Paleomagnetism of Archean rocks from northwestern Ontario: V. Poohbah Lake alkaline complex, Quetico Subprovince

1985 ◽  
Vol 22 (1) ◽  
pp. 27-38 ◽  
Author(s):  
David J. Dunlop
Keyword(s):  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Reverse Polarity ◽  
Alkaline Complex ◽  
Field Reversal ◽  
Polar Wander ◽  
Fine Grained ◽  
Rock Types ◽  
Northwestern Ontario ◽  
Normal Polarity

The Poohbah Lake alkaline complex is a late synkinematic Kenoran pluton in the Quetico gneiss belt of the western Superior Province. Three units of the complex, porphyritic syenite (PS), malignite (M: a nepheline–clinopyroxene–K-feldspar rock), and hornblende syenite (HS), as well as baked Archean schists near the intrusive contact, have a predominantly reverse-polarity R magnetization with mean direction D = 198° I = −22.5° (k = 62, α95 = 5°, N = 13 sites) and a paleopole at 60°E, 50.5°N. Pyrrhotite and coarse primary magnetite are carriers of the R remanence. PS, M, and biotite pyroxenite (BP) exhibit also a predominantly normal N magnetization carried by fine-grained, probably secondary magnetite. N is systematically steeper than R: its mean direction is D = 359.5° I = +55.5° (k = 28, α95 = 8°, N = 13 sites) with a paleopole at 90°E, 77.5°N. R and N do not record an asymmetric field reversal, since reverse-polarity N vectors and normal-polarity R vectors are occasionally found. R resembles in polarity and direction the natural remanent magnetization (NRM) of the 2630 Ma Matachewan diabase. It is probably the primary NRM of the Poohbah Lake pluton, with an age of about 2650 Ma in approximate agreement with the K/Ar isochron age of 2700 ± 25 Ma. N resembles in polarity and direction NRM's from the 2580 Ma Shelley Lake granite and the late Archean Burchell Lake granite. It is probably a secondary NRM about 2550 Ma in age, as suggested by updated K/Ar mica ages. The characteristic NRM of HS samples and secondary magnetizations in other rock types have poles on the Grenville Track of the polar wander path but there is no evidence for Grenvillian-age events in the area.

Paleomagnetism of Archean rocks from northwestern Ontario: IV. Burchell Lake granite, Wawa–Shebandowan Subprovince

1984 ◽  
Vol 21 (10) ◽  
pp. 1098-1104 ◽  
Author(s):  
David J. Dunlop
Keyword(s):  
Natural Remanent Magnetization ◽  
Reverse Polarity ◽  
Polar Wander ◽  
Apparent Polar Wander Path ◽  
Northwestern Ontario ◽  
Normal Polarity ◽  
The Mean ◽  
Low K

The late Archean Burchell Lake granite of the Shebandowan greenstone belt in northwestern Ontario has a characteristic natural remanent magnetization (NRM) resembling the type 1 NRM of the nearby Shelley Lake granite of the Quetico gneiss belt. Of 36 stably magnetized samples, 21 had predominantly normal polarity (1N) NRM and 15 had reverse polarity (1R). The mean direction based on stable end-point and vector subtracted directions is D = 2.3 °I = 48.9 °(k = 23.9, α95 = 10.0°, N = 10 sites). Intersecting remagnetization circles gave a similar direction. The corresponding paleopole BL1 lies at 83.2°E, 71.1°N, near track 6 of the Laurentian apparent polar wander path around 2600 Ma. Although the Burchell Lake pluton is not dated radiometrically, neighbouring granites give K/Ar biotite ages of 2550–2600 Ma, and it is reasonable to suppose that the NRM dates from the time of intrusion or shortly thereafter. A type 2 NRM like that of the Shelley Lake granite was isolated in 12 samples. Its mean direction is D = 80.9 °I = −11.1°, but the precision is unacceptably low (k = 7.7).

