THE GEOCHEMISTRY, Lu-Hf ISOTOPE SYSTEMATICS, AND PETROGENESIS OF LATE MESOPROTEROZOIC A-TYPE GRANITES IN SOUTHWESTERN FENNOSCANDIA

A suite of 920-1170 Ma undeformed granitic intrusions in southern Norway have consistent, ferroan, high-K, A-type major- and trace-element characteristics. Two different groups of intrusions (Groups 1 and 2) are distinguished by different ranges of Sr and Ba concentrations, Rb/Sr values and REE patterns, but overlap in most other geochemical parameters. Results of LA-ICP-MS Lu-Hf analyses of zircon reveal ranges of ca. 18 epsilon units for the suite as a whole and 6 to 10 epsilon units for individual samples. Inherited zircon is scarce, but its composition indicates contributions to the magma from three different crustal sources: (1) ca. 1700 Ma granitoids similar to the younger members of the Transscandinavian Igneous Belt in terms of Hf isotopes, (2) ca. 1500 Ma felsic igneous rocks, and (3) Early Sveconorwegian (ca. 1200 Ma) granitoids with juvenile initial Hf. The initial Hf isotopic composition of igneous zircon is compatible with mixing of Paleoproterozoic and Mesoproterozoic Fennoscandian crustal material with juvenile material derived from the sublithospheric depleted mantle. The low concentrations of Sr and Ba and the elevated Rb/Sr values characterizing Group-2 granites are inherited from ca. 1.5 Ga felsic metaigneous rocks in the source region. Plutons that contain enclaves of mafic to intermediate rocks have discontinuous major-and trace-element trends that are due to mixing of mantle-derived mafic magmas with anatectic melts, followed by imperfect homogenization.