Microgranular enclaves in the Nordingra rapakivi granite, east central Sweden - the early rapakivi development

Quartz-syenitic, brownish red, rounded enclaves occurring in the Nordingra 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 su ite. They are essentially hypersolvus with ferroedenite as the major mafic mineral. At one of the localities, clinopyroxene is a major mineral. Mg/(Fe -tot+Mg) ratios are low, 0.15 +/-0.02 (1 sigma) but not so low as in the ho st granites. The K2O/Na2O ratio obtained from the same rocks is higher in t he enclaves (mean = 2.52 +/-0.15, 1 sigma) than in the host granite. The tr end of Ga/Al against Zr obtained from the enclaves is significantly offset from the corresponding trend obtained from the Nordingra rapakivi granite. epsilon (Nd1578) values of the enclaves vary between -5.0 and -1.7. The enc laves and the host granite define different trends in DEBON & LE FORT PQ-di agrams. The trend of the enclaves is offset towards more potassic compositi ons than that of the granite. Rb-Ba-Sr relations are identical in enclaves and host granites. The ratio Zr/Nb (mean 24; sigma = 5), is significantly d ifferent in enclaves and granites. A general similarity in mineral and chem ical compositions is noted between enclaves and host granites occurring at the same localities. The source rock of the enclaves is a mixture of the so urce 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 Nordingra area.