Northeastern Section - 49th Annual Meeting (23–25 March)

Paper No. 1
Presentation Time: 8:00 AM-12:00 PM


HONSBERGER, Ian W., Department of Earth Sciences, University of New Hampshire, Durham, NH 03824, COISH, Raymond, Geology Department, Middlebury College, Middlebury, VT 05753 and LAIRD, Jo, Earth Sciences, University of New Hampshire, James Hall, Durham, NH 03824,

Whole rock major and minor element geochemistry of barroisite-, tschermakite- and magnesio-hornblende-bearing greenstone within the Rowe-Hawley Zone, Vermont is utilized in both geothermobarometric modeling and tectonic discrimination to describe a Wilson Cycle. Metamorphosed tholeiitic basalts of oceanic origin record a chemical history beginning in the Late Precambrian during the final breakup of Rodinia and culminating in the Ordovician with subduction of the Iapetus Ocean. Ti-Zr-Y, V-Ti, Zr/Y-Zr and Cr-Y igneous tectonic discrimination indicates that these metabasalts formed from a magma source transitional between MORB and island arc tholeiite, implying rift-related generation at the onset of Iapetus Ocean development. Despite overarching geochemical similarities between greenstones in central VT, those not in contact with ultramafics resemble more closely island arc basalts, as they are higher in Cr and lower in Ti, Zr and Y, than other greenstones that are members of fault bounded subduction packages.

Original spatial distribution of source region between the two types of basalts influenced subsequent metamorphism along P-T-t subduction and exhumation paths. Bulk chemistry and amphibole compositional data integrated in Perple_X (Connolly, 2005) through pseudosection and isopleth calculations indicate that Taconian subduction metamorphism of an ultramafic-mafic-pelitic unit in Stockbridge reached 535-540°C and exceeded 8.8 kbar. Under these types of conditions, chlorite-zoisite-phlogopite-rutile and quartz are stable with two amphiboles indicating miscibility along the subduction zone at about 26 km depth. Exhumation of these mafic rocks and polymetamorphism at lower pressures and temperatures is predicted theoretically by decreasing NaM4SiCa-1Al-1 and AlVIAlIVMg-1Si-1 substitution from core to rim in amphiboles preserving three distinct chemical zones. Projections of electron microprobe data using Na and Al compositional isopleths calculated in Perple_X indicate metamorphism for one amphibole equilibria at 445-475°C and 6.3-7 kbar, corresponding to 20-21 km depth. A final phase of exhumation is marked by actinolite growth at ~10 km depth during greenschist facies metamorphism at 400°C and 3 kbar, which may correspond to Acadian intraplate deformation.