2 Timing and Nature of Deformation in the Adirondack Mountains I

Monday, 21 March 2016
8:00 AM - 12:00 PM
Meeting Room 6

PRE OR SYN-GRENVILLIAN DEFORMATION OF THE PARAUTOCHTHONOUS BELT, CENTRAL GRENVILLE PROVINCE (MANICOUAGAN RESERVOIR AREA)?

Paper number: 2-4
Program: Symposia Sessions
Time: 8:50 AM – 9:10 AM

Authors

Abstract

Evidence for pre-Grenvillian deformation is largely preserved in the western Grenville Province, whereas earlier structures are regionally transposed in the eastern Grenville Province. New field-based structural, metamorphic and geochronological data from the Parautochthonous Belt in the Central Grenville Province constrain this contradiction.

D1 and D2 structures indicate reverse, top-to-the-NNE shearing with a SSE and SSW plunging lineation, respectively. D1 is folded by D2 and becomes completely transposed towards the overlying Allochthonous Belt. Local D3 structures indicate oblique-normal, down-to-the-SW shearing. We constrained the age of D2 and D3 by combining results of laser ablation (LA-ICP-MS) and chemical abrasion isotope dilution (CA-ID-TIMS) U-Pb geochronology on zircon. Group 2 zircon in an early-D2 leucosome yielded a weighted mean (WM) 206Pb/207Pb date of 1009 ± 18 Ma (LA-ICP-MS) and, two analyses from tips yielded a WM 206Pb/238U date of 1002 ± 2 Ma (CA-ID-TIMS). Group 1 zircon yielded a WM 206Pb/207Pb date of 992 ± 22 Ma (LA-ICP-MS) and, one analysis of a tip yielded a 206Pb/238U date of 986 ± 4 Ma (CA-ID-TIMS). A late D2 pegmatite yielded a WM 206Pb/207Pb date of 961 ± 22 Ma (LA-ICP-MS). D2 is thus bracketed between 986 and 961 Ma. This timing is synchronous, within error, with a syn-D3 leucosome that yielded a WM 206Pb/238U date of at 993 ± 3 Ma (CA-ID-TIMS).

The age of D1 can be inferred by linking deformation, metamorphism and geochronology. S1 is defined by kyanite, a first generation of biotite and kyanite-bearing leucosome. The oldest 1002 Ma zircon group in the early-D2 leucosome is depleted in HREE, suggesting that growth occurred after or synchronously with a phase that preferentially sequestered these elements, most likely garnet. These observations are most compatible with D1 forming along a prograde path in the kyanite field, first crossing the muscovite-dehydration melting reaction to produce the kyanite-bearing leucosome, then the biotite dehydration-melting reaction to produce garnet and zircon depleted in HREE. This latter melt would then have crystallized on the retrograde path during D2.

D1 is therefore most likely Grenvillian, and there is no evidence for pre-Grenvillian deformation or metamorphism in this part of the Parautochthonous Belt.

Geological Society of America Abstracts with Programs. Vol. 48, No. 1

doi: 10.1130/abs/2016NE-272455

© Copyright 2016 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.