Paleomagnetism of Archean rocks from northwestern Ontario: Wabigoon gabbro, Wabigoon Subprovince

1983 ◽  
Vol 20 (12) ◽  
pp. 1805-1817 ◽  
Author(s):  
David J. Dunlop
Keyword(s):  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Blocking Temperature ◽  
Virtual Geomagnetic Pole ◽  
Polar Wander ◽  
Apparent Polar Wander Path ◽  
Northwestern Ontario ◽  
Paleomagnetic Pole ◽  
Geomagnetic Pole ◽  
Chemical Remanent Magnetization

The Wabigoon gabbro of the Archean Wabigoon greenstone belt in northwestern Ontario preserves a univectorial natural remanent magnetization (NRM) with D = 246°, I = 12° (k = 19.5, α95 = 10.5°, N = 11 sites). The precision is reduced if sample means are averaged, however (k = 9.3, α95 = 9.2°, N = 29 samples). The paleomagnetic pole falls either at 160°W, 11°S (δp = 5.3°, δm = 10.6°), corresponding to an age of ~1300 Ma on the Laurentian apparent polar wander path, or the reverse of this, 20°E, 11° N, corresponding to a late Archean age (~2800 Ma). No ~1300 Ma igneous or metamorphic event is known in the area; a major west-northwest-trending dike about 9 km south of the gabbro yields a virtual geomagnetic pole at 122°W, 45°N and seems to be of Abitibi age (~2150 Ma) rather than Mackenzie age (~1250 Ma). A few gabbro samples and some greenstones from the intrusive baked zone have hybrid remanences in which a higher blocking temperature Kenoran-age (~2600 Ma) NRM is superimposed on the gabbro characteristic NRM. However, the Kenoran component may be a younger chemical remanent magnetization (CRM) residing in hematite. The hypothesis that the gabbro characteristic remanence is itself a hybrid of Kenoran and Keweenawan (~1100 Ma) NRM's, which would explain both the high between-sample scatter and the lack of a ~1300 Ma remagnetizing event, is considered but rejected because fewer than 10% of the gabbro samples exhibit multivectorial swings during alternating field or thermal cleaning. Two geomagnetic field reversals are recorded at interior sites, but only one or none is recorded near the margin of the intrusion. The different cooling histories of margin and interior, as well as the bulk of the other evidence, favour magnetization during initial cooling in late Archean time.

A palaeomagnetic analysis of Miocene fluvial sediments at Pertusa, near Huesca, Ebro Basin, Spain

1984 ◽  
Vol 121 (4) ◽  
pp. 279-290 ◽  
Author(s):  
P. Turner ◽  
J. P. P. Hirst ◽  
P. F. Friend
Keyword(s):  
Natural Remanent Magnetization ◽  
Northern Spain ◽  
Ebro Basin ◽  
Northern Margin ◽  
Fine Grained ◽  
Reducing Conditions ◽  
Secular Variations ◽  
Normal Polarity ◽  
Higher Temperature ◽  
Reversed Polarity

AbstractIn the Miocene fluvial system of the Huesca area, in the Ebro Basin, northern Spain, rivers radiated outwards, to the south and west, from a small sector of the northern margin of the basin. The deposits of the system extend about 60 km radially from this sector and then pass into calcareous and gypsiferous deposits.The magnetostratigraphy of two logged sections, 1 km apart, within this system consists of an upper zone of normal polarity and a lower zone of reversed polarity. The lithostratigraphy and magnetostratigraphy are parallel.The sands and silts are dominated by angular quartz, intraformational clay lithograins and calcite lithograins. They are mainly pale yellow brown to dusky yellow, and magnetic tests indicate that the magnetization is carried dominantly by ferric oxyhydroxides (haematite and gôethite). The higher temperature Natural Remanent Magnetization is probably due to detrital haematite which would have been partially aligned during fluvial deposition. Post-depositional modification (PDRM) may have occurred during dewatering.Haematite pseudomorphing pyrite framboids indicates that early reducing conditions were succeeded by a more oxidizing regime which produced secondary magnetizations associated with fine-grained haematite and goethite.Complete polarity zones are not delineated, but the thicknesses present are not excessive compared with other continental Miocene deposits. The average palaeolatitude of 22° is lower than would be expected for the Miocene of northern Spain, probably due to incomplete averaging of secular variations and other sources of error including possible ‘inclination error’.

