The sub-Jotnian Stromsbro granite complex at Gavle, Sweden

The Stromsbro granite complex in eastern south-central Sweden occurs an the north side of the Gavle graben, which is filled with Jotnian (Mesoproterozoic) sandstones. It has the form of a half oval of which the southern part is covered by the Jotnian rocks, thus rendering the sub-Jotnian label adequate. The granites are undeformed, red, and coarsely porphyritic. A few dolerite dykes are interpreted as associated with the granite. The Stromsbro granites show considerable geochemical similarity with the rapakivi granites of the Fennoscandian Shield. They are metaluminous, within-plate, A-type granites. The dolerites are alkali-rich quartz tholeiites showing marked similarities with other rapakivi-associated basic rocks. The U-Pb zircon age of the granite massif is 1500+/-19 Ma (MSWD=1.7), which is similar to other sub-Jotnian complexes in Sweden. The low initial epsilon(Nd) values of the Stromsbro rocks, and especially the dolerite, suggest the involvement of Archaean crustal material in the magmas. The granites are interpreted as derived by melting of non-depleted, tonalitic granodioritic, early Svecofennian (and partly Archaean), lower crustal protoliths in response to heat brought by mantle-derived magmas, now represented by the dolerites.