are la Nb cannot be used in the geochemical discussion. Se, V belled depending on which of the main outcrop areas and Cr will be used with the above results in mind. they are sampled; Handol (n = 6) and Bunnersjoama The results of the chemical analyses of ultramafic (n = 5) plus Jarpån (n = 1). The mafic rocks from the rocks (n = 8) and metacumulate rocks (n = 5) are pre Bunnran Formation (Table 2D) have been divided into sented in Table 2A and the results for amphibolites three groups; extrusive rocks (n = 10), intrusive fine (n = 9) and plagiogranites (n = 2) in Table 2B. The fine- grained (n = 4) and gabbroic rocks (n = 4), respectively. Tab/e 2B. Whole-rock chemical analyses of amphibolites (A) and plagiogranites (PG) from the Riidberget ultramafic-mafic Complex. 125 126 156 157 280 281 282 283 286 124 155 A A A A A A A A A PG PG Si02 51.7 50.5 50.4 51.9 49.4 46.9 49.5 55.7 49.5 77.9 76.4 Ti02 2.24 1.13 2.74 2.40 2.00 1.86 2.03 1.44 1.39 0.15 0.21 Al203 14.7 15.5 14.9 13.9 15.0 15.4 14.9 15.2 15.5 11.7 11.8 Fe203 14.0 11.9 14.7 14.4 14.8 13.9 16.2 11.3 12.6 2.1 3.5 MnO 0.21 0.19 0.25 0.15 0.25 0.23 0.26 0.09 0.20 0.04 0.05 MgO 4.94 6.50 5.35 5.25 5.15 6.51 4.88 2.70 6.82 0.31 0.49 Ca O 7.34 9.36 6.79 7.19 8.35 11.1 8.18 11.6 9.75 0.5 1.4 Na20 5.02 4.35 5.12 5.06 3.92 3.38 4.45 1.43 3.77 6.87 6.33 K20 0.26 0.24 <0.01 0.16 0.36 0.30 0.35 0.13 0.15 <0.01 <0.01 P20s 0.32 0.12 0.33 0.32 0.22 0.17 0.20 0.20 0.14 <0.01 0.06 l: 100.8 99.7 100.6 100.8 99.6 99.7 101.0 99.7 99.8 99.6 100.2 LO! 0.2 0.6 0.6 0.3 0.3 0.5 0.5 0.7 0.5 0.4 0.2 FeO, 12.62 10.68 13.22 13.00 13.36 12.48 14.56 10.15 11.30 1.88 3.12 FeO,/MgO 2.56 1.64 2.47 2.48 2.60 1.92 2.98 3.76 1.66 6.16 6.37 Se 35 42 37 33 40 47 41 36 39 6 7 V 422 336 334 419 437 423 508 155 389 <5 <5 Cr 9 26 31 13 173 36 14 13 29 <5 8 Ni 25 23 16 Il 35 28 12 <5 22 65 <5 Sr 212 182 102 178 167 306 216 401 83 52 84 Ba 14 20 <2 8 34 35 33 lO 18 <2 <2 y 32 20 34 36 36 29 30 30 25 56 52 Zr 70 49 62 83 91 71 75 59 52 125 133 NORSK GEOLOGISK TIDSSKRIFT 73 (1993) Ma.fic-ultramafic rocks, Handol 31 Table 2C. Whole-rock chemical analyses of mafic dykes (H - Handi\1, BJ - Bunnersji\ama plus Jiirpån) from the Ri\dberget ultramafic-mafic Complex. 170 171 197 259 298 299 210 211 212 267 268 284 H H H H H H BJ BJ BJ BJ BJ BJ Si02 48.5 49.0 48.4 48.6 48.5 48.7 51.2 50.0 54.5 49.4 50.2 49.5 Ti02 1.52 1.23 0.95 1.27 1.45 1.48 2.18 1.83 2.10 1.57 2.27 1.67 A1203 16.6 16.3 19.5 16.0 17.1 17.1 14.2 15.0 14.9 15.6 14.6 15.9 Fe203 9.63 9.35 7.44 9.73 9.55 9.92 12.6 11.4 10.9 10.5 12.8 10.4 MnO 0.13 0.13 0.12 0.16 0.15 0.14 0.20 0.19 0.20 0.18 0.22 0.18 MgO 10.7 9.97 8.47 11.2 8.80 9.03 6.54 6.88 5.03 7.55 6.28 6.71 Ca O 7.78 9.83 10.8 8.69 9.49 7.06 8.50 10.4 7.74 12.2 10.0 10.3 Na20 4.28 3.35 3.17 4.10 5.01 5.04 4.50 4.03 3.74 2.56 3.76 4.67 K20 0.09 <0.01 1.06 0.26 0.27 0.99 0.20 0.19 0.05 0.34 0.82 O. l l P20s 0.19 0.13 0.15 0.14 0.22 0.19 0.16 0.17 0.33 0.22 0.26 0.28 99.5 99.3 100.2 100.1 100.5 99.7 100.2 100.1 99.4 100.2 101.3 99.8 LO! 3.2 2.6 2.3 2.4 3.3 2.3 0.7 I.l 1.7 1.3 0.9 1.9 8.67 8.42 6.70 8.76 8.60 8.93 11.33 10.28 9.80 9.48 11.53 9.36 0.81 0.84 0.79 0.78 0.98 0.99 1.73 1.50 1.95 1.26 1.84 1.39 Se 31 29 21 34 33 34 34 33 30 39 41 39 V 196 187 136 210 216 225 293 249 251 266 326 244 Cr 452 412 416 430 258 268 55 180 65 257 59 139 Ni 217 196 183 237 124 122 32 49 15 55 36 40 Sr 208 240 265 223 217 327 171 167 190 196 164 284 Ba 81 24 403 43 32 164 37 40 43 168 141 39 y 23 23 17 22 26 26 41 34 39 29 42 30 Zr 75 71 50 79 103 101 137 109 150 104 158 127 The uncertainty in some cases, whether a fine-grained trachybasalts-basaltic trachyandesites. This apparent al mafic rock is intrusive or extrusive, is indicated by paren kalinity is most probably due to the addition of sec theses in Table 20. The results fo r the mafic extrusive ondary sodium. This is also the case for the apparent and intrusive rocks from both units will be discussed calc-alkaline signature on the Fe01-Mg0-(Na20 + first, thereafter the remaining rocks from the Rodberget K20) diagram (Fig. 4A). Nearly all samples plotting in ultramafic-mafic Complex. the calc-alkaline field have a Na2 0/Ca0 ratio higher than normal unaltered basalts. More significant is the Zr-P205 plot (Fig. SF; Floyd & Winchester 1975) in Mafic extrusive and intrusive rocks volving less mobile elements on which most samples have subalkaline characteristics. The Zr/Ti02 ratio indicates Element mobility and c/assification. - Many elements, that nearly all samples are subalkaline basalt or andesite/ including Ca, Sr, Ba, K and Na, are susceptible to basalt (Winchester & Floyd 1977). mobility during hydrothermal alteration, low- to medium-grade metamorphism and weathering. However, some high field strength and transition elements, which Variation diagrams. - The chemical variation of igneous are of petrogenetic· interest (e.g. Ti, P, Zr, Y, Nb, V, rocks suites is conveniently displayed on variation dia Cr, Ni, Se), are considered to be relatively little affected grams where oxides, elements or ratios are plotted by metamorphism below amphibolite facies conditions against an index of differentiation. The variation of (e.g. Cann 1970; Coish 1977; Winchester & Floyd 1977; Fe01, Ti02, P2 05, MgO, Cr, Ni, Al203, MnO, Se, Zr, Y Humphris & Thompson 1978; Seyfried & Mottl 1982; and V with