1 Geology, Structure, Geochronology, and Tectonics in Southern New England I: An Eclectic Session in Honor of Robert P. Wintsch

Sunday, 17 March 2019
8:00 AM - 12:00 PM
New Hampshire Room

EARLY SILURIAN IGNEOUS U-PB ZIRCON AGES IN COVER ROCKS CONSTRAIN TIMING OF TECTONIC EVENTS WEST OF THE WATERBURY DOME, CT

Paper number: 1-3
Program: Symposia Sessions
Time: 8:25 AM – 8:45 AM

Authors

Abstract

New SHRIMP U-Pb zircon ages from a meta-igneous rock in the basal member of the Taine Mountain Formation and the late-tectonic Nonewaug Granite provide constraints on pre-Acadian deformation within cover rocks west of the Waterbury dome. Zircon grains separated from a medium-gray, fine-grained, weakly foliated but unlayered, titanite-hornblende-biotite-quartz-plagioclase gneiss, mapped as a metaigneous rock of intermediate composition, yielded an age of 442 ± 3 Ma. The zircon grains are euhedral, have aspect ratios of ~8:1, exhibit oscillatory zoning, and lack metamorphic rims. Whole rock geochemistry indicates the rock is andesitic/dioritic in composition. The unit as mapped has a limited extent but occurs adjacent to amphibolite in a NW-terminating belt of meta-igneous rocks and paragneiss west of the Waterbury dome that is surrounded to the NE, N, and W by rusty pelitic schist (basal Taine Mountain Fm. of Rodgers, 1985). The zircon morphology suggests rapid crystallization, and the 442 Ma age is interpreted as dating intrusion of the protolith during a late-Taconic (D2) phase of regional metamorphism and tectonism.

The Nonewaug Granite is a medium- to coarse-grained, weakly-foliated to massive, muscovite-biotite-microcline-quartz-plagioclase granite that intruded well-foliated staurolite-kyanite-grade schist (Ratlum Mtn Fm. of Rodgers, 1985) and complexly-deformed, migmatitic, kyanite- and Kspar-bearing (and staurolite-absent) schist of above-mentioned basal Taine Mtn. affinity. Zircons separated from the Nonewaug have oscillatory-zoned dark CL cores with Th/U ratios of > 0.1 typical of igneous zircon, and yield an age of 431± 5 Ma, interpreted to be the time of crystallization.

The Nonewaug Granite crosscuts a contact between the staurolite-kyanite and kyanite-Kspar-bearing rocks that is interpreted as a low-angle thrust (decollement) of pre- or early-D2 age. The granite forms foliation-parallel, weakly-foliated sills in S2/L2-deformed Ratlum Mtn. Fm. schist, suggesting late-D2 intrusion of the Nonewaug at 431 Ma. Together the field relationships and geochronology indicate that D2 deformation, including tectonic juxtaposition and high-grade metamorphism, occurred between 442 and 431 Ma in the rocks west of the Waterbury dome.

Geological Society of America Abstracts with Programs. Vol. 51, No. 1

doi: 10.1130/abs/2019NE-327949

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.