Disseminated layering or syn-shearing multi-phase petrogenetic model of Cu-Ni-PGE-bearing Khudolaz Differentiated complex in the Southern Urals: A case study of collisional ultramafic-mafic magmatism

Abstract

The Khudolaz Differentiated Complex (KDC) with Cu-Ni-PGE mineralization is located in the central part of the Southern Urals. Its geology is marked by numerous minor intrusions of diverse composition from schriesheimites to diorite scattered over a wide area and corresponding to individual horizons within larger layered massifs. We analyzed the morphologies and structural-geological constraints of the intrusion emplacement. Based on the mineralogical, geochemical and Sr-Nd isotopic data, we developed a petrogenetic model of KDC that explains the disseminated layering. According to our model a deep-seated magma chamber formed beneath the Khudolaz Trough within a local extension zone at the boundary between the ophiolitic mélange and continental metasedimentary complexes during the syn-collisional shearing. The magma ascended from a deep-seated chamber through a network of syn-shearing faults. The process was accompanied by assimilation of diverse country rocks composing the continental crust beneath the Magnitogorsk island-arc terrane. Smaller intermediate chambers developed in the upper crust, facilitating further melt differentiation. A fault network constraining emplacement of the intrusions occurred about 330 Ma ago during the convergence of the Kazakhstania and Laurussia paleocontinents and their surrounding terranes. A syn-collisional thrust instigated the deformation the deep-seated fractionating chamber, driving the extrusion of its individual horizons into the upper crustal level. As a result, a schriesheimite belt formed, which origin cannot be explained solely by processes of intra-chamber differentiation or finger-like magma flow. Melt differentiation in deep-seated chamber and intermediate chambers was associated with olivine cumulus formation and sulfide precipitation following early sulfide-silicate immiscibility. At the advanced stage of the magmatic system evolution, the melt became enriched in water under crustal conditions, promoting extensive formation of taxitic gabbro and pegmatites. The Carboniferous volcano-plutonic belt of the Magnitogorsk terrane could have formed by the emplacement of oncoming asthenospheric diapirs into tectonic windows. This process was initiated by slab break-off beneath the Magnitogorsk arc and continued after subsequent slab break-off beneath the East Uralian continental block.