Abstract: Magnetic fabric research on the emplacement of the Taimyr fold-belt is still absent. Therefore, we have performed magnetic fabric studies on samples collected from 23 sites from mafic igneous rocks (75ºN, 100ºE) in South Taimyr. This study indicates a correlation between the anisotropy of magnetic susceptibility (AMS), magma flow and fold-related compression in the rocks. AMS measurements on 183 unheated and 122 heated samples reveal the magma flow directions. The magma flow direction is mainly parallel to the ESE-WNW and secondarily parallel to the SSW from the NNE in the mafic sills. The unheated and heated basaltic flows reveal a NE-trending flow vector. In some of the sills, AMS shows a tight clustering of the maximum K1 axes close to the bedding pole, which is not thought to define the flow direction. Corresponding AMS measurements on igneous rocks allow us to infer the existence of magnetic fabrics of tectonic origin linked to the main folding episode that occurred subsequent to the Mid-Triassic magmatic event in latest Triassic-Early Jurassic times. In the sills, the distribution of most maximum K1 axes is close to NS that corresponds to the maximum compressive stress or folding directions; contrarily, the minimum K3 axes have an elongated distribution along the EW direction or parallel to the fold axis before exchanging K1 and K3 axes for inverse fabrics, but close to the NS stress direction after exchanging K1 and K3 axes for inverse fabrics. In the basaltic flows, the minimum K3 axes almost parallel to the NS folding direction. The structural interpretation of all AMS data taken from the igneous bodies is in accordance with a NNW-SSE stress-related folding taking place around 198 Ma in the Taimyr Peninsula (Arctic Siberia). The relationship of susceptibility axes to bedding surfaces and magnetic foliation planes is a criterion that permits differentiation of normal magnetic fabrics from inverse fabrics in the igneous samples. Single-domain (SD) and small pseudo-single-domain (PSD) magnetite crystals in the sill samples are the main carriers of the AMS fabrics, implying the abundance of inverse magnetic fabrics. PSD and multidomain (MD) titanomagnetite (TM) grains as well as minor hematite are identified in the basalts flows, suggesting the occurrence of more normal magnetic fabrics in the basalts than in the sills.
Abstract: This study examined spatial variations in the concentration, grain size and heavy mineral assemblages on Cedar Beach (Lake Erie, Canada). Magnetic studies of heavy mineral-enriched, dark-reddish sands present on the beach showed that magnetite (~150 μm) is the dominant magnetic mineral. Surficial magnetic susceptibility values defined three zones: a lakeward region close to the water line (Zone 1), the upper swash zone (Zone 2) and the region landwards of the upper swash zone (Zone 3). Zone 2 showed the highest bulk and mass susceptibility (κ, Ï) and the highest mass percentage of smaller grain-size (<250 μm) fractions in the bulk sand sample. Susceptibility (i.e. κ and Ï) values decreased and grain size coarsened from Zone 2 lakewards (into Zone 1) and landwards (into Zone 3), and correlated with the distribution of the heavy mineral assemblage, most probably reflecting preferential separation of large, less dense particles by waves and currents both along and across the beach. The eroded western section of Cedar Beach showed much higher concentrations of heavy minerals including magnetite, and finer sand grain sizes than the accreting eastern section, suggesting that magnetic techniques could be used as a rapid, cost-effective way of examining erosion along sensitive coastline areas.
Notes: At present environmental magnetic studies are very limited in a lake-beach environment due to the fast changing conditions. This study used a combination of ESEM-EDAX and environmental magnetism methods: (1) to assess the use of environmental magnetic methods on the coastal area; and (2) to investigate the moving regime of beach sediments (the dominant Grain-Sorting processes by water and currents) on Cedar Beach, western Lake Erie, Canada. The results show that such environmental magnetic investigations can provide valuable information on the migration behavior of the beach sediments. This study is useful for decision-making and coastal managing. The Great Lakes. (Zhang SW et al., 2010; Zhang Shuwei et al., 2010)
Abstract: Magnetic measurements were performed on apparently deformed igneous rocks of 23 sites from the
southeastern part of the Taimyr Peninsula. Rock magnetism and reflected light microscopy analyses
reveal that fine-grained titanomagnetites up to pure magnetites mainly carry the majority of magnetic
fabrics in the sills, and that the slightly coarser Ti-poor or -medium titanomagnetites carry most magnetic
fabrics in the basaltic flows. Magnetic anisotropies were determined by applying anisotropy of
low-field magnetic susceptibility (AMS) on 180 unheated samples and 128 samples that had been previously
heated to 600°C during a paleomagnetic study to detect heating effects on the anisotropy of
magnetic susceptibility (AMS) properties of volcanic rocks. Laboratory heating significantly affects
anisotropy variations of these igneous rocks corresponding to the mineralogical changes during the
heat treatment.
