Contrib Mineral Petrol (1996) 124:126 —138 Springer-Verlag 1996 Alison S. Lathrop · Joel D. Blum · C. Page Chamberlain Nd, Sr and O isotopic study of the petrogenesis of two syntectonic members of the New Hampshire Plutonic Series Received: 13 September 1994 / Accepted: 31 January 1996 Abstract Nd, Sr and O isotope systematics were used to investigate the petrogenesis of two adjacent plutons of the Bethlehem Gneiss (BG) and the Kinsman Quartz Monzonite (KQM), exposed within the Central Maine Terrane (CMT) of New England. Both are Acadian- aged (413 Ma) synmetamorphic and syntectonic members of the New Hampshire Plutonic Series (NHPS). Potential source rocks analyzed for this study include Silurian and Devonian metasedimentary rocks of the CMT, and Ordovician metasedimentary rocks and granitic gneisses of the Bronson Hill Anticlinorium (BHA), which border the CMT to the west. The (413), (413) and O values for the KQM range from 56.3 to 120.0, 2.8 to !6.4, and 7.6 to 12.9, respectively; values for the BG range from 7.4 to 144.7, 0.6 to !9.3, and 8.3 to 11.3, respectively; and values for possible source rocks range from 38.1 to 654.2, !10.7 to 5.4, and 6.2 to 14.1, respectively. Both the BG and KQM have extremely heterogeneous initial isotopic compositions consistent with mixing of multiple crustal source rocks, and neither cont- ains a volumetrically significant (i.e., 510%) mantle- derived component. Overlapping values of (413), (413) and O values for both the BG and KQM samples resemble values for metasedimentary host rocks of the CMT and BHA. We observe no systematic correlations between and values for either the BG or the KQM. The and O values for the BG do not form any simple mixing trends, nor is there any direct correlation between the isotopic com- positions of contact BG samples and their adjacent A.S. Lathrop ( ) · J.D. Blum · C.P. Chamberlain Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA Present address Department of Earth Sciences, Millersville University, P.O. Box 1002, Millersville, PA 17551-0302, USA Editorial responsibility: J. Patchett host rocks, in contrast to our observations for the KQM (Lathrop et al. 1994). We propose that the KQM and BG magmas were generated through anatexis of metasedimentary rocks from both the BHA and CMT in response to crystal thickening during the Acadian orogeny. Melting may have been initiated within CMT metasediments in response to high heat production in these mid-crustal rocks combined with crustal thicken- ing, whereas melting of BHA rocks with normal crustal heat production, which were located at lower-crustal levels than CMT rocks, is likely to have been driven by crustal thickening alone. Following upward advection of mobile BHA magmas, BHA- and CMT-derived magmas may have mingled during complex Acadian deformation in the CMT, thus accounting for the iso- topic similarities we observe between the BG and the KQM. Introduction Development of orogenic belts in collisional environ- ments frequently includes the production of large vol- umes of mid-crustal granitic rocks, and the origin of these magmas remains controversial (e.g., Strong and Hanmer 1989; Barton 1990). Recent heat-flow models suggest that without a significant lower-crustal- or mantle-derived component, anomalous thermal condi- tions such as high mantle heat flux or high mid-crustal concentrations of heat-producing radioactive elements are required to produce in situ anatectic granitic mag- mas at mid-crustal levels (England and Thompson 1984; Wickham and Oxburgh 1987; Zen 1988; Cham- berlain and Sonder 1990). Petrogenetic investigation of granitic rocks and surrounding wall-rocks within a ter- rane can provide both a test of thermal modeling and a possible constraint upon the thermal evolution of the orogenic belt in which the granitic rocks are exposed. The Sr, Nd, and O isotopic compositions of mid- to upper-crustal metasedimentary material differ