Re-evaluation of Rapakivi Petrogenesis: Source Constraints from the Hf Isotope Composition of Zircon in the Rapakivi Granites and Associated Mafic Rocks of Southern Finland

The 1650-1540 Ma rapakivi granites and associated basic rocks (e.g. gabbro-anorthosites) in southern Finland intrude the local juvenile Paleoproterozoic crust with little isotopic contrast to the contemporary mantle. Conventional isotopic studies have not been able to provide a satisfactory answer to the long-running controversy over the sources involved in their petrogenesis. In situ zircon laser ablation inductively coupled plasma mass spectrometry analysis of Lu-Hf isotopes and trace elements has been used to resolve the source and magmatic evolution of the rapakivi granites and associated massif-type anorthositic rocks in southern Finland. Initial Hf isotope compositions of zircon extracted from the rapakivi granites are relatively homogeneous and overlap with the modeled evolution of the Paleoproterozoic crust of southern Finland. Within-sample variation in zircon of the basic rocks is considerably greater than that of the granites and the initial Hf isotope compositions vary from crustal values (epsilon(Hf) similar to 0) to depleted mantle (epsilon(Hf) similar to+9) values. Furthermore, Ti abundances that correlate with Hf isotope compositions record significant crustal assimilation during crystallization of the mafic magmas. Our in situ Hf isotope results enhance the resolution of isotopic methods in pursuit of the sources of the Precambrian rapakivi suites. The new data argue for the truly bimodal nature of rapakivi magmatism, involving at least two distinct magma sources-a relatively homogeneous Paleoproterozoic crustal component for the granites and a depleted, mid-ocean ridge basalt source-like mantle component for the basic rocks.