Rock Magnetic Results Research Articles

Timing and origin of orogenic remagnetizations in Mississippian carbonates, Sawtooth Range, Montana

Paleomagnetic data and rock magnetic results suggest that a widespread orogenic remagnetization caused a pervasive chemical remanent magnetization (CRM) that resides in magnetite in the Mississippian Madison Group in the Sawtooth Range, Montana, during the Late Cretaceous to Early Tertiary. The CRM is similar to a CRM reported by workers from equivalent units in the southern Canadian Cordillera. The CRM is interpreted to be related to alteration by fluids, and there are two likely fluids: hydrocarbons which migrated into the unit and externally derived radiogenic fluids.

Magnetic fabric and rock magnetic studies of metasedimentary rocks in the central Okcheon Metamorphic Belt, Korea

Abstract Anisotropy of magnetic susceptibility (AMS) and rock magnetic studies have been carried out for the metasedimentary rocks in the central Okcheon Metamorphic Belt. The study area is divided into three metamorphic zones: the biotite zone, the garnet zone, and the sillimanite+andalusite zone from southeast to northwest. Magnetic foliation dipping to the northwest is the dominant magnetic fabric in the biotite zone. Magnetic lineation plunging down-dip of the vertical cleavage plane is appeared in the southeastern part of the garnet zone, while magnetic lineation plunging to southeast prevails in the middle and northwestern parts of the garnet zone. It is interpreted that this apparent synform structure, defined by AMS fabric data, was formed by successive top-to-the-southeast vergent thrusting followed by back-steepening process during the regional metamorphism in the Late Paleozoic. In the sillimanite + andalusite zone, AMS fabric is clearly defined and may reflect a tectonic fabric, recorded during the thermal metamorphism in the middle Jurassic, despite the scarcity of rock fabric in the field. The spatial distribution of magnetic mineralogy, defined by the rock magnetic results, implies the 500°C isotherm at the boundary between the biotite and garnet zones, which is associated with the transformation of pyrrhotite into magnetite above 500°C under the oxidizing condition.

Rock magnetic studies on the hominoid-bearing sediments at Zhupeng, Yuanmou Basin, southwestern China and its paleoclimatic significance

It has been proposed that paleoclimatic changes and tectonic events strongly affect hominoid evolution. The Zhupeng section at Yuanmou Basin, southwestern China, with the hominoid-bearing fluvial-lacustrine sediments, is an ideal natural laboratory to test this hypothesis. This study provided an integrated magnetic study, including low-fre-quency susceptibility, the anisotropy of magnetic susceptibil-ity, temperature-dependent susceptibility, hysteresis loops, isothermal remanent magnetization, and anhysteretic rema-nent magnetization, on samples from the Baozidongqing section at Zhupeng. Results show that the dominant mag-netic carrier is hematite, with minor amount of magnetite. Both the composition and concentration of magnetic miner-als strongly correlate with the lithostratigraphy. At least eight short-term events defined by higher concentrations of magnetite were identified. These short events reflect that the subtropical dry-hot climate is similar to today’s climate. Basing on the lithostratigraphic and rock magnetic results, we suggest that Yuanmou Basin was in a steady deposition environment from about 11 to 7 Ma, which is characterized by alternating of long-term torrid-humid climate and short-term dry-hot climate. Our study provides invaluable environment information for understanding the climate shift and the relationship between paleoenvironment and homi-noid evolution in southwestern China during late Miocene.

