Paragenetic relationships, geochemistry and petrogenetic significance of primary Fe[sbnd]Mn phosphates from pegmatites: The case study of Cañada (Salamanca, Spain) and Palermo (New Hampshire, USA) pegmatites

Abstract

Primary FeMn phosphates from four pegmatites (Cañada, Palermo#1, Palermo#2 and Palermo#11) typical of the beryl-columbite-phosphate subtype have been characterized texturally and geochemically. Most of the common FeMn phosphates (triphylite, sarcopside, ferrisicklerite, wagnerite and wolfeite) are extremely poor in trace elements, with the exception of Zn for all them, Li for sarcopside, and Nb and Ta for wagnerite and wolfeite. Calcium-bearing FeMn phosphates (graftonite and johnsomervilleite) are the richest in trace elements, with contents usually over the mean continental crust for Zn, Y, REE and Pb. Crystal-chemical constraints, combined with paragenetic relationships, geochemical and geological processes, are thought to control the content, distribution and behavior of trace elements in FeMn phosphates. Zinc contents in graftonite and triphylite increase with fractionation, whereas Y, Nb, Ba, REE, U and Pb contents in graftonite decrease. Crystallization of REE-bearing phases, such as xenotime-(Y), strongly affects the REE content of the FeMn phosphates. Recrystallization processes also seem to affect the trace element contents in phosphates. The earliest primary phosphate associations from Cañada and Palermo pegmatites (those containing graftonite), show Y/Ho and Zr/Hf ratios inside or close to the CHARAC (Charge-and-Radius-Controlled) field. This could indicate that such associations crystallized in a magmatic stage without a high H2O activity. Phosphates from the latest associations plot outside the CHARAC field, suggesting that fluid activity became important during the crystallization of these latest associations. Ti-in-biotite, garnet-biotite and triphylite-sarcopside geothermometers have yielded relatively high temperatures of crystallization, between 633 and 386 °C, for the earliest associations (presumably crystallized under magmatic conditions). In contrast, the latest associations, whose crystallization presumably proceeded with a higher H2O activity, are in the range of 281–360 °C.