Palaeomagnetism of Precambrian rocks from Southeast Norway and South Sweden

Precambrian amphibolite and hyperite rocks from the Bamble and Kongsberg areas in SE Norway, and amphibolite rocks from SW Sweden were investigated for evidence of remagnetization by the Sveconorwegian metamorphic episode. The similarity of the characteristic natural remanent magnetization directions, shown by the various rocks from the Bamble and Kongsberg areas, indeed supports the idea of remagnetization on a regional scale. Therefore the average pole position at 3°S, 153°W, determined from six sites in these areas, is considered to reflect the average virtual pole position for the post-Sveconorwegian period of uplift and cooling (1,120–975) · 10⁶ year ago. The pole positions determined from the characteristic natural remanent magnetization directions of amphibolite rocks in SW Sweden are indicative of being somewhat younger.In addition, two hyperite dikes were studied near Karlshamn in SE Sweden. Their characteristic natural remanent magnetization is consistent with that of the hyperite dikes in central south Sweden (Mulder, 1971).The Precambrian apparent polar wandering path for Europe is reconstructed on the basis of twenty-three pole positions from the Baltic Shield and three pole positions from Great Britain. This pole path requires an average angular rate of apparent polar wandering of 0.2–0.3° per 10⁶ year.     

Remanent hysteresis study of dolerite dykes

Saturation remanent hysteresis studies were carried out on numerous dolerite dyke samples from the peninsular India. These studies result in four types of remanent hysteresis curves which indicate that the magnetic material is magnetite or titanomagnetite with variable grain-size having remanent coercive forces (H _cr ) of 8 to 30 mT, requiring saturating fields (H _s ) up to 250 mT. Two extreme types of samples with (1) low coercive forces requiring high saturating fields and (2) high coercive forces requiring low saturating fields are noticed along with the generally observed ones. The Granulometric and Lowrie-Fuller Tests on these samples indicated that the magnetic material i.e., magnetite or titanomagnetite in these rocks is in the form of Multi-Domain (MD), Cation Deficient (CD) and a mixture of these two forms (MD+CD) within.     

Palaeomagnetism of (late Vendian-earliest Cambrian) minor alkaline intrusions, Fen Complex, southeast Norway

Emplacement of the Fen central complex (603-565 Ma) within the Fennoscandian Shield in southeast Norway was preceded by the emplacement of numerous minor alkaline intrusions into the surrounding gneisses. A palaeomagnetic sample of 28 of these bodies has identified a predominant SSE negative remanence carried by magnetite in some bodies and hematite in others. A sporadic high blocking temperature component appears to record localised effects associated with the development of the Oslo rift and igneous province to the east, but no major magnetic overprinting by post-emplacement events is recognised. The stable magnetisation vectors for twenty sites comprise a coherent population with those for two sites reversed with respect to the remainder; they yield a mean direction ofD = 210°,I = 44° (₉₅ = 6.4°) and a palaeomagnetic pole at 324°E, 50°S (dpdm=4.9°7.9°). The difference between the pole position for this early phase of the Fen magmatism and that for the late metasomatic rødberg (322°E, 63°S) in the interior of the complex is interpreted in terms of continental movement during the late Vendian-earliest Cambrian interval of alkaline activity here. The defined direction of APW movement continues a motion recognised from other Vendian data but subsequent movements during Lower Cambrian times are unclear.     

Palaeomagnetism of some Precambrian rocks in south-east Greenland

A palaeomagnetic pole is established at 25.1°N 273.9°E (dp = 10.6°, dm = 14.3°) from the norite-charnockite complex at Angmagssalik, emplaced at 1800 Ma. A somewhat older palaeomagnetic pole at 4.2°S 246.7°E (dp = 4.2°, dm = 8.3°) is obtained from Archaean gneisses close to the northern boundary of the Nagssugtoqidian mobile belt; reversals of magnetization are present here. Both magnetizations were imposed during slow cooling following the (late) Nagssugtoqidian metamorphism.In general the gneisses, dyke amphibolites and granite of the Nagssugtoqidian mobile belt are unstably magnetized; their magnetization is attributable to the Earth's present field, and is often extremely weak.A pseudotachylyte within the Archaean gneisses has had a long cooling history. A fragment of the remanence reflects the magnetization characteristic of the Archaean gneisses, whereas most of the magnetization corresponds to a palaeomagnetic pole near that of the Angmagssalik complex. The pseudotachylyte is much older than its magnetizations.An apparent polar wander path is presented for Greenland at ca. 1750 Ma based on the above results and data from west Greenland.     