Notes: Zhang Shuwei et al., 2008 (1)
Zhang SW et al., 2008 (1)
Abstract: Studies of rock magnetism and anisotropy of magnetic susceptibility (AMS) on folded igneous rocks from South Taimyr Peninsula, Arctic Russia are presented. The magnetic remanence is principally carried by fine grained, low-temperature oxidized titanomagnetite, and hematite is the secondary carrier in the basalts. The sills display remarkably homogeneous magnetic properties and contain larger grains with ilmenite lamellae, attesting to slow cooling. Basaltic flows possess slightly more variable magnetic properties, and homogeneous skeletal titanomagnetite suggests rapid cooling. Differences in magnetic properties in sills and basalts are caused by titanomagnetite concentration as well as grain size. Stepwise thermal demagnetization shows a stable primary remanence in the sills and basaltic flows; statistic analysis reveals that the characteristic remanence components are isolated with small within-site dispersion of the mean direction. AMS study was performed on untreated samples and samples that had been previously heated to 600°C during a paleomagnetic study to investigate the AMS properties. Laboratory heating did not lead to obviously consistent axis orientations.
Notes: Zhang Shuwei et al., 2007;
Zhang SW et al., 2007
Abstract:
The use of magnetic methods to examine sediment transport and motion
processes on beaches has been studied on Cedar Beach (Western Lake Erie), in order
to evaluate their applicability in understanding natural and anthropogenic coastal
erosion and accretion. Magnetic studies of the heavy mineral-enriched, dark-reddish
sands present on Cedar Beach (41.68°N, 82.40°W) showed that (low-Ti) magnetite
(dominant PSD-MD and some SD-SP) is the dominant magnetic mineral and the hard
magnetic mineral (HM) (i.e. maghemite, hematite or goethite) is the secondary in
these sands. This study reveals spatial variations in the concentration, sand grain size
(SGS), magnetic grain size (MGS) and heavy mineral assemblages, on the beach, and
indicates a correlation between them. It also examines the influence of seasonal
changes on the beach, and hightlights chemical (biochemical) effects on the beach
sand, especially on the larger size ones in the areas close to water.
Surficial magnetic susceptibility values defined three zones: a lakeward region
close to the waterâs line (Zone 1), the upper swash zone (Zone 2) and the landward
region beyond the upper swash zone (Zone 3). The slightly higher upland areas or
sand dunes (UD) neighbour to Zone 3 and back the beach face. Three groups of
cross-shore transects (West, Middle and East) were chosen to investigate the profile
changes in the western, central and eastern sections of the beach.
Across the shore, Zone 2 showed the highest bulk susceptibility (κ), mass
percentage of smaller grain-size (<250 μm) fractions (i.e. heavy mineral assemblage)
in the bulk sand sample, mass-normalized susceptibility (Ï), mass-normalized
saturation isothermal remanent magnetization (SIRMmass) and mass-normalized
anhysteretic remanent magnetization (ARMmass). All magnetic concentration values
decrease both towards the lake (Zone 1) and towards the land (Zone 3) on the West
and Middle, while these values are slowly increasing from Zone 1 to Zone 3 on the
East. The variations of magnetic concentration parameters (i.e. κ, Ï, SIRMmass and
ARMmass) correlate with the distribution of the heavy mineral assemblage and the
small SGS of the beach sand. From Zone 1 to Zone 2, the percentage of heavy
minerals increased and SGS fined on three (West, Middle and East) sections. From
Zone 2 to Zone 3, the percentage of heavy minerals decreased and SGS coarsened on
the West and Middle, while the percentage of heavy minerals increased and SGS fined
on the East. Contrary to the variability of SGS, MGS slightly increased from Zone 1
(PSD-small MD and more SD-SP) to Zone 3 (large PSD-MD and less SD-SP). Zone 1
had slightly more HM than Zone 2 and Zone 3 on the West and Middle, while Zone 1
and Zone 2 had more HM than Zone 3 on the East. UD showed similar features to
their neighbouring Zone 3.
Along the shore, the West showed the highest magnetic concentration (i.e.
percentage of heavy minerals). From the West to East, magnetic concentration values
decreased, and thus the percentage of heavy minerals decreased and SGS increased.
MGS somewhat decreased from the West (large PSD-MD and less SD) to the East
(PSD-small MD and more SD). The West showed less HM than the Middle and East
transects.
This spatial variation results from the preferential separation of large, less dense
particles by waves and currents both along and across the beach. The prevailing
strong west-southwest winds generated dominant east-northeastward directed waves.