Fe01/Mg0 and Zr is shown in Figs. 5-7 for Shervais 1982). The results of this study indicate that the mafic intrusive and extrusive rocks in the Rodberget these considerations are also valid for the rocks analysed ultramafic-mafic Complex and the Bunnran Formation. here; the elements Ca, Sr, Ba, K and Na show significant By using several variation indices, the trends can be scatter on variation diagrams (not shown), while the compared and evaluated. Diagrams with Y as variation more stable elements correlate well with each other (e.g. index (not shown here) are very similar to those with Zr. Figs. 50, 7F, H), which shows that their concentrations An important fe ature that is immediately apparent on are not severely altered by post-magmatic processes. the variation diagrams is the almost complete overlap of Of the 30 analysed mafic intrusive and extrusive rocks rocks from the Rodberget ultramafic-mafic Complex and (Tables 2C, D), 18 are classified as basalts on the total the Bunnran Formation. The weak tendency for higher alkali-silica plot (Le Bas et al. 1986; diagram not shown Zr/Y-ratios (Figs. 7F, 8B) in the latter is not significant here), and four are basaltic andesites. Higher total alkali within the analytical uncertainties. An effort aimed to content classify the remaining eight samples in the range disprove a cogenetic relationship between the two groups w N � � � � § Table 2D. Whole-rock chemical analyses of mafic igneous rocks (E - extrusive, I - intrusive, G - metagabbro) from the Bunnran Formation. 256 257 258 260 261 262 292 293 295 296 168 184 185 263 177 183 186 196 (E) (E) (E) E E E (E) E E (E) I (l) (I) I G G G G Si02 48.7 46.6 47.9 50.0 49.3 51.7 47.6 55.9 51.1 51.2 51.3 53.8 54.6 52.2 49.8 48.5 49.5 47.9 Ti02 2.38 1.29 1.67 1.71 1.89 1.70 1.41 2.19 2.38 2.62 2.01 1.66 1.72 1.70 1.03 0.797 1.41 0.96 Al203 17.4 17.6 16.7 16.2 16.0 15.3 17.3 14.5 15.5 15.0 15.3 15.4 14.3 15.3 17.0 20.2 16.7 18.6 Fe203 12.7 10.8 10.4 9.99 11.3 10.2 9.54 11.5 12.0 13.1 11.2 9.71 10.2 10.0 8.29 6.80 9.14 7.89 MnO 0.49 0.19 0.16 0.18 0.17 0.19 0.17 0.16 0.14 0.20 0.18 0.19 0.18 0.18 0.13 0.11 0.15 0.13 MgO 5.05 8.51 7.82 8.27 6.30 5.93 9.14 3.57 4.73 6.10 6.65 5.88 5.61 6.07 10.4 6.43 8.14 9.20 Ca O 10.4 12.6 11.8 7.89 8.41 10.19 12.8 5.43 8.27 4.37 10.9 8.33 6.87 8.23 8.56 13.5 9.40 12.2 Na20 3.28 2.98 3.97 4.73 4.36 5.02 2.44 4.77 5.20 4.61 2.76 4.61 3.79 4.91 3.93 2.33 4.39 2.44 K20 0.26 0.33 0.42 0.92 1.75 0.78 0.40 0.89 1.00 1.90 0.45 0.60 1.80 1.57 0.06 0.34 0.46 0.15 0.22 0.14 P20s 0.27 0.25 0.24 0.31 0.37 0.25 0.23 0.52 0.47 0.49 0.21 0.21 0.22 0.27 0.21 0.08 Total 100.9 101.1 101.0 100.2 99.9 101.2 101.0 99.5 100.7 99.7 100.9 100.4 99.3 100.5 99.4 99.0 99.5 99.7 LO! 1.0 2.4 1.7 0.5 2.8 5.1 1.3 0.3 4.9 4.1 1.4 0.8 1.1 2.9 2.6 1.4 3.2 1.7 FeO, 11.45 9.68 9.34 8.99 10.18 9.17 8.59 10.38 10.78 11.82 10.04 8.73 9.19 9.00 7.46 6.12 8.23 7.10 FeO,/MgO 2.27 1.14 1.19 1.09 1.62 1.55 0.94 2.91 2.28 1.94 1.51 1.49 1.64 1.48 0.72 0.95 1.01 0.77 Se 49 41 39 37 33 34 37 29 27 29 35 32 31 35 22 25 33 22 V 374 284 243 245 242 242 226 167 255 301 264 228 228 232 137 139 196 144 Cr 61 76 260 83 50 65 288 38 16 19 211 193 117 68 350 302 261 288 Ni 39 48 74 23 14 23 145 25 lO 9 41 16 99 22 171 72 65 174 Sr 104 294 230 355 266 317 253 134 200 194 204 190 199 348 516 290 242 245 Ba 13 52 108 207 654 194 58 132 190 446 80 57 189 318 60 138 94 54 y 40 25 30 32 34 28 26 61 39 43 36 35 36 27 18 16 25 18 Zr 175 89 122 149 171 129 99 361 189 209 132 145 146 127 79 50 96 52 � � � Cl � � :l � "' � "' � <:! :g � NORSK GEOLOGISK TIDSSKRIFf 73 (1993) Mafic-ultramafic rocks, Handol 33 FeOt A) B) 12 . . . 10 • . . · . 'h Fe0t : • l:' . � � 6 C) D) .. • . • .• . • . • .. .. # • D � Ti02 DA lb ,p• � . o E) F) 0.5 . 0.4 0.3 1'205 Naz<>+KzO Mg O O.l 0.0 G) H) lAT MORB • 300 . . ••• ·f.. . " • V \� "'. . . Ab03 100 o o 2 100 300 FcOt/MgO Fig. 5. Variation diagrams showing the variation of FeO,, Ti02, P205 and V with FeO,/MgO and Zr for metadolerites from the Riidberget ultramafic-mafic Complex (Handiil - open squares, Bunnersjiiama and Jårpån - open triangles) and mafic extrusive rocks (filled circles), metadolerites (filled triangles), and metagabbros (filled squares) from the Bunnran Formation. Volcanic rock series of abyssal tholeiites (A) and Macauley Island tholeiites (M) on the FeO,-FeO,/ MgO and Ti02-Fe0,/Mg0 diagrams are from Miyashiro (1975). The line dividing alkaline and subalkaline basalts on the Zr-P205 diagram is from Floyd & Winchester (1975). The dividing line between MORB (mid-ocean ridge basalt) and istand are tholeiites (lAT) on the Zr-V diagram is from Hawkins (1980). on the Fe01-Fe01/Mg0 diagram (Fig. 5A). A general decrease in Al20J> MgO, Cr and Ni (Figs. 6A-H), also more or less pronounced in the more mobile CaO and Sr (not shown), indicates that olivine, plagioclase, Cr-spinel and/or clinopyroxene were the major crystallizing phases CaO during differentiation. The vague curvatures of the trends show that the separating phases were changing composi Fig. 4. A. Fe0,-(Na20+ K20)-Mg0 and B. AI203-Ca0-Mg0 triangular tion during the crystallization process. A break in slope diagrams showing dunite and serpentinite (crosses), talc-schist (bold crosses), metapyroxenite (filled square), layered cumulate rocks (filled circles), Fe-Ti-am at Fe01/Mg0 = 1.3 and Zr = 