Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny? Aaron S. Yoshinobu Calvin G. Barnes Department of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA
Øystein Nordgulen Norwegian Geological Survey, Leiv Eirikssons 39, 7491 Trondheim, Norway
Tore Prestvik Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
Mark Fanning Research School of Earth Sciences, Australian National University, Canberra ACT-0200, Australia
R.B. Pedersen Geological Institute, University of Bergen, Allegaten 41, N-5007 Bergen, Norway
ABSTRACT Magmatism, contractional deformation, and extension associated with the exhumation of high-pressure rocks in the Scandinavian Caledonides are commonly attributed to the Silurian-Devonian Scandian orogeny, in which eastward thrusting of allochthonous terranes over Baltica was followed by extensional collapse and exhumation. New fieldwork and U-Pb geochronology coupled with recent pressure-temperature estimates within the highest thrust sequence of the Caledonian orogen indicate that an earlier phase of westdirected contractional deformation was punctuated by migmatite-producing events and voluminous magmatism ca. 477–466 Ma and ca. 447 Ma, followed by exhumation in the Late Ordovician. Al-in-hornblende and GASP thermobarometry indicate that emplacement of a suite of 448–445 Ma plutons caused partial migmatization at pressures of 700– 800 MPa. Subsequent isothermal exhumation to pressures of 400 MPa occurred while the host rocks were still partially molten. Rates of exhumation may have ranged from 2 to 11 mm·yr21 or greater. These data provide evidence for a previously unrecognized phase of exhumation in the Caledonides and for aerially extensive west-vergent deformation. Deformation and magmatism associated with these events may be related to Taconic-age orogenesis near Laurentia, where the highest nappe sequences of the Scandinavian Caledonides probably resided during early Paleozoic time. Keywords: exhumation, Taconian orogeny, Caledonides, migmatite, SHRIMP U-Pb.
INTRODUCTION Extension and exhumation in the Caledonian orogen of Norway are generally attributed to postorogenic collapse of overthickened crust following Silurian-Devonian collision of Laurentia and Baltica (e.g., Andersen, 1998; Braathen et al., 2000). However, the kinematics of earlier episodes of deformation, as recorded in the Middle and Upper Allochthons (Andre´asson, 1994), are generally attributed to east-directed thrusting and nappe emplacement during the Late Cambrian to Early Ordovician Finnmarkian phase. Deformation during the Late Silurian to Early Devonian Scandian phase (e.g., Stephens et al., 1985) was also east to southeast directed, so that the tectonic evolution of central Norway is inferred to reflect repeated eastward emplacement of nappes onto Baltica. New U-Pb (zir-
con) ages and structural observations document Ordovician crust-melting events, west-vergent contractional deformation, and Late Ordovician exhumation. These features have closer affinities with the Taconic orogen of northeastern North America than with the Caledonian orogen as exposed in Norway. GEOLOGIC SETTING The Caledonides of central Norway consist of four distinct nappes (Lower, Middle, Upper, and Uppermost Allochthons) that were emplaced on the Baltoscandian crystalline platform during the Silurian-Devonian Scandian orogeny along east-directed thrust faults (Stephens et al., 1985). In Norway, the Lower and Middle Allochthons consist of Neoproterozoic to Ordovician sedimentary and highly deformed Precambrian crystalline rocks meta-
morphosed under subamphibolite-grade conditions. The Upper Allochthon consists of low- to high-grade volcanic, igneous, and sedimentary sequences, many clearly representing arc and ophiolite settings. Together, these three allochthons are interpreted to have originated in part within basins along the Baltoscandian margin of the Iapetus Ocean, although portions of the ophiolitic and arc terranes are probably exotic (Stephens et al., 1985). Between 658N and 668N, the Uppermost Allochthon consists of two composite nappes, the Ro¨dingsfja¨llet nappe complex and the structurally higher Helgeland nappe complex (Fig. 1A). Qualitative structural restorations (e.g., Stephens et al., 1985; Roberts and Gee, 1985) place the Uppermost Allochthon farthest outboard of the margin of Baltica prior to Scandian thrusting. Stephens et al.
q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. Geology; October 2002; v. 30; no. 10; p. 883–886; 2 figures; Data Repository item 2002106.
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Figure 1. A: Simplified tectonic map of Upper and Uppermost Allochthons in central Norway. B: Generalized geologic map displaying locations of U-Pb analyses and pressure-temperature determinations. GASP—garnet 1 aluminosilicate 1 silica 1 plagioclase geobarometer; CORD—reaction biotite 1 sillimanite 5 garnet 1 cordierite 1 melt (Spear et al., 1999; see text for discussion).
