Sm-Nd isotope systematics and the derivation of granitic pegmatites
in southwestern Maine

Paul B. Tomascak*, Eirik J. Krogstad** and Richard J. Walker

Isotope Geochemistry Laboratory, Department of Geology,
University of Maryland, College Park, MD 20742 USA

Canadian Mineralogist 1998, v. 36, p. 327-337

* corresponding author, currently at: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd., NW, Washington, D.C. 20015 USA
** current address: Dansk Lithosfærcenter, Øster Voldgade 10, 1350 København K, Denmark

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 pegmatite sources. 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) = -2.2 to -1.4, (2) the Standpipe Hill series, distinguished by an enrichment in rare earth element minerals, displays epsilon Nd(270 Ma) = -3.4 to -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)= -3.9 to -3.7]. The Northern series pegmatites have Nd isotopic characteristics similar to both migmatites that they intrude [epsilon Nd(270 Ma) = -2.9 to +0.8], and fine-grained biotite granites located ca. 15 km east of the pegmatites [epsilon Nd(270 Ma) = -2.5 to -1.7]. The isotopic data demonstrate that spatially- and temporally-related pegmatites need not be derived from identical sources.

Fig. 1a, Fig. 1b, Fig. 2, Fig.3, Fig. 4

U-Pb Monazite Geochronology of Granitic Rocks from Maine:
Implications for Late Paleozoic Tectonics in the Northern Appalachians

Paul B. Tomascak, Eirik J. Krogstad and Richard J. Walker

Isotope Geochemistry Laboratory, Department of Geology,
University of Maryland, College Park, MD 20742 USA

The Journal of Geology, 1996, v. 104, p. 185-195

ABSTRACT

Crystallization ages of monazites from peraluminous granites and granitic pegmatites in southwestern Maine constrain the timing of final ductile motion of a major shear zone to approximately 280 Ma. U-Pb monazite ages of granite at Brunswick (278 ± 2 Ma) and evolved granitic pegmatites in Topsham (275-269 Ma) provide evidence that synkinematic intrusion was followed by final emplacement at the end of ductile deformation. These granitic rocks were evidently generated during the final high temperature event experienced by this portion of the Appalachian orogen. During this time, the host rocks record initial rapid cooling (c. 16oC/Ma) followed by slower cooling (c. 4oC/Ma) into the Mesozoic. The similarity of ages of granite in Brunswick and pegmatites in Topsham permit the rocks to be related to the same magmatic system. The spatially-associated Sebago batholith, the largest exposed pluton in New England, has a concordant U-Pb monazite age of 293 ± 2 Ma. Thus the Sebago batholith cannot be the source of regional high-temperature metamorphism during the Mississippian, as previous models have suggested. These disparate ages also do not permit the Sebago batholith to be genetically related to granite magmatism in the Brunswick area, although they most likely were generated during different parts of the same tectonic event.