THE SUB-JOTNIAN STROMSBRO GRANITE COMPLEX AT GAVLE, SWEDEN

The Stromsbro granite complex in eastern south-central Sweden occurs a n the north side of the Gavle graben, which is filled with Jotnian (Me soproterozoic) 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 coarsel y porphyritic. A few dolerite dykes are interpreted as associated with the granite. The Stromsbro granites show considerable geochemical sim ilarity with the rapakivi granites of the Fennoscandian Shield. They a re metaluminous, within-plate, A-type granites. The dolerites are alka li-rich quartz tholeiites showing marked similarities with other rapak ivi-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 comp lexes in Sweden. The low initial epsilon(Nd) values of the Stromsbro r ocks, and especially the dolerite, suggest the involvement of Archaean crustal material in the magmas. The granites are interpreted as deriv ed by melting of non-depleted, tonalitic granodioritic, early Svecofen nian (and partly Archaean), lower crustal protoliths in response to he at brought by mantle-derived magmas, now represented by the dolerites.