Protogine Zone Research Articles

Cleaning up the record – revised U-Pb zircon ages and new Hf isotope data from southern Sweden

ABSTRACT Ten samples of felsic plutonic rocks from the Eastern Segment of the Sveconorwegian Orogen in southern Sweden, previously dated by ID-TIMS on zircon, have been dated anew using SIMS spot analysis of individual zircon grains, leading to more reliable and in most cases also more precise revised magmatic crystallization ages. A gneissic monzonite within the Protogine Zone in Småland yields a revised U-Pb age of ca. 1725 Ma, four samples of orthogneiss from Skåne all yield revised ages between 1700 and 1690 Ma, while two samples of coarse-grained granitic gneiss in the same region yield ages between 1690 and 1680 Ma. These revised ages are between 15 and 250 m.y. older than previously obtained TIMS ages. Two samples of the Gumlösa-Glimåkra granite along the Protogine Zone in northern Skåne and one sample of related syenite yield ages around 1220 Ma, similar but more precise compared to the previous ages. The U-Pb zircon data have been complemented by Hf isotope analysis by LA-ICP-MS on the same grains, and previously obtained initial Sr and Nd whole-rock isotope data have been recalculated to the revised crystallization ages. The Sr isotope data scatter, while the revised initial εNd values fall between +1 and +2 for the older rocks, and close to 0 for the younger intrusives along the Protogine Zone. Initial εHf in magmatic undisturbed zircons shows relatively little spread within each sample, between 2 and 4 Epsilon units, disregarding a few outliers, with average values for the 1725 to 1680 Ma rocks between +3 and +5.5 and for the 1220 Ma rocks at ca. +1.5. Covariation between initial εNd and initial εHf in the older rocks suggests either mixing between two isotopically distinct magma sources or one magma source which was isotopically heterogeneous. The isotopic signatures of the 1220 Ma intrusive rocks along the Protogine Zone are indicative of juvenile mantle input to their magmas, rather than pure crustal melting.

An 18th century find of an erratic lazulite-andalusite-quartz boulder in Södermanland, Sweden, and its implications

ABSTRACTAt some point in the 1750s, a jeweller-apprentice by the name Jacob Hässelgren found an erratic boulder next to the Eskilstuna country road in the neighbourhood of Ärla in Södermanland. It contained a deep blue mass of lazulite, at the time an unknown mineral. Pieces of the find eventually reached Daniel Tilas, Torbern Bergman and Axel Fredrik Cronstedt ˗ renowned natural scientists in Sweden ˗ but no detailed studies of the material seem to have been carried out by them. Two fragments of the original boulder are still preserved, and a recent examination shows them to consist of mainly lazulite, andalusite, quartz, pyrophyllite, augelite and svanbergite. The average composition of lazulite is Mg0.700Fe2+0.261Mn0.003Al1.954Fe3+0.017 P2.031O8(OH)2. The mineral assemblage is characteristic of known occurrences of phosphate-Al silicate-quartz appearing along the Protogine Zone in southern Sweden. Transportation of the boulder from its source rock, likely to be located somewhere along the Protogine Zone, ought to have occurred in connection with the development of the Fennoscandian ice sheet during the final Weichselian deglaciation, and the material was possibly discharged from floating ice on the Yoldia Sea.

Mesozoic rift magmatism in the North Sea region: 40Ar/39Ar geochronology of Scanian basalts and geochemical constraints

