Geochemical behaviour of trace elements during fractional crystallization and crustal assimilation of the felsic alkaline magmas of the state of Rio de Janeiro, Brazil.

Akihisa Motoki,Dean P Melo,Thais Vargas,Kenji F Motoki,Susanna E Sichel

doi:10.1590/0001-3765201520130385

Abstract

This paper presents geochemical behaviour of trace elements of the felsic alkaline rocks of the state of Rio de Janeiro, Brazil, with special attention of 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.

Highlights

Geochemical behaviour of trace elements are sometimes different from major elements
This paper presents the behaviour of trace and rare earth elements of these alkaline rocks with additional analytical data
The behaviour of trace elements of the Cretaceous to Early Cenozoic 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 K 2 O/(K 2 O+Na 2 O)

Summary

INTRODUCTION

Geochemical behaviour of trace elements are sometimes different from major elements. Absolute and relative abundance of determined trace elements, such as Zr, Y, Nb, Ga, and Sc, are stable during alteration processes of metamorphism, Correspondence to: Kenji Freire Motoki. REEs, convex pattern, and negative Eu anomaly This sample has positive SSI of 56 and SiO 2oversaturated with strong effects of continental crust assimilation. The Rb-2 is marked by low Ba. The major element behaviour indicate that the former sample is under strong effect of continental crust assimilation and the latter, originated from less fractionated nepheline syenite magma. The variety of country rock types and of partial melting degree should be complexly related to the Figure 14 - Schematic illustration for the contact zone of the nepheline syenite intrusive bodies and its geochemical interpretations on the diagrams of (Na+K) vs SSI, SSI vs Ba, SSI vs Ba/Sr, and SSI vs Zr/TiO2, which are related, respectively, to Fig. 4, 8b, 8c, and 13c. Chemical composition of the felsic alkaline rocks with continental assimilation effects

CONCLUSIONS

The nepheline syenitic rocks of Tanguá and