29 Petrologic, Structural, and Tectonic Interpretations in Northern New England I: A Session Honoring the Work of Jo Laird and Peter J. Thompson

Monday, 19 March 2018
8:00 AM - 12:00 PM
Emerald Ballroom III

AMPHIBOLES AND ISOTOPES BELOW AND ABOVE THE PROSPECT ROCK THRUST, CENTRAL AND NORTHERN VERMONT: INTERCONNECTED AND/OR DISCONNECTED HISTORIES OF MAFIC SUBDUCTION ZONE ROCKS?

Paper number: 29-12
Program: Theme Sessions
Time: 11:15 AM – 11:35 AM

Authors

Abstract

Metamorphic and geochronological studies by Jo Laird, and mapping and structural studies by Peter Thompson, are foundational to understanding Taconian subduction-related tectonometamorphic processes in Vermont. Interpretation of their works suggests that the Prospect Rock thrust (PRT) is a key marker of high pressure amphiboles associated with Taconian subduction, and that lithologic and metamorphic contrasts across the PRT provide unique opportunities for analyzing the tectonic evolution of the Laurentian margin and Iapetus Ocean basin.

This contribution interprets Nd isotopic data from mafic whole-rocks above and below the PRT in the context of existing amphibole compositions, modal space paths, and geochemical data to determine if magmatic origins have bearing on subsequent metamorphic pressure-temperature-time paths. This may help resolve tectonometamorphic links between sodium and calcium-sodium amphibole-bearing mafic bodies throughout central and northern Vermont. Magmatic similarities between mafic bodies would be more consistent with a shared subduction history and imply that barroisite-bearing amphibolites and greenstones may represent different parts of a coherent terrane-scale P-T-t loop. Magmatic contrasts on the other hand favor an interpretation where individual mafic bodies preserve unique P-T-t paths that are not part of the same metamorphic loop.

Mafic rocks below the PRT associated with medium-high to high pressure facies amphiboles preserve lower age-corrected εNd (t) values than mafic rocks above the PRT in the Rowe/Prospect Rock slice (R/PRS). A range of εNd (t) values is observed in mafic rocks of the R/PRS and there does not appear to be a correlation with depth within the slice. Contrasting εNd (t) values and amphibole compositions for mafic rocks intercalated over 10s of meters within the R/PRS supports a petrologic model where individual mafic units preserve independent P-T-t paths. Comparable amphibole compositions are observed in mafic rocks above and below the PRS that display distinct εNd (t) values, suggesting that P-T-t paths may be locally independent of magmatic origin. Perhaps the most appropriate terrane-scale tectonometamorphic model is one that allows for both interconnected and disconnected P-T-t paths and loops for mafic rocks of diverse origins.

Geological Society of America Abstracts with Programs. Vol. 50, No. 2

doi: 10.1130/abs/2018NE-311266

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