Evidence for 1.82 Ga transpressive shearing in a 1.85 Ga granitoid in central Sweden: implications for the regional evolution

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1 Precambrian Research 105 (2001) Evidence for 1.82 Ga transpressive shearing in a 1.85 Ga granitoid in central Sweden: implications for the regional evolution Karin Högdahl a, *, Håkan Sjöström b a Swedish Museum of Natural History, Laboratory for Isotope Geology, Box 50007, SE Stockholm, Sweden b Department of Earth Sciences, Uppsala Uni ersity, Villa ägen 16, SE Uppsala, Sweden Received 28 February 2000; accepted 15 July 2000 Abstract Two crustal-scale shear zone systems, the Storsjön Edsbyn deformation zone and the Hassela shear zone join in the western part of central Sweden and form an anastomosing pattern of ductile shear zones. Several of these, up to 1 km wide, zones truncate an intrusion previously defined as a component of the major, ca Ga magmatic suite (the Revsund granitoids) in north central Sweden. However, within the southern part of the intrusion, titanites that are components of the fabric in a pervasively deformed shear zone (Forsaån) have a U Pb TIMS age of Ma. Zircon porphyroclasts in the same fabric yield a U Pb SIMS age of Ma, overlapping with the U Pb TIMS age of Ma for titanites from an undeformed granitoid. These results are interpreted to define the ages of ductile shearing and magma emplacement, respectively. This interpretation is supported by microstructures as well as the regional structural pattern characterised by shear zones enveloping virtually undeformed pods of the granitoid. Previously reported and locally existing magmatic flow structures indicate some syn-emplacement deformation. The kinematic results, characterised by a high proportion of pure shear within the granitoid and oblique dextral shearing along its margin, reflect strain partitioning under transpressive conditions. The kinematic and geochronological results fit into the regional framework of late-orogenic localised deformation along crustal scale shear zones. Recent, absolute datings of shear fabrics defines ca Ga as a period of shear zone activity in central Sweden. Temporally, this period overlaps with the emplacement of S-type granites and granitoids derived from deeper crustal levels. A similar situation exists in southern Finland. Structural, geochronological and geophysical data suggest that the crustal scale shear zones in central Sweden and southern Finland may be related temporally and spatially. The age and the post-tectonic nature (on a regional scale) of the dated 1.85 Ga granitoid have important regional implications. As both the age of the intrusion and the ductile shearing are older than the established age range ( Ga) of the Revsund granitoids, either an extended intrusive history for the suite is indicated, or the granitoids in this part of the type areas should be excluded from the Revsund suite. Their post-tectonic nature implies that the pervasive Svecokarelian deformation in the area occurred earlier than generally assumed ( Ga). On the other hand, approximately coeval, calc-alkaline granitoids to the south-east of the investigated area are folded and foliated * Corresponding author. Fax: addresses: karin.hogdahl@nrm.se (K. Högdahl), hakan.sjostrom@geo.uu.se (H. Sjöström) /01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S (00)

2 38 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) pervasively, indicating a later Svecokarelian evolution in that area, i.e. that the granitoids are located in domains with separate orogenic evolutions Elsevier Science B.V. All rights reserved. Keywords: Palaeoproterozic; Shear zone; Svecofennium; Transpression; U Pb geochronology 1. Introduction 1.1. Background and aim A spatial connection between granitic intrusions and large-scale deformation zones is common and well-established, while evidence for coeval intrusive and tectonic activities is a more recent finding (Hutton, 1988; D Lemos et al., 1992; Saint Blanquat et al., 1998; Brown and Solar, 1998a,b). The study of such conditions has led to refined models concerning e.g. diagnostic internal structures with respect to tectonic environment and feed-back relations between melt transfer and shear zone activity (e.g. Brown and Solar, 1998a,b). In the central part of the Fennoscandian shield, the relationship between shear zones and granitic intrusions is fundamental for the understanding of the Palaeoproterozoic crustal evolution, granitic intrusions of various ages and origins are frequent (Stephens et al., 1994, 1997; Lundqvist, 1995) and the number of identified shear zones is increasing (e.g. Ehlers et al., 1993; Kärki et al., 1993; Bergman and Sjöström, 1994; Stephens et al., 1994, 1997; Wijbrans et al., 1995; Beunk et al., 1996). So far, the few published ages of tectonic fabric in ductile shear zones, as well as indirect evidence, show that shearing overlapped the periods of granitic plutonism temporally (Högdahl and Sjöström, 1999; Högdahl et al., 1995, 1996; Korja and Heikkinen, 1995; Sjöström and Bergman, 1995; Stephens and Wahlgren, 1995, 1996; Wahlgren and Stephens, 1996; Lindroos et al., 1996). This general condition is valid both for intrusions derived from the deeper levels of the crust as well as S-type granites derived from shallower crustal levels. However, in the Svecofennian domain, the relationship between specific intrusives and shear zones is not well understood, which emphasises the need for combined geochronological and structural investigations. Our study focuses on such a relationship by defining the magmatic age of a granitic rock as well as the age of a ductile shear zone within the intrusion. Microstructures are studied to find out the physical conditions during deformation, and the role of minerals in the deformational fabric, which are suitable for U Pb analyses, e.g. titanite and zircon. Combined with structural analysis in the field, the microstructures are used to sort out the kinematic conditions along the shear zone and along one of the margins of the granitoid. Ages and characteristic features of adjacent shear zones within the granitoid are presented briefly and the local conditions are integrated into the regional picture. Three main factors outline regional implications of this work, (1) The area represents the junction of the two largest shear zones in central Sweden, one of which may continue to southern Finland (Sjöström et al., 2000). (2) The investigated intrusion has previously been referred to as a part of the Revsund granitoids (Högbom, 1894; Lundegårdh et al., 1984). These granitoids make up a major Palaeoproterozoic intrusive suite of north central Sweden and compared with their areal extent, existing age determinations are scarce. Our results have consequences for the definition of a Revsund granitoid in one of the type areas. (3) The results also indicate that pervasive Svecokarelian deformation in the area may have been earlier than generally assumed for the Palaeoproterozoic evolution of north central Sweden The Re sund granitoids: regional occurrence and local character Large massifs of Revsund granitoids (Högbom, 1894) occur in the Palaeoproterozoic paragneisses of the Bothnian Basin in north central Sweden (Fig. 1). The granitoids, dated previously at Ga (Patchett et al., 1987; Skiöld, 1988; Claes-

