STRUCTURE AND GEOCHRONOLOGY OF THE JURASSIC PLINY RANGE CALDERA COMPLEX: 7.5’ JEFFERSON QUADRANGLE, NORTHERN NEW HAMPSHIRE

The southern part of the Jefferson, NH 7.5’ quadrangle was recently remapped for the USGS/NHGS StateMap program. The study area contains two key Appalachian lithotectonic elements: 1) Ordovician granitic gneisses of the Jefferson Oliverian Dome in the Bronson Hill Anticlinorium (BHA); and 2) Jurassic igneous intrusive rocks of the Pliny Range Caldera Complex (PRCC) that were emplaced by ring-diking associated with caldera collapse. Models for the formation of BHA domes call for arc (?) plutonism associated with the Taconic orogeny and elements of Ganderia followed by tectonism during the Salinic, Acadian and Neo-Acadian orogenies as the Oliverian Jefferson Dome diapirically rose through its younger cover.

This study uses U-Pb geochronology and detailed mapping of a 6-acre clear-cut with 50% exposed bedrock located within the PRCC to address relative-age and structural ambiguities in the PRCC and BHA. Mapping in the quadrangle also revealed a previously unrecognized tuffaceous rhyolite, part of the PRCC. Zircons from the rhyolite and two PRCC granites were analysed by LA-ICP-MS (GeoSep Services, Moscow, ID) to determine U-Pb crystallization ages. The granites are 187.3±1.1 Ma and 188.3±1.0 Ma with the latter cutting the former. Within error, the intrusions are essentially contemporaneous. The rhyolite yielded two grain-age populations with concordant ages of 184.9±2.3 Ma and 430.6±2.6 Ma. The younger age is believed to mark the crystallization of the rhyolite and the older the crystallization of a suite of xenocrystic zircons from the older coarse syenite country rock, cut by the rhyolite.

The syenite was thought to be Ordovician in age but the new zircon ages, although indirect, suggest a Silurian age. The syenite is not visibly deformed yet cuts strongly foliated Ordovician Oliverian gneisses suggesting that some deformation in the Jefferson Dome may actually be older (Ordovician?) than previously thought. The granite ages confirm the Jurassic timing of the ring dikes. Xenoliths of the Ordovician gneisses and Silurian syenites within the ring dike complex show only slight deflection of foliations suggesting a less forceful yet complex pattern of ring dike intrusion. The discovery and new age of the rhyolite suggests the last phase of ring diking was extrusive and near contemporaneous with the intrusive ring dikes.