New paleomagnetic, rock magnetic, and petrographic results from the Valle Group, Baja California, Mexico: Exploring the causes of anomalously shallow paleomagnetic inclinations

A detailed paleomagnetic, rock magnetic, and sedimentologic study was conducted on Cenomanian‐age strata of the Valle Group at El Campito, Baja California, to examine the burial compaction effects on the paleomagnetic inclination of marine sedimentary rocks. Alternating field and thermal demagnetization isolated a characteristic remanence at 16 of 18 sites with a mean of Dec = 342.6°, Inc = 19.1°, and α95 = 3.4°. The inclination is 38.7° shallower than that predicted by the reference paleopole for North America. Laboratory experiments indicate that compaction could account for only 7.5° of the inclination shallowing. Fold tests at different scales yield both positive and negative results, making a tight constraint on the remanence's age impossible. A primary postdepositional remanence is supported by bedding‐parallel foliations in the anisotropy of anhysteretic remanence fabric. Although primary anomalously shallow inclinations could indicate significant southerly, then northerly, paleolatitudinal offset, a more likely scenario is a late Cenozoic low‐temperature remagnetization, which is suggested by alteration along the edges of some detrital silicate grains and a bimodal magnetic grain size distribution revealed by isothermal remanence data. The synthesis of the data indicates that the Vizcaino terrane was contiguous with the Cordillera margin during remagnetization and underwent a subsequent 13.5 ± 5.4° clockwise rotation. We tentatively interpret that the remagnetization resulted from burial and/or Miocene volcanism, which caused an acquisition of thermochemical remanent magnetization (TCRM). This study reveals a complicated remanence that resists easy interpretation and suggests caution in studies of similar rocks.

Paleomagnetic dating of burial diagenesis in Mississippian carbonates, Utah

The objective of this study is to test models for the origin of widespread secondary magnetizations in the Mississippian Deseret Limestone. The Delle Phosphatic Member of the Deseret Limestone is a source rock for hydrocarbons, and modeling studies indicate that it entered the oil window in the Early Cretaceous during the Sevier orogeny. Paleomagnetic and rock magnetic results from the Deseret Limestone and the stratigraphically equivalent Chainman Shale in central and western Utah indicate that the units contain two ancient magnetizations residing in magnetite. Burial temperatures are too low for the magnetizations to be thermoviscous in origin, and they are interpreted to be chemical remanent magnetizations (CRMs). Fold tests from western Utah indicate the presence of a prefolding Triassic to Jurassic CRM. Geochemical (87Sr/86Sr, δ13C, and δ18O) and petrographic analyses suggest that externally derived fluids did not alter these rocks. This CRM was acquired at the beginning of the oil window and is interpreted to be the result of burial diagenesis of organic matter. A second younger CRM in western central Utah is apparently postfolding and is probably Late Cretaceous to early Tertiary in age. On the basis of the thermal modeling, the timing overlaps with the oil window. These results are consistent with a connection between organic matter maturation and remagnetization. Modeling of the smectite‐to‐illite transformation in the Deseret Limestone suggests a mean age prior to acquisition of both CRMs, although the range for illitization overlaps with the Triassic to Jurassic CRM. The results of this study support the hypothesis that pervasive CRMs can be related to burial diagenetic processes. In addition, paleomagnetism can be used to determine the timing of such processes, which can benefit hydrocarbon exploration efforts.

Rock magnetic results from Ocean Drilling Program Leg 192: implications for Ontong Java Plateau emplacement and tectonics of the Pacific

Abstract A rock magnetic study has been performed on rock samples recovered at Ocean Drilling Program Leg 192 sites on the Ontong Java Plateau in the western Pacific. Igneous rocks from the five Leg 192 sites displayed variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Titanomagnetite and titanomaghemite are present in the Ontong Java rocks. Samples with titanomagnetite exhibit Verwey transitions in the vicinity of 120K. Low-temperature curves for samples with multiple magnetic phases do not clearly show the Verwey transition. The inversion of titanomaghemite to a strongly magnetized magnetite is shown by the irreversible thermomagnetic-cooling curve. Despite the geographically widespread locations of the drill sites, variations in rock magnetic properties closely resemble each other, consistent with the fundamental results of the leg that the basement rocks were derived from homogeneous Kwaimbaita-type magma with a single age of approximately 120 Ma. The rock magnetic investigation provides constraints to evaluate the fidelity of the natural magnetic memory in the basalt rocks and corroborates the palaeomagnetic palaeolatitudes determinations for the Ontong Java Plateau. The generally good quality of rock magnetic data exhibited by Leg 192 rocks supports the inference that the characteristic directions of magnetization isolated from the Cretaceous Ontong Java Plateau sites were acquired near the onset of the Cretaceous Long Normal Superchron about 120 Ma. The portion of the Pacific plate containing the Leg 192 sites was in the southern hemisphere during the mid-Cretaceous volcanism.

