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Journal of the Geological Society, London, Vol. 150, 1993, pp. 771-783, 11 figs. Printed in Northern Ireland U-Pb zircon ages from the Bindal Batholith, and the tectonic history of the Helgeland Nappe Complex, Scandinavian Caledonides O. NORDGULEN 1, M. E. BICKFORD 2, A. L. NISSEN 1 & G. L. WORTMAN 2 I Geological Survey of Norway, PO Box 3006-Lade, N-7002 Trondheim, Norway 2Department of Geology, Heroy Geology Laboratory, Syracuse University, Syracuse, New York, 13244, USA Abstract: The Helgeland Nappe Complex forms part of the Uppermost Allochthon in the central Scandinavian Caledonides. It consists of metasupracrustal rocks of assumed Precambrian age which are juxtaposed with metasupracrustal rocks present as non-conformable cover sequences to subjacent fragments of Early Ordovician ophiolitic rocks. All of these rock units have been intruded by the Bindal Batholith, which ranges in composition from mafic gabbro to leucogranite. In this paper, U-Pb zircon ages from five plutons in the southern part of the batholith are presented. These are a porphyritic granite of the Andalshatten pluton (447 + 7 Ma), a tonalite from the Kr~kfjellet pluton (443 + 7 Ma), a granodiorite west of G~svassfjellet (437 + 4 Ma), the Kalvvatnet monzonite (435 + 10 Ma), and a monzodiorite near Tosbotn (430 :k 7 Ma). The dates are essentially concordant and are considered to reflect the crystallization ages of the intrusions. Based on regional correlations and the age of the oldest of the dated plutons, it is concluded that internal accretion and polyphase tectonometamorphism of the Helgeland Nappe Complex took place in Ordovician times. The final Silurian (Scandian) thrusting of the nappe stack across the continent of Baltica appears to post-date emplacement of the Bindal Batholith. The Scandinavian Caledonides are characterized by a series The Bindal Batholith and its regional context of generally westward-dipping nappe complexes commonly referred to as the Lower, Middle, Upper and Uppermost The Bindal Batholith is located within the Helgeland Nappe Allochthons (Roberts & Gee 1985; Gee et al. 1985). The Complex (Fig. 1), which is part of the Uppermost Lower and Middle Allochthons consist mainly of Protero- Allochthon in the central Scandinavian Caledonides zoic crystalline and sedimentary rocks which can be related (Ramberg 1967; Gustavson 1978, 1981, 1988; Gee et al. to the continental margin of Baltica. The Seve Nappes of 1985). In the east, the Helgeland Nappe Complex lies the Upper Allochthon are thought to represent a transition structurally above low-grade rocks (K61i Nappe Complex) of zone between this continent and Iapetus (Gee 1975), the Upper Allochthon (Foslie & Strand 1956; Lutro 1979; whereas the Uppermost Allochthon and the KSli Nappes of Dallmann 1986, 1987). In the west it lies above Proterozoic the Upper Allochthon contain rock units which are exotic to orthogneisses and medium-grade metasedimentary rocks of Baltica (Roberts 1988; Stephens & Gee 1989). The present uncertain age; this region constitutes the northern part of configuration of the nappe pile is a result of episodic the Western Gneiss Region and is commonly referred to as orogenic processes starting in the Late Cambrian to Early Vestranden (Fig. 1; Kollung 1967; Roberts et al. 1983; Ordovician and culminating with Scandian collision in the Husmo & Nordgulen 1988; Schouenborg 1989). Silurian to Early Devonian (Gee 1975; Sturt 1984; Roberts The rocks of the Helgeland Nappe Complex are & Gee 1985; Stephens & Gee 1985). commonly highly deformed and generally record medium- Granitoid plutonism of Caledonian age has been grade metamorphic assemblages. In a few areas, however, recorded in several parts of the orogen, but is essentially greenschist-facies rocks are present (Gustavson 1975; confined to the Upper and Uppermost Allochthons T0rudbakken & Mickelson 1986; Bucher-Nurminen 1988). (Stephens et al. 1985a). The rocks in the nappes are Thorsnes (1987), Nordgulen & Schouenborg (1990) and commonly intensely deformed and metamorphosed, and Thorsnes & L0seth (1991), have identified two types of fossils are extremely scarce. Consequently, lithostratigraphic metasedimentary complexes, which occur in different nappe correlation is severely hampered, and the dating of rocks units. One of these comprises migmatitic gneiss, marble and and tectonometamorphic events must largely be based on calc-silicate gneiss which may correlate with rocks of isotopic age determinations. Precambrian age further north in the Uppermost Allochthon U-Pb zircon dates on five plutons of the Bindal Batholith (e.g. Riis & Ramberg 1981; Brattli et al. 1982). These rocks in north-central Norway are presented here. Together with contrast with sequences of conglomerate, schist, psammite, earlier age determinations, the data suggest that a major calc-silicate schist and marble which are interpreted as an part of the batholith was emplaced in the period from Late unconformable cover sequence to variably-sized, mafic to Ordovician to Early Silurian. The data also provide a basis ultramafic lensoid bodies of ophiolitic rocks (Fig. 1). For upon which the tectonic evolution of the Helgeland Nappe reasons discussed by Nordgulen & Schouenborg (1990) and Complex can be assessed. Thorsnes & LOseth (1991), these deposits are interpreted as 771 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/4/771/4892568/gsjgs.150.4.0771.pdf by guest on 26 September 2021 772 O. NORDGULEN ET AL. I MO RNC ROSSVATN FIG.2 r-- HNC BOLV/ER FIG.5 e LEKA ~!!!!iiiiiiiii~iiiY ...................iii!!!ii!!!NAMSSKOGAN ...ii!!ii!iiiiiiiii ~iiiiii!i:::::::::............ iiiiiiiiiiiiiiii::: iiiiiii~i~iiiii~; t BINDAL BATHOLITH Fig. 1. Geological setting of the Bindal Batholith. The locations of Figs 2, 4 and 5 are shown on the map. Abbreviations: i I ULTRAMAFIC ROCKS HNC, Helgeland Nappe Complex; • ~ ROCKS BELOW THE KNC, K61i Nappe Complex; RNC, HNC AND RNC 0 10 25 Km R6dingsfj~illetNappe Complex; HP, t t t I THRUST Heilhornet pluton; OM, Oksdal Massif; VM: Visttindane Massif; R: ROdOy. Early Ordovician or younger in age. The rocks suffered study (Sm-Nd, Rb-Sr and Pb-Pb in feldspar) shows that polyphase deformation and metamorphism, including isotopically different crustal and sub-crustal source regions imbrication and juxtaposition with the older gneisses, prior were involved in the generation of the batholith (Birkeland to being cut by Late Ordovician plutons of the Bindal et al. 1993). Rb-Sr whole-rock dates from various rock types, Batholith (see below). and a U-Pb zircon date on the Heilhornet Pluton (Fig. 1), The Bindal Batholith embraces a wide spectrum of rock indicate ages between Late Cambrian and Silurian (Priem et types ranging from mafic gabbro to leucogranite (Kollung al. 1975; Gustavson & Prestvik 1979; Nissen 1986, 1988; 1967; Myrland 1972; Nissen 1974; Gustavson 1988; T0rudbakken & Mickelson 1986; Nordgulen & Schouenborg Nordgulen & Mitchell 1988; Barnes et al. 1992; Nordgulen 1990). However, the U-Pb dates presented here suggest that in press). Equigranular and porphyritic granites and some of the Rb-Sr dates are not reliable and that the main granodiorites are the most common rock types, but intrusive activity took place in the Late Ordovician to Early tonalites, monzonites and more basic rocks are also notable. Silurian. In the western part of the batholith, anatectic granitoids and tourmaline granites are present. The great majority of the rocks in the batholith are The geology of the dated plutons metaluminous and high-K calc-alkaline (Nordgulen in press). There is a general westward increase in 875r/86Sr initial ratios from fairly low values in the east (0.704-0.705) The Andalshatten pluton to intermediate (0.705-0.71) and high values (>0.71) in the The Andalshatten pluton (see Appendix) is located in the west (Nordgulen & Sundvoll 1992). A combined isotopic region between Velfjord in the south and Visten in the Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/4/771/4892568/gsjgs.150.4.0771.pdf by guest on 26 September 2021 AGE OF THE BINDAL BATHOLITH 773 north (Fig. 2). It consists of porphyritic granodiorite to strong, polyphase, ductile deformation. In areas of granite and has a fairly wide range in composition from comparatively low strain, an approximately layer-parallel mafic hornblende-biotite granodiorite (c. 58% SiOz) in the pressure-solution cleavage (S1) is present in calcareous northeast, to biotite granite (c. 70% SiOz) in the south and metasandstone. Generally, this structure has been obliter- southwest. Chemical data (Nordgulen, in press) show ated by a strong NE-SW, steeply dipping foliation ($2) smooth variation trends. Sr-isotope data from a number of which is axial planar to close or tight, high-amplitude folds samples plot on a Rb-Sr errorchron (MWSD=29.5) of with sharply defined closures. The long axes of inclusions 448+48 Ma with an initial ratio of 0.70855+0.00031 are generally parallel to $2 and to transposed layering in the (Nordgulen & Sundvoll 1992). Nd-isotope data give similar metasedimentary rocks, and these structures as well as tight e~,~-values between -4 and -5 for one fairly basic and one F2 folds, are cut by dykes related to the Andalshatten evolved sample (Birkeland et al. 1993). These data are pluton (Fig. 3). Thus, the structures in the metasedimentary consistent with the interpretation that the pluton constitutes inclusions, in the western part of the intrusion, show that a single magmatic unit in which internal chemical variation they were subjected to a polyphase tectonometamorphic is probably controlled by crystal-liquid processes. evolution prior to being incorporated in the Andalshatten The pluton contains small mafic enclaves which are pluton. elongate parallel to the mineral alignment. Granitic and The metasedimentary rocks can be followed along strike aplitic dykes are rare and mostly occur adjacent to or cutting to the Horn area south of Velfjord (Fig. 2), where similar rafts of metasedimentary rocks. A number of N-S-trending, rocks are present as part of the Br~nn0ysund Group grey, microporphyritic granitic dykes, which may be up to (Heldal & Hjelmeland 1987).

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