Tectonophysics 421 (2006) 161 – 171
www.elsevier.com/locate/tecto
Paleomagnetism of the middle Cretaceous Iritono granite
in the Abukuma region, northeast Japan
Ken-ichi Wakabayashi a , Hideo Tsunakawa a,⁎, Nobutatsu Mochizuki a ,
Yuhji Yamamoto b , Yutaka Takigami c
a
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
b
Geological Survey of Japan, AIST, Tsukuba, Ibaraki 305-8567, Japan
c
Kanto Gakuen University, 200 Fujiagu, Ohta-city, Gunma 375-8515, Japan
Received 5 September 2005; received in revised form 12 April 2006; accepted 26 April 2006
Available online 12 June 2006
Abstract
We have studied the paleomagnetism of the middle Cretaceous Iritono granite of the Abukuma massif in northeast Japan together with
Ar–39Ar dating. Paleomagnetic samples were collected from ten sites of the Iritono granite (102 Ma 40Ar–39Ar age) and two sites of its
associated gabbroic dikes. The samples were carefully subjected to alternating field and thermal demagnetizations and to rock magnetic
analyses. Most of natural remanent magnetizations show mixtures of two components: (1) H component, high coercivity (Bc N 50–
90 mT) or high blocking temperature (Tb N 350–560 °C) component and (2) L component, relatively low Bc or low Tb component. H
component was obtained from all the 12 sites to give a mean direction of shallow inclination and northwesterly declination (I = 29.9°,
D = 311.0°, α95 = 2.7°, N = 12). This direction is different from the geocentric axial dipole field at the present latitude (I = 56.5°) and the
typical direction of the Cenozoic remagnetization in northeast Japan. Since rock magnetic properties indicate that the H component of the
Iritono granite is carried mainly by magnetite inclusions in plagioclase, this component probably retains a primary one. Thus the shallow
inclination indicates that the Abukuma massif was located at a low latitude (16.1 ± 1.6°N) about 100 Ma and then drifted northward by
about 20° in latitude. The northwesterly deflection is attributed mostly to the counterclockwise rotation of northeast Japan due to
Miocene opening of the Japan Sea. According to this model, the low-pressure and high-temperature (low-P/high-T) metamorphism of
the Abukuma massif, which has been well known as a typical location, would have not occurred in the present location. On the other
hand, the L component is carried mainly by pyrrhotite and its mean direction shows a moderate inclination and a northwesterly
declination (I = 42.8°, D = 311.5°, α95 = 3.3°, N = 9). Since this direction is intermediate between the H component and early Cenozoic
remagnetization in northeast Japan, some thermal event would have occurred at lower temperature than pyrrhotite Curie point (∼ 320 °C)
during the middle Cretaceous to early Cenozoic time to have resulted in partial remagnetization.
© 2006 Elsevier B.V. All rights reserved.
40
Keywords: Paleomagnetism; Abukuma massif; Cretaceous; Granite; Remagnetization;
40
Ar–39Ar age
1. Introduction
⁎ Corresponding author. Fax: +81 3 5734 3537.
E-mail address: [email protected] (H. Tsunakawa).
0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2006.04.013
The paleomagnetism of the Japanese Islands are important for the study of tectonic evolution in the island
arc. There are two major pre-Tertiary terrains in northeast Japan: one is the Kitakami massif occupying the
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K. Wakabayashi et al. / Tectonophysics 421 (2006) 161–171
northeastern part and the other the Abukuma massif in the
southeastern part (Fig. 1). Extensive paleomagnetic
studies infer that the Kitakami massif was subjected to
northward drift of more than 24° in latitude during the
Cretaceous to early Cenozoic period (Otofuji et al., 1997)
and then a thermal event of low temperature occurred
there between 62 and 16 Ma probably at the stage of its
suturing to the Asian continent accompanied with
intensive remagnetization (Otofuji et al., 2000, 2003).
The Japan Sea was opened as a back-arc basin in Early to
Middle Miocene in association with about 40° counterclockwise rotation of northeast Japan and about 60°
clockwise rotation of southwest Japan (e.g. Otofuji and
Matsuda, 1983, 1987; Otofuji et al., 1985, 1994). Thus the
middle Cretaceous to early Cenozoic tectonic evolution of
the Kitakami massif has been well clarified at present.
