SILICEOUS, HIGH-MAGNESIAN PARENTAL MAGMA COMPOSITIONS OF THE PGE-RICH EARLY PALEQ-PROTEROZOIC LAYERED INTRUSION BELT OF NORTHERN FINLAND

摘要

The Early Paleoproterozoic (approximately 2.44 Ga) mafic intrusion belt which crosses the Fennoscandian shield has geochcmical signatures that resemble those of modern boninites, and of Archean siliceous, high magnesium basalts (SI 1MB). Major and trace element compositions were determined for chilled margin and preintrusion dike samples of the Penikat, Portimo, and Koillismaa Complexes. Similarities to boninitic magmas include: high MgO contents at intermediate SiO, contents, low TiO2 and HFSE contents (mostly < 0.5 wt.% TiO2), high Cr contents, and high CnO/TiO2 and Al2O/TiO2 ratios combined with low Ti/Zr ratios. Although the presence of a negative normalized Ta anomaly has yet to be proven, a negative Nb anomaly is present, Sc/Y ratios are also close to those of modern boninites. The Nd isotope values, however, show negative eNJ values similar to those of Archean contaminated komatiites and to the Bushveld and Stillwater Complexes, but different from the positive values of modem boninites. The genesis of the intrusion belt's parental magmas can be explained either by processes that form boninites in incipient rift environments, or by assimilation of crust by komatiitic magma and subsequent fractional crystallization of ultramafic components. Earlier Archean tholciitic magmatism, and late-Archean arc volcanism and craton assembly may have produced depleted mantle underneath the Fennoscandian shield crust. Rift volcanism due to a rising mantle diapir resulted in the emplacement of the malic intrusion belt. Geoehemical modelling of contamination of idealized komatiite by granite-gneiss basement for Sc, Zr, La and Nb matched the measured Finnish parental magma compositions at values of 20 - 30% contamination and 35 - 45% fractionation. Modelling of Y and Nd indicated much higher values for both parameters. Trace element ratios support the boninite model of genesis, while the contaminated komatiite model is only partially supported by trace element modelling and by Nd isotopic evidence. If contamination occurred, it may have been with crust other than the granite-gneiss basement into which the intrusions were placed.