Crustal reworking and orogenic styles inferred from zircon Hf isotopes : Proterozoic examples from the North Atlantic region

Abstract

Zircon Hf evolutionary patterns are powerful tools that are frequently used to elucidate magma petrogenesis and crustal evolution. After an initial rock forming fractionation event, the 176Hf/177Hf isotopic signature of a rock is modified through ingrowth of radiogenic Hf, dependent on the 176Lu/177Hf ratio. This fractionation process can be modelled to derive an estimation of the time when mantle extraction may have occurred. Additionally, Hf evolution trends can be used to diagnose closed system reworking where successive samples through time define an Hf evolution array dependant on the source Lu/Hf ratio. However, it is widely recognized that many magmatic events require new mantle addition as the thermal impetus for melting pre-existing crust. In this situation, rather than simply reflecting reworking, the isotopic signature indicates mixing with contributions from both reworked crust and new radiogenic input. Different geodynamic settings have different propensities for either reworking or addition of new mantle-derived magma. Hence, Hf-time trends carry within them are cord, albeit cryptic, of the evolving geodynamic environment as different tectonic configurations recycle and add new crust at different rates, magnitudes, and from different sources. As an example of the difference in apparent Hf evolution slopes, we present Hf-time compilations from three geographically distinct orogens in the North Atlantic Region whose geodynamic configurations remain speculative. We use the εHf/Ma trajectory to assist in understanding their evolution. The εHf/Ma trajectory of the Sveconorwegian Orogen corresponds to a 176Lu/177Hf ratio of 0.012, which implies a process driven primarily by reworking of pre-existing crust that is balanced with input from the depleted mantle resulting in a relatively shallow εHf/Ma slope. The Valhalla Orogen reveals a similar comparatively shallow εHf/Ma path. In stark contrast to these patterns is the steep εHf/Ma trajectory of the Grenville Orogen that requires a mixing process involving a greater contribution of old crust of at least ∼1.8 Ga age. The degree of reworking required to produce the εHf/Ma trend of the Grenville Orogen is consistent with a continent-continent collisional orogeny whereas both Sveconorwegian and Valhalla orogens appear more consistent with accretionary margins.