The eroded western section of Cedar Beach showed much higher concentrations of
heavy minerals including magnetite, and finer sand grain sizes than the accreting
eastern section, as the waves and longshore currents transported the eroded sand
alongshore and deposited it in the east section. Lake-level changes induced by climate
factors such as strong winds and storm waves, have probably influenced both the
distribution of the beach sands and the orientation of the shoreline due to erosion and
accretion. Effects of secondary beach processes (Chemical actions including
biochemical actions) during sediment transport and deposit might be helpful for the
enrichment of MGS ranges and creation of HMs, and grain-sorting processes by
waves and currents can further promote the more regular distribution of MGSs and
hard minerals on the beach. This study has led to a better understanding of the beachâs
dynamics, thereby enabling more useful information for a lake-beach ecological
system and better environmental management of this recreational region.
Notes: Conclusionsï¼
(1) The Curie temperature of 575°C~585°C, as well as the strong attraction to a
handheld magnet indicates that the dominant magnetic mineral is (low to non-Ti)
magnetite, and ESEM-EDAX analysis proved the presence of Ti. These magnetite
particles are dominantly PSD-MD and sometimes SD-SP in character. More or less
SD-SP particles, and larger PSD-MD or smaller PSD-MD may depend on different
areas or zones to some degree. This is consistent with hysteresis measurements,
pARM spectra and bilogarithmic diagrams results. Some sand samples (especially
large sand fractions) show Curie temperatures of 630°C~675°C, indicating hard
minerals (i.e. hematite or maghemite) are secondary magnetic minerals. For all TMS
curves of different size fractions, the larger the sand grain size (SGS), the lower the
heating curves and the higher the cooling curves. Thus, the largest sand grains (i.e.
>1000 μm) had the maximum growth of magnetic susceptibility durin a
heating-cooling cycle. The largest sand grains (i.e. >1000 μm) also showed the
highest and narrowest Hopkinson peaks before curie temperatures, indicating they
may have the most iron-bearing paramagnetic minerals and a tiny fraction of fine
magnetic minerals (i.e. SD magnetites), consistent with ESEM-EDAX results.
(2) Statistics derived from the sieve data suggested that the magnetite had an
average solid grain size (SGS) of ~160 μm, consistent with the statistics results using
ESEM and software (Scandium). Observation under a microscope that the <250 μm
fractions contained the highest percentage (80%) of magnetite particles.
(3) High magnetite concentrations correspond to high concentrations of heavy
minerals in the dark-reddish or purple sands. Zones 2-3 (in the west section) and Zone
2 (in the middle section) have the highest concentration of dark minerals, indicating
the most heavy mineral contents. However, Zone 1 (in the West) and Zones 1 and 3
(in the Middle) show a much lower concentration of dark (heavy) minerals, indicating
a very high concentration of light minerals. It is consistent with observations of
different size fractions under the microscope.
(4) Along the beach, the east section of Cedar Beach shows much lower
susceptibility values (and thus lower magnetite concentration) and a larger physical
grain size than the west section, and the west section has the highest magnetic
concentrations. Across the beach, Zone 2 shows the highest magnetic concentrations
and the smallest sand grain sizes, while Zone 1 has the lowest concentration and the
largest sand grain sizes. For size fractions (<500 μm), sand particles in the same size
(from different areas) shows similar magnetic features to the bulk sands along/across
the beach. Visual and historical observations indicate that the western section of the
beach is eroding while the eastern section is accreting. The magnetic measurements
are consistent with this observation.
(5) Along the beach, the east section shows finer magnetic grain sizes (MGS) in
contrast with the largest sand grain sizes (SGS), the west section has coarser MGS but
the smallest SGS. Across the beach, Zone 1 shows finer MGS and the largest SGS,
Zones 2-3 and UD show coarser MGS but smaller SGS. Generally, the MGS of large
sand grains is fine while it is coarse in small sand grains. It is significant that MGS
has a contrary variability to the SGS on the beach.
(6) Along the beach, the east section shows a slightly higher concentration of hard
minerals (i.e. hematite or maghemite), while the west has a lower concentration.
Across the beach, Zone 1 has more hard magnetic minerals while Zones 2-3 and UD
have less such minerals. In general, large sand grains contain more hard magnetic
minerals.
(7) The spatial variations in magnetic concentration, sand grain size and heavy
mineral concentration are associated with grain-sorting processes that are most likely
controlled by patterns of waves and alongshore currents, and sediment transport,
although aeolian transport cannot be completely ruled out. Although lakeshore
chemical actions have influence on both concentrations of and spatial variations in
hard minerals, patterns of waves and alongshore currents enable a more regular
distribution of the beach sand (with different magnetic particle size and hard magnetic
concentration) in space.
(8) The present study implies that environmental magnetic techniques can be used
to study the mechanism of the lake sediment movement and to monitor a variety of
beach processes, including erosion and accretion, and can thus provide information to
policy makers and the wider public that may assist in resource and environmental
decision making. Lake-beach is an environment ecological system. Environmental
magnetism method can provide valuable informatio for researches on ecological
environment on Cedar Beach.
