Based on the COMAGMAT-3.65 crystallization model a set of phase equilibria calculations called geochemical thermometry have been conducted at P =1kbar and closed conditions with respect to oxygen for 65 rocks representing the principle units of the Layered Series of the Skaergaard intrusion. It allowed us to define the range of initial temperatures 1145 to 1085o C and oxygen fugacities 1-1.5 log units above QFM to slightly below QFM of the original crystal mush from which the rocks from LZa to UZa crystallized. In parallel, average major-element compositions of residual interstitial liquid were calculated demonstrating a trend of continual enrichment of FeO up to ~18wt.% and TiO 2 up to ~5.5wt.% with only minor variations in the SiO 2 contents 48 to 50wt.%. Projection of the compositions onto the OLIV - CPX - QTZ diagram provides evidence that most of the Layered Series crystallized on the Ol - Pl - Cpx -oxide cotectic. Systematic differences between the calculated residual liquid compositions for LZa/LZb and LZc to UZa which are unlikely to reflect fractional crystallization are within the accuracy of the COMAGMAT model, but may be also indicative of a late-stage process involving migration and re-equilibration of interstitial liquids. Estimated amounts of interstitial melts trapped in the Skaergaard "cumulates" range around 50wt.%. Wager's compositions inferred from simple mass-balance were found to lie too far from the Ol - Pl - Cpx boundary to represent a realistic approximation of the low-pressure Skaergaard magma evolution. The main problem of genetic interpretations of the Skaergaard intrusion is a strong misbalance between the parental compositions followed from contact rocks and the results of geochemical thermometry and that of the whole differentiated body. It is most apparent for TiO2 and P2O5 which are almost twice as high in the average intrusion composition compared to the proposed parents. Moreover, the intrusion composition has of 2-4wt.% less SiO2 and much more iron. One possible explanation is to assume the Skaergaard magma came to the chamber with an amount of crystals Ol+Pl equilibrated with the calculated parental liquid. However, even if some amount of "hidden" troctolitic material exists, it is unlikely that crystallization in a closed system could produce large volumes of rocks rich in Fe-Ti oxides without complementary more felsic differentiates.
COMAGMAT, crystallization model, evolution of differentiated liquids, geochemical thermometry.
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