Isotopic evidence for closed‐system anatexis at midcrustal levels: An example from the Acadian Appalachians of New England

Abstract

We have investigated the Sr and O isotope systematics of granitoid and metasedimentary samples from the Central Maine Terrane (CMT) of New England. Granitoid samples were taken from interior and contact zones within the Acadian‐aged (∼410 m.y.), synmetamorphic and syntectonic Kinsman Quartz Monzonite (KQM), which is a member of the New Hampshire Plutonic Series. Metasedimentary samples were taken from Silurian and Devonian formations hosting the KQM. Initial Sr isotope ratios (Sri) and δ18O values for the KQM range from 0.70799 to 0.71246 and 1.6‰ to 12.9‰, respectively, and Sri and δ18O values of the metasedimentary rocks range from 0.70770 to 0.75008 and 6.2‰ to 14.1‰, respectively. We observe a linear and slightly positive correlation between Sri and δ18O for interior KQM samples that can be duplicated by a mixing curve calculated for metasedimentary endmembers, whereas the Sri and δ18O values of contact KQM samples cluster near the Sri and δ18O values of the metasedimentary rocks with which they are in contact. Mixing calculations provide no evidence for a measurable primitive mantle component in either interior or contact KQM samples, and we conclude that the Sr‐O isotopic composition of the KQM is most likely a reflection of isotopic heterogeneities inherited from a complex package of midcrustal metasedimentary source rocks. We propose that the KQM is the product of midcrustal partial melting that was initiated due to excess thermal energy from the decay of anomalously high concentrations of heat‐producing elements in Silurian source rocks within the CMT. Because we see no isotopic evidence for a lower‐crustal or mantle component in the KQM, we suggest that midcrustal anatexis may have occurred as a closed‐system process, requiring no accompanying mantle‐derived magma or above normal mantle heat flow.