3 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) son and Lundqvist, 1995; Delin, 1996; Delin and Aaro, 2000), are interpreted to have a deep crustal origin (Claesson and Lundqvist, 1995) with input from the mantle (Gorbatschev et al., 1997). Their emplacement is assumed to have occurred after Svecokarelian peak metamorphism and deformation and they have, therefore, been classified as post-orogenic (Gaál and Gorbatschev, 1987), or post-kinematic. It has been suggested that the Revsund suite belongs to the Transscandinavian Igneous Belt (TIB) (Gorbatschev and Bogdanova, 1993) extending from southern Sweden to northern Norway, partly below the Scandinavian Caledonides (Fig. 1). The Revsund granitoids in Jämtland county constitute the southernmost component of this intrusive suite. Based on differences in major element composition, the granitoids in the two areas first described by Högbom (1894), were treated subsequently as separate massifs (Gorbatschev et al., 1997), a northern (Fjällsjö) and a southern massif (Fig. 2). Both intrude older Svecofennian granitoids and metasupracrustal rocks. The southern of these massifs is divided into an eastern and a western part (Gorbatschev et al., 1997) by a boundary more or less coinciding with the extension of the Hassela shear zone (HSZ), while the western margin is affected by the Storsjön Edsbyn deformation zone (SEDZ, Bergman and Sjöström, 1994) (Fig. 2). Along these zones, the granitoid has been affected by ductile deformation that has been overprinted partly by brittle structures. The composition of the Revsund granitoids ranges regionally from granite sensu stricto to granodiorite with a metaluminous affinity (Gorbatschev, 1990; Claesson and Lundqvist, 1995). In the northern Fjällsjö massif, there are also varieties with quartz-monzodioritic composition (Persson, 1978). The colour varies successively from pale grey to red and a typical feature is a coarse porphyritic texture (cf. Fig. 4a). In contrast to other areas, the Revsund granitoids in the southern massif of Jämtland are associated with Fig. 1. Simplified geological map of the Fennoscandian shield including the ductile shear zones in central Sweden. Thick, broken line shows the inferred continuation of the HSZ towards the shear zone system in southern Finland (modified after Gaál and Gorbatschev, 1987; Bergman and Sjöström, 1994; Stephens et al., 1994; Sjöström et al., 2000).

4 40 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) Fig. 2. The Revsund granitoids in county Jämtland consist of the northern Fjällsjö Massif and the Southern Massif, which is divided into a western (investigated here) and an eastern part. This boundary approximately coincides with the HSZ and the SEDZ outlines the western margin of the massif. These zones envelope the Ga Ljusdal batholith. Both zones display bulk dextral kinematics (horizontal component). In the western part of the Southern Revsund Massif the SEDZ and the HSZ converge and form a ca. 50-km wide pattern of anastomosing shear zones (modified after Bergman and Sjöström, 1994; Lundqvist, 1995).

