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_Journal of Structural Geology 32 (2010) 1125e1136_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Magnetic fabrics as constraints on the kinematic history of a pre-tectonic granitoid intrusion, Kristineberg, northern Sweden Pietari Skyttä*, Tobias Hermansson b, Sten-Åke Elming a, Tobias Bauer a Division of Geosciences, Luleå University of Technology, SE-97187 Luleå, Sweden b Boliden Mineral AB, SE-93681 Boliden, Sweden Article history: Received 4 February 2010 Received in revised form 24 June 2010 Accepted 29 June 2010 Available online 8 July 2010 Keywords: Palaeoproterozoic Skellefte district Crustal evolution Anisotropy of magnetic susceptibility (AMS) VMS Abstract Outcrop-scale correlations of deformation fabrics with low-field anisotropy of magnetic susceptibility (AMS) measurements revealed a two-stage structural evolution of the pre-tectonic, Palaeoproterozoic Viterliden intrusion in the Skellefte District, Sweden. The first deformation event reflected wN-S compression during basin inversion and comprised reverse dip-slip shearing along major wE-W faults, whereas the low-strain lenses in between experienced penetrative deformation with a component of N-E-SW elongation along the main foliation. This event is largely responsible for the present structural geometry regionally and locally, and also for the magnetic fabric of the rocks. In particular, the sub-vertical maximum principal susceptibility axes (Kmax) within the high-strain zones are related to early dip-slip deformation, and were virtually unaffected by subsequent dextral strike-slip reactivation, which is recorded by sub-horizontal rock lineations. The strike-slip deformation reflects wE-W bulk shortening and may regionally be correlated with reverse faulting along a wN-S trending major shear zone east of the study area. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction The Kristineberg deposit is one of numerous volcanogenic massive sulphide (VMS) deposits in the Skellefte District (SD), Sweden (Fig. 1). The geometry of the Palaeoproterozoic crust within the SD is largely controlled by deformation along major fault zones, both during volcanism and sedimentation (Allen et al., 1996) and during the subsequent crustal shortening events (Bergman Weihed, 2001). However, the understanding of the geology is largely restricted to the area closely surrounding the deposits where intense ore-related alteration locally complicates the deformation pattern, just like in the Kristineberg area (?reb?ck et al., 2005). Therefore, improved constraints about regional-scale deformation processes, including deformation kinematics, are needed to understand the structural evolution of the polydeformed area. Of particular interest are constraints about tectonic movements that affected the Kristineberg deposit (Fig. 2), where sub-horizontal tectonic flow is interpreted to be responsible for the transposition of the ore body into gently west-plunging orientation at depth (Skyttä et al., 2009). More generally, knowledge about structural control on the emplacement of ore deposits and their transposition during subsequent deformation is essential for targeting new deposits. The Kristineberg deposit is hosted by felsic metavolcanic rocks that structurally overlie the composite, pre-tectonic Viterliden intrusion (1907–13 Ma; Bergström et al., 1999), which is not largely affected by ore-related alteration and appears structurally less complicated than the metavolcanic rocks. Measurements on the low-field anisotropy of magnetic susceptibility (AMS) of the Viterliden intrusive rocks, a widely used method to determine either magmatic fabrics in granitoids or solid-state fabrics in a variety of rock types (Kligeld et al., 1977; Goldstein, 1980; Rathore, 1980; Hrouda, 1982; Borradaile, 1988; Bouchez et al., 1990; Borradaile et al., 1992; Bouchez, 1997; Elming and Mattsson, 2001; Mattson and Elming, 2001; Pares and van der Pluijm, 2002, 2003; Evans et al., 2003), were therefore considered suitable for providing constraints about the regional crustal evolution. It is generally agreed that the orientation of the minimum principal axis of magnetic susceptibility (Kmin) is normal to the observed foliation plane (Khan, 1962; Hrouda, 1982; Borradaile, 1991), whereas correlating the orientations of the maximum principal susceptibility axes (Kmax) with the rock fabric is not equally straightforward. In rocks with a weak pre-deformational fabric, Kmax parallels the direction of the regional extension direction, whereas in rocks with a strong initial fabric it may parallel the initial, or be intermediate