Microgranular enclaves in the Nordingrå rapakivi granite, east central Sweden - the early rapakivi development
published: Sep 26, 2001
ArtNo. ESP154017603003, Price: 29.00 €
Quartz-syenitic, brownish red, rounded enclaves occurring in the Nordingrå rapakivi granite have been shown to be equal in age to their host granite. The enclaves form an early, hitherto unknown part of the felsic rapakivi suite. They are essentially hypersolvus with ferroedenite as the major mafic mineral. At one of the localities, clinopyroxene is a major mineral. Mg/(Fetot+Mg) ratios are low, 0.15 ± 0.02 (1 σ) but not so low as in the host granites. The K2O/Na2O ratio obtained from the same rocks is higher in the enclaves (mean = 2.52 ± 0.15, 1 σ) than in the host granite. The trend of Ga/Al against Zr obtained from the enclaves is significantly offset from the corresponding trend obtained from the Nordingrå rapakivi granite. εNd1578 values of the enclaves vary between -5.0 and -1.7. The enclaves and the host granite define different trends in Debon & Le Fort PQ-diagrams. The trend of the enclaves is offset towards more potassic compositions than that of the granite. Rb-Ba-Sr relations are identical in enclaves and host granites. The ratio Zr/Nb (mean 24; σ = 5), is significantly different in enclaves and granites. A general similarity in mineral and chemical compositions is noted between enclaves and host granites occurring at the same localities. The source rock of the enclaves is a mixture of the source of the Nordingra granite and an older, probably Archaean igneous rock. According to earlier suggestions, Archaean rocks once were exposed to the west of the present Baltic Shield and now occurs in its lower crust. It is proposed here that remnants of Archaean rocks underlie the Svecofennian as far east as the Nordingrå area.