Paleomagnetism of the Fond du Lac Formation and the Eileen and Middle River sections with implications for Keweenawan tectonics and the Grenville problem

1981 ◽  
Vol 18 (5) ◽  
pp. 829-841 ◽  
Author(s):  
Doyle R. Watts
Keyword(s):  
Remanent Magnetization ◽  
Lake Superior ◽  
Natural Remanent Magnetization ◽  
Blocking Temperature ◽  
Tectonic Deformation ◽  
River Section ◽  
Orthogonal Projections ◽  
Chemical Leaching ◽  
Polar Wander ◽  
The Times

Overlying the Keweenawan lavas of northern Wisconsin and Michigan is a thick sequence of terrestrial sandstone, shale, and siltstone that has undergone some tectonic deformation associated with movement along thrust faults and the development of the Lake Superior syncline. Thermal and alternating field demagnetization, chemical leaching, and multivector analysis using orthogonal projections reveal a trivector structure of the natural remanent magnetization (NRM) of the Fond du Lac Formation and Middle River section (Amnicon and Orienta Formations), and a bivector structure of the NRM of the Eileen section (Eileen Formation). The components may be classified by their physical properties as revealed by demagnetization. A population of high blocking temperature components, K1, is found in all three sections and gives poles as follows: Fond du Lac, 16°N, 160°E; Middle River, 25°N, 148°E; Eileen, 20°N, 156°E after structural correction is applied. A population of intermediate blocking temperature components, K2, is post-tectonic and found only in the Fond du Lac Formation and Middle River section. Poles calculated from K2 fall among the Grenville type poles (Fond du Lac, 9°S, 145°E; Middle River, 24°S, 162°E). A third population of components, K3, has low blocking temperature and coercivity and is isolated only by chemical leaching. K3 has steep positive inclination, northern declination, and is post-tectonic. It is interpreted as a recent magnetization.Any interpretation of the path of apparent polar wander for North America must accommodate the sequence of magnetization K1 to K2. The timing of tectonism in the Keweenawan basin is bracketed by the times of acquisition of K1 and K2. These results reconfirm some recent interpretations that include Grenville poles on the polar wander track of interior Laurentia.

The Mid-Atlantic ridge near 45° N. VII. Magnetic properties and opaque mineralogy of dredged samples

1970 ◽  
Vol 7 (2) ◽  
pp. 239-256 ◽  
Author(s):  
C. M. Carmichael
Keyword(s):  
Magnetic Properties ◽  
Remanent Magnetization ◽  
Complex Structure ◽  
Magnetic Anomalies ◽  
Natural Remanent Magnetization ◽  
Coarse Grained ◽  
X Ray Diffraction ◽  
Field Polarity ◽  
Fine Grained ◽  
Mid Atlantic Ridge

Measurements of the magnetic properties, paleomagnetic field intensity, and the inferred paleomagnetic field polarity have been made using fine grained basalt and coarser grained rock samples dredged from the mid-Atlantic ridge near 45° N and supplied by the Geological Survey of Canada. The opaque mineralogy of the samples was studied by microscope, Curie point, and X-ray diffraction techniques. The natural remanent magnetization of the basalt is of the order of 5 to 10 × 10−3 e.m.u./cm3 with some values from the center of the median valley reaching 10−1 e.m.u./cm3. Magnetic anomalies over the ridge can be accounted for by the remanent magnetization of a few hundred meters of this basalt. The coarse grained rocks were relatively weakly magnetized, and while they contribute little to the magnetic anomalies, their diverse character suggests that the major portion of the oceanic crust, below a thin veneer of fine grained basalt, has differentiated into a complex structure.

Paleomagnetism of Archean rocks from northwestern Ontario: III. Rock magnetism of the Shelley Lake granite, Quetico Subprovince

1984 ◽  
Vol 21 (8) ◽  
pp. 879-886 ◽  
Author(s):  
David J. Dunlop ◽  
Larry D. Schutts ◽  
Christopher J. Hale
Keyword(s):  
Remanent Magnetization ◽  
Rock Magnetism ◽  
Natural Remanent Magnetization ◽  
Thermal Decay ◽  
Magnetic Phases ◽  
Northwestern Ontario ◽  
Component Type ◽  
Thermal Overprint