between these two (Borradaile and Tarling, 1981; Borradaile and Henry, 1997). The Kmax orientation may also reflect the finite strain state of the rock. Although the magnetic fabric in low-grade sedimentary rocks is governed by dewatering and compaction-related layer-parallel shortening (Sagnotti and Speranza, 1993), Kmax becomes parallel with the bedding cleavage intersection with increasing strain (Hrouda and Janák, 1976), indicating interference between two Metavolcanic rocks Fault surfaces Kristineberg deposit A N Profile A-A' A (N) A'(S) lineation Regional bulk shortening direction ~3 km A' Viterliden intrusion Fig. 2. A schematic block-drawing illustrating the geometric relationship between the Kristineberg ore deposit and the Viterliden intrusion, Skellefte Group metavolcanic rocks and the regional faults. Adapted from Skyttä et al. (2009). Table 1 Petrological properties of the main lithological units within the Viterliden intrusion, including the mode of occurrence of magnetite grains in the studied AMS samples. Location* Rock type Opaque minerals Mt mode of occurence** a b c d e f Ma?c minerals Amount of ma?c minerals (_) Style and or intensity of tectonic fabric Reference to ?g. I Granite mt, py, cpy, sph x (x) bt, chl w10 II Hbl-tonalite mt, py, cpy, sph, hm x (x) x bt, hbl, chl, tita w30 III Hbl-tonalite mt, py, sph IV Qtz-plg porphyritic tonalite mt, py, sph V Hbl-tonalite mt, py, hm VI Hbl-tonalite mt, py VII Qtz-plg porphyritic sph tonalite Hbl-tonalite mt, sph VIII Chl-qtz-mylonite mt, py, sph x xa xb x (x) xx xx x bt, hbl hbl chl hbl, bt hbl (x) (x) chl w30 w10 25 15e20 15 15e20 20 IX Hbl-tonalite mt, py x bt, hbl, chl, epi w20 Moderate-intense, banded Intense, sheared (dip-slip) Moderate intense, sheared (strike-slip) Weak weak to moderate intense Fig. 4c, h Fig. 4g, k Fig. 4a, i Fig. 4d Fig. 4j Figs. 4e and 7c (S-part) Weak Intense, sheared (strike-slip) Moderate Fig. 7c (N-part) Fig. 4f, l * Roman numerals refer to AMS sites, shown in Fig. 3. ** Occurence of mt: (a) equant grains, (b) elongate grains, (c) aggregates of elongated grains, (d) aggregates with no specific preferred orientation, (e) lamellae along bt and chl basal cleavage planes, (f) inclusions along bt-cleavage planes. mt ? magnetite, py ? pyrite, hm ? hematite, cpy ? chalcopyrite, sph ? sphalerite, bt ? biotite, ms ? muscovite, chl ? chlorite, hbl ? hornblende, epi ? epidote, tita ? titanite, qtz ? quartz, plg ? plagioclase. a Predominant in horizontal section. b 50% of the mt in vertical section. 1127 Ключевые слова: vicinity, shear zone, evolution, nite strain, bergman weihed, magnetite, ams signature, deposit, bouchez, grain, tectonophysics, hornblende-tonalites, viterliden, strain partitioning, shortening, parallel, deformed, ore, low-strain, mylonitic layer, scale, reverse, geology, hrouda, result, low-eld anisotropy, journal structural, rathore, tectonic lens, reverse kinematics, locality viii, ams, lens, shear, major, precambrian, dip-slip deformation, variation, basin inversion, palaeoproterozoic, fennoscandian shield, subsequent, tarling, hornblende, orientation, mt, shape, skellefte district, northern, jelinek, intrusion, deformation history, locality iii, journal, vertical, loc, vii, western, magnetic fabric, ruf, crustal, biotite, magnetic susceptibility, susceptibility, skellefte, wa, si, mineral, tectonic fabric, geophysics, neesw elongation, anisotropy degree, weak, ?reb?ck, stage, zone, ax, weihed, structural evolution, kristineberg deposit, eld, dip, ga, measured, locality, kmin, specimen, clustering, khan, tectonic, foliation, iv, rock lineation, granite, paramagnetic mineral, sub-horizontal, unit, compression, klig?eld, sample, anisotropy magnetic, wnes, locality horizontal, skytt?, kinematics, mineral lineation, geological, constraint, kristineberg area, earth, locality vii, reviews, kmax, strike-slip, oblate ellipsoid, viterliden intrusion, rock foliation, table, borradaile, area, ii, structural geology, fault, kmax orientation, trending, magnetic signature, allen, rock fabric, elongated grain, lineation, degree, low-strain lens, site, magnetic foliation, early, rochette, ellipsoid, py, deformation mechanism, equant grain, district, dextral, scatter, regional, fabric, high-strain zone, csd area, anisotropy, wnes compression, kristineberg, skytt? journal, elming, main, horizontal, horizontal level, bergman, journal structural geology, dip-slip, sub-vertical, observed, bulk, geometry, rock, event, magnetic anisotropy, equant, figs, strain, main foliation, viii, high, strike-slip deformation, billstr?m, goldstein, deformation, sweden, slip, high-strain, structural, geophysical, magnetic