NORWEGIAN JOURNAL OF GEOLOGY Caledonian anatexis of Grenvillian crust 173 Caledonian anatexis of Grenvillian crust: a U/Pb study of Albert I Land, NW Svalbard Per Inge Myhre, Fernando Corfu & Arild Andresen Myhre, P.I., Corfu, F. & Andresen, A.: Caledonian anatexis of Grenvillian crust: a U/Pb study of Albert I Land, NW Svalbard. Norwegian Journal of Geology, Vol. 89, pp. 173-191. Trondheim 2008. ISSN 029-196X. Dating by U-Pb ID-TIMS of zircon, titanite and monazite has been carried out on orthogneiss, granite and migmatite from Albert I Land Terrane, Northwest Svalbard, to investigate the origin of this terrane and its role within the North Atlantic Caledonides. Detrital zircons in a migmatized metasedimentary rock of the Smeerenburgfjorden Complex indicate deposition after about 1070 Ma. Zircon and titanite from orthogneiss within the same complex yield an upper intercept age of 967.9 ± 4.7 Ma interpreted to date crystallization of the igneous protolith. Monazite ages of 419.7 ± 0.5 Ma from the orthogneiss are interpreted to date Caledonian reworking. Other migmatite and granite samples of the Smeerenburgfjorden Com- plex record monazite growth during a 6–8 My long period commencing at c. 430 Ma. This period concluded with the intrusion of granitoids with ages of 421.7 ± 0.6 Ma and 418.8 ± 0.7 Ma. The latter is coeval with the Hornemantoppen Batholith emplaced at 418.4 ± 0.8 Ma. The identification of late Grenvillian (Rigolet) magmatic and Caledonian (Scandian) magmatic and metamorphic events, combined with other stratigraphic analogies, supports a link with Svalbard’s Nordaustlandet Terrane as well as with Laurentian terranes in NE Greenland and the Scandinavian Caledonides. Per Inge Myhre, Department of Geosciences, University of Oslo N-0316 Oslo, Norway, now at Department of Geology, University of Tromsø, Dram- sveien 201, N–9037 Tromsø, Norway ([email protected]); Fernando Corfu and Arild Andresen, Department of Geosciences, University of Oslo N-0316 Oslo, Norway. Introduction affinities of Svalbard’s Caledonian terranes is given by Gee & Teben`kov (2004). The evolution of the western The Svalbard archipelago consists of variably deformed belts remains, however, poorly understood. The present and metamorphosed pre-Devonian basement rocks study was aimed, therefore, at resolving the metamorphic unconformably overlain by Devonian or younger cover and plutonic evolution of Albert I Land, and at establish- rocks, which continue into the Barents Sea. The base- ing a more firm basis for tectonic correlations within the ment rocks have been referred to as the “Hecla Hoek” North Atlantic realm. unit (Harland 1971) and have traditionally been inter- preted to represent the northernmost segment of the Pal- aeozoic Caledonide orogen. The appearance of distinctly Regional geology different pre-Devonian provinces within Svalbard led Harland (1971) to subdivide the Hecla Hoek unit into an NW Composite Terrane and Biscayarhalvøya Terrane Eastern, a Central and a Western Province (Harland & Wright 1979) bounded by the Billefjorden, the Central– The NW Composite Terrane comprises the Biscayar- Western and the Svalbard–Greenland fault zones. The halvøya Terrane and the Albert I Land Terrane, separated different provinces have now been renamed the Eastern, by the Raudfjorden Fault (Fig. 1). The two terranes have the North-western and the South-western Terranes (Gee similar characteristics, as they both include Grenvillian & Teben’kov 2004), which we, in line with these authors, orthogneiss and Caledonian HT/LP metamorphic rocks consider to be composite. and granites. The Biscayarhalvøya Terrane, however, also contains an upper amphibolite to eclogite-facies There have been widespread debates on the affinity of complex, known as the Richardalen HP-Complex, (Gee the E Composite Terrane of Svalbard with terranes in 1966), and deformed upper Silurian–Devonian (“Old eastern Laurentia (NE-Greenland) (Gee et al. 1995, 1999; Red Sandstone”) deposits (McCann 2000), both absent Gee & Page 1994; Johansson et al. 2001, 2005; Lyberis from Albert I Land (with the exception of some possi- & Manby 1999; Witt et al. 1998). Similar relationships bly Devonian red beds within olistostromes; Thiedig & have also been proposed for the NW Composite Terrane, Manby 1992). The HP-complex has an inferred pro- based for the most part on the Neoproterozoic supra- tolith-age of 670–645 Ma (Gromet & Gee 1998; Peucat crustal record, but also on the nature of the metamorphic et al. 1989) and underwent HP-metamorphism in the rocks (Gromet & Gee 1998; Ohta et al. 2002; Peucat et Ordovician based on an U-Pb ID-TIMS titanite age al. 1989). A thorough review of the geology and terrane of c. 455 ± 5 Ma, and similar Ar/Ar-ages, from upper 174 P. I. Myhre et