5 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) minor pegmatites and fine-grained uranium- and thorium-rich granitic dykes (Högdahl et al., 1998). 2. Regional deformation zones: the Storsjön Edsbyn deformation zone and the Hassela shear zone On aeromagnetic anomaly maps, the SEDZ (Bergman and Sjöström, 1994) appears as a km wide and 300-km long zone between Edsbyn in the south and Lake Storsjön in the north (Fig. 2). There is independent evidence that the SEDZ continues to the north-northwest beneath the Caledonides (Bergman and Sjöström, 1994). The SEDZ essentially separates 1.8 Ga rocks (mainly Ljusdal and Revsund granitoids and older Svecofennian supracrustal rocks) to the east from a younger, ca. 1.7 Ga TIB intrusion to the west. Structural data and the recognition of various kinds of mylonites show that the SEDZ has been active repeatedly (Bergman and Sjöström, 1994). Ductile, retrograde and brittle ductile mylonites are the most common types along this deformation zone. Dextral, transpressive shearing resulting in steep stretching lineations, has been suggested either to be connected to the emplacement of the Rätan intrusion, or a post-emplacement phenomenon. In the Revsund granitoid affected by SEDZ-deformation, a coarse, dextral C/S-fabric is the dominating structure. Sinistral shear zones occur as conjugate sets, or occasionally as later overprinting structures. The HSZ is localised along the boundary between the Ljusdal Batholith and the older Svecofennian metasedimentary rocks (greywacke-schist) of the Bothnian Basin to the north (Bergman and Sjöström, 1994; Sjöström and Bergman, 1996; Fig. 2). Previously, this boundary has been referred to as a primary feature. It is a steep, WNW- to NW-striking dominantly dextral shear zone formed under wrench conditions. Narrow sinistral zones, conjugate to the dextral pattern, were probably formed during progressive bulk dextral deformation. Some of the former display a retrograde character reflecting shear during a late stage of the deformation. The dextral rotation of the HSZ into the SEDZ indicates either that the HSZ is older, or that they formed simultaneously (Bergman and Sjöström, 1994). The timing of the main ductile deformation along HSZ is bracketed by its imprint on the Ga Ljusdal Batholith (Delin, 1993; Welin et al., 1993), and its syn-metamorphic relation to the regional low-pressure metamorphism (LPM) at ca Ga (Claesson and Lundqvist, 1995). 3. Local shear zone network In the area where the HSZ joins the SEDZ, there are several local, ductile deformation zones dividing the Revsund granitoid into internally undeformed, or only slightly deformed lenses (Figs. 2 and 3). This pattern is apparent on various scales (regional to outcrop), and the deformation zones are characterised by a coarse C/S-fabric. The existence of anastomosing faults (Lundegårdh et al., 1984) coinciding partly with ductile shear zones, indicates that brittle reactivation was favoured by ductile structures. The deformation zone along River Forsaån at the southern end of Lake Locknesjön is the most pervasively deformed of the local shear zones (Fig. 3). It will be described in detail below, after a brief summary of the characteristic features of other zones in the area. East of the Forsaån zone, there is a less persistent mylonite zone lacking C/S-fabric (Fig. 3, (2)). To the west, there is a complex, ca. 5-km wide belt of anastomosing deformation zones, consisting of several localised shear zones that truncate older Svecofennian rocks as well as Revsund granitoids (Fig. 3, (4)). The foliation within the zones is steep and strike in a NNW SSE direction whereas the foliation between the zones is rotated around an axis subparallel to the stretching lineation, which plunge gently to the SSE. The indicated fold structure is probably a result of rotation of pre-existing folds into the adjacent shear zones. The most prominent mylonites in this belt of deformation zones are characterised by a strong sub-horizontal lineation and a weak, steep foliation, i.e. an L S-fabric. In areas where dif-

6 42 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) Fig. 3. Geological map of the western part of the Southern Massif in Fig. 2 showing the shear zones described in the paper (modified after Lundegårdh et al., 1984; Lundqvist, 1996; Lundqvist and Korja, 1997; Sturkell et al., 1998), (1), dextral, west-side-up shear zone at the boundary between metasedimentary rocks and the Revsund granitoid; (2), discontinuous mylonite zone; (3), Forsaån zone; and (4) a belt of anastomosing deformations zones dominated by L S-mylonites. ferent lithologies have been juxtaposed, the mylonites are typically banded. Such a mylonite, affecting early Svecofennian rocks in the central part of the deformation zone, has been dated at Ma(Högdahl et al., 1996). A prominent shear zone bounds the deformation zone to the east. In its northern part, this shear zone separates the Revsund granitoid (to the east) from metasedimentary rocks. Along with this study, recent mapping by the Geological Survey of Sweden shows that there has been intense deformation within both the granitoid and the adjacent metasedimentary rocks to the east (Lundqvist, 1996; L. Lundqvist Uppsala,

7 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) personal communication, 1997). Within the coarse Revsund granitoid east of Lake Börjesjön (Fig. 3(1)) steep, metre-wide ultramylonites occur, which strike ca. 330 and have a strong, oblique stretching lineation plunging ca. 45 to the southeast. Kinematic indicators (shear bands, rotation of gneissosity) verify oblique dextral- and southwest-side-up displacement. Such kinematic conditions have been recorded also in intensely deformed, very planar, steeply dipping metagreywackes ca. 200 m from the contact to the granitoid. In this case, the kinematics are verified by asymmetric boudinage and shear bands (C ), combined with a pronounced stretching lineation plunging ca. 45 to the southeast. A weak, shallow plunging lineation is developed locally on platy quartz. The differences in plunge between the lineations may either be the result of two distinct episodes of shearing or represent two stages of progressive shearing in a transpressive environment (cf. Tikoff and Teyssier, 1994) The Forsaån shear zone The deformation zone along Forsaån (Fig. 3, (3)) was first recorded by Högbom (1894) and interpreted as a penetrative foliation in the Revsund granitoid. It was later re-interpreted as an enclave dominated by felsic metavolcanic rocks with a minor proportion of early-orogenic granitoids and some lenses of Revsund granitoid (Lundegårdh et al., 1984; Gorbatschev et al., 1997). The re-interpretation was based on the assumption that ductile structures should not exist in rocks classified as post-orogenic. Our study supports Högbom s interpretation, i.e. the protolith is a coarsely porphyritic granitoid (cf. Fig. 4A and B). Although strain variations exist, the Forsaån section is very homogeneous compositionally, lacking lithological variations. The deformation zone is ca. 1-km wide and can be traced for more than 10 km in a NNW SSE to NW SE direction. The deformational fabric is more or less continuous through the width of the zone and the boundaries to undeformed rocks are distinct (Fig. 4B). Towards the northwest, the deformation zone follows Lake Locknesjön where it affects early Svecofennian metavolcanic rocks (Mansfeld et al., 1998). This part is recorded on Bouguer anomaly maps as a local gravity low (Sturkell et al., 1998). Phanerozoic rocks and Caledonian thrust sheets cover the continuation farther northwest. The dominating structure in the granitoid along the deformation zone at Forsaån is a coarse, penetrative, subvertical C/S-fabric (Fig. 4C), sometimes grading into pervasively deformed gneiss zones without C/S-fabric (Fig. 4D). With increasing strain, the C/S-fabric is transformed into millimetre- to metre-wide mylonites. Locally, the C/S-fabric is cut by mylonites indicating that the latter are slightly younger. Subordinate ultramylonites have been recorded, in which the foliation is defined mainly by platy quartz (i.e. recrystallised ribbons) and thin mica-rich bands. The bulk sense of shear is not obvious in the steep, NE-dipping Forsaån shear zone and kinematic indicators are contradictory. Dextral shear zones truncating a sinistral C/S-fabric exist (Fig. 4E), as well as sinistral shear zones truncating a pervasive gneissic foliation, or tensile quartz veins indicating sinistral rotation (Fig. 4F, Fig. 5A and D). Altogether, these examples indicate a sequential formation of sinistral and dextral kinematic patterns. In pervasively deformed gneissic parts, there is, at least locally, a faint asymmetric pattern indicating dextral sense of shear in sub-horizontal sections. Still, sinistral shear bands and minor shear zones dominate among the data collected (Fig. 5A). However, more important is that dextral and sinistral shear bands (C ) and minor shear zones are symmetrically arranged with respect to the pervasive, partly mylonitic (C) foliation (Fig. 4E and Fig. 5A). Stretching lineations are generally weak and dominated by gentle to moderate plunges (Fig. 5B). Constructed intersection lineations between minor shear zones or C and the mylonitic foliation (or C) tend to be perpendicular to the stretching lineations (Fig. 5C). The former also show a variation in plunge from steep to moderate within the mylonitic foliation, comparable in amount to the variation in plunge of the stretching lineations (Fig. 5B and C).