More than 100 volcanic necks composed of basanites and melanephelinites occur in Scania, southern Sweden, at the junction of two major tectonic lineaments, the Phanerozoic Sorgenfrei-Tornquist Zone (STZ) and the Proterozoic Protogine Zone. New 40Ar/39Ar isotope analyses of whole rock fragments of nine selected basalt necks suggest that the Mesozoic alkaline volcanism in the Scanian province commenced earlier than previously reported and comprised three separate volcanic episodes that span a total period of ca. 80 Myr: a first Jurassic (191–178 Ma), a second at the Jurassic/Cretaceous boundary (ca. 145 Ma), and a final middle Cretaceous episode (ca. 110 Ma). The new results allow for precise time correlations between eruption events in the Scanian and those in the North Sea volcanic provinces. The older, early Jurassic event in Scania is largely synchronous with that in the Egersund Basin and the Forties field whereas the event at ca. 145 Ma is correlated with activity in the Central Graben. These volcanic episodes also correlate in age with Kimmerian tectonic activity. Volcanic activity in the middle Cretaceous period has also been dated in the triple junction in the North Sea and offshore in the Netherland Sector. The correlation of basalt volcanism in Scania with the Egersund nephelinites strongly suggest that volcanism was triggered by repeated tectonic activity along the STZ. Geochemical data of alkaline mafic rocks in the Scanian and the North Sea volcanic provinces imply that different provinces have largely unique geochemical signatures in favour of a heterogeneous mantle in the North Sea volcanic region. However, basalts of different generations in one and the same province cannot be readily separated on the basis of geochemistry, suggesting that the same lithospheric mantle was the source of repeated volcanism over time in each province. The data suggest a low degree of melting of a volatile-bearing mantle lherzolite enriched in incompatible elements with the exception of the Forties basalts in the rift centre, produced by larger degree of melting and evolved by fractional crystallization.

Mesoproterozoic bimodal magmatism along the Protogine Zone, S Sweden: three magmatic pulses at 1.56, 1.22 and 1.205 Ga, and regional implications

The Protogine Zone is a major tectonic zone situated at the eastern boundary of the Sveconorwegian orogenic belt in southern Sweden. This structure hosts Mesoproterozoic dolerite dykes and plutonic rocks including ultramafic cumulate, syenite, monzonorite and granite. Pb–Pb zircon evaporation ages are reported for seven samples of rocks along the Protogine Zone, including the Vaggeryd syenite pluton. Two samples of the main syenite facies of the Vaggeryd pluton yield ages of 1220±3 and 1219±3 Ma. Three samples of anorthosite gabbro, aplite and pegmatite, all of which form dykes in the Vaggeryd pluton, yield equivalent ages of 1220±3, 1221±3 and1218±3 Ma. They demonstrate that the different facies recorded in the pluton belong to a single magmatic pulse at 1220 Ma, significantly older than previous estimate of 1204±4 Ma (Jarl 2002). Samples of the Gumlösa-Glimåkra granite in NE Scania and the Åker metabasite, located immediately west of the Vaggeryd syenite, were dated at 1204±3 and 1567±3 Ma, respectively, hence reproducing earlier reported ages for these intrusions determined by conventional TIMS and SIMS U–Pb techniques (1204−15 +16 Ma, Johansson 1990 and 1562±6 Ma, Söderlund et al. 2004a). The 1575–1562 Ma suite of mafic intrusions along the Protogine Zone shows temporal correspondence with rapakivi magmatism in SW Finland and suggests that the Protogine Zone first may have originated as a zone of crustal weakness, which later was reactivated (e.g. at 1224–1215 and ca. 1204 Ma) in an extensional, possibly back-arc, tectonic setting prior to the 1.13–0.95 Ga Grenvillian-Sveconorwegian orogeny.

U–Pb baddeleyite ages and Hf, Nd isotope chemistry constraining repeated mafic magmatism in the Fennoscandian Shield from 1.6 to 0.9 Ga

The Palaeoproterozoic (1.90 - 1.60 Ga) crust of central Fennoscandia was intruded repeatedly by dolerite dikes and sills during the Neo- and Mesoproterozoic eons. We report 17 new baddeleyite U - Pb dates comprising six generations of dolerites ( in Ma): Blekinge-Dalarna dolerites 946 - 978 Protogine Zone dolerites 1,215 - 1,221 Central Scandinavian Dolerite Group 1,264 - 1,271 Tuna dikes and age equivalents in Dalarna 1,461 - 1,462 Varmland dolerites similar to 1,568 Breven-Hallefors dolerites similar to 1,595 The favoured tectonic model implies that the majority of these suites were related to active margin processes somewhere west ( and possibly south) of the Fennoscandian Shield. Dolerite intrusions are interpreted to reflect discrete events of back-arc extension as the arc retreated oceanward. Initial Hf and Nd isotope compositions of the dolerite swarms fall between CHUR and normal-depleted mantle, and suggest a variably depleted and re-enriched mantle as the source for the here investigated 1.6 to 0.95 Ga old mafic rocks. Repeated recycling of older crustal components, mainly sediments ( dominated by material with short residence ages) in earlier subduction systems may have been very efficient at producing geochemically and isotopically variably enriched lithospheric mantle sections beneath the Fennoscandian Shield.

