PLUTONIC TO VOLCANIC ROCKS OF THE VOYKAR OPHIOLITE MASSIF (POLAR URALS): STRUCTURAL AND GEOCHEMICAL CONSTRAINTS ON THEIR ORIGIN

Abstract

Tectonised mantle ultramafic rocks constituting the major part of theVoykar ophiolite massif are intruded by two rock series: (1) the Trubaju complex (mainly pyroxenites and norites), and (2) the Izshor complex (mainly gabbros, diabases and troctolites). The Trubaju rocks were emplaced earlier and at deeper levels with respect to the Izshor ones. Geochemical criteria, such as the relative REE, HFSE and LILE abundances and major-element variations, suggest that the rocks from both these complexes crystallised from magmas very similar in many geochemical parameters, originating from a highly depleted mantle source. The studied rocks are extremely depleted in Y, Zr, Hf and Nb as compared to normal MORB, thus their parental magmas can be interpreted as generated in supra-subduction environments of an inter-arc basin that split an older island-arc structure during the Early Devonian. The existence of the basin was short, and prior to the Eifelian; relic fragments of the basin crust were emplaced as the Voykar allochthonous ophiolite into the composite arc structure whose tectonic evolution terminated in the Late Devonian prior to its collision with the continental margin of the East European platform.