130 ppm for MnO and in phibolites ( open circles) and felsic rocks ( open squares). Shaded field covers the particu1ar Se (Figs. 7A-D) is thought to reflect the distribution of mafic extrusive and intrusive rocks from the ROdberget ultramafic beginning of extensive crystallization of clinopyroxene. mafic Complex and the Bunnran Formation. Fields for metamorphic peridotites (MP) and ultramafic and mafic cumulates (UC, MC), positions of average This pattem of olivine and plagioclase crystallization mid-Atlantic ridge basalt (large open circle) and the Skaergaard liquid trend prior to pyroxene is typical for low pressure fractionation (SLT) are from Coleman (1977). Dividing line between tholeiitic (TH) and in ocean-floor basalts (e.g. Bender et al. 1978) and has calc-alcaline (CA) composition is from lrvine & Baragar (1971). been verified both experimentally and from natural sam using the variation diagrams ( Russell & Stanley 1990) ples. The Si02-content has been shown to be constant or has failed. The significance of the chemical similarity will to slightly increase during early differentiation. be discussed later. Relatively tight positive correlations between Zr, Y A differentiation trend similar to that of abyssal tholei and Ti02 are consistent with their behaviour as incom ites is indicated on the Ti02-Fe01/Mg0 diagram (Fig. patible elements. The most evolved extrusive rock falls 5C; Miyashiro 1975), but is considerably less apparent clearly off the trend for Ti02, and also for Fe01 and V 34 S. Bergman 12 D A) ,o B) � lO D � . � " "li" · . . . . . MgO . · . · . . "!� ·· . ,. . • . l . 4 . ' . 2 .. o C) "'. D�4 ...... • . 100 ...... , ...... Cr . . .. lO E) F) " ... oee. 100 " . . . • .,· . • .... . Ni •t •• ...... 10 .. . .. G) H) 20 . . D D . . 18 . . . .· A120:3 " � . 16 . q, • . 6 • • ·. " ...... • ·� to. • . . • ...... 14 . o l 2 100 300 feOt/MgO Zr Fig. 6. Variation diagrams showing the variation of MgO, Cr, Ni and Al203 with Fig. 7. Variation diagrams showing the variation of MnO, Se, Y and Zr with FeO,/MgO and Zr. Symbols as in Fig. 5. FeO,/MgO and Zr. Lines of Zr/Y are from LeRoex et al. ( 1985), where N·type MORB has a Zr/Y-ratio of 2.2-4.2; T-type has 3.1-4.7 and E-type 7.1. Trends for N-type and E-type MORB on the Y-Ti02 diagram are from Perfit et al. 1980). 5. (Figs. 5A-D, G-H, 7H), suggesting saturation of the ( Symbols as in Fig. melt with respect to an Fe-Ti-bearing phase at minimum concentrations of Zr = 200 ppm and Ti02 = 2.6% (cf. all samples plot within the appropriate MORB field, DeLong & Chatelain 1990). These considerations are although the different fields overlap in many diagrams. based on only one sample and must therefore be viewed Some of the more evolved samples tend towards having with caution. There is no evidence for apatite removal in an apparent within-plate basalt-signature (Figs. 8A, B, the P205-diagrams (Figs. 5E-F). D). The analytical uncertainties for V and Cr (see above) do not affect the conclusions based on the discrimination Discrimination diagrams. - The use of discrimination diagrams (Figs. 5H, 80-F). diagrams is a convenient way to characterize mafic ig Some MORBs deviate fr om normal (N-type) MORB neous rocks and to compare them with recently fo rmed by being enriched (E- or P-type) in fo r example the rocks in well known tectonic environments. Several dia incompatible elements K, Rb, Ba, Nb and light rare grams have been presented (e.g. Pearce & Cann 1973; earth elements and by having distinct isotopic signatures Floyd & Winchester 1975; Miyashiro 1975; Pearce 1975, (e.g. Schilling 1975; Sun et al. 1979; Wood 1979; LeRoex 1980; Shervais 1982) using both incompatible (e.g. et al. 1983). These features are indicative of heterogenei Ti, Zr, Y, Nb, V) and compatible (e.g. Cr, Ni) elements. ties in the suboceanic mantle. The slopes on the Zr/Y The diagrams are empirical, and some caution should be and Y /Ti02-diagrams ( Figs. 7F, H) suggest closest taken in applying them, although explanations fo r the affinity to N- or T-(transitional-) type MORB (Perfit et element variations in different tectonic environments are al. 1980; LeRoex et al. 1983, 1985). given in terms of different source characteristics, melting conditions, etc. (e.g. Pearce & Norry 1979). Further more, the fields in many diagrams overlap and may not Ultramafic rocks give an unequivocal result. In the discrimination diagrams ( Figs. 5H, 8A-F) the The distribution of all analysed ultramafic-felsic samples rocks in both the Rodberget ultramafic-mafic Complex is shown on the triangular dia grams in Figs. 4A-B. The and the Bunnran Formation give a consistent indication best preserved ultramafic rocks are characterized by high of affinity to mid-ocean ridge basalt (MORB). Most or amounts of MgO, Cr and Ni and low Al203 contents Mafic-ultramafic rocks, Handol 35 NORSK GEOLOGISK TIDSSKRIFT 73 (I993) 10 �------� A) • 1� �------� Ti <>l D) l 1000 Cr 100 lA 10 lO B) l lO 100 l 000 l� Zr/Y y 10 �------� E) !+---��----� MORB 10 100 1000 ·� . Zr Ti02 a• 6Ai' a • ø l •"'J • Ti/100 lAB C) O.l l lO 100 l 000 l� Cr 600 F) 500 Zr 400 V 300 200 100 o o 4000 8000 12000 16000 Ti Fig. 8. Discrimination diagrams (symbols as in Fig. 5): A. FieJds of are Javas (lAB), MORD and within-pJate Javas ( WPB) on the Zr-Ti01 diagram (Pearce 1980). 