Petrology and ion microprobe U‐Pb chronology applied to a metabasic intrusion in southern Sweden: A study on zircon formation during metamorphism and deformation

The Åker metabasite occupies a key position in a major tectonic lineament in southernmost Sweden, the Protogine Zone, which coincides closely with the eastern boundary of the late Mesoproterozoic Sveconorwegian orogen of southwest Scandinavia. Metamorphic reactions, associated with the transformation from isotropic gabbro to foliated garnet amphibolite, were identified from disequilibrium textures of which some involved release of zirconium (Zr) and growth of metamorphic zircon. Ion microprobe dating of igneous zircon gave 1562 ± 6 Ma, whereas metamorphic zircons yielded ages of 1437 ± 21, 1217 ± 75, and 1006 ± 68 Ma. The presence of baddeleyite pseudomorphs made up of saccharoidal zircon and a higher abundance of older rather than younger metamorphic zircons suggest redistribution of Zr into new zircon, first by the breakdown of baddeleyite (ZrO2) and later by the consumption of igneous phases containing trace amounts of Zr. Several generations of metamorphic zircon and the presence of 1.56 and 1.22 Ga mafic intrusions along the Protogine Zone call for a complex tectonic history probably reaching back to at least ∼1.56 Ga. Growth of metamorphic zircon at ∼1.44 Ga may relate to a regional, compressional event. The WNW trending deformational structures on both sides of the Protogine Zone may possibly relate to that event. The ∼1.22 Ga metamorphic zircons are coeval with the emplacement of numerous granitic, syenitic, and mafic intrusions along and parallel to the Protogine Zone. The age around 1.0 Ga, finally, marks Sveconorwegian metamorphism for which thermobarometry of the Åker garnet‐amphibolite suggests 1000–1200 MPa at 600°C–630°C. Thereafter, significant relative uplift of the rocks to the west of the Protogine Zone occurred on nearly vertical, north‐south trending deformation zones.

Age and emplacement of late Sveconorwegian monzogabbroic dykes, SW Sweden

The monzogabbroic (jotunitic) Göteborg dykes in SW Sweden display igneous mineralogy and texture, with chilled margins that crosscut all ductile structures in the wall rock. Morphological features suggest emplacement into brittle crust along a set of pre-existing WNW- to W-trending fractures. U–Pb dating of baddeleyites from the largest of these dykes, the Tuve dyke, yields a late Sveconorwegian (Grenvillian) age of 935±3 Ma, which is considered as the crystallisation age of the dyke swarm. The Göteborg dykes and the Hakefjorden Norite–Anorthosite Complex on the Swedish west coast show many similarities with the jotunitic rocks in the Rogaland Anorthosite Complex of south-western Norway, in terms of age, major element composition and Fe–Ti oxide concentrations. The Blekinge–Dalarna dyke swarm east of and along the Protogine Zone in south-eastern Sweden, and the Hunnedalen dykes in south-western Norway, also record the characteristic high Ti–P–K, evolved “jotunitic” character. A distinct feature of all these late Sveconorwegian mafic rocks is the low content of normative diopside (low Ca), but in other aspects they resemble modern continental flood basalts. Ages around 930 Ma for many late Sveconorwegian intrusions correspond in time with the latest phase of extensional movements along crustal-scale shear zones, suggesting that this magmatism is related to extensional collapse and exhumation of the Sveconorwegian orogen.

The timing of the Protogine zone deformation – a discussion

The timing of the Protogine zone deformation – a discussion

The timing of the Protogine zone deformation – a reply

The timing of the Protogine zone deformation – a reply

The timing of the Protogine zone deformation

The timing of the Protogine zone deformation

U-Pb zircon ages from the Vaggeryd syenite and the adjacent Hagshult granite, southern Sweden

South of Lake Vättern a series of syenite bodies have intruded in an elongated area subsequently affected by the Protogine Zone deformation. The by far largest of these bodies, the Vaggeryd syenite, has been dated by the U-Pb zircon method together with the TIB granite immediately to the east of the syenite, in this paper temporarily called the Hagshult granite. The crystallization age of the syenite sample is determined to 1204±6 Ma. The possibility that the Vaggeryd syenite consists of magmas from two separate intrusion events is discussed. Since the Vaggeryd syenite is transected by the ductile deformation of the Protogine Zone, this deformation must be younger than c. 1205 Ma. The result from the Hagshult granite yields an intrusion age of 1673±19 Ma. The granite thus belongs to the youngest group of TIB intrusions.