8 44 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) Fig. 4. (A) The undeformed, coarsely porphyritic granitoid sampled for U Pb TIMS titanite analyses. The sample site, ca. 1 km west of the Forsaån zone, is shown in Fig. 3. (B) Characteristic ductile mylonite truncating the porphyritic granitoid. The change from faintly gneissose granitoid to mylonite is abrupt. (C) Pervasive sinistral C/S-fabric in the granitoid along the Forsaån deformation zone. (D) High strain gneiss zone at Forsaån. The light-coloured bands are made up by polygonised, former K-feldspar phenocrysts. Typical microstructures are shown in Fig. 6B. Sampling site for the U Pb analyses (cf. Fig. 3). (E) Intensely deformed granitoid dominated by a sinistral C/S-fabric. Sinistral shear bands (C ) running from upper right to lower left are distinct in the upper right part of the picture. These C show a dextral rotation into the mylonite in the central part of the picture. The mylonite contains asymmetrically folded quartz veins demonstrating dextral shear. Some dextral C running upper left to lower right are also visible (above lens cap). (F) Tensile quartz veins displaced by a thin sinistral shear zone. Forsaån deformation zone.

9 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) The orientations of strain axes, indicated by two, conjugate dextral and sinistral shear zones, are NNW-trending (close to horizontal) X-axes (stretching), WSW-trending (close to horizontal) Z-axes (shortening) and steep Y-axes (Fig. 5C). X and Y thus plot in the fields of stretching- and constructed intersection lineations, respectively, and Z close to the field of poles to the mylonitic foliation, i.e. Z is more or less perpendicular to that foliation (Fig. 5B and C). In terms of strain axes, the distribution of stretching and intersection lineations indicates that X and Y rotate within the shear plane, while Z is less variable and more or less orthogonal to that plane. Apparently, pure shear predominated during the development of the deformation zone and resulted in the development of the S L tectonites reflected by the poorly developed stretching lineations. Such conditions would also explain a sequential development of minor shear Fig. 5. Structural data from the Forsaån deformation zone presented in an equal-area, lower hemisphere, stereographic projection. (A) Poles to steep, sinistral and dextral shear bands and minor shear zones define two groups that are symmetrically arranged with respect to the foliations shown in Fig. 5B. Predominantly pure shear is indicated. (B) The poles to foliations define a steep NE-dipping zone. Stretching lineations rotate in the shear zone but are dominated by shallow to intermediate plunges. (C) Constructed intersection lineations have steep to intermediate plunges in the shear zone. The intersection lineations plot in the field where stretching lineations are few in Fig. 5B, suggesting a perpendicular relationship between the lineations. Strain axes based on conjugate minor shear zones indicate subhorizontal stretching (X), steep intermediate axis (Y) and subhorizontal shortening (Z). Note that X plots among stretching lineations in Fig. 5B and that Y plots within the field of intersection lineations. Z is perpendicular to the shear zone, which is consistent with predominantly pure shear. (D) Schematic presentation of structures in the Forsaån section.