(1985) and Grenne et al. (1999) suggested a Laurentian affinity for the Uppermost Allochthon. We focus on structural relationships and magmatism in the Helgeland nappe complex and develop a hypothesis for its precollisional tectonic setting and assembly. Helgeland Nappe Complex and Bindal Batholith The southwestern Helgeland nappe complex can be divided into four nappes juxtaposed along east-dipping, west-vergent shear zones. From structurally lowest to highest these are the Sauren-Torghatten, Lower, Middle, and Upper nappes (Fig. 1B; Thorsnes and Løseth, 1991). Shear zones separating the Sauren-Torghatten from the Lower nappe, and the Lower from the Middle nappe, show down-to-the-east, normal displacement reactivation. The Sauren-Torghatten and Middle nappes consist of dismembered ophiolites (e.g., Leka ophiolite, Prestvik, 1980) unconformably overlain by metaconglomerate, pelitic schist, psammite, minor amphibolite, minor carbonate, and felsic volcanic rocks (Thorsnes and Løseth, 1991). The contact between ultramafic rocks and metasedimentary rocks dips eastward, indicating that both the 884
Sauren-Torghatten and Middle nappes are upright. The peak prograde metamorphic mineral assemblage in metapelites in these two nappes is biotite 1 muscovite 1 staurolite 1 garnet 6 sillimanite (or kyanite; Hjelmeland, 1987), indicating amphibolite facies conditions. In contrast, the Lower and Upper nappes consist of stromatic and diatexitic migmatite, pelitic schist, quartzo-feldspathic gneiss, and marble. The entire nappe sequence is intruded by the 447 Ma granodioritic Andalshatten pluton, which contains stoped blocks (k1 km2) of rocks identical to those found in the Helgeland nappe complex (Fig. 1B; Nordgulen et al., 1993). The Bindal batholith, including the Anhalshatten pluton, intrudes the Helgeland nappe complex (Fig. 1; Nordgulen et al., 1993). It spans an age range of 477–430 Ma, an interval that also encompasses at least one regional migmatization event (discussed subsequently). The oldest plutons (ca. 477–468 Ma; this paper) are peraluminous to strongly peraluminous and predominantly granitic; they were emplaced during a regional migmatization event that affected the Lower nappe. These older granites have isotopic compositions consistent with crustal sources. From 448 to 430
Ma, a variety of plutons was emplaced; they range from granite to gabbro and display chemical and isotopic evidence for mixed crust and mantle sources (Nordgulen and Sundvoll, 1992; Nordgulen et al., 1993; Birkeland et al., 1993). U-Pb (zircon) ages1 were determined for five samples from the Velfjord region by thermal ionization mass spectrometry (TIMS) methods (Table DR1; Figs. DR1–DR5; see footnote 1) and additional samples were dated by using sensitive high-resolution ion microprobe (SHRIMP I) at the Australian National University (Table DR2; Fig. DR6; see footnote 1). Two samples from the Vega pluton (Fig. 1B) were dated. This pluton carries abundant metasedimentary inclusions and has isotopic signatures indicative of a crustal source (Birkeland et al., 1993). The bestestimate SHRIMP ages (Table DR2; see footnote 1) are 468.9 6 3.2 Ma and 477 6 3 Ma (SHRIMP ages at 95% confidence interval). A strongly peraluminous, two-mica granite west of Velfjord was dated by TIMS methods. This pluton is cut by the shear zone that juxtaposes the Sauren-Torghatten and Lower nappes (Fig. 1B; Table DR1; see footnote 1). Two dated fractions result in a discordia plot (Fig. DR1; see footnote 1) with a lower intercept of 470.4 6 3.6 Ma and an upper intercept of 1308 6 140 Ma. The lower intercept is taken to represent an igneous age that is similar to the age of the Vega pluton. South of Velfjord, three large dioritic plutons yielded nearly concordant TIMS U-Pb ages of 448–445 Ma (Fig. 1B; Table DR1; Figs. DR2–DR4; see footnote 1). They intrude migmatitic gneiss and carbonate rocks of the Lower nappe (Fig. 1B), but do not intrude the Middle nappe. Barnes and Prestvik (2000) demonstrated that heat released from these dioritic plutons resulted in remelting of the host migmatites to form diatexitic contact migmatites and associated small, peraluminous granitic bodies. Two of these granitic bodies were dated, one by conventional methods and one by SHRIMP. The former yielded a 207Pb/206Pb age of 477 6 9.3 Ma (Fig. DR5; see footnote 1). This age, although consistent with the early peraluminous magmatic event, is not consistent with an origin by remelting proposed by Barnes et al. (2002). SHRIMP analyses of zircons from the second body (Fig. DR6; Table DR2; see footnote 1) show a cluster of six ages, the mean value of which is 447.1 6 3.7 Ma, and a cluster of five ages at 467.8 6 4.9 Ma. The younger age cluster is identical to 1 GSA Data Repository item 2002106, U/Pb SHRIMP and TIMS data tables, concordia plots, histograms, and Al-in-hornblende data table, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA, editing@ geosociety.org, or at www.geosociety.org/pubs/ ft2002.htm.
