Ancient and juvenile components in the continental crust and mantle: Hf isotopes in zircon from Svecofennian magmatic rocks and rapakivi granites in Sweden

Abstract

The sources of igneous rocks in the continental crust are elusive, but they may be traced by radiogenic isotopes, which convey a message about the age and composition of the concealed parts of the continent. We investigated the Hf-isotope composition of zircon in ten rocks from central and southern Sweden. Two felsic metavolcanic rocks and two metagabbros (ca. 1.89 Ga) from Bergslagen, southern Sweden, show ε Hf (t) ranges of -1.8 to +5.1 and +2.6 to +6.8, respectively, suggesting that juvenile sources have contributed to both. A 1.85 Ga granite from southern Bergslagen shows a ε Hf (t) range of - 2.6 to +4.6 for magmatic zircons, but both highly negative and positive values for inherited grains, providing evidence for both Archean and juvenile crustal sources. These and previous data confirm the existence of juvenile proto-Svecofennian crust (<2.2-1.9 Ga) with a minor Archean component, from which later crustal magmas were generated. The Hf-isotope evolution curve for this crust can be approximated by ε Hf (1.90) = 3 ± 3 and 176 Lu/ 177 Hf = 0.018. Similarly, the present data, together with data for younger mafic intrusions, can be used to infer the presence of a "mildly depleted" sub-Svecofennian mantle evolution curve with ε Hf (1.90) = 4.5 ± 2.5 and 176 Lu/ 177 Hf = 0.0315. Zircons from four out of five rapakivi intrusions (1.53-1.50 Ga) in central Sweden yield negative ε Hf (t) in the range -9.8 to -4.6, suggesting mixed Archean and juvenile Svecofennian sources. One intrusion farther south ranges between ε Hf (t) of -4.1 and -1.6, and has a larger contribution from Svecofennian crust. The data suggest that the crust in Bergslagen, southern Sweden, is dominantly Paleoproterozoic, while higher proportions of Archean Referencesmaterial are present below central Sweden. © 2011 Geological Society of America.