RAPAKIVI GRANITES OF THE BALTIC SHIELD - THE NORDINGRA GRANITE, ITS CHEMICAL VARIATION AND SM-ND ISOTOPE COMPOSITION

The Nordingra rapakivi massif is intruded into Paleo-Proterozoic rocks belonging to the Svecofennian Orogen. It is almost entirely composed of granite, gabbro and anorthosite. The granite is a red, orthoclase-h ornblende-clinopyroxene rock with minor Ca-poor pyroxene and fayalite. Biotite is rare. The composition is bimodal with a modal class around 72 wt% SiO2 and a secondary mode at approximately 66 wt% SiO2. The gr anite is an A-type granite. It is iron extreme with (FeO+Fe2O3/MgO+FeO +Fe2O3) varying between 0.90 and 0.98. The(K2O/K2O+Na2O) ratio is high varying between 0.62 and 0.69. Compared to other rapakivi massifs, no extremely evolved rocks have been found. The Rb/Ba ratio does not exc eed 0.61. The granite is moderately high in F; F varies between 240 an d 2110 ppm. The chondrite normalized REE patterns are characterized by gentle slopes and moderate negative Eu-anomalies. These anomalies bec ome more negative with an increasing Rb/Ba ratio. La/Lu varies between 4.2 and 7.6, epsilon(Nd) between -2.5 and -3.0 and depleted-mantle mo del ages between 2.16 and 2.31 Ga. The preferred genetic model suggest s formation of a gabbroic magma in the upper mantle, underplating and partial fusion of the continental crust. The protolith is a rock simil ar in composition to the Early Svecofennian granitoids found in the ne ighbourhood. It was probably less contaminated with metasedimentary ma terial than the majority of granitoids found at the present erosion su rface.