Peculiarities of ultrabasic-alkaline magma differentiation from gigantic Guli intrusion (Polar Siberia)

Kogarko L.N., Gerasimova E.I.

Vernadsky Institute of Geochemistry and Analytical Chemistry Russian Academy of Sciences, Moscow, Russia


In the Polar Siberia there is the biggest in the world Gulinskiy ultramafic-alkaline complex. It was formed during 7 phases. According our data the age of the massif is 250 Ma, sinhronous with the formation of Siberian Trapps.

We investigated  the chemical composition of Guli rocks and dyke series. The obtained data permit to distinguish some petrological trends (Figs). The sharp increase of SiO2 content is observed when the MgO reach about 10%. Silica is  practically constant during the long interval of Guli magma evolution. FeO shows maximum also in the same region MgO concentrationcon. Al2O3, Na2O, K2O concentrations increase  with decrease of MgO content, but when MgO reach 10-15 %, more marked increase of alkalies and aluminium contents occure (fig. 2 c-h). Decrease of Al2O3, Na2O, K2O concentrations occures in the latest evolution stages (MgO concentration is in interval from the first to tenth percent) (fig. 2 c, e). The Cr2O3 concentration falls gradually and becomes extremely low from 10 % MgO (fig. 2 g). CaO and TiO2 concentrations also pass through the maximum (on fig. 2 h showed diagram for TiO2) when MgO approach close to 10 %.

Established regularities show the processes of crystallization differenciation of initial ultrabasic-alkaline magma, the composition of which is close to meymechites and picrites (SiO2 content is nearly 40 %). Olivine and chromite fractionate in the first evolution stages; SiO2 concentration stays practicaly constant (nearly 40 %) with a fast decrease of MgO content. This process is quite long and received data witness about very wide field of olivine crystallization in the process of ultrabasic-alkaline magma differentiation. The crystallization of spinel (chromite-titano-magnetite), perovskite, apatite, melelite and to all appearance calcite is started in a melt when MgO content reaches nearly 10 % and lower till first percent. This is confirmed by sharply decrease of CaO, TiO2, FeO contents and a growth of Al2O3, Na2O and K2O concentrations. The fractionization of feldspars starts on the latest evolution stages, resulting in the sharply decrease of Al2O3, Na2O, K2O contents (fig. 2 c, e).

Thereby, we can determine  the order of crystallization and mineral fractionization in the way of the primary magma evolution of Guli massif. This is olivine and chromspinelide, titano-magnetite, perovskite, apatite, melelite, calcite and feldspars.

One should note, the strong resemblance of petrological trends of Guli massif rocks and dike series.  The evolution of  dike rocks is showed on fig. 1 b, fig.2 b, d, f, h. The evolution of Guli massif rocks and dikes was a result of crystallization differentiation processes and fractionation of olivine, chromit, titano-magnetite, perovskite, apatite, melelite, calcite and feldspars. Such paragenesis of fractionated minerals showed the low pressure crystallization, which had passed in shallow intermediate magma chamber under the similar physico-chemical parameters and besides compositions of Guli massif primary magma and dikes series were the similar.


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