Further rock magnetic and chronostratigraphic results on reversal excursions during the last 50 ka as derived from northern high latitudes and discrepancies in precise AMS14C dating

A total of five sediment cores from three sites, the Arctic Ocean, the Fram Strait and the Greenland Sea, yielded evidence for geomagnetic reversal excursions and associated strong lows in relative palaeointensity during oxygen isotope stages 2 and 3. A general similarity of the obtained relative palaeointensity curves to reference data can be observed. However, in the very detail, results from this high-resolution study differ from published records in a way that the prominent Laschamp excursion is clearly characterized by a significant field recovery when reaching the steepest negative inclinations, whereas only the N–R and R–N transitions are associated with the lowest values. Two subsequent excursions also reach nearly reversed inclinations but without any field recovery at that state. A total of 41 accelerator mass spectrometry (AMS) 14C ages appeared to allow a better age determination of these three directional excursions and related relative palaeointensity variations. However, although the three sites yielded more or less consistent chronological as well as palaeomagnetic results a comparison to another site, PS2644 in the Iceland Sea, revealed significant divergences in the ages of the geomagnetic field excursions of up to 4 ka even on basis of uncalibrated AMS 14C ages. This shift to older 14C ages cannot be explained by a time-transgressive character of the excursions, because the distance between the sites is small when compared with the size of and the distance to the geodynamo in the Earth's outer core. The most likely explanation is a difference of reservoir ages and/or mixing with old 14C-depleted CO2 from glacier ice expelled from Greenland at site PS2644.

Late Carboniferous to Permian remagnetization of Devonian limestones in the Ardennes: Role of temperature, fluids, and deformation

To test various remagnetization mechanisms in unmetamorphosed to very low grade metamorphic carbonates, we integrated paleomagnetic and rock magnetic data of Devonian carbonates with geological observations in the well‐studied Rhenohercynian Belt of France and Belgium. Paleomagnetic results from 42 sites show that there are three separate remanence components. Hysteresis measurements that are consistent with unremagnetized limestones and the low paleotemperature of the Boulonnais area (<55°C) indicate that component B is a primary Middle Devonian natural remanent magnetization (NRM). Prefolding to postfolding Carboniferous component C and postfolding Early Permian component P both occur throughout the Ardennes and Brabant Massif. Rock magnetic results indicate that component P is carried by pyrrhotite and component C is carried by a magnetite mix that straddles the single‐domain‐multidomain grain‐size range (10–30 nm). The relative intensity factor, Ri, defined as ln(intensity P/intensity C), shows a marked spatial correlation with the southern of two Mississippi Valley‐type (MVT) ore districts. This, in combination with the timing of remagnetization, suggests that component P pyrrhotite formed as a result of percolation of MVT fluids through these carbonates. Fine‐grained magnetite formation, as a by‐product of smectite‐to‐illite conversion in the presence of host‐rock‐buffered internal fluids during deformation, is the most likely remagnetization mechanism for component C. Pressure solution deformation processes may have enhanced the smectite‐to‐illite conversion and hence remagnetization.

