Abstract

The Nd isotopic compositions of monazite and apatite are used to assess the initial isotope systematics of approximately 270 Ma granitic pegmatites in the Topsham area of southwestern Maine. The isotopic compositions are compared to values of spatially associated granites and country-rock migmatites to constrain potential sources of the pegmatites. The pegmatites form two groups: (1) the Northern series, which comprise the majority of the pegmatites exposed in the area, lack abundant rare-earth-element-enriched minerals, and have epsilon Nd (270 Ma) in the range -2.2 to -1.4; (2) the Standpipe Hill series, distinguished by an enrichment in rare-earth-element minerals, displays a epsilon Nd (270 Ma) in the range -3.4 and -3.3. Data for each group are internally consistent and suggest that the different pegmatite series were not derived from a single isotopically uniform source. The source of the Standpipe Hill series resembles adjacent biotite leucogranite [epsilon Nd (270 Ma) between -3.9 and -3.7]. The Northern series pegmatites have Nd isotopic characteristics similar to both migmatites that they intrude [epsilon Nd (270 Ma) between -2.9 and +0.8], and fine-grained biotite granites located ca. 15 km east of the pegmatites [epsilon Nd (270 Ma) between -2.5 and -1.7]. The isotopic data demonstrate that spatially and temporally related pegmatites need not be derived from identical sources.

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