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The Siberian Traps - Geochemistry of Lavas

The bulk of the published data for the Traps are on lavas and intrusive samples from Noril'sk (Lightfoot et al., 1990, 1993; Sharma et al., 1991; Naldrett et al., 1992; Wooden et al., 1993; Hawkesworth et al., 1995; Fedorenko et al., 1991, 1996), from Putorana (Sharma et al., 1992; Ryabchikov et al., 2001) and from boreholes in the West Siberian Basin (Medvedev et al., 2003; Al'Mukhamedov et al., 2004 ; Reichow et al., 2005).
Naldrett et al. (1992) ascribed the lavas of the Noril'sk area to five main magma types. These are described in the table opposite, which incorporates the revisions by Fedorenko et al. (1996).
The majority of the lavas are evolved basalts with Mg numbers (100Mg/Mg+Fe) less than 60. Picritic lavas in Gudchikhinsky and Tuklonsky Formations have Mg numbers over 70, but these mostly, if not all, reflect olivine accumulation.
Both high- and low-Ti variants are present in the Noril'sk successions. High-Ti lavas dominate the earlier lavas (Ivakinsky to Gudchikhinsky), and low-Ti lavas dominate the later lavas.
There are other pronounced temporal trends in lava composition at Noril'sk, which are illustrated and discussed further here. The bulk of the Noril'sk lavas have high Th/Ta ratios (much higher than a primitive mantle value of 2.3), indicative of contamination of the magmas by continental crust (Wooden et al., 1993; Sharma, 1997). This is consistent with the high La/Sm and 87Sr/86Sr ratios (and low 143Nd/144Nd ratios) of many of the lavas, with the main exception of the Gudchikhinsky magma type.

Magma Type Characteristics Ti REE profiles Epsilon Nd (t) Broad interpretation
Mokulaevsky magma type Similar to Tuklonsky type Low Similar to Tuklonsky type >0 Shallow source (or extensive melting), and with little crustal contamination
Lower Nadezhdinsky magma type Light-REE-enriched basalts with high Sr (and low Nd) isotope ratios Low High La/Sm and low Sm/Yb less than -4 Shallow source (or extensive melting), and with substantial crustal contamination
Tuklonsky magma type Includes picritic basalts. Tend to be Ni-depleted Low Unfractionated, 'flat' REE patterns, with low La/Sm and low Sm/Yb 0 to -4 Shallow source (or extensive melting), and with little crustal contamination
Gudchikhinsky magma type Includes picrites and Ni-rich suites High Low La/Sm but high Sm/Yb +2 to +6 Deeply sourced, but with little crustal contamination
Ivakinsky/Sverminsky magma type Alkalic and subalkalic basalts High Fractionated, with high La/Sm and high Sm/Yb 0 to -4 Deeply sourced, and with significant crustal contamination

Table summarising the main compositional characteristics of the five main magma types at Noril'sk. From Naldrett et al. (1992), Fedorenko et al. (1996) and Sharma (1997).

The lavas and intrusives of the Maymecha-Kotuy Province include pricrites, basalt, trachybasalt, basanite, olivine nephelinite and meimechite. The latter is an unusual alkaline magnesian rock with >18% MgO, > 1% TiO2. The original meimechitic magma probably also had a high water content (Arndt et al., 1995). High 3He/4He ratios have been found in olivine nephelinites from the Maymecha-Kotuy region (Basu et al., 1995) indicating involvement of an undegassed mantle source in the formation of at least that part of the province.
Basalts from the central and eastern parts of the West Siberian Basin are compositionally similar to the Lower Nadezhdinsky magma type (Reichow et al., 2005).