Magnetic and mineralogical studies to characterize oil reservoirs in Venezuela

High-resolution aeromagnetic surveys over oil fields and magnetic susceptibility (MS) measurements in soils, sediments, and drill cuttings have been proposed as complementary or alternative means of exploration and assessment of hydrocarbon reservoirs. Some micromagnetic anomalies detected by these studies could be the likely result of the existence, at shallow depths, of authigenic magnetite and/or Fe-sulphides. It has been hypothesized that such magnetic minerals are the chemical byproducts of primary Fe-oxides exposed to a reducing environment induced by the underlying reservoir. Secondary magnetite could appear as spherical aggregates (framboids) of submicronic crystals identifiable by scanning electron microscopy (SEM). The formation of these framboids seems linked either to inorganic chemical processes associated with the presence of hydrocarbons or to oil biodegradation. In the last few years, we have examined the causal relationship between framboids of magnetic minerals and hydrocarbons via rock magnetic and electron paramagnetic resonance (EPR) analyses of drill cuttings. The samples used have been taken at different depths (within the first 1500 m) of producing and nonproducing wells from two oil fields in southwestern Venezuela. We have complemented MS results with S-ratio logs, SEM, X-ray energy dispersion (EDX), IRM acquisition curves, and high-temperature susceptibility measurements. More recently we have also studied soil samples from an oil prospective area in western Venezuela, where rock magnetic and EPR results were compared with ethane gas analyses and remote sensing data. Rock magnetic and EPR studies have been performed in 14 oil wells from La Victoria (3) and Guafita (11) fields (Figure 1). Since our research has focused at the near-surface expression of hydrocarbon migration, the samples analyzed (drill cuttings) were taken from different depth at intervals of about 15 m between 100 and 1500 m. All samples belong to a single group of molasses from strata that lie far above the producing levels …

Stable Eocene Magnetization Carried by Magnetite and Iron Sulphides in Marine Marls (Pamplona-Arguis Formation, Southern Pyrenees, Northern Spain)

In order to establish the magnetic carriers and assess the reliability of previous paleomagnetic results obtained for Eocene marine marls from the south Pyrenean basin, we carried out a combined paleo- and rock-magnetic study of the Pamplona-Arguis Formation, which crops out in the western sector of the southern Pyrenees (N Spain). The unblocking temperatures suggest that the characteristic remanent magnetization (ChRM) is carried by magnetite and iron sulphides. The ChRM has both normal and reversed polarities regardless of whether it resides in magnetite or iron sulphides, and represents a primary Eocene magnetization acquired before folding. Rock magnetic results confirm the presence of magnetite and smaller amounts of magnetic iron sulphides, most likely pyrrhotite, in all the studied samples. Framboidal pyrite is ubiquitous in the marls and suggests that iron sulphides formed during early diagenesis under sulphate-reducing conditions. ChRM directions carried by magnetic iron sulphides are consistent with those recorded by magnetite. These observations suggest that magnetic iron sulphides carry a chemical remanent magnetization that coexists with a remanence residing in detrital magnetite. We suggest that the south Pyrenean Eocene marls are suitable for magnetostratigraphic and tectonic purposes but not for studies of polarity transitions, secular variations and geomagnetic excursions, because it is difficult to test for short time differences in remanence lock-in time for the two minerals. The presence of iron sulphide minerals contributing to the primary magnetization in Eocene marine marls reinforces the idea that these minerals can persist over long periods of time in the geological record. K e y wo r d s : paleomagnetism, rock magnetism, marine sediments, early diagenesis, magnetic iron sulphides, Pyrenees

Thellier palaeointensity experiments on Faroes flood basalts: technical aspects and geomagnetic implications

Based on palaeomagnetic and rock magnetic results, 108 samples from 27 flows from Palaeogene flood basalts of Faroe Islands were chosen for whole-rock Thellier palaeointensity experiments. Altogether 90 samples were rejected due to either chemical alterations or typical multidomain (MD) behaviour evidenced by pTRM-tails. AF pre-treatment was used to reduce the effect of MD grains on Thellier experiments. Only five flows (18 samples) yielded acceptable palaeointensity estimates, with flow mean VDMs ranging from 3.5 to 7.4×10 22 A m 2. Modest selection criteria imposed on all published 5–160 Ma palaeointensity data left only 15 palaeomagnetic dipole moments: eight from whole-rock samples, six from submarine basaltic glass and one from single plagioclase crystals. More data are needed before the intriguing differences between results from different materials can be put into a geomagnetic context.

