Chemical zoning of muscovite megacrystal from the Brazilian Pegmatite Province

Abstract

Macroscopically homogenous muscovite plate from the Cruzeiro pegmatite, located in the Eastern Pegmatite Province in Minas Gerais, may show complex distribution patterns of some trace elements. In geochronological and petrological studies, as for example in the distinction of magmatic and post-magmatic mica, the cause of zoning could be taken into consideration. The complex chemical zoning in the studied mica plate can be best explained by growth in an evolving magma followed by alteration due to percolation of hydrothermal fluids. Enrichment of Rb towards the border is interpreted as resulting from the chemical evolution of the residual magma during crystal growth. The depletion in ( IV Al+ VI Al) as well as the increase in (Fe+Mg) and Si along a fracture could be due to the hydrothermal celadonitic substitution of muscovite. This alteration also caused depletion in the contents of Rb, Ga, Y, Nb, Sn, and Zn and residual concentration of Ti. Elements such as Ga, Y, Nb, Sn, and Zn, rarely considered in the discussion of differentiation or alteration processes in micas, have been shown to be as significant as the alkali-elements.

The magmatic–hydrothermal transition in rare-element pegmatites from southeast Ireland: LA-ICP-MS chemical mapping of muscovite and columbite–tantalite

“…These studies document white micas which are zoned in major elements; are macroscopically zoned, or show zoning by epitactic overgrowth of different mica species. Only Clarke and Bogutyn (2003) Rb in the study of Viana et al (2007), but not in the present study.…”

“…The homogeneity is evident from the lack of optical zoning (macroand microscopically), homogenous BSE intensities and low standard deviations for average concentrations of K, Al and Si as determined by SEM-EDS. However, high-resolution LA-ICP-MS imaging demonstrated that minor and trace element concentrations vary strongly over several growth zones.Studies on minor-and trace-element zoning in white mica from granites and pegmatites are sparse Viana et al (2007). present chemical data for a single muscovite megacryst (40x80 cm) from a rare-element pegmatite from the Eastern Pegmatite Province in Minas Gerais, Brazil.…”

Microstructures of epidote-prehnite bearing damaged granitoids (northern Victoria Land, Antarctica): clues for the interaction between faulting and hydrothermal fluids

“…chlorite, epidote and white mica) give temperatures between 300 and 350 • C (Bruhn et al, 1994), at low greenschist facies conditions; this temperature range agrees with the values proposed for the formation of Fe-rich epidote (Liou et al, 1983). The phengitic composition of white mica after biotite points to a high temperature of crystallization (Morad et al, 2010), confirming its formation in a hydrothermal system (Dempster et al, 1994;Gomes and Neiva 2000;Viana et al, 2007).…”