The Abukuma massif is composed mainly of Cretaceous plutonic rocks and metamorphosed pre-Cretaceous
rocks and has been regarded as a typical location of the lowpressure and high-temperature (low-P/high-T) metamorphism (e.g. Miyashiro, 1958). As for the post-Cretaceous
tectonics, it is inferred from the paleomagnetism of the
Miocene volcanic rocks that the Abukuma region
underwent the Miocene counterclockwise rotation due to
opening of the Japan Sea (e.g. Takahashi et al., 1999).
The metamorphic belt of the Abukuma massif was
interpreted as an extent of that in southwest Japan on the
basis of the structures and microstructures (Faure et al.,
1986). However no clear evidence for petrological identity of those metamorphic belts has been found (Tagiri
et al., 1988). Reconnaissance paleomagnetic surveys of
Cretaceous granites in the Abukuma region (Kawai et al.,
1971; Ito and Tokieda, 1986) show northwesterly deflection of natural remanent magnetization (NRM) of
I = 54.6°, D = 328.1° and α95 = 8.5° (N = 8). Since this
mean NRM inclination is almost the same as the inclination of the geocentric axial dipole (GAD) field at the
present location (I = 56.5°), it may be suggested that the
Abukuma granite was formed at the present latitude in the
Cretaceous time. However it should be noted that the
NRM directions of the Abukuma granite by the previous
studies are indistinguishable from the early Cenozoic remagnetization direction of the Kitakami massif (I = 56.5°,
D = 321.2°, α95 = 5.2°; Otofuji et al., 2003). Besides magnetic cleaning would be insufficient for other Abukuma
granitic rocks in the previous studies because those AFD
treatments were accomplished at 10–120 mT. These suggest that the previous NRM data of the Abukuma granite
were heavily affected by the secondary component due
to the remagnetization in northeast Japan reported by
Moreau et al. (1987) and Otofuji et al. (2003). Therefore
the pre-Miocene tectonic evolution of the Abukuma
massif has not been revealed well. The main objective in
this study is to obtain more reliable paleomagnetic data
Fig. 1. Locality maps of the sampling sites of the Abukuma massif, northeast Japan. Left map: the tectonic map shows two terrains of northeast Japan.
Middle map: the Abukuma massif consists mainly of the Cretaceous granites (gray-colored area) and the metamorphosed pre-Cretaceous rocks
(hatched area), which are sometimes bounded by faults (solid line). Right map: ten sites of the Iritono granite are located along the road (ITG05-11,
closed circles), and two sites at its associated gabbroic dikes (ITD01 and 04, thick bars).
K. Wakabayashi et al. / Tectonophysics 421 (2006) 161–171
of the Abukuma massif with special reference to the
granite formation.
2. Geology, sampling and
40
Ar–39Ar dating
2.1. Geology and sampling
The precise petrological and geochronological study
revealed that the low-P/high-T metamorphism (N700 °C)
in the Abukuma region occurred with very fast burial and
exhumation rates during 112–122 Ma according to U–Pb
zircon ages (e.g. Hiroi et al., 1998). The lower-pressure
contact metamorphism (500–600 °C) subsequently took
place due to emplacement of the late- to post-kinematic
granites (Tagiri et al., 1993), and then the metamorphic
event is considered to have ceased 96–98 Ma (Hiroi et al.,
1998) on the basis of K–Ar biotite ages of those granites
(Shibata and Uchiumi, 1983).
A granite body of several km in size is located at
Iritono area in the southern part of the Abukuma massif
(37.1°N, 140.7°E; Fig. 1). The Iritono granite is grouped
into the younger post-kinematic granite (e.g. Hiroi et al.,
1998) and was unaffected by the main metamorphism.
Therefore the Iritono granite is suited for the paleomagnetism to investigate the post-metamorphism tectonics of
the Abukuma massif. As the Iritono granite is partly
weathered, we selected fresh outcrops along the road cut.
