THE PETROGENESIS OF THE HIGHLANDCROFT AND OLIVERIAN PLUTONIC SUITES, NEW HAMPSHIRE: IMPLICATIONS FOR THE STRUCTURE OF THE BRONSON HILL TERRANE

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. Isotopic data, especially ε_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 identical, both are members of the same arc complex. Amphibole geobarometry suggests a higher emplacement pressure for the Oliverian suite, ~ 4.6 Kbars for the Jefferson Batholith of the Oliverian Plutonic Suite compared to ~ 2.7 Kbars of the Lost Nation pluton of the Highlandcroft suite. Amphibole-plagioclase and zircon saturation geothermometry give temperatures of 750 - 800^o for both suites. Bulk-rock Zr concentrations are not supportive of inheritance in either suite and the presence or absence of inherited zircons may not distinguish one suite from the other.