Complex proterozoic to paleozoic history of the upper mantle recorded in the Urals lherzolite massifs by Re–Os and Sm–Nd systematics

Abstract

Re–Os and Sm–Nd isotope systematics for the Nurali and the Mindyak lherzolite massifs have been determined in conjunction with their whole-rock major and trace element contents. The data suggest that the peridotites represent residues after the extraction of up to 25% of partial melt from the fertile mantle protolith. Later melt percolation and associated fluid-rock reaction events have modified the Sm/Nd, Re/Os and Pd/Os ratios of peridotites, but didn't affect significantly their major element compositions. The Re–Os and Sm–Nd isotope data strongly suggest that several magmatic events are involved in the evolution of these bodies. The mantle sections of these complexes formed during Proterozoic times, represent the first reported evidence for Precambrian peridotites in the Southern Urals. The oldest Re–Os age of 1250 ± 80 Ma for the Nurali cumulates records the separation from the convective upper mantle. Multiple partial melting of the peridotites followed by fractional crystallisation produced layered cumulates which were subsequently stored in the sub-continental lithosphere over ∼ 0.8 Ga. The age of the Nurali ophiolite coincides with the development of an epicontinental rift basin on the passive margin of the Baltica proto-continent. The younger Sm–Nd age of Mindyak peridotites (882 ± 83 Ma) and Re–Os age of associated gabbros (804 ± 37 Ma) record another tectonic event responsible for the separation of the Mindyak massif from convective mantle. Between ∼ 850 and 650 Ma, the margins of the paleo-Asian ocean became the site of island-arc formation. This ophiolite then evolved in an intra-oceanic island-arc setting. The Mindyak lherzolite massif could be the first record of a Neoproterozoic Cadomian arc in the Southern Urals. A later island-arc formation event has then affected both massifs in different ways. The Mindyak massif was incorporated into a rifting zone at ∼ 500 Ma, producing a second partial melting event of peridotites, cross-cutted by mafic dykes. The Nurali massif has also been cross-cut by gabbro-diorite dykes at Devonian time during the subduction event leading to Urals island-arc formation. The combination of 187Re– 187Os and 147Sm– 143Nd systematics for peridotites, mafic–ultramafic cumulates and mafic dykes reveals the complex history of ophiolite complexes, including isolation from the convective upper mantle at Proterozoic time, storage in sub-continental lithospheric mantle and re-activation during later tectonic events, such as island-arc formation.