Paleomagnetic samples were collected from 10 sites of the
Iritono granite (ITG05, ITG06, ITG07, ITG08, ITG09a,
ITG09b, ITG09c, ITG09d, ITG10 and ITG11) and also
from 2 sites of gabbroic dikes of 0.1–2 m width (ITD01
and ITD04) intruded into the Iritono granite in WNW
strikes (Fig. 1).
2.2.
40
163
40
Ar–39Ar plateau age of 105.3 ± 2.6 (1σ) Ma from a
whole rock sample of the gabbroic dike adjacent to ITD04
(Takigami et al., 1984; Fig. 2b). Since those plateau ages
are indistinguishable at 2σ level, both of the Iritono granite
and dikes were formed almost simultaneously. According to the time sequence of metamorphic events in the
Abukuma region (e.g. Hiroi et al., 1998), it is thought
that the Iritono composite mass and its associated gabbroic
dikes were formed as the post-kinematic granitic rocks
about 100 Ma and presumably cooled down in several
million years.
3. Rock magnetic and paleomagnetic experiments
and results
3.1. Experiments
We carried out alternating field demagnetization (AFD)
up to 140 mT and thermal demagnetization (ThD) up to
590–600 °C in air for 95 granite samples (AFD, 47; ThD,
48) and 18 dike samples (AFD, 7; ThD, 11) to identify
primary components of characteristic remanent magnetization (ChRM). Part of the granite samples were subjected
to low temperature demagnetization (LTD) at liquid
nitrogen temperature before AFD treatment in order to
prevent the effect of multi-domain (MD) components
Ar–39Ar dating
We also collected a block sample of the granite near
ITG06 site for 40Ar–39Ar dating. The sample was crushed
and biotite minerals were separated. Biotite sample, age
standard samples and chemical samples were irradiated
by fast neutrons. Ar gas was extracted by stepwise heating
and Ar isotopes were measured by a mass spectrometer
(VG3600, Micromass). Applying the correction for the
mass discrimination and the interfering isotopes from K
and Ca, 40Ar–39Ar ages were calculated. Detailed experimental procedures of the 40Ar–39Ar method are described
elsewhere (e.g. Takigami et al., 1998).
The diagram of the 40Ar–39Ar age spectrum is shown
for the granite sample in Fig. 2a. The age spectrum shows
a good plateau of 840–1200 °C steps yielding a plateau
age of 101.9 ± 0.2 (1σ) Ma with 61.3% fraction of released 39Ar. Preliminary geochronological study gave the
Fig. 2. 40Ar–39Ar age spectra of a biotite sample from the Iritono granite
(a; this study) and a whole rock sample from the gabbroic dike intruded
into the granite (b; Takigami et al., 1984). Plateau ages with 1σ error are
calculated from heating steps of 840–1200 °C and N900 °C for the
granite and the dike, respectively. As those ages are indistinguishable at
2σ level, they were almost simultaneously formed about 100 Ma.
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K. Wakabayashi et al. / Tectonophysics 421 (2006) 161–171
Fig. 3. Thermomagnetic analyses of the Iritono granitic rocks (ITG, granite; ITD, gabbroic dike). Ms, saturation magnetization at 0.2 or 0.5 T dc field;
Ms0, saturation magnetization before heating; T, temperature during heating (rightward arrow) and cooling (leftward arrow) cycles.
(Ozima et al., 1964; Heider et al., 1992). NRM intensities
of the granite and dike samples range mostly in 0.3–
3 × 10− 2 A/m (1–10× 10− 6 Am2/kg) and 3–6 × 10− 2 A/m
(1–2 × 10− 5 Am 2/kg), respectively. Demagnetization
results were analyzed with principal component analysis
(Kirschvink, 1980), and the observed components of
maximum angular deviation (MAD) ≤10° are adopted in
the paleomagnetic analysis.
Thermomagnetic analyses in a helium gas were performed for representative samples using vibrating sample
magnetometer (VSM, MicroMag 3900). Hysteresis parameters were measured with VSM for chip samples from
the granite samples. For ITG09 samples, after chips were
further crushed, 3 fractions of feldspar including plagioclase and alkali–feldspar and 3 fractions of biotite were
separated by handpicking under the microscope for VSM
measurements. We also observed magnetic minerals with
optical microscopy, energy-dispersive X-ray spectroscopy
(EDS) and electron probe microanalyzer (EPMA) to identify magnetic minerals. Details of the experimental procedure are described elsewhere (e.g. Mochizuki et al.,
2004; Yamamoto and Tsunakawa, 2005).