8. FieJds of basalts of island-are tholeiitie, MORD and within-p1ate basalt affinities on the Zr-Zr/Y diagram (Pearce 1980). C. Ti-Zr-Y triangular diagram (Pearce & Cann 1973) where MORD p1ot in field 8, LKT (low-K tholeiites) in fields A and B, CAB (cale-alkaline basalts) in fields B and C and WPB (within-p1ate basalts) in field D. D. Fields of basalts of island-are tholeiite, MORB and within-plate basalt affinities on the Cr-Y diagram (Pearce 1980). E. Dividing line between ocean-floor basalts and island are tho1eiites on the Ti01-Cr diagram (Pearce 1975). F. Fields of lAB (island-are basalts), MORB and alkaline rocks on the Ti-V diagram (Shervais 1982). 36 S. Bergman NORSK GEOLOGISK TIDSSKRIFT 73 (1993) and plot within the field of metamorphic peridotites than in the metadolerites (Table 2B, C). High Fe01/Mg0 ( Coleman 1977). Their chemical characteristics are more ratios and low Cr and Ni contents suggest a higher typical of tectonites than of cumulates following the degree of differentiation, but this explanation is not in classification of ultramafic rocks in the Scandinavian accordance with the generally low abundance of Zr. A Caledonides of Qvale & Stigh ( 1985). The talc-schist higher degree of partial mel ting of the same source as the samples have higher contents of Si02, Ti02, Al203, other groups would dilute the Zr contents but this fails CaO, Sr, Zr, Y, V and are poorer in MgO than the best to explain the high Fe01, V and Ti02 • A mantle source preserved ultramafic rocks. of different bulk chemical and/or mineralogical composi Microprobe analyses of olivines have been carried out tion or a high degree of partial melting including a using a CAMECA SX 50 electron microprobe at the mantle phase rich in high field strength elements could Department of Mineralogy and Petrology, Uppsala. explain the observed pattern fo r these rocks. However, Large, old, strained and small, young, strain-free olivines the preferred interpretation of the origin of this group of fall in the compositional range forsterite = 92.2-92.6% rocks is by accumulation of crystals from an evolving and NiO = 0.34-0.40% in good agreement with previous basaltic liquid. Addition of variable amounts of horn analyses (Filippidis & Annersten 198 1). Such high NiO blende and Fe-Ti-oxide to a differentiated basaltic liquid contents are characteristic of mantle-derived olivines could explain the relatively high contents or steep trends (Sato 1977), which supports a previous study indicating of Fe, Ti, V and Y, the diluted concentration of the that early olivines were deformed at upper mantle condi incompatible element Zr and the small amounts of the tions (van der Kamp 1974). This, together with the compatible elements Cr and Ni. This is supported by almost constant composition of the different olivine gen Figs. 9A-D, where the Fe-Ti-amphibolites may be in erations, argues against the suggestion by Stigh ( 1979) terpreted as complementary cumulate rocks to more that the olivine-rich rocks are metamorphosed serpen evolved liquids along the inferred liquid line of descent. tinite protrusions, since olivines having that origin would The more evolved part of the latter is controlled by only have been more forsterite rich (Filippidis & Annersten one sample, however. 198 1; cf. Sj ostrom 1986). The olivine data also argue Hornblende-ilmenitejmagnetite-bearing gabbros or mi against an origin as a layered intrusion fo r the Rodberget crogabbros chemically similar to the Fe-Ti-amphibolites ultramafic-mafic Complex. have been described from for example the Mid-Atlantic Recent microprobe data yield different compositions Ridge near 24°N (Thompson 1973), the Oceanographer of chromite from dunite compared to chromite from the Fracture Zone in the north Atlantic Ocean (DeLong & Cr-rich quartzite and the melange (P. Palmblad, Upp Chatelain 1989, 1990), the Sarimiento Ophiolite Complex sala, unpublished results). If this is a result of different in southern Chile (Saunders et al. 1979) and from the origins for the chromites, more complex models than Løkken Ophiolite in central Norway (Grenne 1989). those presented in the following section will have to be considered. Fe /sic rocks The felsic rocks have the chemical characteristics of Cumulate rocks differentiates of subalkaline basalts, i.e. plagiogranites The leucocratic metagabbros have high MgO, CaO and (Coleman & Peterman 1975). These typically display low Al203 contents and low amounts of FeO, Ti02 , P205 K20 ( Summary of geochemistry Fe - Ti-amphibolites The results of the geochemical investigation suggest that It is evident on many diagrams (for example Figs. 9A the vast majority of the analysed mafic intrusive and ex D) that this group of rocks differs significantly in chemi trusive rocks are tholeiitic basalts or basaltic andesites with cal composition from the metadolerites of the Rodberget modified contents of mobile elements. Such modifications ultramafic-mafic Complex. At similar Zr-contents, Fe01, could have occurred shortly after solidification by sub Ti02, MnO, V and Y are significantly higher in most seafloorhydrothermal alteration and metamorphism and/ Fe-Ti-amphibolites and Mg<}, Al203, Cr, Ni are lower or during later regional syndeformational metamorphism. NORSK GEOLOGISK TIDSSKRIFT 73 (1993) Mafic-ultramafic rocks, Handol 37 20 �------� ) o C) A o 0o • 12 FeOt Mg O 8 10 4 o 0+-----,-----�----�----� o o 100 200 300 400 o 100 200 300 400 3 �------10000 B)o D) o o 1000 2 Cr Ti02 100 l 10 04------r----��----�----� 1+-----�----.