Internal zonation and chemical evolution of the Black Mountain granite pegmatite, Maine

The crystallization history of the Black Mountain granitic pegmatite, near Rumford, W Maine, is evaluated from the compositions of its minerals. The pegmatite is strongly zoned, developing from consolidation of a rare-element-enriched melt. Field and trace element data show that the wall zone crystallized first from an initially B-enriched melt. Subsequent crystallization of three intermediate zones reflects changes in melt composition, including a general decrease in Mg, Ca and Fe and an increase in Be, Nb, Ta, Sn and P. Significant enrichment in Li, Rb, Cs and F occurred during the latest stage of pegmatite crystallization prior to core development. Late albite-dominant units enriched in B, Nb > Ta, Sn and Zr replace primary zones; their chemical trends deviate from normal fractionation trends, e.g. in late enrichment in Fe.