3.2. Thermomagnetic analysis
Thermomagnetic curves of the granite (Fig. 3a, b and c)
show two major phases with Curie temperatures (Tc) of
300–400 °C and 550–600 °C. The lower Tc phase is
clearly observed for ITG10-09 (Fig. 3c) associated with
irreversible behaviors in cooling curves, whereas it is a
little amount in ITG09a-03. Combining the microscopic
observations discussed later, the higher and lower Tc phases
are identified as titanium-poor titanomagnetite (almost
magnetite) and pyrrhotite, respectively. In comparison
K. Wakabayashi et al. / Tectonophysics 421 (2006) 161–171
165
Fig. 4. Representative orthogonal plots in alternating field demagnetization (AFD) and thermal demagnetization (ThD) of the Iritono granite. The
intensity curve (J) is in unit of 10− 5 Am2/kg. Solid (open) symbols are for the horizontal (vertical) projection in the geographic coordinates. Triangles
indicate NRM before the low temperature demagnetization (see the text).
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K. Wakabayashi et al. / Tectonophysics 421 (2006) 161–171
Table 1
Summary of NRM directions of high Bc–Tb components (H components) and low Bc–Tb components (L components) for the Iritono granite and its
associated gabbroic dikes of the Abukuma massif, northeast Japan
Site
ThDtype
H component
L component
AFD
ThD
I [°]
D [°]
α95 [°]
AFD
ThD
I [°]
D [°]
α95 [°]
Iritono granite
ITG05
M
ITG06
M
ITG07
M
ITG08
M/P
ITG09a
M
ITG09b
M
ITG09c
M
ITG09d
M
ITG10
P/M
ITG11
M
3/17
4/7
2/2
0/2
6/7
0/0
0/0
4/6
0/2
0/4
4/4
3/5
3/3
2/2
6/9
4/4
4/4
4/6
5/5
5/6
33.2
34.6
24.1
26.7
25.2
29.5
27.0
26.2
33.5
30.7
311.4
312.8
308.3
306.4
312.9
310.0
310.9
314.9
315.2
306.7
1.8
3.1
5.5
–
3.3
4.0
2.3
6.3
2.9
7.4
17/17
7/7
0/2
1/2
7/7
0/0
0/0
6/6
2/2
2/4
0/4
0/5
0/3
1/2
1/9
0/4
0/4
1/6
4/5
0/6
40.6
39.6
–
41.3
40.3
–
–
37.1
42.7
47.4
315.2
314.6
–
309.0
310.7
–
–
312.4
310.7
303.9
3.5
2.8
–
–
6.4
–
–
9.1
5.0
–
Gabbroic dikes
ITD01
–
ITD04
–
0/2
0/5
5/5
5/6
32.4
35.5
305.2
317.8
8.4
6.9
1/2
1/5
4/5
6/6
51.1
44.0
304.8
320.3
6.2
9.8
Mean
N = 12
29.9
311.0
2.7
N=9
42.8
311.5
3.3
The ThD-type describes the main carriers of granite NRMs (M, magnetite with no or very small amount of pyrrhotite; M/P, magnetite and some
amount of phyrrhotite; P/M, pyrrhotite and some amount of magnetite). AFD, “Number of samples yielding H (L) components by AFD”/“Number of
samples measured by AFD”; ThD, “Number of samples yielding H (L) components by ThD”/“Number of samples measured by ThD”; I, inclination;
D, declination; α95, 95% confidence circle.
between those two phases, the ratio of amount of pyrrhotite
to magnetite increases in the order of ITG09a-03, ITG0814 and ITG10-09. As for the dike sample ITD01-06,
significant increase in saturation magnetization (Ms) is
observed for the cooling curve (Fig. 3d), suggesting that
much amount of pyrrhotite is contained and that the Ms
increasing around 300 °C in heating cycle can be elucidated by thermal alteration of pyrrhotite to magnetite due to
the laboratory heating.