10 46 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) zones and the symmetric kinematic pattern shown in Fig. 5A C. An important inference is that the kinematic data from the Forsaån section (within the granitoid) deviate considerably from those recorded along the margin of the granitoid. This difference is significant for the interpretation that strain was partitioned. Some narrow, sinistral shear zones truncating the pervasive shear fabric, have a sub-horizontal lineation and tend to indicate lower magnetic susceptibility values than the gneissic zones. These shear zones may, therefore, represent a later phase of deformation under more oxidised conditions. Further diagnostic patterns will be described in the following section. Late brittle deformation is widespread in the area. It resulted in the formation of cataclasites in zones ranging from millimetres to several centimetres in width, occasionally accompanied by pseudotachylite melts. Revsund granitoids that are deformed by brittle deformation have typically a deep red colour. Open cavities sporadically host small crystals of quartz, locally together with epidote and fluorite and in places calcite Microstructures in the Forsaån shear zone Microcline porphyroclasts in the pervasive C/Sfabric at Forsaån locally show rather coarse coreand-mantle structures (Fig. 6A), formed by dynamic recrystallisation along the margins of the microcline megacrysts. Such fabrics are common in feldspars affected by deformation at temperatures of C (Passchier and Trouw, 1996), i.e. low- to medium-grade conditions. This temperature range is also indicated by other feldspar microstructures, e.g. the porphyroclasts generally lack fracturing and micro-kink-bands typical of low-grade conditions ( C). They also contain flame perthites typical of low- to mediumgrade conditions ( C), while myrmekites characteristic of medium- to high-grade conditions are absent (Passchier and Trouw, 1996). These temperature estimates are, thus, comparable to the 500 C suggested for the development of C/S-fabric in granites affected by shearing (Gapais, 1989). In more deformed parts, the porphyroclasts are polygonised entirely with well-developed triple points between the crystals (Fig. 6B). Bands of epidote with allanite cores wrap around some polygonised augen. Coexisting epidote and accessory amounts of chlorite and muscovite probably reflects saussuritisation of plagioclase; most chlorite appears to be a late replacement. The existence of titanites along C and C shows that they are part of the deformational fabric (Fig. 6C). They occur preferably in biotite and in the saussuritised bands, minor amounts are found at quartz- and feldspar grain boundaries. Consequently, the titanites used for dating are part of a thoroughly recrystallised fabric showing some neomineralisation. Both the high degree of recrystallisation and the indicated temperature range of ca C are fundamental conditions when the relationship between the obtained age and the evolution of the shear zone is considered (see below). The narrow sinistral shear zones that truncate the pervasive core-and-mantle fabrics and are characterised by intense grain-size reduction. Biotite (partly chloritised), chlorite and dynamically recrystallised quartz define a C/S-fabric; kinked and bent muscovite occurs locally (Fig. 6D). Large quartz grains show elongate subgrains and contain bands that are recrystallised dynamically. There is evidence of both grain-boundary migration recrystallisation (most common) by interlobate, highly irregular grain boundaries and subgrain rotation recrystallisation. The latter is displayed by the existence of small grains with slightly different optical orientation along grain boundaries. These recrystallisation mechanisms indicate temperatures probably exceeding ca. 400 C. The microstructures and the mineralogy indicate somewhat lower temperatures compared with that of the pervasive pattern. The microstructures of the sinistral zones also appear less evolved, and combined with the recognition in the field that they may be more oxidised (lower magnetic susceptibility values) than the gneissic zones, support the inference of a late development Microstructures along the eastern margin of the granitoid The ultramylonites developed in the granitoid close to the eastern margin (Lake Börjesjön area,

11 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) Fig. 6. Microstructures from the Forsaån section and the sheared margin to the metagreywacke in the east. (A) In the C/S-fabric, K-feldspar porphyroclasts (one in the central part) are surrounded partly by a mantle of dynamically recrystallised K-feldspar (to the left of the large grain). Forsaån shear zone. Length of photograph corresponds to 5.8 mm. X nicols. (B) With higher strain (cf. Fig. 4D) the K-feldspar porphyroclast are often completely polygonised. Locally well-developed triple points exist between newly formed crystals. Forsaån zone. Length of photograph corresponds to 5.8 mm. X nicols. (C) Titanites occur along C- (subhorizontal) and C - planes (SW NE) preferably in biotite and bands of saussurite that occasionally wrap around quartz and feldspar. Bt, biotite; Ti, titanite; Sau, saussuritised plagioclase. The sample is from a high strain gneiss zone within the Forsaån zone. Length of photograph corresponds to 3 mm. (D) The fabric in thin sinistral shear zones along Forsaån is defined by partly chloritised biotite, chlorite and dynamically recrystallised quartz. Length of photograph corresponds to 3 mm. X nicols. (E) In intensely mylonitised granitoid, -porphyroclasts (recrystallised to asymmetric ribbons in the upper central part of the picture) and C at low angle to the mylonitic foliation indicate dextral sense of shear. These microstructures are typical for the mylonites east of Lake Börjesjön (Fig. 3(1), Length of photograph corresponds to 5.6 mm. (F) Muscovite fish (large grains in the lower part of the picture) with internal strain are found in the deformed metagreywackes east of Lake Börjesjön. The crystals in the quartz plate in the upper part of the picture have boundaries indicating grain boundary migration recrystallisation. Length of photograph corresponds to 5.6 mm. X nicols.

