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Geology and Geochemistry of Mafic-Ultramafic Rocks (Koli) in the Handol Area, Central Scandinavian Caledonides

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Geology and Geochemistry of Mafic-Ultramafic Rocks (Koli) in the Handol Area, Central Scandinavian Caledonides 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 � Foliation 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
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