-----r---� o 100 200 300 400 o 100 200 300 400 Zr Zr Fig. 9. Diagrams to illustrate the chemical relations between extrusive and intrusive rocks of the Rodberget ultramafic-mafic Complex and Bunnran Formation (shaded fields) and the Fe-Ti-amphibolites (open circles), layered cumulate rocks (filled circles) and metapyroxenite (filled square). The analysed igneous rocks cover a range of composi dolerite dykes, (2) a melange with mainly ophiolitic tions that are similar to those commonly found in recent de bris and min or continental components, ( 3) mafic ig ocean ftoor and in many ophiolites. The chemical varia neous sheets and ftows interlayered with metasedimen tions can be simply explained by partial fusion of sub tary rocks, formed at < 2000 m depth and with a MORB oceanic mantle, fractional crystallization in shallow chemistry, and ( 4) a varied metasedimentary succession magma chambers resulting in formation of ultramafic with comrnon quartz-rich ( continent-derived) rocks and mafic cumulates and variously differentiated liquids. probable volcanic components. No significant chemical differences have been detected Two models will be considered. One assumes a dose between the metadolerite dykes of the inferred ophiolite genetic relationship between the ROdberget ultramafic and the mafic meta-igneous rocks of the Bunnran For mafic Complex and the Bunnran Formation, the other mation. The original .distinction between these major assumes separate rock-forming events. groups was based on their associated rocks, ultramafic The first model accepts that the dykes in the ophiolite mafic or metasedimentary. In the light of their chemical and the dykes in the Bunnran Formation are equivalent similarities, a cogenetic hypothesis can be considered, and that they fed the high-level intrusions and extru although this is by no means a unique explanation of the sions. Proximity to both a continental and a volcanic data. If the most evolved extrusive sample in the Bun source permitted high sedimentation rates during mafic nran Formation is representative of the later part of the magma formation at a spreading centre. Most of the liquid evolution, then support is given to a complemen magma formed intrusions and extrusions in the overlying tary magmatic relationship with the Fe-Ti-amphibolites metasedimentary rocks rather than sheeted dykes. Geo in the Rodberget ultramafic-mafic Complex. logical and geochemical support for this interpretation has been presented above. The second model involves the formation and early accretion of the Rodberget ultramafic-mafic Complex to Models for the formation of the Koli rocks in a continental margin. This was followed by erosion, the Handol area formation of a melange and deposition of the sedimen A genetic model must account for the presence of: (l) a tary part of the Bunnran Formation during a later phase fragmented ophiolite with a MORB signature for the of rift magmatism. This model would fa vour earlier 73 (I993) 38 S. Bergman NORSK GEOLOGISK TIDSSKRIFT The general environment offormation was characterized by three input sources: an igneous source supplying magma of MORB composition, a source for quartz-rich sediments and a source for tuffaceous material. A plate tectonic scenario with these ingredients is envisaged where a spreading centre was situated dose to a continental margin and within the reach of pyrodastic material from island are volcanoes. A well-known si te of present ocean crust fo rma ti on dose to a continental margin is found in the Gulf of California, where mostly MORB magma (e.g. Saunders et al. 1982a, b) is intruded into rapidly accumulating continent-derived sediments, leading to formation of basaltic sill-sediment complexes (Einsele 1985). The spreading ridges in the Gulf of California consist of short segments separated by long transform faults. A similar situation is found off Burma in the Andaman Sea (Fig. lO; Rodolfo 1969; Curray et al. 1982; Hia Maung 1987), where oblique subduction of the Indian plate has lead to fo rmation of a pull-apart basin o with spreading centres and transform faults behind an island are system and dose to the continental slope off * volcano Burma. Unfortunately, the volcano-sedimentary succes <:!:P Outboard temmes s seamowtt sion in this basin is not well knowli.However, it is assumed • Virisen terrane here that the rock types and their relations in parts of the spreading ridge and Andaman Sea are similar to the conditions in the hetter # transform faults/ a? Tec10nically shonened known Gulf of California (with the addition of volcanic fracture zones margin of Baltica activity) and to those prevailing when the rocks of the Fig. JO. The present situation in the Andaman Sea with oblique subduction of the Handol area were fo rmed. The Andaman Sea environment Indian plate which has led to formation of a pull·apart marginal basin with a is especially appealing as a model because of the high large am ount of fracture zones and transform faults (data are from Rodolfo 1969 and Curray et al. 1982). The diversified environment conceming sources for density of transform faults and