ND, SR AND O ISOTOPIC STUDY OF THE PETROGENESIS OF 2 SYNTECTONIC MEMBERS OF THE NEW-HAMPSHIRE PLUTONIC SERIES

Nd, Sr and O isotope systematics were used to investigate the petrogen esis of two adjacent plutons of the Bethlehem Gneiss (BG) and the Kins man Quartz Monzonite (KQM), exposed within the Central Maine Terrane ( CMT) of New England. Both are Acadian-aged (approximate to 413 Ma) syn metamorphic and syntectonic members of the New Hampshire Plutonic Seri es (NHPS). Potential source rocks analyzed for this study include Silu rian and Devonian metasedimentary rocks of the CMT, and Ordovician met asedimentary rocks and granitic gneisses of the Bronson Hill Anticlino rium (BHA), which border the CMT to the west. The epsilon(Sr)(413), ep silon(Nd)(413) and delta(18)O values for the KQM range from 56.3 to 12 0.0, 2.8 to -6.4, and 7.6 parts per thousand to 12.9 parts per thousan d, respectively; values for the BG range from 7.4 to 144.7, 0.6 to -9. 3, and 8.3 parts per thousand to 11.3 parts per thousand, respectively ; and values for possible source rocks range from 38.1 to 654.2, -10.7 to 5.4, and 6.2 parts per thousand to 14.1 parts per thousand, respec tively. Both the BG and KQM have extremely heterogeneous initial isoto pic compositions consistent with mixing of multiple crustal source roc ks, and neither contains a volumetrically significant (i.e., greater t han or equal to 10%) mantle-derived component. Overlapping values of e psilon(Nd)(413), epsilon(Sr)(413) and delta(18)O values for both the B G and KQM samples resemble values for metasedimentary host rocks of th e CMT and BHA. We observe no systematic correlations between epsilon(N d) and epsilon(Sr) values for either the BG or the KQM. The epsilon(Sr ) and delta(18)O values for the BG do not form any simple mixing trend s, nor is there any direct correlation between the isotopic compositio ns of contact BG samples and their adjacent host rocks, in contrast to our observations for the KQM (Lathrop et al, 1994). We propose that t he KQM and BG magmas were generated through anatexis of metasedimentar y rocks from both the BHA and CMT in response to crystal thickening du ring the Acadian orogeny. Melting may have been initiated within CMT m etasediments in response to high heat production in these mid-crustal rocks combined with crustal thickening, whereas melting of BHA rocks w ith normal crustal heat production, which were located at lower-crusta l levels than CMT rocks, is likely to have been driven by crustal thic kening alone. Following upward advection of mobile BHA magmas, BHA- an d CMT-derived magmas may have mingled during complex Acadian deformati on in the CMT, thus accounting for the isotopic similarities we observ e between the BG and the KQM.