Palaeomagnetism and magnetomineralogy of a Holocene lake sediment from Vågsøy,western Norway

Stable remanent directions encountered in three oriented lake-sediment cores, covering 6 m of gyttja deposits, reveal declination swings with amplitudes of the order of ± 50°. Anomalously low inclinations, scattered around the horizontal in the upper 2 m of core, is attributed to compressional disturbances of the alignment of magnetic grains during the coring procedure. Four ¹⁴C dates obtained (1250, 1940, 4600 and 6330 y B.P.) have enabled a correlation with features ‘c’–‘g’ of the British Geomagnetic Master Curve). For the older part of the curve, features ‘h’ and ‘i’, the correlation is questionable. Magneto-mineralogical parameters and qualitative chemical analysis of individual grains by secondary X-ray emission analysis reveal the presence of pure magnetite grains. Scanning electron micrographs show a dominant proportion of concoidal slightly elongated grains (10–80 μm). A few almost-perfect octahedral magnetite crystals can be related to outcrops of augen gneisses in the vicinity confirming a detrital origin of these grains.     

Complete remagnetization of some Permian dykes from western Norway induced during burial/uplift

Stable paleomagnetic directions in four basaltic dykes and in some associated Caledonian metamorphic rocks define high-latitude, Mesozoic paleomagnetic pole positions which are not compatible with the K---Ar age of 250 My determined for the dykes. A monotonic increase of 40% occurs in the potassium content of samples taken across a dyke 32 cm wide. This is accompanied by only a 1–2% variation in the K---Ar age of the samples, suggesting the absence of any significant level of initial argon. Titanomagnetite grains having bulk compositions around x = 0.6 have suffered extensive low-temperature alteration, forming assemblages of ferri-rutile granules in a matrix of pure magnetite. The complete remagnetisation of both the dykes and the associated country rocks is probably an expression of a VRM acquired at elevated temperatures (150–500°C) at the emplacement depth of the dykes. A stable remanent magnetisation was locked-in during uplift of the area, probably related to the Kimmerian basin development in the adjacent North Sea and the epeirogenic uplift of western Fennoscandia.     

Formation of dykes,feeder-dykes,and the intrusion of dykes from magma Chambers

The paper discussed the formation of dykes, and applies the results to Iceland. It is postulated that dykes follow the pathway of least work and of least tensile strength and thereby intrude the subvertical joints in lava flows. It is suggested that dykes form in magma pulses, where each dyke is split in two by the next magma pulse and so on. In Iceland the estimated time between successive magma pulses is of the order of several hundred days. Statistical considerations indicate that in Iceland the probability of seeing dykes end upwards is only 1–2%, which agrees with observations. It is concluded that many and probably the majority of dykes are non-feeders. This, together with the low probability of finding the connection between feeder and lava flow, explains the scarcity of observed feeder-dykes. It is concluded that overpressure in shallow magma chambers needed to drive magma through crustal fractures in Iceland is usually smaller than the tensile strength of the host rock (several MPa), which thereby is the critical factor in dyke intrusion.     

Palaeomagnetism of the monzonite porphyry from Milton,New South Wales

Summary Directions of natural remanent magnetization aftermagnetic cleaning of specimens from monzonite porphyry at Milton are given. Thermal and alternating magnetic field stability tests indicate that the resultant direction is that of the earth's magnetic field at the time of cooling. Comprison of the pole position calculated from this result with other pole positions from rocks of known age confirms the probable Permian age of the intrusion and the wide divergence of Australian from European and North American pole positions of this age.     