The Shelley Lake granite of northwestern Ontario contains five magnetic phases: deuteric and post-crystallization hematites, which are relatively abundant but carry only 1–4% of the natural remanent magnetization (NRM); primary magnetite in coarse (50–500 μm) grains, both optically homogeneous and subdivided by hematite lamellae; micrometre-size secondary magnetite in chloritized biotites; and submicrometre-size magnetite, whose presence is inferred from low blocking temperatures in thermal decay curves of the NRM. The NRM is a composite of type 1 and type 2 remanences, which differ in direction by about 90° (see companion paleomagnetic paper). Both NRM components occur in normal (N) and reverse (R) polarities. Type 1 remanences (1N/1R) have the hallmarks of multidomain (MD) behaviour: high blocking temperatures but low coercivities, exponential alternating field (AF) decay curves, generally MD results of the Lowrie–Fuller test, and MD to transitional values (0.3–10) of the Koenigsberger Qn ratio. Furthermore, intensities of 0.6 Oe (0.06 mT) laboratory thermoremanent magnetizations (TRM's) match those of 1R and some 1N NRM's. We argue on this evidence that 1R and at least part of 1N NRM's are TRM's residing in coarse MD-size primary magnetite. This primary TRM dates from initial cooling of the Shelley Lake pluton around 2580 Ma. Thermal decay spectra of single-component type 2 NRM's (2N/2R) resemble those of 1R. However, the considerable overlap of 2N/2R and 1R blocking temperatures in multivectorial NRM's demonstrates that type 2 remanence must be a chemical or thermochemical rather than a thermal overprint.

PALEOMAGNETISM OF THE ALLARD LAKE ANORTHOSITE SUITE

1967 ◽  
Vol 4 (3) ◽  
pp. 357-369 ◽  
Author(s):  
R. B. Hargraves ◽  
D. M. Burt
Keyword(s):  
Magnetic Field ◽  
Lake Area ◽  
Magnetic Vector ◽  
Soft Magnetic ◽  
Hard Component ◽  
Polar Wander ◽  
Rock Types ◽  
Normal Polarity ◽  
Rock Suite ◽  
Structural Blocks

Analysis of paleomagnetic data on 112 samples of Precambrian anorthosite, norite, pyroxene–syenite and hemo-ilmenite ore from four main localities in the Allard Lake area reveals: (1) A relatively soft magnetic component resides in magnetite, which is of variable abundance in all rock types. The magnetite remanent magnetic vector is of normal polarity, and is more or less directly opposed to a hard component residing in hemo-ilmenite, the predominant oxide. Self-reversal of the permanent magnetism in the hemo-ilmenite is the favored explanation. (2) Disregarding polarity, the stable remanent magnetic vector has similar orientation in all rock types from all four localities (N = 4, α95 = 18.0). This precludes significant differential rotation of individual structural blocks since the rocks became magnetized. Furthermore, either no polar wander occurred during the emplacement and cooling of the entire igneous rock suite, or all these rock types acquired their magnetism simultaneously during subsequent metamorphism. (3) On the centered-axial-dipole model for the earth's magnetic field, the remanent vector in the Allard Lake rocks gives a pole southeast of New-foundland, at coordinates 38°36′ N., 39°36′ W. (α95 = 18.0).

Magnetic Properties of Coring Samples from Aceh Basin West Off Sumatera Island

2014 ◽  
Vol 896 ◽  
pp. 434-439
Author(s):  
Eddy Z. Gaffar
Keyword(s):  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Magnetic Polarity ◽  
Isothermal Remanent Magnetization ◽  
Reverse Polarity ◽  
Geologic Time ◽  
The Third ◽  
Geologic Time Scale ◽  
The Last Glacial Maximum ◽  
The Last Glacial

