Russian Journal of Earth Sciences
Vol. 6, No. 6, December 2004
Continuous record of geomagnetic field variations during cooling
of the Monchegorsk,
Kivakka and Bushveld Early Proterozoic layered intrusions
D. M. Pechersky1, V. S. Zakharov2, A. A. Lyubushin1
1Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia
2Moscow State University, Moscow, Russia
Abstract
A continuous record of the geomagnetic field direction during of cooling of the Monchegorsk
(2.5 Ga),
Kivakka (2.45 Ga) and Bushveld (2.06 Ga) layered intrusions was conducted
for the first time.
The low-Ti titanomagnetites ( Tc = 530-580oC) are the main natural remanent
magnetization (NRM) carriers.
The high temperature NRM component according to petromagnetic characteristics is
thermoremanent and was acquired at
the cooling phase of the intrusion.
The magnetic grains above 540oC are close to a single-domain state. The
temperature versus time and cooling
velocity versus temperature and time dependences during cooling of intrusions were
calculated on the basis of
solution of the Stefan problem and the nonstationary heat conduction problem.
The geothermal gradient value was taken as 20o km
-1. The unblocking temperatures in the course of
thermal demagnetization of samples ( Td ) and the blocking temperatures during cooling from
high
temperatures ( Tb ) rate dependences were used for conversion of the
Td to
Tb [Dodson and McClelland, 1980].
A detailed thermal demagnetization at 2o-3o interval from
530oC up to 580oC was made and a pattern
of behavior of the geomagnetic field direction was obtained (a) for the time
of
"running" the Curie points over the section and (b) for the cooling time
of the intrusion at each sampling point from
Tb = 580oC.
A wavelet analysis has been used to study the field variation etc.
The main rhythms of the direction variation spectrum are 3-4.5, 5-7, 8-10, 12, 15-17,
19-20, 30-40, 50-60,
and 90-100 kyr and they change in time. The length of rhythms is different and
ranges from 1-2 oscillations
("splashes") up to 10-12.
The Monchegorsk intrusion. The pole 265.3oE, 1.3o
N. The length of record is ~70 kyr.
The geomagnetic excursion of the reversal polarity lasting for less than 2000 years
and the R-subchron of ~60 kyr
were recorded.
Kivakka intrusion. The A
1 prefolded and A
2 synfolded components which
occurred at the stage of cooling of the intrusion are isolated.
The body started to tilt approximately 85 kyr after intrusion emplacement and
it lasted for 20-25 kyr.
The pole of the A
1 is 17.8oS and 247oE.
Duration of the record is 35 kyr and only one R-polarity existed during this
period.
Bushveld intrusion. Orientation of the horizontal plane of the samples has
been restored from
viscous magnetization and a paleomagnetic pole (12oN, 35.4oE) has been determined from a high-temperature
N-component of the NRM. A paleomagnetic record for ~500 kyr was obtained; polarity
of the field has changed only once during this period.