DIRECT OBSERVATION OF BORO-ALUMINOSILICATE MELT COMPOSITIONS: INSIGHTS FROM RAMAN SPECTROSCOPY OF MELT INCLUSIONS IN PEGMATITIC TOURMALINE OF THE GATUMBA-GITARAMA AREA (RWANDA)

This study presents Raman spectroscopic analyses of melt inclusions in tourmaline from tourmaline-quartz-(muscovite) assemblages of common pegmatites of the Gatumba-Gitarama area (Rwanda). The melt inclusions show a main mineralogy composed of muscovite, alpha-quartz, moganite, dickite, and minor feldspars which demonstrate, in combination with the observation of dawsonite, nahcolite, jeremejevite, and childrenite daughter minerals, a CO2-, H2O-, B2O3-, and P3O4- enriched peraluminous boro-aluminosilicate composition for the trapped melt. The variable amount of acidic interstitial fluids inside the melt inclusions resulted mainly from heterogeneous trapping of omnipresent, exsolved aqueous fluids during melt inclusion entrapment. Aliquots of this exsolved alkali-rich aqueous fluid phase are preserved in the numerous coexisting fluid inclusions in tourmaline. The observed mineralogy and composition of the melt inclusions deviates strongly from a bulk single-phase melt crystallization model for pegmatite formation. Based on reported experimental, theoretical, and natural constraints, an alternative hypothesis can offer an explanation for the formation of the anomalous but omnipresent residual melts trapped in tourmaline: i.e., the immiscibility of a hydrous fluid and a boro-aluminosilicate melt from the residual bulk aluminosilicate melt. The chemically anomalous composition of the immiscible boro-aluminosilicate melt trapped inside the melt inclusions can explain the mineralogical transition from a granitic mineral mode towards a schorl-quartz-(muscovite) assemblage within the common mineralogical zonation of the pegmatite dike.