28 The Grenville Orogen in Eastern North America

Saturday, 21 March 2020
8:00 AM - 12:00 PM
Regency Ballroom B

PRIMARY CO2 FLUID INCLUSIONS FROM THE TRANSITION ZONE OF THE GORE MOUNTAIN GARNET DEPOSIT, CENTRAL ADIRONDACKS, NEW YORK, USA

Paper number: 28-4
Program: Symposia Sessions
Time: 9:00 AM – 9:20 AM

Authors

Abstract

The Gore Mtn. garnet deposit is characterized by megacrystic garnet hosted by amphibolite and hornblendite. A transition zone between garnet ore and metagabbro contains 0.5 to 1 cm garnets accompanied by coarse-grained plagioclase, hornblende and orthopyroxene. Several researchers have proposed that influx of H2O fluid along the boundary between metagabbro and metasyenite led to garnet formation. However, no aqueous fluid inclusions have ever been described in Gore Mtn. garnet. Furthermore, recent rehomogenization of cristobalite + kumdykolite-bearing polyphase inclusions as well as the confirmation of primary glass (Ferrero et al., 2019, AGU abs) in Gore Mtn. megacrystic garnets favors an origin in the presence of tonalitic to trondhjemitic melt.

To date, only polyphase crystalline inclusions have been reported from Gore Mtn. megacrystic garnet. However, transition zone garnet from the number eight pit of the Barton Mine contains small (10 to 30 μm), isolated, fluid inclusions having a negative rhombohedral crystal shape, and are inferred to be of primary origin. These fluid inclusions contain one phase at 20°C, however, birefringent solids were observed in some. Upon cooling, all fluid inclusions froze between -70°C and -88°C. Melting of CO2 solid occurred from -57.7°C to -56.6°C (n = 51), indicating CO2 as a dominate phase but not the only volatile. The inclusions homogenized by CO2 vapor bubble disappearance between -11.2°C and +5.0°C, but the average and standard deviation of 49 measurements were -3.0°C ± 3.7°C. Calculated densities (assuming pure CO2) over the standard deviation range are 0.965 to 0.923 gms/cm3.

Preliminary laser Raman analysis confirms the presence of CO2 but also reveals small amounts of CH4. Birefringent solids show peaks for Mg-calcite and pyrophyllite. These phases are interpreted as step-daughter phases, formed after trapping by a reaction between the host garnet and the inclusion fluid. No liquid H2O was detected but the presence of pyrophyllite confirms that H2O was at one time a fluid component, at least in the transition zone at Gore Mtn. We infer that H2O entering the metagabbro generated tonalitic to trondhjemitic melt that aided garnet growth. The primary CO2-rich fluid inclusions in the transition zone may represent trapping of a distal fluid depleted of its H2O component.

Geological Society of America Abstracts with Programs. Vol. 52, No. 2

doi: 10.1130/abs/2020SE-344592

© Copyright 2020 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.