fracture zones where sediments, igneous rocks, ophiolite fragments and serpentinites may be compar· diapiric and/or sedimentary melanges are likely to be able to the situation when the rocks in the Handol area were formed. Map of formed (e.g. Saleeby 1984). Sea-water circulation and Scandinavia (same scale, simplified from Stephens & Gee \985 and Stephens 1988) with positions of various localities discussed in the text. hydrati on in these zones will promote forma ti on of serpen tinite protrusions, and uplifted oceanic crust along fault scarps will be eroded to fo rm sedimentary deposits (cf. correlation (Stephens & Gee 1985) of the Koli rocks in the Karson & Dewey 1978; Simonian & Gass 1978). These Handol area with those in the Otta area in Norway, where zones of weakness may have been used to accommodate Sturt et al. ( 1991) recently described a major unconformity strain during later orogenic events and are likely to have separating a fragmented ophiolite from the overlying been obscured by overprinting. Some of them could fo ssiliferous (late Arenig-early Llanvirn) rocks. This cover presently be expressed as ultramafic beits, deformation sequence could be a facies variation correlative of the zones and terrane boundaries. Bunnran Formation. Such a history would fit some of the The Andaman Sea has already been used as an analogue isotopic evidence from the Seve Nappes (e.g. Dallmeyer for the Solund- Stavfjord Ophiolite Complex (Skjerlie & et al. 1985; Dallmeyer & Gee 1988; Mørk et al. 1988; but Furnes 1990; Furnes et al. 1990) in the southwestern see Williams & Claesson 1987) of early Caledonian tec Norwegian Caledonides (Fig. lO). Other areas which have tonothermal activity in the Baltoscandian margin. In the been compared with the Andaman Sea are the Sierra Handol area, however, other models are possible, since: (l) Nevada ophiolite belt (Saleeby 1982), where multiple separate magmatic episodes are not required by the ensimatic rifting events and sedimentation patterns record geochemical data, (2) unconformable deposition of the the production of a marginal or inter-are basin system Bunnran Formation on top of the Rodberget ultramafic adjacent to both primitive volcanic are and continental mafic Complex cannot be demonstrated, and (3) dolerite source regions, and the Devil's Elbow ophiolite remnant dykes would be expected in the dunites and serpentinites and cover in the Klamath Mountains of California (Wyld if the Bunnran Formation magmatism was a later event, & Wright 1988). but no such dykes are known. The approach taken here is that a model should not be more complex and involve a larger amount of magmatic Regional implications and tectonic events than required to explain the available information. Consequently, the firstmodel is preferred and Several authors have correlated the tectonostratigraphi will be further explored below. cally lowest Koli units in the Tannforsen region with NORSK GEOLOGISK TIDSSKRIFT 73 (1993) Mafic-u/tramafic rocks, Handol 39 500 450 400Ma (Fig. 10; Kulling 1933) in the county of Vasterbotten, Sweden. It is a volcano-sedimentary complex with a wide C bria am n Ordovician range in age (Early Ordovician to Early Silurian). The Am Lin � �in,�.:thon, major components are, from its base and upwards: (a) 1-·······- -·-·.·1 ;:te�} solitary and detrital serpentinites (with Ordovician gas Otta (post-ophiolitc sequence) o tropods; Holmqvist 1980; Bruton & Harper 1981), (b) Ankarede- - Solund-Stavfjord transitional within-plate basalts ( Stephens et al. 1985), Rangeof late Range of earl y (c) subduction-related igneous rocks (U-Pb zircon age Caledonian rift· Caledonian ophi<>- - ----- n:latcd magmatism 488 ± 5 Ma; Claesson et al. 1983), ( d) clastic rocks (Mid litcs in Scandinavia Scandinavia in Ordovician) derived from a platformaljcratonic source Fig. l/. Diagram showing time of deposition of turbidites and bentonite in the with minor mafic-ultramafic detritus, and (e) Ashgill Lower Allochthon in Jiimtland (Karis in Gee & Kumpulainen 1980), time of limestones and Llandovery phyllites with rift-related & deposition of the Otta Conglomerate (Bruton Harper 1981) unconformably basalts (Stephens et al. 1985). The clastic rocks (turbid overlying the Vågåmo Ophiolite (Sturt et al. 1991) and age data for the Ankarede Volcanite Formation in the Virisen terrane (Claesson et al. 1983), the Solund ites) were presumably easterly-derived and have been Stavfjord Ophiolite Complex (Dunning & Pedersen 1988) and the ranges in age interpreted to reflect uplift of the outer margin of Baltica for Scandinavian ophiolites and other rift-related magmatic rocks (from compila· due to an early orogenic event (Stephens & Gee 1985). ti on in Tucker et al. 1991 and Pedersen et al. 1991) . The time-scale is from Snelling ( 1985). Deposition of turbidites is also recorded in the Lower Allochthon in Jamtland where there is good stratigraphic control (Karis; in Gee & Kumpulainen 1980). units at a comparable tectonostratigraphic level further In the Scandinavian Caledonides, ultramaficbodies are north in the Swedish counties of Jamtland and Vaster mainly concentrated in the lower and upper parts of the botten (Sjostrand 1978; Beckholmen 1978, 1984; Gee & Seve Nappes and in the Lower Koli (e.g. Zachrisson Kumpulainen 1980; Sandwall 1981; Sjostrom 