Department or Program

Geology

Abstract

The southern half of the Jefferson, NH 7.5’ quadrangle was remapped for the USGS/NHGS StateMap program. The study area contains two key Appalachian lithotectonic elements: 1) Ordovician granitic orthogneisses 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 cone-sheeting associated with caldera collapse. Models for the formation of BHA domes call for arc (?) plutonism associated with the Taconic orogeny and elements of Gander followed by tectonism during the Salinic, Acadian and Neo-Acadian orogenies as the Oliverian Jefferson Dome diapirically rose through its younger cover. This study used U-Th-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 analyzed by LA-ICP-MS to determine U-Th-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 orthogneisses suggesting that some deformation in the Jefferson Dome may actually be older (Ordovician?) than previously thought. Alternately, the syenites may be related to the Salinic Orogeny, have a rheology such that they are resistant to ductile deformation, and/or they are related to a separate plutonic suite (Highlandcroft?) from the foliated orthogneisses. Xenoliths of the Ordovician orthogneisses within the ring complex show consistent deflection of foliations with strike and dip averaging 304°, 62° relative to trends identified distal from the caldera (257°, 43°). The two distinct foliation trends combined with caldera-centric dipping, ring-parallel hornblende veins (278°, 61°) identified cutting only the Oliverian orthogneisses and Silurian syenites suggests a faulted relationship for re-orientation of foliations. The granite ages confirm the Jurassic timing of the cone-sheets and identification of new relative age relationships amongst the PRCC units suggest two temporally separate phases of caldera collapse and intrusion. The combination of intense breciation and consistent foliation among Oliverian orthogneisses within the ring complex suggests a less forceful yet complex pattern of cone-sheet intrusion during the second phase of collapse. The new rhyolite age suggests a third and final dominantly extrusive (?) phase of caldera collapse near contemporaneous with the intrusive cone-sheeting of the first and second phases.

Level of Access

Open Access

First Advisor

Eusden, Jr., Dykstra

Date of Graduation

Spring 5-2016

Degree Name

Bachelor of Science

Number of Pages

152

Components of Thesis

1 PDF

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