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Abstract
The Habo Volcanic Suite is situated in the southwestern part of the Småland-Värmland batholiths of the Transscandinavian Igneous Belt (TIB), Sweden. It comprises mainly intermediate to mafic pyroclastic and redeposited volcanic rocks and is distinct from other, more felsic, volcanic rocks of the Småland-Värmland batholiths. The Habo Volcanic Suite has been intruded by a TIB-granite, here dated at 1660±9 Ma. U-Pb ages of zircon from the Habo Volcanic Suite fall into two groups; 1795±14 Ma for domains with Th/U >0.2 and 1694±7 Ma for rims with Th/U <0.2. The 1795±14 Ma age is interpreted as the crystallization age of the Habo Volcanic Suite, linking it to other Småland volcanic rocks, whereas the 1694±7 Ma rims are interpreted to reflect a thermal event related to intrusions of younger TIB rocks. These interpretations are supported by LA-ICPMS REE analyses which show flatter REE patterns and Th/U <0.2 for metamorphic zircon; and positive Ce anomalies for magmatic zircon. Two geochemically different groups of the Habo Volcanic Suite can be distinguished. The first consists of primitive, alkaline mafic rocks whereas the second group comprises felsic to intermediate, subalkaline compositions. Mixing or crustal assimilation between a juvenile basaltic magma and an upper-crustal component is likely to have formed the second group. The mixing, as well as the geochemical signatures and the great abundance of intermediate volcanic rocks strongly suggest an active continental margin setting for the Habo Volcanic Suite, supporting previously suggested theories of an Andean-type convergent continental margin setting for the TIB.