3.3. Demagnetizations of the Iritono granite
Demagnetization results of the granite samples except
ITG08 and ITG10 show median destructive fields (MDFs)
of 30–50 mTand sharp peaks of blocking temperature (Tb)
spectra around 550 °C, indicating that main carriers of
NRMs are single-domain (SD) grains of magnetite. It is
also suggested that small amount of pyrrhotite grains are
contained in some samples because slight decrease in
NRM intensity (less than 10%) was sometimes found at
300–350 °C. Representative examples of demagnetization
results are shown for ITG09 sites in Fig. 4a and b. AFD
result of ITG09a-04-2 sample shows two major components of a high Bc component of shallow inclination and a
low Bc component of moderate inclination. From principal
component analysis, its high coercivity fraction
(Bc ≥ 80 mT) gives a shallow and northwesterly direction
of I = 21.4° and D = 314.0° (MAD = 2.0°), while its low
coercivity fraction (Bc ≤ 20 mT) yields I = 43.4° and
D = 309.9° (MAD = 3.2°). The intermediate coercivity
fraction is interpreted as a mixture of high and low Bc
components. On the other hand, ThD result of ITG09c-441 has a dominant component of high Tb spectra with little
component of low Tb (Fig. 4b). The high Tb fraction
(Tb ≥ 500 °C) yields a shallow and northwesterly direction
(I = 28.5°, D = 308.1°, MAD = 0.5°), whereas a segment of
Tb ≤ 400 °C shows a relatively steeper one (I = 38.2°,
D = 300.4°, MAD = 16.6°) though its MAD is larger than
10°.
ITG08 and ITG10 samples exhibit different manners
of AFD and ThD from the other granite samples in this
study (Fig. 4c–f). ITG08 samples are characterized by
high MDFs (N50 mT), some amount of hard remanences
remaining after AFD and 10–30% decrease in NRM by
about 350 °C heating temperature of ThD. NRMs of
ITG10 samples were very hard against AFD (MDF
∼ 100 mT), resulting in relatively large components of
Bc N 140 mT. In ThD treatment of ITGIO, more than half
of NRM decayed around 350 °C and the remaining
NRMs rapidly decreased above 550 °C. These large
drops of NRM intensity around 350 °C together with the
thermomagnetic analyses indicate that ITG08 and ITG10
samples contain significant amount of pyrrhotite, high
magnetocrystalline anisotropy of which possibly causes
the remaining components after AFD (Clark, 1984;
Dekkers, 1988). The samples of ITG08 and ITG10
K. Wakabayashi et al. / Tectonophysics 421 (2006) 161–171
exhibit shallow and northwesterly components of high
Tb spectra and relatively steep and northwesterly one of
low Tb spectra, which are similar to those of the other
sites (see Table 1).
LTD treatment erased 10–20% of NRM for the granite
samples (Fig. 4). Since the component erased by LTD
treatment is directionally similar to the component of the
intermediate coercivity fraction, effects of viscous remanent magnetization (VRM) and some artificial isothermal
remanent magnetization (IRM) are insignificant.
According to the ThD behaviors and the thermomagnetic analyses, the granite samples in this study can be
categorized to three types: (1) ThD-type M with magnetite
and no or small amount of pyrrhotite (ITG05, ITG06,
ITG07, ITG09a–d and ITG11), (2) ThD-type M/P with
magnetite and some amount of phyrrhotite (ITG08) and
(3) ThD-type P/M with pyrrhotite and some amount of
magnetite (ITG10). The measured remanences consist
mainly of two components: a shallow and northwesterly
component of high Bc–Tb spectra (Bc N 50–90 mT or
Tb N 350–560 °C; hereafter called the ‘H component’) and
a relatively steeper and northwesterly one of low Bc–Tb
spectra (‘L component’).
3.4. Demagnetizations of the gabbroic dikes
Gabbroic dike samples from ITD01 and ITD04 were
also measured with AFD and ThD, and representative
examples are shown in Fig. 5. NRM of ITD01-01-a was
unstable in AFD treatment (MDF ∼ 5 mT) and H component was not isolated (Fig. 5a). In ThD treatment, a
stable component of shallow inclination was often recog-
167
nizable for Tb ≥ 400 °C though NRM was extensively
demagnetized by 350 °C as shown for ITD01-06-b (Fig.
