Published:January 01, 2004
Zircon grains extracted from anatectites in the New Russia gneiss complex have been dated to establish the timing of very high-temperature metamorphism in the northeastern Adirondack Highlands, southwestern Grenville Province. The isotopic and chemical systematics of the zircons studied indicate that (1) zircon grains extracted from anatectic segregations in metagabbro are isotopically homogeneous, implying that only zircon that grew during anatexis is present, and (2) zircons from an anatectic charnockite are complex and preserve evidence of a multistage thermal evolution for the gneiss. This complex history can be deciphered by combining knowledge acquired from backscatter electron images and electron microprobe chemical analyses to guide selection of grain components for isotopic dilution thermal ionization mass spectrometry (ID-TIMS) U-Pb geochronology. The analysis indicates that melt from the metagabbro crystallized at 1054 ± 0.4 (2σ) Ma at T = 915 ± 50 °C. Zircon from the charnockitic gneiss yielded ages that date an inherited component at 1143 ± 2 Ma (207Pb/206Pb); the emplacement of the protolith at 1112 ± 2 Ma; and the crystallization of the anatectic melt at 1040 ± 1 Ma (T ≥ 825 ± 50 °C). Late, retrograde metamorphic zircon yielded a range of ages including a concordant age at 1003 ± 4 Ma.
The method described allows precise dating of anatexis that typically marks the metamorphic peak in granulite-facies terranes and so can be used to constrain model pressure-temperature-time (P-T-t) paths. The timing of anatexis and a previously reported age of 1026 Ma for andraditic garnets from skarn adjacent to the Marcy meta-anorthosite establish rapid, nearly isobaric cooling from 915–750 °C in ∼30 m.y.