Geochemical and geochronological studies in the Stora Le-Marstrand belt of Orust, S.W. Sweden

Abstract

The Stora Le - Marstrand belt in Orust consists of a basement complex of semi-pelitic and psanmitlc para-gneisses with minor tholeiitic amphibolite bodies which exhibit widespread migmatite development in two stages. Several intrusive bodies cut the basement complex between the two migmatisations; these include the Halleviksstrand amphibolite and the Assmunderod-Myckleby augen granite. A suite of granite sheets followed by basic dykes and finally pegmatites cut the complex after the second migmatisation.
The geochemistry of the para-gneisses offers some support to the geological evidence for a sedimentary origin. The raigmatitic para-gneisses grade into areas of nebulite which may originate by closed-system anatexis and localised collection of the resulting melt. The disequilibrium of the Rb-Sr isotopic systems of these rocks is probably due to the effects of the later period of migmatisation and partly to secondary recrystallisation of muscovite.
Rb-Sr whole-rock emplacement ages for the Halleviksstrand amphibolite (1432 - 92 m.y.) and the AssmunderSd-Myckleby augen granite (1379 - 46 m.y.) provide a younger age limit for the first migmatisation, which they cut, and an older age limit for the second migmatisation and subsequent events. The low initial Sr/ Sr ratio (0.7032 - 5) of the amphibolite implies a primitive (e.g. upper mantle) source , while that of the augen granite (0.713 - 3) indicates a crustal source.
Geochemical variation in the younger granites suite is probably the result of differentiation of a basaltic or tonalitic magma, implying an upper mantle source. It is likely that metamorphic disturbance rather 87 86 than variation in initial Sr/ Sr ratio has been responsible for the non-equilibrated Rb-Sr isotopic systems in these rocks.
The geochemical data for the basic (and minor acidic) dyke suite is consistent with fractionation of a tholeiitic magma. Relatively narrow deformed dykes record isotopic equilibration ca. 1060 m.y. ago. This Sveconorwegian age is interpreted as the age of the first post-dyke metamorphism and it implies that the important regional post-dyke deformation is Sveconorwegian in age. A temperature of 590°C is estimated for this metamorphism using garnet-biotite geothermometry.
Muscovites from two bodies of deformed granitic pegmatite give a Rb-Sr age of ca. 950 m.y.