Palaeomagnetic evidence for large-scale dextral movement along the Trollfjord-Komagelv Fault,Finnmark, north Norway

Palaeomagnetic data from Late Precambrian dykes from the northern part of Varanger peninsula, north Norway, suggest a two-axis magnetization structure. The dominant component is considered to be syn- to late-tectonic and probably acquired at around 640 m.y. B.P. Superposed on this magnetization is a minor component which is compatible with the relative Lower-Middle Palaeozoic field; i.e. it was most likely imposed during the climax of the Caledonian orogenic movements in north Norway. The estimated relative Late Precambrian palaeopole cannot easily be reconciled with the European Late Precambrian polar path. This disagreement can be accounted for by assuming a post-magnetization dextral megashear, of the order of 500–1000 km, along the Trollfjord-Komagelv fracture zone. This type of displacement is in line with geological evidence and the palaeomagnetic reconstruction supports the long-held view of there having been continuity between the depositional environments of the Varanger Peninsula Barents Sea Group, the Eleonore Bay Group of east Greenland and the Hecla Hoek Formation of east Spitsbergen. The character and age of the horizontal displacement, post-640 to pre-500 m.y. B.P., is seen in conjunction with the opening up of the lapetus Ocean and reactivation of ancient deep-seated fractures during both the spreading and the contraction phases of ocean development.     

Palaeomagnetism of middle Ordovician volcanic rocks from Quebec

This palaeomagnetic study is centered on agglomerates and volcanic rocks from the western margin of the Appalachian belt in the Drummondville-Actonvale-Granby area, Quebec (long.: 72°30′W, lat.: 46°00′N). It involves a total of 36 oriented samples (111 speciments) distributed over eleven sites. Both thermal and AF cleaning techniques were used to isolate residual remanent components. The dispersion of the directions is slightly reduced after AF cleaning and thermal treatment.The palaeopole position obtained is 191°E, 6°N (d_m = 14°, d_p = 7°) after thermal treatment and 164°E, 19°N (d_m = 11°, d_p = 6°) after AF cleaning. The polarity of most of the sites (two exceptions) are reversed. The thermal-treated data appear to be relatively stable and an approximate value of the primary magnetization is extracted from them. The palaeopole obtained does not lie close to the tentatively defined position of the Cambrian and Ordovician poles from rocks of the North American plate; it is located near the Upper Cambrian and Lower Ordovician poles from eastern Newfoundland and the Lower Ordovician pole from the Caledonides in Europe.     

Stability of remanent magnetization of some dykes from Mysore State,India

Summary The magnetic properties of some dykes from Mysore State, India, have been studied in detail. The rocks were found to have aQ _n ratio varying from 1.5 to 11.6, a remanent coercive force varying from 100 to 250 Oersteds, Curie temperature varying from 250 to 480°C and were found to have lamellae of ilmenite oriented in (111) plane of magnetite. The stable natural remanent magnetization of the rock seems to be of TRM origin with titanomagnetite and low grade titanomaghemite being the main carrier of remanent magnetization.N.G.R.I. Contribution No. 70-215.     

The primary magnetic remanence of a dolerite sill from northeast Spitsbergen

An Upper Jurassic dolerite sill from Lomfjord, northeast Spitsbergen, has been submitted to rock- and palaeomagnetic studies. In the cooling stage the original titanomagnetite underwent a high-temperature alteration forming exsolution of ilmenite lamellas. Due to rapid cooling the border zones in part escaped the high-temperature alteration but instead low-temperature oxidation, producing at least a low-temperature metastable magnetic mineral, is fairly pronounced in these parts of the sill. Thermal demagnetization has established that both mineral phases carry the same general magnetization direction suggesting the total remanence (reverse) to be of deuteric origin. Consideration of the cooling rate indicates that the accumulated geomagnetic secular variation may cover a time span of the order of 10³ years. The relative pole position is at 61°N, 210°E, a result which is in agreement with some other Mesozoic results from Spitsbergen, but in rather marked disagreement with other published data for the Mesozoic of Europe.     

