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

The crystallization history of the Black Mountain granitic pegmatite. near Rumford, western Maine, is evaluated using compositions of rock-forming and accessory minerals. The pegmatite is strongly zoned; the zonation develope d from the consolidation of a rare-element-enriched melt. Field and trace-e lement data show that the wall zone crystallized first from an initially B- enriched melt. Subsequent crystallization of three intermediate zones refle cts changes in melt composition, which include a general decrease in levels of Mg, Ca, and Fe, and increases in levels of Be. Nb, Ta. Sn, and P. Signi ficant enrichment in Li, Ph, Cs and F occurred during the latest stage of p egmatite crystallization prior to core development. Late albite-dominant un its enriched in B, Nb > Ta, Sn and Zr replace primary zones; their chemical trends deviate from normal trends of fractionation (e.g., late enrichment in Pe).