Palaeomagnetism of Mesoproterozoic dykes from the Protogine Zone, southern Sweden and the enigmatic Sveconorwegian Loop

A palaeomagnetic study of samples from dykes in the Protogine Zone (PZ) south of Lake Vättern has yielded two characteristic magnetic components, A and B. A comparison with previous data and the Sveconorwegian Loop of the Fennoscandian Apparent Polar Wander Path (APWP) make it possible to estimate the age of the corresponding palaeomagnetic poles. They are situated close to the ‘Loop’, which indicates ages between ca 1100 and 850 Ma. The Loop is based mainly on three clusters of palaeopoles, two situated at low latitude and one at intermediate to high latitude. The possible configuration of the Loop is discussed.

An array study of lithospheric structure across the Protogine zone, Värmland, south-central Sweden — signs of a paleocontinental collision

A small seismological array was installed on both sides of the Protogine Zone (PZ) in Värmland, south-central Sweden, to study the structure of the mantle lithosphere and lateral variations of its anisotropy. No distinct isotropic velocity anomalies were detected by tomography in the upper mantle around the PZ. The observed velocity variations depending on the direction of propagation can be explained by anisotropy within the subcrustal lithosphere on both sides of the suture. The best solution of a joint analysis of anisotropic inferences from teleseismic P-residual spheres and an inversion of shear-wave splitting parameters, resulted in 3D self-consistent anisotropic models of blocks of the subcrustal lithosphere. The anisotropic structures within the lithosphere are approximated by hexagonal models (kP=5%) with low-velocity symmetry axes. The high-velocity planes dip to the E in a region westward of the PZ and to the NW eastward of the PZ. The PZ can be interpreted as a steep and narrow suture cutting the whole lithosphere and separating the two anisotropic blocks of different origin.

Mineralogical and genetical aspects of the Smålands Taberg Fe-Ti-V ore, Protogine Zone of southern Sweden

The Fe-Ti-V deposit in Smålands Taberg is ca. 1200 Ma in age and composed of olivine, titanomagnetite, and minor plagioclase cumulates. The ore is massive in hand specimens, however, weak foliation caused by orientation of tabular plagioclase laths indicates magmatic lamination or a crystallization process characteristic for layered intrusions. More than 500 microprobe analyses of cumulates from profiles normal to the igneous lamination in the plagioclase-poor part of the ore reveal compositional uniformity of olivine (Fo70), and plagioclase (An58). On the other hand, titanomagnetite which is the only primary Fe-Ti oxide in the ore shows a variety of exsolution modifications but no compositional cryptic variation among the compatible elements Fe, Ti, Al, Mg, Mn, and V. It is assumed that the cumulates of the Smålands Taberg are products of density-controlled, early crystallization of basaltic magma. The displacements and fractures within the primary ore body as well as oxidation and exsolution processes are discussed in the context of the formation of both the Protogine Zone and the Vättern rift.

Composition and U–Th–total Pb model ages of polygenetic zircons from the Vånga granite, south Sweden: An electron microprobe study

U–Th–total Pb dating of zircons from the Vånga granite, south Sweden, by electron microprobe (EMP) revealed three different zircon generations. U–Th–Pb analyses of finely zoned zircons, existing both as rounded cores and overgrowth rims, yielded a well defined U–Th–total Pb model age of 1448±25 Ma (1[sgrave]). This age is interpreted to date the intrusion of the Vånga granite, which is notably older than previously suggested. Hf- and F-rich overgrowths yielded a U–Th–total Pb model age of 1128±51 Ma (1[sgrave]). Based on the Hf- and F-enrichment and morphological features, it is suggested that the overgrowths formed by a dissolution–precipitation process in a hydrothermal environment. In few cases magmatic zircon overgrows clear, unzoned cores, which could hardly be dated by EMP due to low U, Th, and Pb contents. A single analysis of one of these cores, however, yielded a U–Th–total Pb model age of c. 1700 Ma, which fits with a previously determined age of refractory zircons in a c. 1445–1465 Ma old Karlshamn-type granite. This agreement supports the previous interpretation that the Vånga granite crystallized from a residual liquid separated from the Karlshamn granite magma. The new intrusion age of c. 1450 Ma indicates that the Vånga granite intruded contemporaneously with an extensive thermomagmatic activity further west in the Eastern Segment of the Sveconorwegian orogen. The c. 1130 Ma age of zircon overgrowths has tectonic implications on the N–S trending Protogine Zone; a major shear zone that marks the eastern boundary of Sveconorwegian reworking. Since equally old syenites and dolerites occur within the Protogine Zone, the age of these hydrothermal overgrowths indicates that the crust affected by Protogine Zone deformation is not exotic relative to the crust of Blekinge, nor is it conceivable to relate the metamorphic contrast across the Protogine Zone to deformation east thereof.

