CONTINUOUS RECORD OF GEOMAGNETIC FIELD VARIATIONS DURING COOLING OF THE MONCHEGORSK, KIVAKKA AND BUSHVELD EARLY PROTEROZOIC LAYERED INTRUSIONS

Abstract

A continuous record of the geomagnetic field direction during of cooling of the Monchegorsk (2.5Ga), Kivakka (2.45 Ga) and Bushveld (2.06 Ga) layered intrusions was conducted for the first time. The low-Ti titanomagnetites ( Tc = 530-580° C) 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 540° C 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 20° 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 2°-3° interval from 530°C up to 580°C 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 = 580°C. 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 265.3° E, 1.3° 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 A1 prefolded and A2 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 A1 is 17.8°S and 247°E. 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 (12°N, 35.4°E) 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.