Petrography and geochemistry of hydrothermally altered volcanic rocks metamorphosed at granulite-facies conditions: an example from the central Grenville province

Hindemith, Marisa Ann (2014) Petrography and geochemistry of hydrothermally altered volcanic rocks metamorphosed at granulite-facies conditions: an example from the central Grenville province. Masters thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (80MB)

Abstract

A layered bimodal sequence (LBS) was recently recognized in the Canyon domain in the central Grenville Province, and inferred to be the extrusive products of riftrelated magmatism at ~1.25 Ga. The LBS consists of alternating felsic and mafic rocks with, locally, pre-metamorphic hydrothermally altered zones (HAZ), and were metamorphosed to mid-P granulite-facies conditions during the Grenvillian orogeny (~1 Ga). In the HAZ, characteristic felsic rocks are pink and white gneisses, grading into varieties of aluminous gneisses and garnetites. This thesis is a petrographic and geochemical study of these rocks, and provides new insights on the protoliths, the hydrothermal alteration, and the subsequent metamorphic overprint. The most informative microstructures were observed in the aluminous gneisses, as for instance: (a) relics of bipyramidal quartz phenocrysts indicative of a volcanic precursor; (b) aluminous nodules, seams of sillimanite, and concordant quartz veins representing relict former pathways of hydrothermal fluid; and (c) quartz-K-feldsparplagioclase- garnet-sillimanite-biotite mineral assemblages with textural evidence of partial melting, and consistent with a mid-P granulite-facies metamorphic overprint. In addition, slightly elevated contents of Mn in garnet, Zn in spinel, and Ba in biotite and Kfeldspar in some rocks are consistent with hydrothermal fluid activity. Geochemical data placed the following constraints on the protoliths: (a) in terms of immobile elements, the chemical protoliths are characterized as rhyolite/dacite to basalt/andesite, and most likely formed in an arc to rifted arc setting; (b) alteration indices (>50) and evidence in most rocks showing K-metasomatism and Na-depletion attest to hydrothermal alteration; and (c) the overall increase in contents of Al, K (with the exception of the garnetites), immobile and trace elements suggests seafloor alteration, most common to VMS environments.

Actions (login required)

View Item