Seismic imaging in the frontal part of the Sveconorwegian orogen, south-western Sweden

Reflection seismic data have been acquired along a 17 km profile in the eastern frontal part of the Sveconorwegian orogen, south-western Sweden. Receiver spacing was 25 m and nominal shot spacing 100 m. Shots ranging in size from 0.5 to 1.0 kg were fired in 3 m deep shot holes. The data image the structure of the upper part of the crust from depths of a few 100 m down to depths of approximately 10 km. The profile crosses the central axis of an inferred fan-like structure, indicated from earlier, detailed surface mapping results. Deformational fabrics within this structure dip approximately 15–40°E in the western part of the profile and 60–80°W in its eastern part. The seismic data display a bivergent geometry with the central axis situated approximately 8 km east of a major fault (Protogine Zone in earlier studies). Surface structural data predict the hinge axis to be located about 3 km farther east. Despite this discrepancy, there is good general agreement with the structure inferred from the surface data. The image obtained from the reflection seismic data establishes that the fan-like structure is a major, upper crustal feature. The present data in combination with previous deep seismic data sets exclude a single-stage extensional model, with an east-dipping extensional deformation zone protracting eastwards from the western part of the profile. The combined seismic data provide support for a two-stage compressional model for the development of the fan-like structure, with crustal thickening and stacking prior to 955 Ma and later compression with a stronger horizontal component of displacement during 930–905 Ma. The latter gave rise to the development of the complex system of retrogressive deformation zones referred to as the Sveconorwegian Frontal Deformation Zone (SFDZ), a tectonic analogy to the Grenville Front in the eastern part of North America. The SFDZ dips westwards, may become listric at depth and possibly sole in a zone of lower crustal reflectivity west of the present profile. The kinematics and geometry of the SFDZ suggest that reverse displacements (thrusting) may be responsible, at least in part, for the exhumation of the medium- to high-grade rocks in the western part of the fan-like structure. Both the seismic data and the surface structural geology show many similarities to the Grenville Front.

GPS measurements of crustal deformation in Skåne, Sweden, between 1989 and 1996

We estimate a network of crustal deformation in Skåne using observations of the Global Positioning System (GPS) from 1989, 1990, 1992, and 1996. The network straddles the Tornquist Zone, potentially one of the most active fault zones in Sweden. It includes 5 GPS sites spaced approximately 80 km apart in the south of Sweden. The precisions of the relative horizontal components for these stations are described by repeatabilities with approximately 3 mm in the north-south direction and about 1 mm in the east-west direction in 1992 and 1996, but with a precision of only about 10 mm for the horizontal components in 1989 after the improvement of broadcast orbits, keeping three fiducial station (Onsala, Wettzell, and Tromsö) fixed at the ITRF94 system. About 72% of the GPS integer carrier phase ambiguities were resolved for the 1992 and 1996 campaigns and merely 43% in the 1989 campaign. Three stations, Dalby, Limhamn, and Blentarp, moved with a rate of 4 to 5±0.5 mm/yr toward the SSE and ESE relative to the station Stavershult. SSE and ESE displacements of the three stations mean left lateral strike-slip shear along the Tornquist Zone in Skåne, which is consistent with geological results. The 2.0±0.7 mm/yr S displacement of station Kivik indicates right lateral displacement along the southern aseismic portion of the Protogine Zone. The results were similar when the Onsala station was fixed at the ITRF94. For a more detailed investigation of the present tectonic deformation in Skåne we suggest the GPS network be enlarged to more than 10 stations.