GEOLOGY, October 2002
Figure 2. Major tectonic elements in Ordovician evolution of Helgeland nappe complex and Uppermost Allochthon. A: Proposed location of continental slivers proximal to Laurentia (L) and formation of suprasubduction-zone Leka ophiolite above east-dipping subducting slab. B: Collision of continental fragments, preservation of ophiolitic rocks, and generation of early peraluminous plutons (e.g., Vega). C: Continued amalgamation of Uppermost Allochthon with Laurentia and loading of lower nappes prior to 448 Ma during Taconic orogenesis. Loading of nappe pile is shown schematically in pressure-temperature diagram. D: Exhumation of Helgeland nappe complex ca. 447 Ma, followed closely by initiation of west-dipping subduction and eventual closure of Iapetus during Scandian phase of Caledonide orogeny. Schematic pressure-temperature diagram illustrates known conditions for prograde metamorphism and exhumation based on this study and Barnes and Prestvik (2000); K— kyanite, S—sillimanite, A—andalusite, B—Baltica. See text for details.
that of the adjacent pluton (448 Ma) and is consistent with formation of the granitic body by contact anatexis. We interpret the older cluster to represent zircon crystallization at the end of the thermal event responsible for the older peraluminous granites and interpret the age range 477–468 Ma as a period of regional migmatization. According to this interpretation, the 477 Ma age (Fig. DR5; see footnote 1) results from zircon that grew during the older migmatite event. Thermobarometric Constraints in the Helgeland Nappe Complex Al-in-hornblende pressures of crystallization for the Velfjord plutons range from 700 to 800 MPa based on the Anderson and Smith (1995) calibrations and are consistent with pressure-temperature estimates from enclosing pelitic aureole rocks (Barnes and Prestvik, 2000). Estimates (see footnote 1) from the crosscutting Andalshatten pluton, which contains the necessary mineral assemblage for Alin-hornblende barometry, yield pressures of 670 MPa (Table DR3; see footnote 1). Contact migmatite host rocks adjacent to the Velfjord plutons provide evidence for exhumation from mid-crustal depths while the leucosomes were still partially molten. Near the pluton contacts, early kyanite is overprinted by multiple generations of sillimanite. At near-solidus conditions, cordierite (commonly included in poikilitic alkali feldspar) was a stable magmatic phase (Barnes and Prestvik, 2000). This sequence of aluminosilicate stability, combined with the Al-in-hornblende and GASP thermobarometry, indicates synmagmatic decompression and a clockwise pressure-temperature path, as shown in Figure 2 (Barnes and Prestvik, 2000). The pressuretemperature path consequently indicates that the migmatites remained partially molten durGEOLOGY, October 2002
ing exhumation to pressures of 400 MPa. New Al-in-hornblende pressure estimates from the Tosbotn pluton (Fig. 1B; Table DR3; see footnote 1) indicate that by 435 Ma, the Helgeland nappe complex was at pressures of ;400 MPa. Timing of Deformation and Rates of Exhumation The timing of juxtaposition of the nappes in the Helgeland nappe complex is bracketed by crosscutting relationships between the nappes and the plutons within the batholith. The oldest regionally deformed pluton in the Velfjord region is the 470 Ma two-mica granite, which is cut by the shear zone between the Sauren-Torghatten and Lower nappes (Fig. 1B). Therefore, contraction associated with northwest-vergent imbrication and recorded as the shear zone that cuts the two-mica granite and extension, recorded as exhumation of the contact migmatites, were complete by the time the crosscutting Andalshatten pluton solidified at 447 Ma. Exhumation of the Velfjord plutons from 700 to 400 MPa most likely occurred at 448–447 Ma, because the contact migmatites were still partially molten. Rates of exhumation can be evaluated if the temperature interval over which the contact granites and plutons crystallized and the duration of crystallization can be defined. Locally abundant retrograde muscovite in the contact granites and migmatitic leucosomes suggests final crystallization at an H2Osaturated solidus. At ;700 MPa, solidus temperatures are in the 640–680 8C range (e.g., Johannes and Holtz, 1996). Maximum temperatures for the contact granites were ;800 8C, on the basis of phase relationships and zircon saturation temperatures (Barnes and Prestvik, 2000). Because melting and formation of the contact granites and related leucosomes