12 48 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) Fig. 3, (1)) have a pronounced foliation made up by alternating platy quartz and bands containing fine-grained, brown biotite. The quartz plates have internal C/S-fabrics defined by optically oriented, dynamically recrystallised quartz crystals, i.e. microstructures typical of dislocation creep. Kinematic indicators are fairly common, including composite asymmetric augen, -porphyroclasts and locally existing C at a low angle to the mylonitic foliation (C, Fig. 6E). K-feldspar porphyroclasts generally show intense (static?) sericitisation. An important condition is that the ultramylonites lack chlorite and epidote minerals, i.e. exhibit no records of retrograde, low temperature deformation. In the metagreywackes ca. 200 m from the contact to the granitoid, a pseudoclastic texture is still visible in spite of strong shearing. The deformational fabric contains dynamically recrystallised quartz plates in which the crystals indicate grain boundary migration recrystallisation ( cf. above). Fine-grained, brown biotite dominates over white mica, but larger muscovite fish with internal strain (undulose extinction and rare kinkbands) exist (Fig. 6F). The microstructures in the adjacent metasedimentary rocks are, thus, comparable with those in the mylonitised granitoid, suggesting that they formed simultaneously along a major shear zone. The comparatively low metamorphic grade of the greywacke close to the intrusion, including a pseudoclastic texture, indicates that the greywacke has been juxtaposed by post-emplacement shearing. 4. U Pb geochronology 4.1. Analytical method A sample of the recrystallised C/S-mylonite from the Forsaån shear zone (Swedish national grid, /145656) was collected for U Pb titanite thermal ionisation mass spectrometry (TIMS) and zircon secondary ion mass spectrometry (SIMS) analyses to investigate both the timing of shearing and the protolith age. As a reference sample, the undeformed protolith was also collected for U Pb titanite analysis. The sampling site of the latter is ca. 1 km west of the Forsaån zone (Swedish national grid, / ) within one of the type areas for Revsund granitoids as defined by Högbom (1894) (Fig. 3). The majority of the titanites from the C/S-mylonite are smaller than 100 m and oblate shaped. Due to crushing and milling, many of the crystals were fragmented. Approximately 30 transparent crystals and fragments, free from inclusions and cracks and with a total weight of 460 g were selected for U Pb analyses. They were dissolved in HF:HNO 3 in a Savillex beaker on a hot plate for ca. 70 h. The sample was evaporated and HCl added before it was aliquoted. One aliquot was spiked with a 208 Pb 233 U 235 U tracer, and U and Pb were separated using a HBr and HNO 3 ion exchange technique. The titanites from the undeformed granitoid are distinctly different from those of the mylonite. They are generally larger ( m) and euhedral with well-developed crystal faces with sharp edges. Due to their large size, the crystals are almost opaque and to avoid problems with hidden impurities and inhomogeneous parts, only clear and inclusion free, ca. 150 m large fragments ( ca. 15 corresponding to ca. 1 mg) were selected for analysis. In this case, they were dissolved in an autoclave in 205 C for 50 h and a 205 Pb 233 U 235 U spike were used as a tracer, while the rest of the procedure was as above. The U Pb analyses were carried out on a Finnigan MAT 261 solid source mass spectrometer at the Swedish Museum of Natural History in Stockholm. Corrected isotope values, U/Pb, Pb/ Pb ratios and intercept ages were calculated using the program by Ludwig (1993, 1995). The initial lead correction was made according to Stacey and Kramers (1975) and the decay constants recommended by Steiger and Jäger (1977) were used. The zircons from the Forsaån C/S-mylonite are pale pink in colour and generally have a short prismatic habit. They have well-developed crystal faces with very sharp edges and high lustre. No cores were detected with a binocular microscope, but cathodoluminescence images showed that cores were present in some crystals, while others have regular zonation patterns typical for igneous zircons (Vavra, 1990) or slightly irregular patterns.

13 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) The zircons selected for U Pb SIMS analyses were mounted in transparent epoxy resin together with chips of reference zircon with an age of 1065 Ma (Wiedenbeck et al., 1995). The sample was polished to reveal as much of the mounted zircons as possible and coated with ca. 25 nm of gold. The analyses were performed at the Swedish Museum of Natural History, Stockholm, using the NORD- SIM Cameca IMS 1270 ion microprobe. A 4 na O 2 primary beam producing an ellipsoid analysis spot size of approximately 25 m was used, and a single electron multiplier in an ion counting mode measuring following masses, 90 Zr 2 16 O (196), 204 Pb (204), background (204.2), 206 Pb (206), 207 Pb (207), 208 Pb (208), 238 U (238), 232 Th 16 O (248), 238 U 16 O 2 (270). Detailed descriptions of the analytical and calibration procedures are given by Whitehouse et al. (1997, 1999). Corrected isotope values, using modern common lead composition (Stacey and Kramers, 1975) and measured 204 Pb, U/Pb and Pb/Pb ratios, and intercept ages were calculated using the Isoplot/Ex ver (Ludwig, 1999). The ages are calculated using the decay constants recommended by Steiger and Jäger (1977). For the regression, different line fitting models are recommended by Ludwig (1999). In this case, Model 1 has been used as the MSWD (mean square of weighted deviates) value is relatively low. This means that the scatter is assigned to analytical errors and error correlation only. Fig. 7. (A) U Pb concordia diagram from TIMS titanite analyses showing that the undeformed granitoid and the Forsaån shear zone yield ages of and Ma, respectively. The titanites from the former are 250 m and euhedral with sharp crystal faces, whereas the latter are 100 m and oblate shaped. Microstructures indicate that the titanites from the Forsaån zone grew during deformation and the age is therefore interpreted to represent the ductile shearing. Size of the error ellipsoid and uncertainties are given with 95% confidence. Isotopic data are presented in Table 1. (B) U Pb concordia diagram for SIMS zircon data from the Forsaån zone. A regression through ten analyses in the central part of the crystals yields an age of Ma. The four shaded ellipses show analyses at the margin of the zircons and have been omitted from the regression. The relative probability plot (inserted diagram) shows a slight uneven distribution, indicating a younger overprinting/lead loss. Sizes of the error ellipsoids are given in 1 and uncertainties with 95% confidence. Isotopic data are presented in Table Results The titanites from the C/S-mylonite in the Forsaån shear zone yield an almost concordant result of Ma (Fig. 7A). The best estimate of the age is given by the 207 Pb 206 Pb data on which the diminutive discordance has an insignificant influence. The age is interpreted to be that of the deformation. There are two specific reasons for this interpretation, which are both based on the microstructures recorded. 1. The titanites are part of the deformational fabric, i.e. they grew/recrystallised during the deformation, which resulted in a thorough reworking of the granitoid in the deformation zones.

