A combined geophysical and structural-geological investigation of the Precambrian intrusive Homme granite in south-western Norway

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Abstract
The Precambrian Homme granite has been investigated by structural, gravity and magnetic methods, which reveal a discordant intrusive pluton situated in granulite-facies banded gneiss surrounded by granitic gneiss. The granite is catazonal and intruded during a sequence of deformational episodes and is metamorphosed in amphibolite facies. The mean density of 71 samples of the granite is 2.605–103 kg m−3, which is about 0.16 · 103 kg m−3 less than the mean value of the surrounding inhomogeneous banded gneiss and close to the mean value of the granitic gneiss. The granite is thus associated with a negative gravity anomaly. A three-dimensional gravity interpretation of the granite and the banded gneiss gives a residual anomaly of about -3.5 mgal caused by the granite and a thickness of 500–800 m for the main part of the pluton, and at least 1000–1500 m in the northern part. The other rock units have thicknesses of the same size. The aeromagnetic anomalies in the investigated area are normally small and do not generally reflect variations in the size and shape of the formations. The volume susceptibility and natural remanent magnetization (NRM) have been determined for the rock units. The Homme granite has a mean susceptibility of about 21 · 10−3 (SI units) and a mean NRM of about 0.58 A m−1. With local variations, the net intensity of magnetization of the banded gneiss and the granitic gneiss is close to that of the granite, but there are differences in the ratios between permanent and induced magnetization. An interpretation of positive magnetic anomalies of about 1000 gammas along a banded-gneiss border zone is given. The interpretational models reveal a partly mushroom-shaped pluton, with a root below the northern part, suggesting a feeder zone. This shape and the observed disturbance of the country rocks confirm the diapiric mode of emplacement by forceful intrusion.