occurred at 700 MPa between 447 6 2 Ma and 448 6 2 Ma and crystallization occurred at 400 MPa in the same time interval, the uncertainties in the U-Pb age data can be used to bracket the duration of cooling through the solidus. A maximum duration for crystallization of 5 m.y. yields a time-averaged exhumation rate of 2.2 mm·yr21. Given the overlap in uncertainties of the U-Pb ages, a minimum duration of 106 yr (or less) yields a timeaveraged rate of 11 mm·yr21. If durations of cooling from ;800 8C to ;660 8C are much less than 106 yr, then the exhumation rates were even faster. DISCUSSION Crosscutting relationships and U-Pb zircon ages indicate that juxtaposition of nappes within the Helgeland nappe complex, pluton emplacement, and exhumation must have followed closely in time during the Late Ordovician. This structural and magmatic evolution cannot be due to an early phase of the Scandian orogeny for the following reasons. (1) Structural vergence within the Helgeland nappe complex is predominantly top to the west (Fig. 1B); (2) Scandian, extension-related exhumation is Silurian or younger in age (Andersen, 1998; Braathen et al., 2000); and (3) existing lithologic correlations suggest that rocks of the Uppermost Allochthon are exotic with respect to the paleo–Baltican margin (Roberts et al., 2001). Instead we suggest that the Ordovician tectonic evolution of the Helgeland nappe complex is more consistent with Taconic-style deformation as documented in the northeastern United States and Canada (Stanley and Ratcliffe, 1985). SHRIMP data indicate at least two separate Ordovician zircon-crystallizing events. The first occurred in the ca. 477–468 Ma interval and culminated in generation of widespread migmatites and emplacement of the voluminous, peraluminous, crust-derived granites (e.g., Vega pluton; Fig. 1). The second, which began at 448–447 Ma, involved emplacement of isotopically complex plutons with mixed crust and mantle components, such as the Velfjord and Andalshatten plutons, and associated contact migmatization. We postulate that migmatization and emplacement of the peraluminous, older part of the Bindal batholith occurred during collision and amalgamation of continental fragments outboard of Laurentia above an east-dipping (present geography) subducting slab (Fig. 2). West-vergent structural asymmetry (this paper; Roberts et al., 2001), probable continental-shelf assemblages, faunal correlations (Robinson et al., 2001), and timing relationships are consistent with a paleogeographic setting near Laurentia in the Late Ordovician. Amalgamation of the Helgeland nappe complex with probable Lauren885
tian lithosphere occurred between ca. 468 and 448 Ma, prior to emplacement of the ca. 448– 445 Ma Velfjord plutons (Fig. 2C). Nevertheless, synmagmatic folds in the plutons and their aureoles, and axial-planar leucosomes in 447 Ma contact migmatites, indicate that contraction was also synchronous with solidification of the plutons (Barnes et al., 2002). Following collision, subduction polarity switched but magmatism continued above a west-dipping subduction zone (Fig. 2D). Extension and exhumation (Fig. 2D) were contemporaneous with, or followed closely after, contraction and pluton construction at 447 Ma, because contractional and extensional shear zones appear to be cut by the 447 Ma Andalshatten pluton. Although disagreement exists over the timing and style of Taconic deformation and magmatism (e.g., Karabinos, 2001), the lithology, structural style, and ages of magmatism in the Helgeland nappe complex are similar to those in Newfoundland (e.g., Notre Dame arc, Whalen et al., 1997; Dashwoods block, Waldron and van Staal, 2001) and in the New England Appalachians (e.g., Bronson Hill arc, Robinson et al., 2001). However, magmatism in the time-equivalent North American Taconic arcs noted here is generally metaluminous, intermediate in composition, and has isotopic signatures indicative of a mantle component (e.g., Whalen et al., 1997), in