Updated Pressure and Temperature Determinations for the Metapelites of the Western Hudson Highlands, NY
Date of Award
Master of Science (MS)
College of Science and Mathematics
Earth and Environmental Studies
Thesis Sponsor/Dissertation Chair/Project Chair
Pelitic sedimentary material was deposited along the eastern margin of North America in the Mesoproterozoic (~1300-1200 Ma) in an active continental arc basin setting and was subsequently metamorphosed during the Ottawan Orogeny (~1080-1030 Ma) as a part of a closing ocean basin between two continents (Volkert and Drake, 1999). The resulting metapelitic gneisses now exposed in the western Hudson Highlands in Harriman State Park, New York were analyzed to determine the sedimentary protolith and the peak metamorphic pressure-temperature (P-T) conditions during the Ottawan Orogeny that affected this region during the final assembly of Rodinia. The protolith, a clay-bearing arkosic sandstone, was determined using light microscopy, which also established thin section mineralogy. Bulk rock analysis was performed with an inductively coupled plasma optical emission spectroscope (ICP-OES). Detailed chemical compositions of the garnet, biotite, muscovite, and plagioclase crystals, found in polished thin section, were obtained using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). These results were then processed through eight different gamet-biotite (g-b) net Fe-Mg exchange thermometers to determine temperature, a gamet-aluminosilicate-silica-plagioclase (GASP) barometer and a gametplagioclase- muscovite-biotite (GPMB-Fe) barometer to estimate the pressure.
Previous electron microprobe analysis (EMP) and thermobarometry performed on the metapelites at various outcrops located in Harriman State Park produced regional peak metamorphic P-T estimates ranging from 3.0-5.5 kbar and 700-750° C (Dallmeyer and Dodd, 1971). A later Dallmeyer paper provides a slightly modified P-T estimate of 4.0-5.5 kbar and 700-750°C (Dallmeyer, 1974). Young (1995) estimated higher peak metamorphic pressures of 6.2-8.0 kbar at similar temperatures of 670-740°C based on the presence of komerupine group minerals (mainly prismatine) in metapelites from the nearby New Jersey Highlands.
Based on a new set of mineral analyses obtained in this research and using more modem calibrations of several different thermobarometers our results suggest peak metamorphic conditions of 5.6 - 7.1 kbar and 700-750°C. This generally supports the original temperature estimates of Dallmeyer (1974) and Young (1995). However, the lack of cordeirite and any konerupine group minerals would further constrain the pressure in the 5.6 - 7.1 kbar range. Results from the four sample locations are as follows:
Lake Welch: LW-1: 756°C (±26°) and 736°C (± 26°) at a pressure of 6.1 kbar (±0.7).
Queensboro Circle: QB-1: 725°C (±28°), QB-4: 736°C (±46°), QB-5: 713°C (±31°) QB-7: 612°C (±47°), QB-7B: 611°C (±14°), QB-7C: 617°C (±32°).
Sebago Beach: SB-1: 661°C (±39°), SB-2: 750°C (±37°), SB-5: 627°C (±49°), SB-6: 752°C (±33°) and 743°C (±61°) at a pressure of 7.1 kbar (±0.6), SB-8: 702°C (±35°).
Torne Brook: TB-1: 741°C (±68°) and 776°C (±21°) at a pressure of 5.6 kbar (±0.7), TB-2: 696°C (±27°), TB-3: 715°C (±57°).
This P-T data is consistent with upper amphibolite to lower granulite facies metamorphism and burial depth in the 20 - 25km range. The depth translates to a geothermal gradient of ~ 30 - 40°C/km which would place these samples nearly directly in line with the gradient for the Barrovian type metamorphism in the Scottish Highlands. This would additionally support the theory of an early mountain building episode as a result of a continent-continent collision in the region.
Sonnenwald, Eric W., "Updated Pressure and Temperature Determinations for the Metapelites of the Western Hudson Highlands, NY" (2013). Theses, Dissertations and Culminating Projects. 987.