Magma differentiation and mineralisation in the Siberian continental flood basalts

Abstract

New major, trace element and Sr and Nd isotope data are presented for selected lavas from the three uppermost formations in the Siberian Trap, and on over 60 samples of the associated intrusive rocks. The lavas from a 1400 m section are remarkably homogeneous and, apart from four samples of basaltic andesites, SiO 2 = 48.4–49.6%, MgO = 8.1–6.3%, and Mg ∗ = 54–58, TiO 2 = 1.05-1.6%, ϵ Sr = 1–7 and ϵ Nd = 3.8-1.3 . There is no significant depletion in Ni and Cu, or coupled increase in SiO 2 and La/Sm, which so characterise the underlying Nadezhdinsky formation rocks. The intrusive rocks are considered in 5 groups, following Naldrett et al. The alkaline rocks (Group 1), dolerites with a range of Ti contents (Groups 2 and 3), and differentiated intrusions not associated with ore junctions (Group 4), all exhibit restricted initial ϵ Sr and ϵ Nd values of 3–32 and 3.5 to −3.2, respectively. In contrast, the intrusions related to ore junctions (Noril'sk- and Lower Talnakh-types, 5A and 5B) trend towards higher ϵ Sr and lower ϵ Nd , with ϵ Sr = 17–59 and ϵ Nd 2.9 to −3.4 in the Noril'sk-type, and 41–66 and −3.7 to −6.2 respectively in the Lower Talnakh-type. The roles of crustal contamination and partial melting in the continental mantle lithosphere are briefly reviewed. A minimum of three components are required to explain the basalt data, which are therefore inconsistent with simple mixing between plume derived magmas and crustal material. Rather, magmas were derived from both the mantle lithosphere and the underlying asthenosphere, and crustal contamination modified the composition of specific magma types. The minor and trace element characteristics of the contaminant appear to have been similar to those of an inferred deep-seated crustal melt, rather than an upper crustal melt, or a bulk sediment. The between-suite variations in the intrusions are similar to those in the lavas but, in addition, there are within-suite variations attributed to late stage, open system differentiation within the Noril'sk and Lower Talnakh-type intrusions. In the preferred model in which sulphide precipitation occurred in response to the crustal contamination processes responsible for the elevated La/Sm and lower ϵ Nd of the Lower Talnakh and Nadezhdinsky rocks, sulphide precipitation took place before the crystallization of the silicate phases currently preserved in the intrusive rocks.