Carboniferous palaeomagnetism of the Rocky Creek Block, northern Tamworth Belt, and the New England pole path

Palaeomagnetic, rock magnetic and magnetic fabric results are presented for a Carboniferous (Visean to Westphalian) succession of felsic, mainly ignimbritic, volcanic and volcaniclastic rocks from the Rocky Creek Block of the northern Tamworth Belt, southern New England Orogen. Detailed thermal demagnetisation of 734 samples from 64 sites show three groups of magnetic components with low (<300°C), intermediate (300–600°C) and high (500–680°C) unblocking temperature ranges. Well‐defined primary magnetisations have been determined for 28 sites with evidence of four overprint phases. The overprints arise from a mid‐Tertiary weathering event (or possibly recent viscous origin), and from fluid movements associated with the Late Cretaceous opening of the Tasman Sea, thrusting during the Middle Triassic main phase of the Hunter‐Bowen Orogeny, and latest Carboniferous — Early Permian formation of the Bowen‐Gunnedah‐Sydney Basin system. Rock magnetic tests establish that the primary magnetisation carriers in the volcanic rocks are mainly magnetite (predominantly single domain, or pseudo‐single domain, and little or no multidomain) and hematite. Optimal magnetic cleaning is achieved at high to very high temperatures, with subtle, but systematic, directional and statistical differences between primary components derived from the mainly hematite fraction and pseudo‐components derived from the mainly magnetite fraction. The 28 primary magnetisation results are presented as six mean‐site results, summarised below and representing 25 sites, and three single‐site results. Fold tests could be applied to five mean‐site results. These are all positive, but one of these results may represent a secondary magnetisation. The primary magnetisation results define a Visean to Westphalian pole path. This long pole path indi cates extensive latitudinal and rotational movement for the Rocky Creek Block, and potentially for the New England Orogen, as follows: (i) Yuendoo Rhyolite Member (Caroda Formation, Visean) pole 235.8°E, 27.7°S, ED95 = 9.0°, n = 3; (ii) Peri Rhyolite Member/Boomi Rhyolite Member (Clifden Formation, Namurian, 318.0 ± 3.4 Ma) pole 177.4°E, 63.4°S, ED95 = 5.2°, n = 3; (iii) tuffaceous beds above Boomi Rhyolite Member (Clifden Formation?, Namurian) pole 162.2°E, 59.1°S, ED95 = 10.2°, n = 3; ((iv) upper Clifden Formation/lower Rocky Creek Conglomerate (Namurian/Westphalian) pole 95.3°E, 49.6°S, ED95 = 8.1°, n = 3 (possible overprint)); (v) Rocky Creek Conglomerate (Westphalian) pole 136.5°E, 57.6°S, ED95 = 5.3°, n = 5; (vi) Lark Hill Formation (Westphalian) pole 127.0°E, 50.4°S, ED95 = 4.8°, n = 8.

Paleomagnetism of early Paleozoic volcanic rocks from the Beishan area, Gansu of northwest China: Preliminary insight into early Paleozoic kinemics of the Beishan terrane

We report rock magnetic and paleomagnetic results for Middle Silurian Gongpoquan and Middle Ordovician Huaniushan volcanic samples collected from the Mazongshan and Hongliuyuan areas of Gansu Province, north-west China. The results indicate that the main magnetic mineral of the Gongpoquan and Huaniushan volcanic rock is magnetite, and it may remain characteristic remanent magnetization acquired at the time of formation of the rocks. However, with the exception of samples from site go20, most of Middle Ordovician Huaniushan volcanic samples have a very weak nature remanent magnetization, and generally show an erratic demagnetization behavior, which does not allow isolation of characteristic remanent magnetizations from these samples. Comparison of paleomagnetic results obtained from the Beishan area of Gansu with Paleozoic apparent polar wander paths for Tarim and Siberia suggests that the Beishan tarrane, the southern part of the Beishan area of Gansu beyond the Mingshui-Shipanjin-Xiaohuangshan zone, and adjacent Tarim block and Siberia plate could not have been connected in their present configuration by the Middle Silurian time. On the other hand, we infer that the early Paleozoic Mingshui-Shipanjin-Xiaohuangshan oceanic basin may open from west to east during period from the Ordovician to Silurian, and the post-Middle Silurian closing of this oceanic basin may result from a progressed eastward scissor-like rotation

