Marks, and 5 co-authors, reported in 2008 that trace element data were obtained for HFSE-rich minerals (titanite, eudialyte, Ti-bearing andradite, zircon) from plutonic rocks of the Eocene Tamazeght complex, High Atlas Mountains, Morocco. In most lithologies significant amounts of titanite are present and as such, titanite fractionation played a major role in the evolution of the HFSE budget of the whole complex. By combining our data with published titanite–melt partition coefficients we explore the effects of titanite fractionation on the evolution of geochemical parameters (Zr/Hf, Nb/Ta, La/Lu and Th/U ratios) during differentiation of silicate melts in detail.
Based upon this, the authors also discussed the genetic relationships between the various rock types present in the Tamazeght complex and show that the above-mentioned element ratios can potentially be useful for distinguishing geochemically distinct melt reservoirs within a given plutonic rock suite. Zircon, eudialyte and Ti-bearing andradite are generally rare or are restricted to localized rock units. Despite their scarcity, each of them can have a significant effect upon the HFSE budget in their evolving magmas. The authors developed first estimations of eudialyte–melt partition coefficients for REE. Application of these data to eudialyte-bearing rocks of the Ilímaussaq complex (South Greenland), and comparing the results to published estimations for the parental melts of these rocks yields consistent results and demonstrates the reliability of their estimations.
Thje authors concluded that Zr and Hf behave nearly identical in terms of geochemical behavior and, consequently most of the crust maintains near chondritic Zr/Hf ratios of ~35–40. In contrast, the calculated melts of the Tamazeght complex, Morocco have super-chondritic and highly variable Zr/Hf ratios of ~50–90. As titanite is the primary reservoir for both Zr and Hf in most of these rocks and preferentially incorporates Hf, the crystallization of titanite leads to these elevated Zr/Hf ratio in the residual melt. Thus, high Zr/Hf is a fingerprint for the fractionation of titanite. In that sense, titanite fractionation in magmatic systems has the opposite effect to zircon fractionation (e.g., Claiborne et al., 2006), and our results support the work of Bea et al. (2006).
Comparable behavior is observed for Nb/Ta and La/Lu, while the opposite (although less obvious) is visible for Th/U ratios. However, if all of the investigated rocks units were to have been derived from a common parental melt, Zr/Hf ratios should systematically increase from oldest to youngest rock types and
excellent correlations between Zr/Hf ratios and Zr concentration in titanite and the other element ratios would be expected. This, however, is not the case and the four investigated element ratios (Zr/Hf, Nb/Ta, Th/U and La/Lu) show variably disturbed behavior.
The authors interpreted this as indication of the influence of geochemically distinct melt batches (both in terms of element concentrations and element ratios), which is in accordance with the interpretations given by Marks et al. (2008) based on mineral chemistry and petrology of the rocks. In general, this means that the investigated element ratios cannot on their own be used to monitor fractionation of HFSE-rich mineral phases within a given magmatic suite, but that they are useful for distinguishing geochemically different melt reservoirs of a given rock series.
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