14 50 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) The microstructures indicate that deformation took place below the C closure temperature for Pb diffusion in titanite of this size (Scott and St-Onge, 1995), which means that no substantial diffusional Pb-loss occurred after the titanite formation. The titanites from the undeformed reference sample of the granitoid yield a considerably higher age than the titanites from the mylonite. The U Pb analysis is slightly discordant with a 207 Pb/ 206 Pb age of Ma (Fig. 7A). Cathodoluminescence images on zircon from the Forsaån C/S-mylonite show a large variation of internal structures including inherited cores. The latter were not analysed and no systematic age variations with respect to different cathodoluminescence images were found among the others. However, analyses (four points) made close to crystal edges tend to yield somewhat lower ages than analyses from the central parts. This trend is visible in the asymmetrical relative probability plot (Fig. 7B). Regression through ten points analysed in central parts of the crystals yield an age of Ma with an MSWD value of 0.58 (Fig. 7B). Some spots are reversed discordant, a phenomenon, which is not uncommon in analyses made by the SIMS technique. This could be an instrumental artefact, but micro-scale heterogeneities, with lead gain or uranium loss in the analysed part of the crystals, cannot be excluded. The zircon age overlaps with the titanite age of the undeformed granitoid. It is interpreted to reflect the minimum magmatic age of the protolith, since some lead loss could have occurred during the subsequent shearing and later reactivations. These events are most likely the reason for the younger overprinting/lead loss in the outer parts of the crystals. 5. Discussion 5.1. Regional implications of geochronological and structural data The magmatic age of the granitoids studied, i.e. in the western part of the southern Revsund massif in the Jämtland county, is constrained to ca Ma by independent and overlapping zircon and titanite analyses. In spite of being collected from a C/S-mylonite, the zircons partly show typical magmatic zonation patterns and the titanites from the reference sample are part of an isotropic magmatic fabric. The titanite age is also in accordance with an MaU Pb SIMS result on zircon from the same undeformed granitoid, and supported by and Ma obtained from other K-feldspar megacryst bearing granitoids in the vicinity (Högdahl and Sjöström, 2000). The large difference in titanite habit between the undeformed reference sample and the C/S-mylonite shows that the titanites in the latter are newly formed/recrystallised. This difference is an additional support for the interpretation that the Ma result is the age of the shear fabric. The character of the microstructures shows that they were arrested at medium to low grade conditions after the solidification of the intrusive. Nevertheless, in the southern massif, locally existing magmatic foliations indicating syn-magmatic deformation have been reported (Gorbatschev et al., 1997). Such structures may represent end members of continuous deformation from magmatic to solid-state conditions, as demonstrated in the Mono Creek granite in the Sierra Nevada batholith (Saint Blanquat and Tikoff, 1997). In that granite, a magmatic fabric changes progressively to a solid-state high-temperature fabric, and finally to a low-temperature fabric within a shear zone. However, in our study, information is lacking on the spatial distribution and frequency of magmatic foliations, and most important, anisotropy of the magnetic susceptibility data (AMS) necessary to decipher magmatic fabrics existing in texturally isotropic rocks, are too scarce. Therefore, the present data is not conclusive for whether the deformation was continuous or discontinuous. In addition, the transpressive conditions, defined by structural data from the shear zones in the southern massif, were arrested in a solid-state environment. The Ma Sierra Nevada batholith shows that kinematic conditions change during the period of magmatism (Saint Blanquat and Tikoff, 1997; Saint