1983; 1969; Stigh 1979). Most of these bodies are solitary, Stephens & Gee 1985) and with units to the south at primitive ultramafic rocks and, although it was not ex Feragen and Otta in Norway (Stephens & Gee 1985). cluded that some are basal parts of ophiolites, an origin They have previously been included in the Virisen terrane as serpentinite protrusions was considered most likely (Fig. 10; Stephens & Gee 1985) and the Lower Koli (Stigh 1979; Qvale & Stigh 1985). In the Lower Koli, Terrane (Roberts 1988). However, the problems concern they are often associated with monomict detrital serpen ing tectonostratigraphic correlation of the Tannforsen tinites. According to Stigh ( 1979), these are 1ocated at Koli rocks along the mountain belt were discussed briefly various stratigraphic levels within the Lower Koli, from by Stephens & Gee (1989). As a consequence, these the lowest part (Early Ordovician or older) to the upper authors proposed a more cautious approach and placed part of the Gilliks Formation (pre-Ashgill) below the the Tannforsen rocks in a separate isolated terrane. Vojtj a conglomerate. Previously published geochronological results that may The partly well-preserved ultramafic rocks at Feragen be of re1evance for the present study are presented in Fig. (Røros area in Norway) have been interpreted to be Il . No definite age of fo rmation of the Rodberget ultra cumulates formed in a back-are environment (Moore & mafic-mafic Complex and the Bunnran Formation can be Hultin 1980). In Vasterbotten, mafic-ultramafic igneous suggested based on this compilation, but it will serve as rocks associated with clastic metasedimentary rocks have a basis for discussion. Caledonian rifting-related mafic been interpreted as having formed in relation to initial magmatism (in a convergent plate setting) is concen island-are development or back-are spreading, associated trated into two main periods at ca. 490 Ma and ca. with the fo rmation of a relatively small ocean basin close 435 Ma (data summarized by Pedersen et al. 1991 and to the margin of the Baltoscandian craton (Stephens Tucker et al. 1991). The older phase of magmatism (and 1977). However, Stephens & Gee ( 1985) found the evi tectonic activity) occurred along both the Baltoscandian dence of proximity of the Virisen terrane (Lower Koli) to and Laurentian margins that were separated at this time the Baltoscandian miogeocline difficult to reconcile with by the Iapetus Ocean. At the time of the younger mag the character of its detrital serpentinites. Therefore, they matic phase, most terranes were dose to each other as proposed that the ultramafic rocks may have originated the lapetus Ocean was in its waning stages. Correlation in the Early Ordovician in a tru1y ensimatic environment, can be made with: (a) terranes which were el ose to remote from the Baltoscandian margin. The u1tramafic Baltica in the Early Ordovician or with (b) successions in rocks were subsequently transported towards the margin the Mid-Late Ordovician-Early Silurian. These are: (a) in the Middle Ordovician, and were tectonically em the older rocks in the Virisen terrane, maybe including p1aced into the depositional basin of the Virisen terrane Otta and (b) the younger part of the Virisen terrane and immediately outboard of the Baltoscandian margin. the rocks associated with for example the Solund However, no unconformity reflecting this emplacement Stavfjord Ophiolite Complex and the igneous complex at has yet been recognized within the type areas of the Sulitjelma (Boyle 1980; Pedersen et al. 1991). Virisen terrane. The Virisen terrane as defined by Stephens & Gee An Andaman Sea-type setting (Fig. 10) will have large ( 1985) has its type area in the Bj orkvattnet-Virisen area facies variations with a dominance in different areas of 40 S. Bergman NORSK GEOLOGISK TlDSSKRIFf 73 (1993) for example are volcanic rocks, continent-derived sedi Acknowledgements. - Dr H. Sjostrom and Professor C. Talbot are gratefully mentary rocks or back-are basin oceanic crust, and differ acknowledged for comments, constructive criticism and support at various stages of the work. Dr M. Beckholmen, Mr U. B. Andersson and especially Dr M. B. ent facies may overlap in time and space. The variations Stephens gave valuable suggestions. Professor D. G. Gee gave enthusiastic in this setting may explain and integrate into a coherent support throughout the project. This study benefited from discussions in the field model the following components of the Virisen terrane: with Dr H. Sjostrom, Professor C. Talbot, Dr M. Beckholmen, Dr R. Kumpu lainen, Dr L. Bjorklund, Dr M. Piasecki and Professor D. G. Gee. Mr O. Wallner {l) the large amount of serpentinite protrusions and is thanked for preparing thin sections. I would like to thank Dr H. Furnes, Dr T. detrital serpentinites, (2) the presence of fragments of Grenne and Dr E. Zachrisson for constructive manuscript reviews. A grant from oceanicjmarginal basin crust (Handol; Feragen; possibly the Swedish Natura! Science Research Council is gratefully acknowledged. Raudfjellet in the Snåsa area in Norway, cf. Stigh 1979); Manuscript received December 1991 ( 3) the mafic intrusions and extrusions among the metasedimentary rocks, ( 4) the continent-derived meta sedimentary rocks, and ( 5) the rifted are intrusive