Motoki and 4 co-authors reported in 2015 on the geochemical behaviour of trace elements in the felsic alkaline rocks of the state of Rio de Janeiro, Brazil, with special attention to the fractional crystallization and continental crust assimilation. Fractionation of leucite and K-feldspar increases Rb/K and decreases K2O/(K2O+Na2O). Primitive nepheline syenite magmas have low Zr/TiO2, Sr, and Ba. On the Nb/Y vs. Zr/TiO2 diagram, these rocks are projected on the field of alkaline basalt, basanite, and nephelinite, instead of phonolite. Well-fractionated peralkaline nepheline syenite has high Zr/TiO2 but there are no zircon.
The diagrams of silica saturation index (SSI) distinguish the trends originated form fractional crystallization and crustal assimilation. In the field of SSI<-200, Zr/TiO2 and Ba/Sr have negative correlations to SSI in consequence of fractional crystallization. In the field of SSI>-200, they show positive correlations due to continental crust assimilation. Total REEs (Rare Earth Elements) is nearly 10 times that of granitic rocks, but LaN/SmN and LaN/YbN
are similar. REE trend is linear and Eu anomaly is irrelevant. The pegmatitic liquid generated by country rock partial melting is SiO2-oversaturated and peraluminous with high Ba, Sr, Ba/Sr, Zr/TiO2, and SSI, with high content of fluids. This model justifies the peraluminous and SiO2-oversaturated composition of the rocks with relevant effects of continental crust assimilation.
The authors concluded in some detail that the behaviour of trace elements of the Cretaceous to Early Cenozoic in felsic alkaline rocks of the state of Rio de Janeiro, Brazil, show the following features:
1. The Rb/K and Zr/TiO2 ratios increase according to the reduction of K2O/(K2O+Na2O). Adversely the cases of non-alkaline granitic magmas, the K2O/(K2O+Na2O) of the nepheline syenite magma decrease by fractional crystallization of felsic minerals.
2. The nepheline syenitic rocks of Tanguá and Rio Bonito complex are less fractionated. On the diagram of Nb/Y vs. Zr/TiO2, these rocks are projected on the areas of trachy-andesite, alkaline basalt, basanite, and nephelinite, because of the low Zr contents.
3. The SSI vs. Zr/TiO2 diagram shows distinct trends of fractional crystallization and continental crust assimilation. In the field of SSI<-200, negative correlation trend is observed because of fractional crystallization. In the domain of SSI>-200, positive trend is found due to continental crustal assimilation.
This diagram distinguishes the two trends with critical SSI value of -200.
4. The Sr and Ba contents have very wide variation and have no relation to fractional crystallization indexes. The SSI vs. Ba/Sr diagram distinguishes the trends of crustal assimilation and fractional crystallization in the similar way of SSI vs. Zr/TiO2 diagram.
5. Total REEs are generally high, about 10 times that of granitic rocks. The LaN and YbN are similar to those of granitic rocks. The REE pattern is linear with small Eu anomaly because of irrelevant plagioclase fractionation. The less fractionated nepheline syenite, has low total REEs and concave REE pattern. According to the magma fractionation, total REEs and (Sm/Sm*)* increase. The samples with strong continental assimilation have high total REEs, concave REE pattern, and slight negative Eu anomaly.
6. The SiO2-oversaturated magma generated by the country rock melting is peraluminous with high Ba, Ba/Sr, Zr/TiO2, and total REEs. The low-degree partial melting of country rock can generate fluid-rich peraluminous pegmatitic melt, even from the orthogneiss country rocks of meta-aluminous composition. The hyper-liquidus temperature of the magma enables the mixture between the magmas of thermodynamically incompatible compositions.
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