Rock magnetic properties of a loess-paleosol couple along an N-S transect in the Chinese Loess Plateau

We have conducted detailed rock magnetic experiments on samples from loess unit 8 (L8) and paleosol unit 8 (S8) in Jingbian, Yichuan and Duanjiapo loess sections along an N-S transect in the Chinese Loess Plateau. Major rock magnetic results are as follows: (ⅰ) An increase of high field susceptibility ( χ h ) in the same level of loess or paleosol from north to south is observed, suggesting an enhancement of pedogenesis. (ⅱ) The low field susceptibility ( χ L ) in loess unit L8 is almost the same in three sections. In contrast, the χ L of paleosol unit S8 in Yichuan is highest, and the χ L of Duanjiapo is lower than that in Yichuan section, suggesting that there is not correlation between the χ L and the degree of pedogenesis in loess-paleosols. (ⅲ) With the increasing of χ L , both the contents of the superparamagnetic (SP) and the ferrimagnetic grains in loess-paleosol increase, however, the enhancement of magnetic susceptibility is probably dependent more on the increase of the ferrimagnetic concentration than on a change in the grain size. (ⅳ) The content of the maghemite in loess unit 8 increases from the northern to the southern part of the Chinese Loess Plateau, and is positively correlated with the pedogenesis of the loess.

Contrasting rock-magnetic characteristics of two upper Paleozoic loessite-paleosol profiles

Rock magnetic results from paleosols in two North American Paleozoic loessite sequences, the Maroon Formation (Colorado) and the lower Cutler beds (Utah), indicate that bulk magnetic susceptibility (χb) variations can be complex. In Maroon Formation profiles, χb increases with increased pedogenic intensity and the major contributor to the augmentation of the χb signal is the ferrimagnetic susceptibility (χf). These trends are consistent with previous studies in which χb has been linked to climatically controlled pedogenesis. Thermal demagnetization of low-temperature saturation isothermal remanent magnetizations (SIRMs) indicates this increase correlates to a greater abundance of superparamagnetic (SP) grains, most likely magnetite, within the paleosols. The presence of coarser magnetite is indicated by the Verwey transition. In contrast, paleosols in the lower Cutler beds show a less predictable pattern in the χb signal. The paramagnetic χ (χp) and χf contribute to varying degrees to the χb within the loessite-paleosol couplets. Thermal demagnetization of low-temperature SIRMs indicates that both SP and coarser magnetite is present. There are no consistent patterns and often no significant variations in the amount of SP material within loessite-paleosol couplets. The thermal demagnetization curves for samples of the lower Cutler beds display a gradual decay of remanence and a weakly developed Verwey transition, both of which probably relate to maghemitization. Most paleosol and loessite samples differ in that there is more remanence-carrying material, predominantly hematite and some magnetite and maghemite, in the loessite. Rock-magnetic differences among loessite-paleosol couplets of the lower Cutler beds may relate to the heterogeneous distribution of pedogenic carbonate and/or to the presence of abundant hematite. This study shows that combining rock-magnetic techniques with sedimentologic evidence is important for deciphering the origin and preservation of χb patterns in ancient strata.

Palaeomagnetic and rock magnetic results from serpentinized peridotites beneath the Iberia Abyssal Plain