5b). These ThD behaviors and the thermomagnetic analyses suggest that the H and L components in ThD are
carried mainly by magnetite and pyrrhotite, respectively.
The unstable AFD manner of gabbroic dike samples may
be attributed to coarse pyrrhotite grains (∼ 100 μm)
observed by reflected-light microscopy and EDS (see
Section 3.5).
3.5. Minerals containing magnetic carriers
Transmitted-light microscopic observations of the
granite samples indicate that plagioclase, alkali–feldspar,
biotite and quartz crystals preserve almost the original
shapes. Samples from ITG09a–d sites show no or little
alteration while samples from the other sites are more or
less altered. In particular, feldspars and biotites of ITG08
and ITG10 samples are partly replaced by small minerals
such as chlorite, sericite, etc.
The demagnetization results indicate that major NRM
carriers of the granite samples in this study are magnetite
and sometimes pyrrhotite. We further investigated which
mineral contains magnetic grains by microscopic observations and EDS/EPMA measurements, especially for
ITG09 samples of ThD-type M. ITG09 samples show that
opaque minerals (≤10 μm, occasionally ∼ 100 μm) are
dispersed in plagioclase, relatively large grains of which
were identified as magnetite by EPMA (Fig. 6a). Some
amount of magnetite grains were also found in alkali–
feldspar while biotite contains small amount of magnetite,
pyrrhotite and sphene. These observations suggest that
Fig. 5. Representative orthogonal plots in alternating field demagnetization (AFD) and thermal demagnetization (ThD) of the gabbroic dike sample
from ITD01. Main remanence carrier is pyrrhotite with some amount of magnetite. Symbols are the same as in Fig. 4.
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K. Wakabayashi et al. / Tectonophysics 421 (2006) 161–171
NRMs of ITG09 samples are carried mainly by magnetite
inclusions in plagioclase.
From the demagnetization behaviors, the H component is probably originated from SD magnetite grains.
The Day plots (Day et al., 1977; Dunlop, 2002) of
hysteresis parameters are shown in Fig. 7a and b. It is
seen in Fig. 7a that data points of non-separated chips
from ITG05, 06, 07, 08, 10 and 11 are broadly distributed
in pseudo-single domain (PSD) to MD region along the
trend of SD–MD magnetite mixture (Parry, 1982; Channell and McCabe, 1994). For ITG09 samples, non-separated chip samples show a cluster in PSD region close to
the trend of SD–MD magnetite mixture (Fig. 7b). Data
points of ITG09 feldspar fractions on the Day plot gather
near the cluster of ITG09 non-separated chips, while
those of biotite fractions are fairly shifted toward MD
region. This result supports that NRMs of ITG09 samples are carried mainly by magnetite inclusions in plagioclase. It should be noted that remanent magnetization
of plagioclase in granitic and basaltic rocks can yield
reliable paleomagnetic data (e.g. Wu et al., 1974; Geissmann et al., 1988; Tarduno et al., 2001).
As pointed out in Section 3.4, unstable AFD
behaviors of the dike sample suggest the presence of
coarse grains of pyrrhotite. Representative example of
reflected-light microscopic picture is shown for ITD04
(Fig. 6b), where the bright and large minerals are identified as pyrrhotite by the EDS observation.
4. Discussions
4.1. H component preserving the primary component
We have measured NRM directions of 95 granite
samples from 10 sites and 18 dike samples from 2 sites.
The H component of shallow inclination was detected
from all of the 12 sites (69 samples), while the L component of moderate inclination was from 9 sites (61
samples). As in the previous chapter, demagnetization
behaviors infer that the H and L components are carried
mainly by magnetite and pyrrhotite, respectively. Mean
directions within site and between sites are summarized
for the H and L components in Table 1 and projected on
equal area net in Fig. 8.
