In the "Itremo sheet" tectonic unit, south of Betafo, Madagascar, the presence of Ca-rich host rocks, and the strongly evolved composition of the pegmatitic melt led to the formation of very uncommon boron-bearing minerals associated with tourmaline. Selected samples containing both three-coordinated (tourmaline, hambergite), and four-coordinated (danburite, rhodisite, behierite) boron minerals, were collected from the Anjanabonoina and the Tetezantsio pegmatites, to study boron isotope fractionation processes. During the evolution of the Anjanabonoina pegmatite, formation of boron-bearing minerals proceeded in three main steps: 1) early crystallization of danburite I + dravite-schorl in the massive zone, 2) late crystallization of zoned tourmalines (from dravite-schorl to elbaite/liddicoatite) ± hambergite in miarolitic cavities, 3) very late crystallization of danburite II + schorl overgrowths, in brecciated miarolitic cavities. Crystallization of the strongly evolved Tetezantsio pegmatite vein, led to the formation of a massive aggregate of coarse grained red liddicoatite + rhodizite + danburite + behierite in the quartz-feldspatic matrix. At Anjanabonoina significant d11B variations were observed between tourmalines growth in early, danburite-rich, massive zones (+4.5?), and late, danburite-free, miarolitic zones (mainly in the range +6.0/+7.0?). In early massive zone, danburite (d11B =-0.99?) shows a significant, negative, shift in d11B respect to the associated dravite-schorl (?tour-danb=5.5?). Preferential partitioning of 10B in four-coordinated boron minerals was confirmed by the study of cogenetic tourmaline and rhodizite from Tetezantsio. Remarkable constant d11B values were measured on continuously zoned (dravite-elbaite) single crystals, from the late, danburite-free, miarolitic cavities. Minor differences in d11B of tourmaline can be observed from pocket to pocket (pockets at +6.0, +6.4, +7.0 ?) with the exception of a single pocket showing very high d11B=+14.0?. Assuming a modal proportion of 3:1 between danburite and tourmaline in the massive zone, (petrographic observations), and using their d11B values, a model for the boron isotopic evolution in the residual fluids was calculated. In this way, the most frequent ?11B values of miarolitic tourmaline (+6.0/+7.0?) can be obtained when about 15-20 % of initial boron is extracted by crystallization of the tourmaline-danburite assemblage. The more extreme d11B value (+14.0?) was attained where larger amount of the mineral assemblage extracted up to 40 % of initial boron. Thus, in complex pegmatitic systems the occurrence of four-coordinated boron minerals, enriched in 10B, strongly influences the boron isotopic evolution of the system and may complicate the petrogenetic interpretation and, in particular, the understanding of possible interactions between pegmatitic fluids and host rocks.
Boron isotopes study of borates and boro-silicates from Anjanabonoina and Tetezantsio pegmatites (Central Madagascar)
Dini A;Tonarini S;
2002
Abstract
In the "Itremo sheet" tectonic unit, south of Betafo, Madagascar, the presence of Ca-rich host rocks, and the strongly evolved composition of the pegmatitic melt led to the formation of very uncommon boron-bearing minerals associated with tourmaline. Selected samples containing both three-coordinated (tourmaline, hambergite), and four-coordinated (danburite, rhodisite, behierite) boron minerals, were collected from the Anjanabonoina and the Tetezantsio pegmatites, to study boron isotope fractionation processes. During the evolution of the Anjanabonoina pegmatite, formation of boron-bearing minerals proceeded in three main steps: 1) early crystallization of danburite I + dravite-schorl in the massive zone, 2) late crystallization of zoned tourmalines (from dravite-schorl to elbaite/liddicoatite) ± hambergite in miarolitic cavities, 3) very late crystallization of danburite II + schorl overgrowths, in brecciated miarolitic cavities. Crystallization of the strongly evolved Tetezantsio pegmatite vein, led to the formation of a massive aggregate of coarse grained red liddicoatite + rhodizite + danburite + behierite in the quartz-feldspatic matrix. At Anjanabonoina significant d11B variations were observed between tourmalines growth in early, danburite-rich, massive zones (+4.5?), and late, danburite-free, miarolitic zones (mainly in the range +6.0/+7.0?). In early massive zone, danburite (d11B =-0.99?) shows a significant, negative, shift in d11B respect to the associated dravite-schorl (?tour-danb=5.5?). Preferential partitioning of 10B in four-coordinated boron minerals was confirmed by the study of cogenetic tourmaline and rhodizite from Tetezantsio. Remarkable constant d11B values were measured on continuously zoned (dravite-elbaite) single crystals, from the late, danburite-free, miarolitic cavities. Minor differences in d11B of tourmaline can be observed from pocket to pocket (pockets at +6.0, +6.4, +7.0 ?) with the exception of a single pocket showing very high d11B=+14.0?. Assuming a modal proportion of 3:1 between danburite and tourmaline in the massive zone, (petrographic observations), and using their d11B values, a model for the boron isotopic evolution in the residual fluids was calculated. In this way, the most frequent ?11B values of miarolitic tourmaline (+6.0/+7.0?) can be obtained when about 15-20 % of initial boron is extracted by crystallization of the tourmaline-danburite assemblage. The more extreme d11B value (+14.0?) was attained where larger amount of the mineral assemblage extracted up to 40 % of initial boron. Thus, in complex pegmatitic systems the occurrence of four-coordinated boron minerals, enriched in 10B, strongly influences the boron isotopic evolution of the system and may complicate the petrogenetic interpretation and, in particular, the understanding of possible interactions between pegmatitic fluids and host rocks.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