15 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) Blanquat et al., 1998). Consequently, the transpressive conditions at 1816 Ma cannot be applied to describe the syn-magmatic evolution of the ca Ma granitoid. On a regional scale, the Revsund granitoids have been interpreted to truncate the structures in the older, pervasively deformed gneisses (Stephens et al., 1994). This pattern has also been observed in the western part of the southern massif (Gorbatschev et al., 1997). Apparently, much of the regional deformation must have preceded ca Ga. Consequently, the peak orogenic deformation in the investigated area appears to be earlier than the generally assumed interval of ca Ga (e.g. Stephens et al., 1997). However, the inferred age of the regional low-pressure metamorphism ( ca Ga, Claesson and Lundqvist, 1995) and the approximately coeval Forsaån shear zone would represent a second tectonometamorphic episode. The Ma age of the protolith in the Forsaån C/S-mylonite and the Maage of the reference sample are both anomalous compared with the previously dated Ga age range of Revsund granitoids. As this time interval is based on a few precise age determinations scattered over ca km 2, our results may reflect that the emplacement period for the suite was more extended than assumed generally. However, the unexpected ages add to other anomalous features of the southern Revsund massif; the untypical association of pegmatites and U- and Thrich dykes and a deviating geochemical signature compared with normal Revsund granitoids (M. Ahl, Stockholm, personal communication, 1997; Gorbatschev et al., 1997). Altogether these anomalies challenge the interpretation that this part of the massif consists of Revsund granitoid according to the presently applied definition, in spite of its location within a type area originally defined by Högbom (1894). Compared with other granitic rocks in the region, our results partly overlap with the Ga age of the granitoids within and to the west of the Ljusdal Batholith (Fig. 2; Delin, 1993; Welin et al., 1993; Delin and Persson, 1999). Contrary to the rocks in the southern Revsund massif in the Jämtland county, the Ljusdal Batholith is foliated and folded in most parts. The structural difference between the approximately 1850 Ma Revsund granitoid and the Ljusdal granitoid may reflect tectonic histories substantially different. If so, the major Svecokarelian deformation pre-dates ca Ga north of the Ljusdal Batholith and post-dates this age within the batholith, and consequently, an important domain boundary would exist between these igneous provinces. This possible difference in timing of the Svecokarelian evolution may be similar to the situation in southern Finland with a metamorphic peak at Ga and a subsequent metamorphic event at Ga in the south (Nironen, 1997; Korsman et al., 1997). The U Pb titanite result from the Forsaån C/S-mylonite is in accordance with inferred early ( ca Ga) deformation along the SEDZ and HSZ (Bergman and Sjöström, 1994). It overlaps partly with the age of other deformation zones occurring in the region. A shear zone ca. 10 km to the SW (Fig. 34) has been dated at Ma(Högdahl et al., 1996) and titanites from the Hagsta and Ljusne high-t shear zones ca. 400 km to the SSE have been dated at and Ma, respectively (Högdahl et al., 1995; Högdahl and Sjöström, 1999). These data fall in the same age range as U Pb columbite tantalite Ma ages for late-kinematic pegmatites in southern Finland, which intruded during transpressive, semi-ductile conditions (Lindroos et al., 1996). This transpressive zone is, tentatively, the eastern continuation of the HSZ, which has recently been suggested to be a coherent structure across the Fennoscandian shield (Sjöström et al., 2000) based on structural (Ehlers et al., 1993; Bergman and Sjöström, 1994; Lindroos et al., 1996; Stålfors and Ehlers, 2000), geochronological (Lindroos et al., 1996; Högdahl and Sjöström, 1999) and geophysical data (Korhonen et al., 1999). Also, kinematically, the Forsaån shear zone fits into the regional pattern defined by the SEDZ and the HSZ that are both characterised by a dextral, horizontal component but deviate in vertical components. The difference in kinematic conditions recorded along the eastern margin of the granitoid (dextral and southwest-side-up) and

16 52 K. Högdahl, H. Sjöström / Precambrian Research 105 (2001) internally in the Forsaån section (dominantly pure shear conditions) demonstrates partitioning of strain (Fig. 4F) as well as the variation in kinematic character of adjacent deformation zones. Most, if not all, described zones have a component of orthogonal pure shear indicating transpressive deformation. In the case of the Forsaån and related zones, the shearing post-dates the magma emplacement. However, in a regional perspective, the shear zone activity dated at Ma temporally overlaps with the Ga emplacements of S- type granites (Claesson and Lundqvist, 1995) and lithium caesium tantalum (LCT) pegmatites (Romer and Smeds, 1994, 1997), and partly with the previously established age range of the Ga Revsund granitoids in the central part of the Svecofennian domain. 6. Conclusions Two crustal-scale shear zone systems, SEDZ and the HSZ, truncate the investigated granitoid of the southern Revsund massif in Jämtland county, central Sweden. They form an anastomosing ca. 50-km wide pattern of steep, ductile shear zones and shear pods of various scales. The age of a Revsund granitoid located in the western part of the Southern massif is ca Ma, constrained by U Pb SIMS analyses on zircons ( Ma) from a sheared granitoid and by U Pb TIMS analysis on titanite ( Ma) from an undeformed granitoid. As these samples were collected within a type area originally referred to as Revsund granitoids, either the previously known age range ( ca Ga) or the definition of these granitoids as part of the Revsund suite must be revised (Table 1). The granitoid in the southern massif is mainly isotropic outside shear zones indicating that the regional, Svecokarelian deformational pattern, truncated by the granitoid, have formed earlier than 1.85 Ga in the area. This is supported by the condition that titanites from the isotropic granitoid give the magmatic and not the metamorphic age. By contrast, the more pervasively deformed ca Ga Ljusdal granitoid to the south indicates that major deformation in that area post-dates 1.85 Ga. In the Forsaån section, the deformational (predominantly C/S-) fabric is arrested at ca C, i.e. at sub-solidus conditions. Titanite is found in C and C planes and yields an U Pb age of Ma. This age is considerably younger than the U Pb titanite age of the granitoid outside the shear zone, and is interpreted to reflect the age of the deformation. The ductile pattern has partly been overprinted by later deformation at brittle conditions, which could explain the lead loss and, thus, younger ages found in the outer parts of the zircons. The local occurrence of a magmatic foliations in the surrounding granitoids indicates that also ca Ga syn-emplacement deformation took place. Kinematic data from the investigated local shear zones indicate strain partitioning, resulting in dominantly dextral simple shear in the marginal parts of the granitoid massif and pure shear conditions in the internal part (Forsaån section). The integrated picture indicates that the deformation was transpressive. The Forsaån and associated shear zones are components of the late orogenic, crustal scale, ductile shear zone system of central Sweden. The Forsaån zone is the oldest hitherto dated ductile shear zone in the region, that together with other data define periods of shearing in the time interval Ma. This interval overlaps with activity along structures kinematically similar in southern Finland. Possibly, the shear zones described here are part of a coherent structure extending across the Fennoscandian shield, from central Sweden to southern Finland. Acknowledgements We are indebted to Dr Lena Lundqvist and her assisting geologists Björn Magnor and Carin Ivarsson at the Geological Survey of Sweden for the introduction to the Forsaån section and other deformation zones in the eastern part of the massif. Dr Erik Sturkell, Nordvulk, contributed with never ending enthusiasm and great knowledge of the local geology. Dr Per-Olof Persson and Pro-