contrast to the diverse suite of 448 Ma and younger plutons of the Bindal batholith. Our working hypothesis is that the Helgeland nappe complex represents orphaned allochthonous, along-strike equivalents of these peri-Laurentian arc sequences (e.g., van Staal et al., 1998) that were stranded on Baltica during final breakup of Pangea. The geologic evolution of the Helgeland nappe complex, including west-directed thrusting and crustally derived magmatism followed by or synchronous with exhumation, may be characteristic of the northern portion of the Taconic orogen. Future geochronologic and field and petrologic studies in East Greenland should provide further insight into the nature of the northern extent of the Taconic orogen. CONCLUSIONS 1. The nappes of the Helgeland nappe complex were imbricated along east-dipping, west-vergent shear zones after 477 Ma and prior to 447 Ma. 2. SHRIMP U-Pb data indicate at least two zircon crystallizing events centered around 477–468 Ma and 448–445 Ma; both events produced voluminous crustal magmas. 3. Construction of large, 448–445 Ma Bindal plutons at ;800–700 MPa was coincident with formation of proximal (contact) migmatites. 4. Exhumation of the Bindal plutons and 886
migmatites while melt was present occurred ca. 448–445 Ma. 5. Estimates of the rates of exhumation range from 2 to 11 mm·yr21; the actual rates may have been much faster. Together, these data represent new evidence for Ordovician orogenic events in the Uppermost Allochthon and establish further links between Taconic orogenesis in North America and the tectonic evolution of exotic rocks in the central Norwegian Caledonides. ACKNOWLEDGMENTS We thank M. Barnes, G. Dumond, K. Reid, P. Beebe, K. Beebe, and T. Foster for field assistance and Petra Aune and Ha˚kon Aune for delightful hospitality in Velfjord. C. van Staal, D. Roberts, and B. van der Pluijm provided constructive and thorough reviews that are greatly appreciated. Research was supported by National Science Foundation grant EAR-9814280 and the Texas Tech University Research Enhancement Fund. Logistical support from the Norwegian Geological Survey is gratefully acknowledged. REFERENCES CITED Andersen, T.B., 1998, Extensional tectonics in the Caledonides of southern Norway: An overview: Tectonophysics, v. 285, p. 333–351. Anderson, J.L., and Smith, D.R., 1995, The effects of temperature and oxygen fugacity on the Alin-hornblende barometer: American Mineralogist, v. 80, p. 549–559. Andre´asson, P.G., 1994, The Baltoscandian margin in Neoproterozoic–early Paleozoic times: Some constraints on terrane derivation and accretion in the Arctic Scandinavian Caledonides: Tectonophysics, v. 231, p. 1–32. Barnes, C.G., and Prestvik, T., 2000, Conditions of pluton emplacement and anatexis in the Caledonian Bindal batholith, north-central Norway: Norsk Geologisk Tidsskrift, v. 80, p. 259–274. Barnes, C.G., Yoshinobu, A.S., Prestvik, T., Nordgulen, Ø., Karlsson, H.R., and Sundvoll, B., 2002, Mafic magma intraplating: Anatexis and hybridization in arc crust, Bindal Batholith, Norway: Journal of Petrology, v. 43 (in press). Birkeland, A., Nordgulen, Ø., Cumming, G.L., and Bjørlykke, A., 1993, Pb-Nd-Sr isotopic constraints on the origin of the Caledonian Bindal batholith, central Norway: Lithos, v. 29, p. 257–271. Braathen, A., Nordgulen, Ø., Osmundsen, P.T., Andersen, T.B., Solli, A., and Roberts, D., 2000, Devonian, orogen-parallel, opposed extension in the central Caledonides: Geology, v. 28, p. 615–618. Grenne, T., Ihlen, P.M., and Vokes, F.M., 1999, Scandinavian Caledonide metallogeny in a plate tectonic perspective: Mineralium Deposita, v. 34, p. 422–471. Hjelmeland, H., 1987, Tektonostratigrafi og strukturell utvikling i omra˚det Torgøya— Brønnøysund, Sørvestlige Nordland [M.S. thesis]: Bergen, Norway, 300 p. Johannes, W., and Holtz, F., 1996, Petrogenesis and experimental petrology of granitic rocks: New York, Springer-Verlag, 335 p. Karabinos, P., 2001, How do orogenies end? An example from the Taconic orogeny in the northern Appalachians: Geological Society of America Abstracts with Programs, v. 33, no. 6, p. 261. Nordgulen, Ø., and Sundvoll, B., 1992, Strontium isotope composition of the Bindal batholith,
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