Metamorphic evolution of anatectic metapelites from the Gabriel high strain zone, Grenville Province

Strowbridge, Susan Leah (2005) Metamorphic evolution of anatectic metapelites from the Gabriel high strain zone, Grenville Province. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Gabriel High Strain Zone, exposed in the Manicouagan Reservoir area, is part of the Allochthonous Belt of the eastern Grenville Province. It was metamorphosed under mid-P granulite facies conditions during the culmination of the Ottawan orogeny (ca. 1050 Ma). Anatectic metapelites from the Gabriel High Strain Zone are characterized by quartz-rich layers that also contain ferromagnesian minerals and, in some cases sillimanite, alternating with granitic layers which likely represent segregated leucosome. -- Samples of the quartz-rich layers have been divided into three groups on the basis of the observed mineral assemblages which in turn are controlled by bulk composition. Group 1 has a moderate to high Alumina Index (0.35--0.66) and the highest XMg (0.51--0.77) and contains the assemblage quartz + garnet + biotite + sillimanite + cordierite ± K-feldspar ± plagioclase; group 2 has a moderate to high Alumina Index (0.22--0.60) and intermediate XMg (0.35--0.51) and consists of the mineral assemblage quartz + K-feldspar + garnet + biotite + sillimanite ± plagioclase; and finally, group 3 has a low Alumina Index (0.10--0.13) and low to intermediate XMg (0.23--0.39) and is comprised of the assemblage quartz + garnet + biotite + plagioclase + K-feldspar. These mineral assemblages provide complementary information that, if put together, allow the P-T field of the thermal peak to be tightly constrained.-- Textural relationships and AFM topologies in the KFMASH system were used to determine the following sequence of reactions that affected these rocks with increasing temperature. Cores of garnet porphyroblasts in groups 1 and 2 rocks contain inclusions of sillimanite needles and are interpreted to have grown by subsolidus reactions in the sillimanite stability field. The absence of muscovite and the coexistence of sillimanite porphyroblasts and K-feldspar in the matrix are indicative of the reaction: muscovite + albite + quartz → K-feldspar + sillimanite + liquid (R1) which marks the onset of dehydration melting in politic rocks. Clear rims of garnet porphyroblasts are consistent with a second phase of garnet growth by the continuous reaction: biotite + sillimanite + quartz → garnet + K-feldspar + liquid (RIIa). In addition, the coexistence of garnet and cordierite in group 1 rocks indicates that the subsequent discontinuous reaction: biotite + sillimanite + quartz → garnet + cordierite + liquid (R2) was crossed. In the rocks of groups 1 and 2 prograde biotite was eliminated by reactions R2 and RIIa, respectively, therefore it was not available for further dehydration melting by the reaction: biotite + garnet + quartz → orthopyroxene + cordierite + liquid (R3). In contrast, group 3 rocks retain peak biotite, therefore the absence of orthopyroxene in them indicates that the P-T conditions for reaction R3 were not reached. In all rocks, texturally late biotite and sillimanite aggregates developed at the expense of garnet and cordierite as a result of reactions RIIa and R2 having taken place in the reverse sense during cooling. -- Garnet porphyroblasts experienced extensive chemical homogenization at high temperatures and only preserve, in the best cases, an incomplete record of their evolution. For instance, growth zoning in terms of grossular, which is preserved in the most Ca-rich samples, attests to a two stage garnet growth and variably developed rim zoning is consistent with retrograde resetting of the composition. Based upon textures, AFM topologies, garnet XFe isopleths and the KFMASH petrogenetic grid it is inferred that the thermal peak occurred between 6.2 kbar (between 820 to 870°C) and ~8.9 kbar (at 900°C) and that the P-T path was clockwise with little decompression between the prograde and retrograde parts of the path. Application of thermobarometry (garnet-biotite and GASP) was of limited use in these rocks because: (a) biotite is in most cases retrograde and in addition experienced extensive resetting of its composition during late cooling; and (b) it was difficult to identify plagioclase that was stable at the thermal peak. However, in group 1 and 2 rocks GASP isopleths are consistent with the P-T field determined with the petrogenetic grid. In addition, GASP isopleths that correspond to different stages of the metamorphic evolution cover a narrow range further supporting the interpretation that the P-T path did not involve significant decompression. -- The metamorphic evolution of the sillimanite-bearing metapelites of the Gabriel High Strain Zone is consistent with a previously proposed tectonic model which suggests that this zone was part of the hangingwall of an extruded high-P unit known as the Manicouagan Imbricate Zone. The results of this study provide additional constraints to this model. For instance the examined rocks were metamorphosed at similar temperatures, but lower pressures than the Manicouagan Imbricate Zone and experienced a P-T path that is consistent with heating and cooling with little decompression in between. These observations indicate that a fast heat transfer from the hot extruding Manicouagan Imbricate Zone to the Gabriel High Strain Zone may have been responsible for the metamorphism of the latter.

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