Mantle plume activity in the Mirovoi ocean: evidence from accreted oceanic slices in North Wales

The composition of the Earth's mantle is influenced by plate tectonic processes, and one of the ways to study the geochemical evolution of the mantle and test plate tectonic and geodynamic models is to investigate the composition of ancient mantle-derived rocks. In this work, we study the Neoproterozoic oceanic basalts that occur as fragments in an olistostromal mélange in the Llŷn Peninsula of North Wales. Based on field, petrographic, and geochemical evidence, we argue that these rocks represent pieces of seamounts. Petrogenetic modelling shows that the basalts can be linked to the same source, with geochemical variations being explained by intraplate plume-related melting under variably thickened lithosphere and differing degrees of fractional crystallisation. The Nd–Hf isotopic data indicate a depleted mantle source, with mean <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Nd(t) and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Hf(t) of 7.0 and 9.4, respectively. The depleted Nd and Hf signatures associated with elevated Pb isotopic ratios suggests that the mantle source tapped by this plume was dominated by a component similar to modern-day PREMA. Monte Carlo simulations utilising constraints on the age of accretion to an active margin and lithospheric thickness at the time of plume impingement yield results that show plume activity between 655–590 Ma. The Late Neoproterozoic timing of accretion of these rocks to the active margin might also help explain the exhumation of blueschists in North Wales. Paleogeographic and geochronological constraints show that, rather than being derived from the Iapetus Ocean, the seamounts preserved in North Wales likely originated in the shrinking circum-Rodinia Mirovoi super-ocean. Records of preserved seamounts elsewhere in the world show that Neoproterozoic plume activity must have been common in the Mirovoi ocean. The isotopic characteristics of the plume-related basalts in this study show that the PREMA component was present in the mantle sources of magmatism in the Mirovoi superocean and support the hypothesis of the longevity of this component. This study shows how a multidisciplinary approach integrating geochemistry and plate reconstruction can help tracking mantle evolution in response to plate tectonics.