Supported in part by grant EAR-0106890 from the National Science Foundation
The Adirondack Highlands are an exposed section of the mid-crust that dates from 1.0-1.3 Billion years ago. This research focuses on constraining the metamorphic conditions that these rocks experienced by using the a variety of geochemical and petrologic tools.
The results of oxygen isotope thermometry in Adirondack quartzites yields metamorphic temperatures 700-800°C, consistent with granulite-facies mineral assemblages. Samples from the Irving Pond quartzite record D(Qtz-Grt)=2.58±0.58‰, corresponding to peak metamorphic conditions of 733±37°C. This agrees well with some new estimates from garnet-biotite exchange thermometry, and similar quartz-garnet temperatures are obtained from three other localities on the periphery of the Adirondack Highlands. All of these temperatures are higher than previous regional temperature estimates. Peak metamorphic temperatures are preserved in rocks with varying quartz:feldspar ratios and garnet sizes. Similar fractionations from all of these slowly-cooled rocks suggest slow rates of oxygen diffusion in quartz, feldspar, and garnet, consistent with the results of anhydrous diffusion experiments and low water fugacities in Adirondack quartzites. These results suggest low water activity in most (if not all) samples of this suite during cooling after metamorphism.
These quartzites contain detrital zircon with ubiquitous metamorphic overgrowths. Zircon cores are resorbed, preserve a variety of internal zoning styles and inclusions, and have ages of 1.3 to 2.7 Ga. Metamorphic overgrowths formed in the quartzites during the Shawinigan orogeny (1.20-1.14 Ga). The average volume of overgrowths has a positive correlation between zircon growth and melt productivity during metamorphism. Crystal-size distributions suggest zircon coarsening by the dissolution of small crystals and Zr transfer via a partial melt, and thus zircon overgrowths date anatexis. These results have implications for provenance studies, as dissolution of small zircons could bias age spectra of metasedimentary rocks. These results have implications for provenance studies, as dissolution of small zircons could bias age spectra of metasedimentary rocks. Matt Quinan (’17) and I have begun to explore these ideas in a project where we are dating zircon from Adirondack and Quebec quartzites. (*indicates student author)
Darling, RS, and Peck, WH, 2016, Metamorphic conditions of Adirondack rocks: Adirondack Journal of Environmental Studies, v. 21, p. 61-79.
Peck, WH, and Quinan, MP, 2016, Links between the Adirondacks and the Morin terrane: New evidence from geochronology: Geological Society of America Abstracts with Programs, v. 48(2), doi: 10.1130/abs/2016NE-272318.
Peck, WH, McLelland, JM, Bickford, ME, *Nagle, AN, *Swarr, GJ, 2010, Mechanism of zircon overgrowth formation of a granulite-facies quartzite, Adirondack Highlands, Grenville Province, New York: American Mineralogist, v. 95, p. 1796-1806.
Peck, WH, and Valley, JW, 2004, Garnet-quartz oxygen isotope thermometry in the southern Adirondack Highlands (Grenville Province, New York): Journal of Metamorphic Geology, v. 22, p. 763-773
Peck, WH, and Valley, JW, 2003 Oxygen isotope thermometry of quartzites, southern Adirondack Highlands, Geological Society of America Abstracts with Programs, v.35, n. 7., p. 592.
Peck, WH, Valley, JW, and Graham, CM, 2003, Slow oxygen diffusion rates in igneous zircons from metamorphic rocks: American Mineralogist, v. 88, p. 1003-1014.