REMOBILIZATION OF THE CONTINENTAL LITHOSPHERE BY A MANTLE PLUME - MAJOR-ELEMENT, TRACE-ELEMENT, AND SR-ISOTOPE, ND-ISOTOPE, AND PB-ISOTOPE EVIDENCE FROM PICRITIC AND THOLEIITIC LAVAS OF THE NORILSK DISTRICT, SIBERIAN TRAP, RUSSIA

The Late Permian to Early Triassic Siberian Traps have been sampled by drill core (core SG-9) and from surface exposure (section 1F) in the Noril'sk region of the Siberian Platform, Russia. Combined major, trac e element, and Nd-, Sr-, and Pb-isotope data on selected samples throu gh the Siberian Trap. offer new chemostratigraphic criteria for the id entification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near Noril'sk. A L ower Sequence of sub-alkalic basalts, tholeiites. and picritic basalts (upwards these are the Ivakinsky, Syverminsky, and Gudchichinsky form ations) arc overlain by an Upper Sequence of picritic basalts and thol eiites interbedded with tuffs (upwards, these are the Khakalichansky, Tuklonsky, Nadezhdinsky, Morongovsky, Mokulaevsky and Kharayelakhsky f ormations). The Gudchichinsky and Tuklonsky formations contain both pi critic and tholeiitic lavas. The Tuklonsky formation tholeiites and pi crites have moderate Gd/Yb (1.6-1.8), low TiO2 (0.45-0.95 wt%), a sign ificant negative Ta and Nb anomaly (Nb/La = 0.42-0.57) and unradiogeni c Nd (epsilon(Nd)CHUR = 0 to - 4.6). In contrast, both the Gudchichins ky formation tholeites and picrites have high Gd/Yb (2.3-3.1), and TiO 2 (1.2-2.3 wt%), no significant Nb or Ta anomaly (Nb/La = 0.8-1.1), an d radiogenic Nd (epsilon(Nd)CHUR = 3.7 to 7.3). The low-Ti and Nb/La, high La/Sm, and unradiogenic Nd-isotope signatures of the picritic Tuk lonsky formation lavas and the tholeiitic lavas of the Upper sequence are characteristic of magmas strongly influenced by material from the continental lithosphere, whereas the high-Ti and Nb/La, low La/Sm and radiogenic Nd-isotope signatures of the Lower Sequence are more compar able to deeper asthenospheric mantle-plume generated lavas similar to oceanic island basalts. The lavas overlying the Tuklonsky formation ha ve mg-numbers of 0.63 to 0.68. and are more evolved than the Tuklonsky (Mg-number < 0.62) and have more radiogenic epsilon(Nd)CHUR (Tuklonsk y: - 0.03 to - 4.66; Mokulaevsky: + 0.60 to + 1.61), but have many of the incompatible trace element features of the Tuklonsky type magma. T hese lavas show a progressive upwards decline in SiO2 (55-49 wt%), La/ Sm (4.6-2.0), and (+ 67 to + 13) which has previously been attributed to epsilon(Sr)UR decrease in the proportion of crustal material contri buted to the magma. This paper explores an alternative model where a c omponent of the crustal contribution might be derived from within an a ncient region of the mantle lithosphere as recycled sediment rather th an from the overlying continental crust.