ISOTOPIC EVIDENCE FOR CLOSED-SYSTEM ANATEXIS AT MIDCRUSTAL LEVELS - AN EXAMPLE FROM THE ACADIAN APPALACHIANS OF NEW-ENGLAND

We have investigated the Sr and O isotope systematics of granitoid and metasedimentary samples from the Central Maine Terrane (CMT) of New E ngland. Granitoid samples were taken from interior and contact zones w ithin the Acadian-aged (approximately 410 m.y.), synmetamorphic and sy ntectonic Kinsman Quartz Monzonite (KQM), which is a member of the New Hampshire Plutonic Series. Metasedimentary samples were taken from Si lurian and Devonian formations hosting the KQM. Initial Sr isotope rat ios (Sr(i)) and deltaO-18 values for the KQM range from 0.70799 to 0.7 1246 and 7.69 parts per thousand to 12.9 parts per thousand, respectiv ely, and Sr(i) and deltaO-18 valUeS of the metasedimentary rocks range from 0.70770 to 0.75008 and 6.20 parts per thousand to 14.1 parts per thousand, respectively. We observe a linear and slightly positive cor relation between Sr(i) and deltaO-18 for interior KQM samples that can be duplicated by a mixing curve calculated for metasedimentary endmem bers, whereas the Sr(i) and deltaO-18 values of contact KQM samples cl uster near the Sr(i) and deltaO-18 values of the metasedimentary rocks with which they are in contact. Mixing calculations provide no eviden ce for a measurable primitive mantle component in either interior or c ontact KQM samples, and we conclude that the Sr-O isotopic composition of the KQM is most likely a reflection of isotopic heterogeneities in herited from a complex package of midcrustal metasedimentary source ro cks. We propose that the KQM is the product of midcrustal partial melt ing 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 midcru stal anatexis may have occurred as a closed-system process, requiring no accompanying mantle-derived magma or above normal mantle heat flow.