Noril'sk- and Lower Talnakh-type intrusions are not conduits for overlying flood basalts: insights from residual gabbroic sequence of intrusions

Abstract

Noril'sk- and Lower Talnakh-type intrusions are widely considered to be conduits that fed the overlying flood basalts. The basalts are interpreted to have formed from residual liquids that were able to escape the chambers and continue their way to the surface, leaving in the intrusions mafic to ultramafic cumulates deposited from olivine phenocryst-charged magmas. This implies that upper gabbroic rocks of the intrusions which crystallised from the residual liquids are direct plutonic equivalents of the basalts. However, this study shows that residual gabbroic intrusions and the flood basalts are dramatically different in composition. The former rocks are of broadly olivine gabbroic composition (e.g. olivine-bearing gabbrodolerites, prismatic gabbro, magnetite gabbro), for which both petrography and chemistry indicate almost orthopyroxene-free compositions. In contrast, the latter rocks are of broadly gabbronoritic bulk composition, with up to ∼25% normative content of orthopyroxene. This indicates that the residual gabbroic intrusions and the flood basalts are produced from two distinctly different magmas. The parental magma of the residual gabbroic intrusion sequence was opx-free silica-undersaturated olivine basalt, whereas that of the basalts was opx-rich silica-saturated tholeiite. Phase equilibrium relationships clearly show that these two types of magmas are not co-magmatic, as there is no way to derive one liquid from another by fractional crystallisation. This makes groundless the popular concept which considers the Noril'sk- and Lower Talnakh-type intrusions as exit conduits for a great volume of basaltic magma. Continued mention of these intrusions as hosts of a classic example of Cu–Ni–PGE deposits formed in a conduit system environment would therefore be highly undesirable.