Sm-Nd Isotope Systematics of Adirondack Lithologies: A Case for Rapid Granulite Exhumation

The difference between ages of anorthosite crystallization and onset of granulite metamorphism in the Adirondacks is now considered much less than previously assumed. Our results indicate a maximum difference of 40Ma, between new U-Pb ages of zircons and the formation of garnets in Snowy Mountain granulites. Here we provide additional Sm-Nd data of various Adirondack lithologies to make a case of coherent exhumation history of the Adirondack granulites. These data include: Diana Syenite Complex (19 wholerock isochron age, 1120+/-57 Ma, initial εNd=3.2); Jay Mountain metagabbro (8 wholerock isochron age, 1099+/-42 Ma, initial εNd=3.5); Snowy Mountain Dome (10 wholerock and garnet isochron age, 1095+/-7 Ma, initial εNd=2.5, 5 mangerite-charnokite initial εNd=2.7-5.1); Gore Mountain garnet-amphibolite (7 internal mineral isochron age, 1059 +/-19 Ma, initial εNd=3.3); Willisboro wollastonite skarn (11 internal mineral isochron age, 1035+/-40, initial εNd=2.9).Additional mangerite-charnokite from the central Adirondacks initial εNd at 1140 Ma ranges from 1-5. An interesting implication of the skarn data is their positive initial εNd value indicating substantive interchange of the anorthositic Nd (REE) with the sedimentary carbonate by hydrothermal fluids in a shallow crustal process.

We interpret the above Sm-Nd data by anorthosite emplacement at 25 Km depth at 1140 Ma, exhumation starting soon after, continuing through lower and upper crust, culminating with contact wollastonite metamorphism at 1035 Ma. Our data are also consistent with the associated mangerite-charnokite bodies enveloping the anorthositic domes as crustal melts, coeval with anorthosite emplacement, and heating effects of the ascending anorthosite body. From these data we estimate Adirondack exhumation rate of 0.28 km/Ma and cooling rate of 5.5^oC/Ma, much faster than previously assumed.