A major change in magma sources in late Mesozoic active margin of the circum-Sea of Japan domain: Geochemical constraints from late Paleozoic to Paleogene mafic dykes in the Sergeevka belt,southern Primorye,Russia

More than 30 mafic dykes crop out in the Sergeevka belt in the coastal South Primorye, Far East Russia, of which geologic settings have been unclear for years. This study conducted major- and trace elements characterization, Sr–Nd isotope analyses, and Ar–Ar amphibole and U–Pb zircon datings for these rocks in order to identify their origin. The results demonstrated that all dykes are characterized by high Ba/Yb and low Nb/Y, Zr/Y, and Th/Yb ratios, which suggest their origin from arc melts derived from thin wedge mantle and shallow-dipping slab. These dykes are clearly separated into two distinct age/geochemistry suites; that is, the Paleogene and Early Cretaceous one with dolerites/basalts and adakitic rocks, and the Permian–Triassic one with high-Mg and high-Al gabbro-dolerite varieties. Their geochemistry suggests that the older suite was sourced from a primitive depleted MORB mantle (DMM)-type mantle, whereas the younger suite from an enriched mantle II (EM2)-type mantle domain. The transition in source type from DMM to EM2 occurred during the Jurassic-earliest Cretaceous time, probably by a strong influence of a mantle plume onto the long-continuing subduction-related magmatism. The plume influence reached the maximum when the unique meimechite-picrite complex formed in the region.     

Palaeomagnetism of Upper Cretaceous volcanic rocks from Cabo de Sto. Agostinho,Brazil

One hundred samples from nine sites in Upper Cretaceous volcanics (K/Ar age 85–99 m.y.) of the magmatic province of Cabo de Santo Agostinho, Pernambuco (8.4°S, 35.0°W) yield a mean direction of magnetizationD = 0.4°, I = ?20.6°withα₉₅ = 4.8°, k = 114 after AF cleaning. All sites have normal polarity with a mean pole, named SAK₁₀, at 87.6°N, 135°E withA₉₅ = 4.5° which is close to other Upper Cretaceous poles for South America. These poles are compared with Upper Cretaceous poles of Africa for various reconstructions of the two continents.     

Palaeomagnetism of the Foyers and Strontian granites,Scotland

Palaeomagnetic data from the “newer granites” of Foyers and Strontian show similar directions of magnetization with southerly declinations and horizontal to intermediate, mostly downward dipping, inclinations. These palaeomagnetic directions which accord with previous data from the Helmsdale granite are thought to be of Siluro-Devonian age. The present results are not sufficiently precise to justify discussion of possible lateral displacements along the Great Glen Fault of the order of a few hundred kilometres, but it is clear that the recent idea of a c. 15–20° latitudinal offset along the Fault in the Carboniferous has to be disregarded.     

Paleomagnetism of Lower Devonian volcanics and Devonian dykes from northcentral New Brunswick,Canada

Some 50 oriented samples (120 specimens) have been collected on eight sites of volcanic rocks from the Lower Devonian Dalhousie Group of northern New Brunswick and Devonian andesitic to basic dykes from central New Brunswick. Univectorial and occasional multivectorial components were extracted from the various samples. Results after AF and thermal demagnetization compare relatively well. In the volcanics and tuffs, two components of magnetization have been isolated: A (D = 33°, I = ?58°, α₉₅ = 7.3°, K = 236) for four sites and B (D = 66°, I = +53°) for three sites. The grouping of component A is improved after tilt correction but the fold test is not significantly positive at the 95% confidence level. Component A is interpreted as being primary while component B is unresolved and appears to be the resultant magnetization of a Late Paleozoic and a recent component. The pole position obtained for tilt corrected component A is 268°E, 1°S, d_p = 6.5°, d_m = 8.8°. The paleolatitude calculated for component A is 39°S. The paleopole of in situ component A is located close to those of the Early-Middle Devonian formations from Quebec, New Brunswick and New England states while the paleopole of tilt-corrected component A is similar to Lower Devonian poles of rock units from the Canadian Arctic Archipelago. If component A is primary (as we believe it to be), then the western half of the northern Appalachians had already docked onto the North American Craton by Early Devonian time. Alternatively, if component A is secondary the same conclusion applies but the juxtaposition took place in Middle Devonian time.     