al. NORWEGIAN JOURNAL OF GEOLOGY amphibolite-facies felsic and mafic gneisses (Dallmeyer Albert I Land Terrane et al. 1990; Gromet & Gee 1998). A low-grade rock unit The Albert I Land Terrane encompasses the area north structurally interlayered with the high-pressure rocks of Kongsfjorden and west of the Raudfjorden Fault zone underwent metamorphism at 430 ± 3 Ma (Gromet & (Figs. 1 & 2). Recorded geological observations from the Gee 1998). South of the eclogite-bearing complex in the area date back to the first half of the 19th century (Blom- Biscayarhalvøya Terrane, Caledonian metamorphism strand 1864), and when systematic mapping efforts com- was characterized by upper amphibolite facies meta- menced in the 1960’s the main geological features were morphism with some areas experiencing partial melting already well known (Holtedahl 1913; Schetelig 1912). (Wyss et al. 1998). Fig. 1. Bedrock map of Svalbard. RF: Raudfjorden Fault, BBF: Breibogen-Bockfjorden Fault, BF: Billefjorden Fault, EOF: Eolsletta Fault, M: Motalafjella blueschist, R: Richardalen eclogites. NORWEGIAN JOURNAL OF GEOLOGY Caledonian anatexis of Grenvillian crust 175 The Albert I Land Terrane is made up of 3 main lithotec- banded gneisses include mafic and felsic varieties as well tonic units (Fig. 2): (i) the Smeerenburgfjorden Complex as quartzite, pelite, calc-silicate and marble layers, which consisting mainly of migmatites and para -and orthog- record upper amphibolite facies conditions (Bucher neisses, (ii) the metasedimentary Krossfjorden Group, 1981). Orthogneiss occurs within the Smeerenburgfjor- and (iii) Caledonian granitoids (Dallmann et al. 2002). den Complex as smaller or larger bodies, and those com- From field evidence it is clear that the Smeerenburgfjor- monly contain xenoliths of deformed pelitic and mafic den Complex comprises migmatized parts of the Kros- lithologies, remnants of the crust into which the orthog- sfjorden Group in addition to orthogneisses presumably neiss originally intruded. The map by Dallmann et al. equivalent to the substrate of the Krossfjorden Group. (2002) places the boundary between the Krossfjorden Therefore, the Smeerenburgfjorden Complex is both Group and the Smeerenburgfjorden Complex along the younger and older than the Krossfjorden Group, and this dashed line indicated in Fig. 2. study aims to resolve some of this complexity by dating the migmatites and orthogneisses, respectively. The Krossfjorden Group The Smeerenburgfjorden Complex The Krossfjorden Group dominates the southern part of Albert I Land. It is subdivided into the Nissenfjella, Rocks assigned to the Smeerenburgfjorden Complex Signehamna and Generalfjella units (Gee & Hjelle 1966). (Fig. 2) include a wide variety of banded ortho- and The two lower units are dominated by metapelitic and paragneisses, and migmatites (Ohta et al. 1996). The metapsammitic rocks with some marbles, whereas the Fig. 2. Geological map of Albert I Land Terrane with sample locations. 176 P. I. Myhre et al. NORWEGIAN JOURNAL OF GEOLOGY upper unit is dominated by marble. The age of deposi- matite-rich (Smeerenburgfjorden Complex) unit forms tion of the metasediments is not well constrained due to a N–S-trending horst between this fault and the Raudf- metamorphic overprint, lack of fossils and lack of dat- jorden fault zone, some 10 km to the east (Dallmann et able volcanic interlayers. The distribution of different al. 2002; Gjelsvik 1979). The appearance of the migma- rock units within the Krossfjorden Group is controlled titic rocks varies from veined melanosome-dominated by large-scale west vergent isoclinal folds and a set of migmatites (Fig 3a) to rocks dominated by granitic leu- younger NNW–SSE-trending faults. A penetrative N–S cosome with only scattered biotite-rich schlieren. The striking axial planar foliation (S1) is associated with the melanosome blocks in Fig. 3a are banded (± garnet) early formed isoclinal folds (D1), and in most places is mica–schists and quartzo-feldspatic rocks and could parallel to the original bedding (S0). The dip of the foli- represent an early deposited metasedimentary unit (pre- ation is variable and appears to be related to a younger cursor). The transition between leucosome-rich and leu- fold phase (D2). The mineral assemblages (micas ± gar- cosome-poor parts of the rock mass seems to be gradual net in schist) associated with the D1 event indicate defor- and not associated with structural discontinuities, sug- mation under upper greenschist to lower amphibolite gesting that different areas in the precursor rock were facies conditions. affected by different degrees of migmatization during the same event. Two samples of leucosome-rich granite were Late granitoids taken in order to constrain the timing of the migmati- zation.