Open AccessSome Curious Magnetic Results From a Precambrian Granite
Author(s) -
Spall Henry,
Noltimier H. C.
Publication year - 1972
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1972.tb06126.x
Subject(s) - precambrian , geology , sill , geophysics , demagnetizing field , magnetite , geochemistry , mineralogy , seismology , magnetization , magnetic field , paleontology , physics , quantum mechanics
Summary The 1320 My Spavinaw granite from north‐eastern Oklahoma is strongly magnetized, averaging about 10 −2 emu cm −3 . The NRM directions are randomly oriented: steep and shallow, positive and negative inclinations are equally represented. Although very fine grained titanomagnetite and haematite are primary Fe‐Ti oxides, demagnetization generally produces no change from the initial NRM directions whether up to 660 °C or in 1400 Oe peak A.F. This tends to rule out large amounts of secondary magnetization and self‐reversal, particularly as there are no apparent compositional variations throughout the granite. We discount lightning because the same magnetic features are shown by samples from all the surface exposures, as well as by subsurface samples from deep wells. Among possible, but unsubstantiated, explanations are: (1) the NRM may be due to stable viscous components which have been acquired throughout the granite over an interval of the order of 10 9 years, (2) the granite may have recorded multiple reversals of the Precambrian field during cooling, and (3) there was some form of random motion in the crystal mush below 600°C, possibly associated with the injection of multiple granite sills.