Palaeomagnetism of silurian lavas of Somerset and Gloucestershire,England

Detailed alternating field demagnetisation of Upper Llandovery volcanics of the Mendip Hills and Gloucestershire has isolated remanence directions interpreted as primary from each of five sites. Well-defined high-coercivity secondary magnetisation is present in six samples of one site and low-coercivity secondary remanence is present in all samples from another site; the former component was apparently acquired in Permo-Triassic times. Primary directions of magnetisation show marked improvement in precision after correction for penecontemporaneous folding, and show a late Llandovery reversal in the sense R → N.The group mean directions of magnetisation isD = 243.5°,I = 47.5° (precision parameterk = 29). Petrographic examination confirms observations from magnetic properties that relict titanomagnetite (oxidation classes 3 to 5) is the remanence carrier in most samples. Hematite, probably mostly late magmatic in origin, is widely developed in all samples, but only the principal remanence carrier where it has thoroughly replaced the titanomagnetite. Low-coercivity remanence is apparently caused by weathering effects but there is no clear visible cause for secondary high-coercivity remanence carried by some samples.The mean virtual geomagnetic pole position is close to Upper Silurian/Lower Devonian pole positions from other parts of Britain and defines a minimum apparent polar shift of 60° between late Ordovician and Upper Llandovery times. Reference to absolute age dates suggests that this shift took place between ca. 447 and 434 m.y. followed by slight polar movement between ca. 434 and 394 m.y.     

Boninitic geochemical characteristics of high-Mg mafic dykes from the Singhbhum Granitoid Complex,Eastern India

The high-Mg mafic dykes from the Singhbhum Granitoid Complex in East India have geochemical characteristics[e.g.,enrichment of the large ion lithophile elements and light rare earth elements(LREEs) relative to high field strength elements(HFSEs):high-MgO(8%),high-SiO_2(52%),low-TiO_2(0.5%),and high CaO/Al_2O_3(0.58)]similar to those found in boninitic/noritic rocks.Their high percentage of orthopyroxene as a mafic mineral and of plagioclase as a felsic mineral,and normative hypersthene content greater than diopside content are also indications of their boninitic/noritic affinity.On a triangular diagram of MgO-CaO-Al_2O_3 and on binary diagrams of Ti/V vs Ti/Sc and TiO_2 vs Zr,these samples show geochemical similarities with Phanerozoic boninites and Paleoproterozoic high-Mg norites.On major and trace element variation diagrams,these dykes show a normal crystallization trend and their Nb/La(0.5) and Nb/Ce(0.21) values lower than average bulk crust(0.69 and0.33,respectively) suggest no crustal contamination.Their low values of Rb/Sr(0.11-0.41) and Rb/Ba(0.10-0.27)also suggest little or no effect of post magmatic processes.Their TiO_2(0.27-0.50),Al_2O_3/TiO_2(19.30-42.48),CaO/TiO_2(12.96-32.52),and Ti/V(12-18) values indicate derivation from a depleted mantle source under oxidizing conditions such as a mantle wedge.Ni vs Zr modeling shows that the studied high-Mg dykes were generated by25-30%melting of a refractory mantle source.Enrichment of Rb,Th,U,Pb,Sr,and LREEs,and depletion of HFSEs—especially Nb,P,Ti,Zr—on primitive mantle—and chondrite-normalized spider diagrams,respectively,are clear signals that the slab-derived component played an important role in the formation of melts for these rocks in a supra-subduction zone setting.