The H component of shallow inclination (I = 29.9°,
D = 311.0°, α95 = 2.7°, N = 12) is originated from high Bc–
Tb spectra while the L component of moderate inclination
(I = 42.8°, D = 311.5°, α95 = 3.3°, N = 9) from low Bc–Tb
spectra. It is seen in Fig. 8 that the mean H component is
25–27° shallower and 10–17° more westerly than the
previous data from the other Abukuma granites (I = 54.6°,
D = 328.1°, α95 = 8.5°; Ito and Tokieda, 1986) and the
Fig. 6. Representative photographs of the microscopic observation. (a) Magnetite inclusions in two plagioclases of granite samples (ITG09b) under
transmitted-light microscope (Mt, magnetite). Coarse opaque particles of ∼ 60 μm (left) and ∼ 10 μm (right) are euhedral crystals of magnetite. It is
recognizable that relatively fine and nearly euhedral magnetite grains are dispersed in both plagioclases. Reflected-light microscopic image is also
shown for the coarse magnetite inclusion of the left photograph. (b) Pyrrhotite in the gabbroic dike (ITD04) under reflected-light microscope (Pr,
pyrrhotite).
K. Wakabayashi et al. / Tectonophysics 421 (2006) 161–171
169
the L component is pyrrhotite, it is suggested that the
Iritono granite has not been reheated above the Curie point
of pyrrhotite (∼ 320 °C). This is consistent with that the
primary component more easily survived as H component
of high Tb (N 350–560 °C). In this model, the Iritono
granite and dikes initially contained both of magnetite and
pyrrhotite grains, part of which were magnetically reset by
reheating at temperature lower than 320 °C or were
produced due to some thermal event of low temperature
(b 320 °C) to have resulted in crystallization remanent
magnetization (CRM; Dunlop and Özdemir, 1997).
4.3. Effects of the tilting and the geomagnetic secular
variation
Tilting effect on the remanent magnetization should
carefully be examined because the bedding correction is
generally difficult for the granite. For example, McCausland et al. (2005) suggest that the low inclinations of some
Baja British Columbia stocks are caused by the tilting due
to the adjacent fault motion. Ito and Tokieda (1986) argue
that the northwesterly declinations of the Cretaceous Abukuma granite are attributed to the titling of about 30° with
NE–SW strike. However those northwesterly magnetization directions can naturally be interpreted as the early
Cenozoic remagnetization in northeast Japan reported by
Otofuji et al. (2003) though the possibility of tilting cannot
be completely rejected. The tilting effect seems to be
Fig. 7. (a) Day plot of ITG05, 06, 07, 08, 10 and 11 samples for nonseparated chips. (b) Day plot of ITG09 samples for 13 non-separated
chips (triangles), 3 fractions of feldspar (circles) and 3 fractions of biotite
(squares). Ms, saturation magnetization; Mrs, saturation remanence; Bc,
coercive force; Brc, remanent coercivity; SD, single-domain; PSD,
pseudo-single-domain; MD, multi-domain. SD, PSD and MD regions are
based on Dunlop (2002). Trend of SD–MD magnetite mixture is also
shown after Channell and McCabe (1994) and Parry (1982).
early Cenozoic remagnetization direction of the Kitakami
massif (I = 56.5°, D = 321.2°, α95 = 5.2°; Otofuji et al.,
2003). The mean L component seems to be intermediate
between the mean H component and the remagnetization
direction. These results imply that primary components of
the Iritono samples are represented by H components and
are often superimposed by secondary components due to
the remagnetization, particularly for L components.
4.2. L component affected by the remagnetization
It is noteworthy that even the inclination of the L
component is shallower than that of the remagnetization
(Fig. 8) and thus part of the L component may preserve the
primary component. Since the main remanence carrier of
Fig. 8. Equal area projection of the paleomagnetic directions from the
Iritono granite and gabbroic dikes of the Abukuma massif in this study.
Iritono-H, the mean direction of H components averaged for all the 12
sites; Iritono-L, the mean direction of L components averaged for 9 sites
(Table 1); GAD, the geocentric axial dipole field at the present location of
the Iritono granite. Related directions are also shown: mean NRM
direction from other Abukuma granites after Ito and Tokieda (1986), and
the Cenozoic remagnetization direction of the Kitakami massif in
northeast Japan after Otofuji et al. (2003). Circles indicate 95%
confidence limits of α95.
170
K. Wakabayashi et al. / Tectonophysics 421 (2006) 161–171
unsuited for the cause of the magnetization deflection of
the Iritono granitic rocks because the L component is
considered to significantly reflect the remagnetization.