and References extrusive rocks (e.g. Ankarede in Vasterbotten in Sweden) and distal tuffites. Andersen, T. B., Skjerlie, K. P. & Furnes, H. 1990: The Sunnfjord Melange, evidence of Silurian ophiolite accretion in the West Norwegian Caledonides. Recent tectonic models of the Scandinavian Cale Journal of the Geo/ogica/ Society, London 147, 59-68. donides are not specific conceming the polarity of the Beckholmen, M. 1978: Geology of the Nordhallen-Duved-Greningen. area in Early Ordovician Virisen are subduction zone (Stephens Jiimtland, central Swedish Caledonides. Geo/ogiska Foreningens i Stockholm Forhand/ingar /00, 335-347. & Gee 1985, 1989), but an easterly dip was preferred by Beckholmen, M. 1984: Structural and metamorphic zonation in Tiinnforsfiiltet, Gee ( 1975) and Stephens ( 1988). The model presented in western Jiimtland, Swedish Caledonides. Medde/anden frdn Stockholms Univer Fig. l O is merely an expanded version of the latter adding sitets Geo/ogiska Institution 258, 82 pp. & the complexities arising from oblique convergence leading Bender, J. F., Hodges, F. N. Bence, A. E. 1978: Petrogenesis of basalts from the project FA MOUS area: experimental study from O to 15 kbars. Earth and to a large concentration of fracture zones in the basin. If P/anetary Science Le/ters 41, 277-302. the Handol rocks are Early Ordovician in age or older, Bergman, S. 1987: A possible ophiolite at Handol, Swedish Caledonides. Geo/o then correlation with the Virisen terrane is supported. giska Foreningens i Stockholm Forhand/ingar 109, 340-343. Bergman, S. 1992: P-T paths in the Handol area, central Scandinavia: record of Another possibility is to compare with the Caledonian Caledonian accretion of outboard rocks to the Baltoscandian margin. Journal events in the Late Ordovician-Early Silurian. The Sol ofMetamorphic Geology 10, 265-281. & und-Stavfjord Ophiolite Complex (Skjerlie & Furnes Bonatti, E., Lawrence, J. R., Hamlyn, P. R. Breger, D. 1980: Aragonite from deep sea ultramafic rocks. Geochimica el Cosmochimica Acta 44, 1207-1214. 1990; Furnes et al. 1990), which has been dated to Bonatti, E., Clocchiatti, R., Colantoni, P., Gelrnini, R., Marinelli, G., Ottonello, 443 ± 5 Ma (Dunning & Pedersen 1988), has important G., Santacroce, R., Taviani, M., Abdel-Meguid, A. A., Assaf, H. S. & El characteristics in common with the rocks of the Rodberget Tahir, M. A. 1983: Zabargad (St. John's) Island: an uplifted fragment of sub-Red Sea lithosphere. Journal of the Geo/ogica/ Society of London 140, ultramafic-mafic Complex and the Bunnran Formation. 677-690. These characteristics include an ophiolite complex overlain Boyle, A. P. 1980: The Sulitjelma amphibolites, Norway: part of a Lower by a clastic sequence which contains extrusive and intrusive Palaeozoic opbiolite complex? In Panayiotou, A. (ed.): Ophio/ites, Proceedings rocks that are cogenetic with the ophiolitic rocks. Detrital International Ophio/ite Symposium, Cyprus, 1979, 567-575. Geological Survey of Cyprus, Nicosia. ultramaficrocks are also known from this area (Andersen Bruton, D. L. & Harper, D. A. T. 1981: Brachiopods and trilobites of the Early et al. 1990). The coeval rocks at Sulitjelma, dated to Ordovician serpentine Otta Conglomerate, south central Norway. Norsk Geol 437 ± 2 Ma (Pedersen et al. 1991), have also been inter ogisk Tidsskrift 61, 153-181. Cann, J. R. 1970: Rb, Sr, Y, Zr and Nb in some ocean-ftoor basaltic rocks. preted to contain an ophiolite (Boy le 1980). Earth and Planetary Science Letters 10, 7-1 1. Carter, N. L. & Ave'Lallemant, H. G. 1970: High temperature ftow of dunite and peridotite. Geo/ogica/ Society of America Bulletin 81, 2181 -2202. Cas, R. A. F. & Wright, J. V. 1988: Volcanic Successions, Modern and Ancient, Conclusions 528 pp. Unwin Hyman, London. Claesson, S., Klingspor, I. & Stephens, M. B. 1983: U-Pb and Rb-Sr data on an The combined geological and geochemical evidence Ordovician volcanic-subvolcanic complex from the Tjopasi Group, Kali Nappes, Swedish Caledonides. Geo/ogiska Foreningens i Stockholm Forhand/in favours a dose genetic relationship between the Rod gar 105, 9-15. berget ultramafic-mafic Complex and the Bunnran For Coish, R. A. 1977: Ocean ftoor metamorphism in the Betts Cove Ophiolite, mation. This implies formation of oceanic crust in a basin Newfoundland. Contributions to Mineralogy and Petrology 60, 255-270. infilled with material from both continental and volcanic Coleman, R. G. 1977: Ophiolites. Ancient oceanic lithosphere? 229 pp. Minerals and Rocks 12. Springer-Verlag, Berlin/Heidelberg. sources. Fracture zones in the oceanic crust are likely sites Coleman, R. G. & Peterman, Z. E. 1975: Oceanic plagiogranite. Journal of for formation of serpentinite diapirs and melange. The Geophysical Research 80, l 099- 1108. complex environment in the Andaman Sea can serve as a Conference Participants 1972: Penrose Field Conference: Ophiolites. Geotimes 17, 24-25. modem analogue for the site of formation of the Koli Curray, 1. R., Emmel, F. J., Moore, D. G. & Raitt, R. W. 1982: Structure, rocks of the Handol area, and can be expanded to be a tectonics, and geological history of the northeastern Indian Ocean. In Nairn, model for the entire complex igneous-sedimentary Lower A. E. M. & Stehli, F. G. 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