Tetrad effects in the rare earth element patterns of granitoid rocks as an indicator of fluoride-silicate liquid immiscibility in magmatic systems

Abstract

This paper focuses on reasons for the appearance of tetrad effects in chondrite-normalized REE distribution patterns of granitoids (Li-F granites, peralklaine granites, ongonites, fluorine-rich rhyolites, and granitic pegmatites). The analysis of published data showed that the alteration of such rocks by high- and/or low-temperature metasomatic processes does not result in most cases in the appearance or enhancement of M-type tetrad effects in REE patterns. These processes are accompanied by the removal or addition of lanthanides, a W-type sag appears between Gd and Ho, and negative or positive Ce anomalies develop sometimes in REE patterns. The formation conditions of peculiar rocks enriched in Ca and F from the Ary Bulak ongonite massif (eastern Transbaikalia) and the character of REE distribution in these rocks and melt inclusion glasses were discussed. Based on the obtained data and the analysis of numerous publications, it was concluded that REE tetrad effects in rare-metal granitoids are caused by fluoride-silicate liquid immiscibility and extensive melt differentiation in the accumulation chambers of fluorine-rich magmas. A considerable increase in fluorine content in a homogeneous granitoid melt can cause its heterogenization (liquation) and formation of fluoride melts of various compositions. The redistribution of lanthanides between the immiscible liquid phases of granitoid magma will result in the formation of M-type tetrad effects in the silicate melts, because the REE patterns of fluoride melts exhibit pronounced W-type tetrad effects. The maximum M-type tetrad effect between La and Nd, which is observed in many rare-metal granitoids, is related to the character of REE partitioning between fluoride and silicate melts and F- and Cl-rich magmatic fluids. The low non-chondritic Y/Ho ratio (<15) of many rare-metal granitoids may be indicative of a contribution of fluoride melts to the differentiation of F-rich silicic magmas, from which these rocks were formed. The influence of high-temperature F-Cl-bearing fluids on melts and/or granitoid rocks results in an increase in Y/Ho ratio owing to the elevated solubility of Ho in such fluids.