Abstract A palaeomagnetic and rock magnetic study has been performed on serpentinized peridotite rocks recovered at Ocean Drilling Program (ODP) Leg 149 and 173 sites, off the west coast of Portugal (Sites 897, 899 and 1070). Stable components of magnetization are revealed after detailed thermal and alternating field demagnetization. Results from a series of rock magnetic measurements with advanced instrumentation all corroborate the demagnetization behaviour and show that (titano)magnetites and maghemite are present in the peridotites. At Leg 149 and 173 sites, the inclinations of characteristic magnetization in the ‘fresher’ lower part of the serpentinized peridotite section show a predominantly reversed polarity in a depth zone of c. 25 m, which is compatible with a reversed event (probably correlated with marine Anomaly M0 at c. 121 Ma) before the Cretaceous Long Normal Superchron (83–120 Ma) and is incompatible with a Holocene field direction. In contrast, the inclinations of samples from the more ‘altered’ upper part are predominantly normal. In view of the polarity of magnetization identified from the overlying Cretaceous and Tertiary sediments and the newest interpretation of the drilling results for the Iberia margin, our results suggest that the Iberian peridotites were emplaced during Aptian-Barremian time and recorded the middle Cretaceous geomagnetic field during Anomaly M0 time ( c. 121 Ma). Our data are consistent with new radiometric dates for the Iberian peridotites at Leg 173 drill sites.

Paleoposition of the northern margin of Armorica in Late Devonian times: Paleomagnetic and rock magnetic results from the Frankenstein Intrusive Complex (Mid‐German Crystalline Rise)

A detailed rockmagnetic and paleomagnetic study of the Frankenstein Intrusive Complex (FIC, 363±7 Ma in age), southwestern Germany, has been carried out in order to test whether the paleomagnetic declinations within the internal parts of the European Variscides follow a common trend controlled by the structural grain of the orogen or whether the distribution of declinations follows a random pattern. In addition, new paleomagnetic data will contribute to further refine the drift history of the Armorican Terrane Assemblage during late Paleozoic times. The unmetamorphosed Frankenstein Igneous Complex forms part of the Mid‐German Crystalline Rise within the Armorican Terrane Assemblage, amalgamated to Avalonia and Baltica during the Variscan orogeny, Stepwise thermal and alternating field (AF) demagnetization experiments identify four components (A, B, Cn, and Cr) of magnetization. Component A is a recent viscous overprint that parallels the present‐day geomagnetic field. The high unblocking temperatures of A in some samples can be explained by the presence of multidomain magnetite. Component B is carried by hematite and is considered to be a late Carboniferous remagnetization. Components Cr and Cn, found in 110 samples from 20 sites, are antiparallel and pass a class C reversal test. Their primary origin is further supported by a contact test. Cr and Cn have maximum unblocking temperatures of up to 580°C and occasionally above over 600°C, indicating magnetite and hematite to carry the characteristic remanent magnetization. Ore microscopy revealed that primary hematite lamellae in ilmenite carry Cr and Cn and that a secondary generation of hematite is the carrier of B. The overall site mean direction of Cr and Cn (20 sites) based on endpoints and remagnetization circle analysis of 198°/40° (declination/inclination) with a 95% confidence limit (α95) of 4.9° and a precision parameter (k) of 45.0 defines a paleopole at 15°S, 9°W. When comparing to the apparent polar wander path for stable Europe this result indicates up to 27° counterclockwise rotation of the FIC region. This is significantly less than reported previously for the Odenwald, Spessart, Black Forest, and the Vosges Massifs of the central European Variscides. Furthermore, the resulting paleolatitude indicates significant separation between Baltica and this sector of the Armorican Terrane Assemblage in the latest Devonian.

Palaeomagnetic results from remagnetized mid-Cretaceous(Albian-Cenomanian) strata of northeastern Venezuela