Miocene sandstones are distributed on south of the
Iritono granite with slight tilting of about 10° in N50°W
strike (Geological Survey of Japan, 1973). If the correction
for this tilting is applied to the Iritono granite, the directions (I, D) of H and L components are calculated to be
(37.3°, 315.0°) and (49.9°, 318.5°), respectively. In this
case, while the direction of L component is closer to that
of the remagnetization, H component still shows significantly shallower inclination. Therefore no tilting effect
is temporarily taken into account in the later discussion.
The Iritono granitic rocks might possibly average out
the geomagnetic secular variation if the cooling rate of the
granite was low. Assuming that the Iritono granite has a
radius (r) of 2 km (see Fig. 1), a thermal conductivity (k)
of 1 W/mK, an isobaric heat capacity (Cp) of 1 × 103 J/kg
K and a density (ρ) of 2.7 × 103 kg/m3, the representative
cooling time (τ) for decreasing to 1/e of the initial temperature is estimated applying the following equation.
seen in Fig. 2a, no significant loss of radiogenic Ar is
observed for the granite even at low temperature steps
of the 40Ar–39Ar age spectrum. On the other hand
the 40Ar–39Ar age spectrum of the dike shows about
30 Ma age at a low temperature step of 700 °C (Fig. 2b).
This age spectrum may support the early Cenozoic remagnetization event in northeast Japan proposed by Otofuji
et al. (2003).
5. Conclusions
Thus TRM of the single granite specimen could average out the geomagnetic secular variation of the period
less than 40 ka to give the mean geomagnetic field of
about 100 Ma.
We detect the shallow and northwesterly remanent
magnetization (I = 29.9°, D = 311.0°, α95 = 2.7°, N = 12) of
high Bc–Tb spectra from the Iritono granite and its associated dikes of the Abukuma massif, which were dated as
about 100 Ma by the 40Ar–39Ar method. For the granite
samples, the high Bc–Tb component is carried mainly by
magnetite inclusions in plagioclase, the direction of which
is obviously different from NRMs of the previous studies
and from the early Cenozoic remagnetization in northeast
Japan. These results suggest that the primary component
retains in the measured samples, particularly in high Bc–
Tb fractions. The shallow inclination of the primary
component implies that the Abukuma massif was located
at a low latitude of 16 °N about 100 Ma and then drifted
northward onto the Asian continent. According to this
model, the low-P/high-T metamorphism in the Abukuma
region would have occurred in about 20° lower latitude
zone than the present. The low Bc–Tb component of
moderate inclination and northwesterly declination was
also detected (I = 42.8°, D = 311.5°, α95 = 3.3°, N = 9),
suggesting that the remagnetization at low temperature
(b 320 °C) took place before Miocene opening of the
Japan Sea.
4.4. Tectonic implications
Acknowledgements
The shallow inclination obtained from the Iritono granite and the dikes (I = 29.9°) infers that the Abukuma massif
was located at a low latitude of 16.1 ± 1.6°N about 100 Ma
and then drifted northward by about 20° in latitude. As a
result, the Abukuma massif would be accreted to the
eastern margin of the Asian continent and undergo counterclockwise rotation due to Miocene opening of the Japan
Sea. This tectonic evolution of the Abukuma massif can be
compared to that of the Kitakami massif (e.g. Otofuji et al.,
1997). In this model the low-P/high-T metamorphism of
the Abukuma massif would have occurred under quite
different tectonic situation from the present.
The thermal event of relatively low temperature
(b320 °C) suggested by this study would have unaffected
K–Ar geochronological system of the Iritono granite. As
Vuslat Tatar, Koichiro Shimura, Iwao Goto and Naoki
Horii helped us in the sampling and the experiment.
Kazuo Amano is gratefully acknowledged for the helpful
discussion on the tectonic implications. We also thank two
anonymous reviewers for their constructive comments.
s ¼ Cp qr2 =k f1 1013 secondsf3 105 years
If the initial temperature is 900 °C and the blocking
temperature spectra range in 500–580 °C, the duration Δt
for cooling down from 580 to 500 °C can be estimated
below.
Dt fs ð580−500Þ=f900 ð1−1=eÞgf4 104 years
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