Radiogenic isotope systematics of the Herefoss granite, South Norway: an indicator of Sveconorwegian (Grenvillian) crustal evolution in the Baltic Shield

Abstract

A new PbPb mineral isochron dates the crystallization of the Herefoss granite (South Norway) to 926 ± 8 Ma, confirming earlier RbSr evidence of a late Sveconorwegian (Grenvillian) age. Initial Sr and Nd isotopic compositions recalculated to ε Sr(926 Ma) ≤ 40, ε Nd(926 Ma) = −3.2 to −0.8 indicate that the magma has originated from a mixture of two distinct source components: (1) A crustal component, which has resided in a moderately LILE-enriched reservoir ( f Rb = +7 to +8, 238 U 204 Pb = 17–22 ) since 1.6–1.9 Ga. This crustal component is clearly distinct from the extremely LILE-enriched metasedimentary country rocks of the granite ( f Rb > 21, 238 U 204 Pb > 24 ). (2): A mantle-derived component ( ε Nd = +4.6 to +5.9, ε Sr = −12 to −9, 238 U 204 Pb = 7.96 ) younger than 1.5 Ga. The ranges of initial Sr, Nd and Pb composition observed in the intrusion indicate that the magma was isotopically heterogeneous at the time of emplacement, and that the crustal contribution amounted to ∼ 35–55%. By the end of the Sveconorwegian orogeny, the southwestern part of the Baltic Shield had acquired a compositionally layered structure, where an “uppermost” zone of the crust with extreme LILE enrichment was underlain by a heterogeneous layer consisting of rocks with “normal upper crustal” degree of LILE enrichment and mafic intrusions younger than 1.5 Ga. There is no evidence of involvement of LILE-depleted “deep continental crust”-source material in the petrogenesis of the Herefoss granite. Rejuvenation of the lower crust by injection of mantle-derived magmas (“underplating”) has been an important process in the evolution of the southwestern part of the Baltic Shield. The absence of a young mantle-derived component from ∼ 1.12-Ga charnockitic intrusions in the Bamble sector and from the 989-Ma Grimstad granite suggests that this process may have been contemporaneous with late Sveconorwegian mafic magmatism in the Rogaland-Vest Agder sector further to the west.