SUMMARY We report palaeomagnetic and rock magnetic results of a sedimentary sequence (Pertigalete cement quarry) located in northeastern Venezuela. Sampling was restricted to the vicinity of the contact between the upper Cretaceous Chimana and Querecual formations. Biostratigraphic evidence reveals an upper Albian age for this formational transition. Profiles of site-averaged NRM intensity of the high-coercivity (over 30 mT) and high-temperature (over 400°C) components appear to be related to the contact and distance from the contact. We interpret this profile as the probable outcome of overlapping thermochemical remagnetization events resulting from hydrothermal activity that was focused along the two formations. Direct spectral analyses performed on the site-averaged stable NRM intensity profile allow the separation of at least two of these remagnetization events. On the other hand, palaeomagnetic results show a considerable streaking of site mean declinations, suggesting that tectonic or structural horizontal movements around a vertical axis have occurred after NRM acquisitions. Horizontal rotation angles, plotted against stratigraphic levels for bedding-corrected data, show some features that seem to coincide with the alteration peaks isolated in the profile of site-averaged stable NRM intensities. Thus, it appears that repeated thermochemical remagnetizations with overlapping unblocking spectra, and horizontal movements around a vertical axis could have been responsible for much of the within-site dispersion. A simple three-stage reconstruction of the possible chain of thermochemical and tectonic occurrences that could lead to the present-day palaeomagnetic and rock magnetic evidence is proposed. These events, including clockwise horizontal rotations around a vertical axis, are tentatively placed in a geological time framework between middle Miocene and Pliocene times according to the main geological and geochemical evidence available.

Rock magnetic properties related to thermal treatment of siderite: Behavior and interpretation

Detailed analyses of rock magnetic experiments were conducted on the oxidation products of high‐purity natural crystalline siderite that were thermally treated in air atmosphere. Susceptibilities increase sharply between 400° and 530°C indicative of some new ferrimagnetic mineral phase generation. Both a drop (between 540° and 590°C) on the heating cycle and a dramatic increase (from 590°C to 520°C) on the cooling cycle occurred and are well consistent with the characteristic of magnetite. A distinct Hopkinson‐type susceptibility peak indicates that hematite is the terminal product if siderite is heated to 700°C over and over. It has been revealed in detail that the original inverse magnetic susceptibility fabric contributed by the crystalline anisotropy of siderite in siderite‐bearing specimens is changed to a normal magnetic fabric during incremental heating over 410°–490°C. This is a result of dominant contributions from the distribution anisotropy of newly transformed ferromagnetic minerals. A strong chemical‐viscous remanent magnetization could be produced during siderite oxidation in an external field. Rock magnetic experimental results show that magnetite, maghemite, and hematite are the transformation products of high‐temperature oxidation of siderite in air. Maghemite was not completely inverted to hematite even at temperature as high as 690°C during incremental thermal treatments. The mineral transformation processes were confirmed by conventional optical microscopic observation, X‐ray diffractometry and Mössbauer spectroscopic analyses. These results indicate that the rock magnetic methods used here are reliable and highly sensitive in detecting very small magnetic phase changes in rocks. We conclude that these temperaturedependent variations of magnetic properties can be used as criteria for identification of siderite in rocks and sediments. Furthermore, it is clear that great care should be exercised in thermal demagnetization of siderite‐bearing rocks in paleomagnetic, magnetic anisotropy, and rock magnetic studies.

Environmental changes since 13,000 cal. BP reflected in magnetic and sedimentological properties of sediments from Lake Holzmaar (Germany)

Late- and Postglacial sediments from a German maar lake (Holzmaar) in the West Eifel Volcanic Field were investigated with rockmagnetic and sedimentological methods. The concentration of magnetic minerals is very variable and zones exist, where magnetic properties are dominated by paramagnetic minerals. Remanence carrying (ferromagnetic) minerals are dominated by a low coercivity fraction, identified by its Curie temperature and by scanning electron microscopy as titanomagnetite with a grain size range between a few microns and ≈25 μm. An additional high coercivity phase, probably hematite, is present throughout the entire record in low concentrations. Rockmagnetic results indicate the possible postdepositional dissolution of (titano-) magnetite and the authigenic growth of greigite in one confined section. A positive correlation between initial magnetic susceptibility ( κ) and sediment accumulation rate and between κ and the non-arboreal pollen percentage is observed while κ and organic carbon content show a negative correlation. These coeval variations are due to a nearly exclusively detrital sedimentary input. The variations show a broad dependence on climatic conditions for times prior to significant human activities in the region and the impact of the human settlement history afterwards. Early Holocene climatic variations, indicating two cooling events, agree with ice rafted debris events of the North Atlantic.