Abstract
Rare-element pegmatites of the lithium-cesium-tantalum (LCT) family originate from crustal melts derived from orogenically-thickened continental crust and yield a minimum estimate for the age of orogenic thickening. Pegmatites of this type from the Palaeoproterozoic Bothnian Basin of central Sweden formed during the short age interval between 1803 and 1794 Ma, even though the dated pegmatites are spread over a large area. In contrast, LCT-family pegmatites from a much smaller area in south-central Sweden seem to encompass a much larger age range between 1820 and 1815 Ma (Mysingen-Utö pegmatites) and ca. 1780 Ma (Skrumpetorp pegmatites). This broad age range is matched by the age distribution of migmatitic granites (1846-1779 Ma). The ages of LCT-type pegmatites and migmatitic granites are compatible with earlier findings that Svecofennian crustal thickening started earlier to the south and, in addition, indicate that this thickened crust persisted during the compression and thickening of the northern area. Niobium-yttrium-fluorine (NYF)-type pegmatites and related granites, which could be related to crustal thinning or could reflect melting at a deeper crustal level, overlap in age with LCT-type pegmatites in south-central Sweden. The NYF-type pegmatites are coeval or younger when both pegmatite types occur in the same subareas.
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