Emplacement ages and metamorphic overprinting of granitoids in the Sveconorwegian Province in V rmland, Sweden - an ion probe study

North of Lake Vänern in the Sveconorwegian province of Sweden, veined and banded grey to red granitoids characterize the Western Segment in western Värmland. Two gneiss samples from Rackstad, north of Arvika, yielded concordant U-Pb emplacement ages of 1596 - 11 Ma and 1590 - 14 Ma, respectively from zircon cores, while metamorphic zircon rims in one of the samples gave a weighted mean Pb-Pb age of 971 - 8 Ma. Patches of metamorphic zircon overgrowth pre-dating the metamorphic rims indicate that an even older metamorphic event influenced this rock. East of the Mylonite Zone, a N-S-trending Sveconorwegian shear zone, more massive reddish granitoids occupy the segment between this zone and another N-S-trending Sveconorwegian shear zone, the Protogine Zone. A sample from Hänsjön, north of Torsby, yielded concordant ages with a weighted mean of 1689 - 12 Ma. One grain has a Pb-Pb age of 1770 - 29 Ma, which is inherited from the source of the granite, and is similar to the age of early magmatism in the Transscandinavian Igneous Belt, east of the Torsby Segment. These results confirm that north of Lake Vänern, the Torsby Segment was formed ca. 100 Ma earlier than the adjoining segment to the west, in accordance with previous results from further south.

Neoproterozoic palaeomagnetic directions in rocks from a key section of the Protogine Zone, southern Sweden

SUMMARY New palaeomagnetic data from the Protogine Zone (PZ) of southern Sweden are presented together with data from east and west of the zone. They confirm previous palaeomagnetic results from the PZ that indicate a diVerence in magnetic properties between rocks north and south of a section at 57°N, here called the Alvesta‐Ljungby Palaeomagnetic Borderzone (ALPB). The results are divided into four groups: group I [declination (D)=297°, inclination (I)=’78.5°, a 95 =2.4°, pole position 42.7°S, 220.9°E]; group II (D=129.4°, I=52.2°, a 95 =4.2°, pole position 9.4°S, 235.5°E); group III (D=155.4°, I=61.3°, a 95 =4.9°, pole position 11.6°S, 211.2°E); group IV (D=125.5°, I=35.4°, a 95 =5.4°, pole position 0.8°N, 244.1°E). A fifth, poorly defined, group has a mean direction D=304.5°, I=5.4°, a 95 =15.1° and pole position 21.3°N, 255.9°E. These palaeopoles fall on the c. 1050‐900 Ma Sveconorwegian Loop of the Fennoscandian apparent polar wander path. Together with previously published data from southern Norway, the group I poles define the high-latitude apex of the loop at c. 950‐930 Ma. The groups II‐V palaeopoles are situated at low latitudes but it is diYcult to place them in chronological order owing to a lack of age data and two possible interpretations, that is of an open or a closed loop. The study shows that the part of the PZ extending south of the ALPB belongs to the Southwest Granulite Region and the rocks became remagnetized during the tectonic stacking and uplift processes that created that region. Dolerites from east of the PZ have primary magnetizations and the data indicate that they belong to the Blekinge‐ Dalarna dolerites, a suite of dykes east of the PZ with ages between 995 and 880 Ma. Dolerites west of the PZ yield low-latitude Sveconorwegian palaeopoles but it is not certain if their magnetizations are primary or are due to remagnetization during the Sveconorwegian Orogeny.

A PIXE study of clouded plagioclase from southern Sweden

Clouded plagioclase is found in mafic dykes occurring along the Protogine Zone in southern Sweden. The clouding intensity varies from slight to more pronounced brown/dark or opaque. In order to understand the trace-element partition/contribution in the clouding process, a PIXE (Particle Induced X-ray Emission) analysis has been conducted on clouded plagioclase samples (Bjära, Bjärnum, Taberg). The experiments were undertaken at the Lund Nuclear Microprobe. For each locality, PIXE spectra and elemental distribution line-profiles (rim to core) are displayed. This study has shown undulated (inhomogeneous), stepwise (semi-homogeneous), and homogeneous clouding at the elemental level. The result suggests that the trace elements partially contribute to the observed plagioclase clouding.