ORIGINS AND AGES OF PROTEROZOIC GRANITOIDS IN THE BOTHNIAN BASIN, CENTRAL SWEDEN - ISOTOPIC AND GEOCHEMICAL CONSTRAINTS

The Paleoproterozoic Bothnian Basin in central Sweden and south-central Finland is dominated by a 10 km thick sequence of metasediments, mainly metaturbidites. These have been intruded by granitoids, which have traditionally been classified as early, late and postorogenic relative to the c. 1.9-1.8 Ga Svecofennian Orogeny. The early orogenic granitoids are part of a calc-alkaline suite ranging in composition from ultramafic to granitic, while the late orogenic granitoids mainly are purely granitic and associated with abundant pegmatites. The postorogenic group forms large massifs of mainly granitic composition, and associated pegmatites are rare. An early orogenic tonalite yields a U-Pb zircon age of 1877+/-6 Ma, which is within the age range for Svecofennian granitoids of this tectonic position elsewhere in Sweden and Finland. A late orogenic granite belonging to the Harno granites from the southern part of the Bothnian Basin gives a U-Pb monazite age of 1822+/-5 Ma. This provides a minimum age for the main phases of Svecofennian deformation in the region, and a correlation with the 1.84-1.83 Ga old granite-migmatite zone of southern Finland is suggested. A postorogenic granite belonging to the Revsund granitoids gives a U-Pb zircon age of 1798+/-8 Ma. Together with previously published age data, this indicates that the Revsund granitoids were emplaced successively during an age interval of at least between 1.80 and 1.77 Ga. REE patterns and trace element model calculations indicate that late orogenic granites could have been produced by partial melting of metasediments similar to those exposed at the surface. A contribution from the early orogenic granitoids is possible but not discerned by these calculations. The postorogenic granites, on the other hand, have too high concentrations of Sr, Ba, Y and HREE to be derived from such a metasedimentary source. The initial epsilon(Nd) values for the early orogenic granitoids range between +2 and -1. Corresponding values for both late and postorogenic granites range between -1 and -3, which is on average slightly higher than those for surrounding metasediments. The initial Sr isotope ratios for the Harno granites are intermediate between those for the metasediments and the early orogenic granitoids, while the initial Sr isotope ratios for the Revsund granites are distinctly lower. The geochemical and isotopic data support a model in which the Harno granites were derived mainly from Bothnian Basin metasediments, with a contribution from early orogenic granitoids. The composition of the Revsund granitoids is, however, inconsistent with such a derivation. For these rocks a deeper, as yet unknown crustal source with a significant pre-1.9 Ga crustal component is indicated.