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The petrogenesis of the Highlandcroft and Oliverian Plutonic Suites, New Hampshire: Implications for the structure of the Bronson Hill terrane

^* Department of Geological Sciences, Brigham Young University, Provo, Utah 84602
^** Keck Isotope Laboratory, Department of Earth Sciences, A232 Earth and Marine Sciences, University of California, Santa Cruz, California 95064

dorais{at}byu.edu

Both Laurentian and peri-Gondwanan origins for the Oliverian and Highlandcroft Plutonic Suites of New Hampshire have been proposed. A peri-Gondwanan origin places the Red Indian Line, the principle Iapetus suture, to the west of the Bronson Hill Anticlinorium and requires the plutons to be allochthonous. In contrast, a Laurentian margin emplacement setting places the Red Indian Line farther to the east in New Hampshire. The combination of _Nd and ²⁰⁷Pb/²⁰⁴Pb values are most compatible with the suites representing mixtures of arc-derived magmas and Laurentian crust, indicating that both suites were derived to the west of the Red Indian Line. The tectonic setting for the arc probably was a westerly dipping subduction zone under the Laurentian margin, resulting from a polarity flip after the collision of the Shelburne Falls arc during the Taconic orogeny. Bulk-rock major and trace elements, as well as Nd, Sr, and Pb isotopic results, indicate that the Highlandcroft and Oliverian Plutonic Suites of New Hampshire are both members of the same arc complex.