Three oriented core samples was taken by Japanese MIRAI Research Vessel in Aceh Basin west of the Sumatera Island. 927 small box samples from 3 coring samples was measure. Core bottom ages of three cores are in the last glacial maximum (around 19 kyBP). We measured Natural Remanent Magnetization (NRM), NRM after Alternating Field Demagnetization up to 800 Oe. Measurement of NRM have done by 2G Enterprice Squid, Isothermal Remanent Magnetization (IRM) and Anhysteretic Remanent Magnetization (ARM) on Paleomagnetic Laboratory of Geological Survey of Japan. Result of NRM after Alternating Field Demagnetization shows that there are reverse polarizaty magnetizations. This reverse polarity seems not usual since in Geologic Time Scale 1989 showed that the first reversal polarity was between Brunches Normal Epoch and Jaramillo Epoch beginning at 0.75 Ma There are some possibility of reverse polarity events contained in the three cores mentioned above. The first possibility is the result of the tsunami so that the sediment that settles on the Aceh Basin was a sediment that remain turbid flows that enable magnetic recording on magnetic sediment did not follow directions when he formed so that it seems a polarity reverse polarity when only superficial. The second possibility is indeed occurs under reverse polarity at age 700.000 years. Research in Argentina, South America show that between 11 thousand to two thousand years ago there was an excursion from the magnetic polarity produces some reverse polarity at the time (Nami, 1999). The third possibility was circular variation magnetic.

scholarly journals Combined In Situ Chemical and Sr Isotopic Compositions and U–Pb Ages of the Mushgai Khudag Alkaline Complex: Implications of Immiscibility, Fractionation, and Alteration

Minerals ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 450
Author(s):  
Fan Yang ◽  
Wei Chen ◽  
Jindrich Kynicky ◽  
Yuancan Ying ◽  
Tian Bai
Keyword(s):  
Oscillatory Zoning ◽  
Alkaline Magmatism ◽  
Alkaline Complex ◽  
Late Mesozoic ◽  
Fine Grained ◽  
Rock Types ◽  
Lree Enrichment ◽  
Limited Variation ◽  
High Level

The Mushgai Khudag complex consists of numerous silicate volcanic-plutonic rocks including melanephelinites, theralites, trachytes, shonkinites, and syenites and also hosts numerous dykes and stocks of magnetite-apatite-enriched rocks and carbonatites. It hosts the second largest REE–Fe–P–F–Sr–Ba deposit in Mongolia, with REE mineralization associated with magnetite-apatite-enriched rocks and carbonatites. The bulk rock REE content of these two rock types varies from 21,929 to 70,852 ppm, which is much higher than that of syenites (716 ± 241 ppm). Among these, the altered magnetite-apatite-enriched rocks are characterized by the greatest level of REE enrichment (58,036 ± 13,313 ppm). Magmatic apatite from magnetite-apatite-enriched rocks is commonly euhedral with purple luminescence, and altered apatite displays variable purple to blue luminescence and shows fissures and hollows with deposition of fine-grained monazite aggregates. Most magmatic apatite within syenite is prismatic and displays oscillatory zoning with variable purple to yellow luminescence. Both magmatic and altered apatite from magnetite-apatite-enriched rocks were dated using in situ U–Pb dating and found to have ages of 139.7 ± 2.6 and 138.0 ± 1.3 Ma, respectively, which supports the presence of late Mesozoic alkaline magmatism. In situ 87Sr/86Sr ratios obtained for all types of apatite and calcite within carbonatite show limited variation (0.70572–0.70648), which indicates derivation from a common mantle source. All apatite displays steeply fractionated chondrite-normalized REE trends with significant LREE enrichment (46,066 ± 71,391 ppm) and high (La/Yb)N ratios ranging from 72.7 to 256. REE contents and (La/Yb)N values are highly variable among different apatite groups, even within the same apatite grains. The variable REE contents and patterns recorded by magmatic apatite from the core to the rim can be explained by the occurrence of melt differentiation and accompanying fractional crystallization. The Y/Ho ratios of altered apatite deviate from the chondritic values, which reflects alteration by hydrothermal fluids. Altered apatite contains a high level of REE (63,912 ± 31,785 ppm), which are coupled with increased sulfur and/or silica contents, suggesting that sulfate contributes to the mobility and incorporation of REEs into apatite during alteration. Moreover, altered apatite is characterized by higher Zr/Hf, Nb/Ta, and (La/Yb)N ratios (179 ± 48, 19.4 ± 10.3, 241 ± 40, respectively) and a lack of negative Eu anomalies compared with magmatic apatite. The distinct chemical features combined with consistent Sr isotopes and ages for magmatic and altered apatite suggest that pervasive hydrothermal alterations at Mushgai Khudag are most probably being induced by carbonatite-evolved fluids almost simultaneously after the alkaline magmatism.

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