MAGMA DIFFERENTIATION AND MINERALIZATION IN THE SIBERIAN CONTINENTAL FLOOD BASALTS

New major, trace element and Sr and Nd isotope data are presented for selected lavas from the three uppermost formations in the Siberian Tra p, 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, SiO2=48.4-49.6%, MgO=8.1-6.3%, and Mg =54-58, TiO2=1.05-1.6%, epsilon(Sr)=1-7 and epsilon(Nd)= 3.8-1.3. Ther e is no significant depletion in Ni and Cu, or coupled increase in SiO 2 and La/Sm, which so characterise the underlying Nadezhdinsky formati on rocks. The intrusive rocks are considered in 5 groups, following Na ldrett et al. The alkaline rocks (Group 1), dolerites with a range of Ti contents (Groups 2 and 3), and differentiated intrusions not associ ated with ore junctions (Group 4), all exhibit restricted initial epsi lon(Sr), and epsilon(Nd) values of 3-32 and 3.5 to -3.2, respectively. In contrast, the intrusions related to ore junctions (Noril'sk- and L ower Talnakh-types, 5A and 5B) trend towards higher epsilon(Sr), and l ower epsilon(Nd), With epsilon(Sr)= 17-59 and epsilon(Nd) 2.9 to -3.4 in the Noril'sk-type, and 41-66 and -3.7 to -6.2 respectively in the L ower Talnakh-type. The roles of crustal contamination and partial melt ing in the continental mantle lithosphere are briefly reviewed. A mini mum of three components are required to explain the basalt data, which are therefore inconsistent with simple mixing between plume derived m agmas and crustal material. Rather, magmas were derived from both the mantle lithosphere and the underlying asthenosphere, and crustal conta mination modified the composition of specific magma types. The minor a nd trace element characteristics of the contaminant appear to have bee n similar to those of an inferred deep-seated crustal melt, rather tha n an upper crustal melt, or a bulk sediment. The between-suite variati ons in the intrusions are similar to those in the lavas but, in additi on, there are within-suite variations attributed to late stage, open s ystem differentiation within the Noril'sk and Lower Talnakh-type intru sions. In the preferred model in which sulphide precipitation occurred in response to the crustal contamination processes responsible for th e elevated La/Sm and lower epsilon(Nd) of the Lower Talnakh and Nadezh dinsky rocks, sulphide precipitation took place before the crystallisa tion of the silicate phases currently preserved in the intrusive rocks .