Eclogite facies metamorphism of mafic and ultramafic rocks in the Tshenukutish terrane, Manicougan Imbricate Zone, Eastern Grenville Province

Cox, Richard Alexander (1999) Eclogite facies metamorphism of mafic and ultramafic rocks in the Tshenukutish terrane, Manicougan Imbricate Zone, Eastern Grenville Province. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Grenville Province of eastern Canada consists of a variety of lithotectonic units (terranes) which formed a large mountain chain along the margin of Laurentia at 1190-1000 Ma. The deepest crustal levels of the Grenvillian orogen are best exposed along a belt of high-PT terranes which form part of the parautochthonous belt. The best exposed section is the Manicouagan Imbricate Zone which contains rocks metamorphosed at conditions up to eclogite facies. The Tshenukutish terrane is the highest structural division of the Manicouagan Imbricate Zone and contains mafic and ultramafic rocks which characterise two distinct lithotectonic units. The structurally lower unit, the Baie du Nord segment, comprises granitic and dioritic country rocks intruded by Fe-Ti gabbro and anorthosite. The highest structural units in the Tshenukutish terrane belong to the Boundary zone, a complex series of tectonic slices, which include lenses of mafic and ultramafic rocks known as the Lac Espadon suite. Cumulate textures suggest that the Lac Espadon suite represents a layered, mafic and ultramafic complex. Geochemical analysis of the gabbroic end-members from both the Lac Espadon suite and the Baie du Nord segment indicate a within-plate tholeiite petrotectonic signature. REE-patterns of the gabbroic rocks and associated cumulates further suggest fractionation from a tholeiitic source and high-crustal level emplacement U-Pb dating of igneous baddcleyite gives an age of 1170+/-5 Ma for emplacement of Fe-Ti gabbro in the Baie du Nord segment. In the Lac Espadon suite, U-Pb ages of igneous zircon and baddeleyite (ca. 1650-1630 Ma) suggest a Labradorian age of emplacement The ages of zircon formed during anatexis (1046+/-3 Ma) and other metamorphic zircon ages (1042+22/-28 Ma, 1030+10/-7 Ma, 1030+/-12 Ma, 1012+/-12 Ma) show the timing of high-PT metamorphic overprint to be Grenvillian in age. The timing of retrogression and cooling is constrained by the ages of titanite (1006-997 Ma) and rutile (960-929 Ma). The Fe-Ti gabbroic rocks from the Baie du Nord segment display mineral assemblages which represent progressive stages of metamorphic transformation resulting in the formation of coronas, granoblastic pseudomorphs (transitional) and eclogites. In addition, the transitional and eclogitic samples exhibit coronas developed locally around igneous xenocrysts. Maximum T-conditions recorded in the Baie du Nord segment are in the range 720-820 °C with recorded P-conditions reflecting increasing eclogitization (14 kbar in coronitic rocks and up to 17 kbar in eclogitic rocks). Estimates of peak PT-conditions recorded by massive corona-bearing rocks in the Lac Espadon suite are in die range 780-930 °C at 16-19 kbar. Reequilibration during the early stages of exhumation at high T-conditions (>700 °C) variably affected all samples in the Tshenukutish terrane. The PT-conditions recorded during retrogression (ca. 700-800 °C and 8-12 kbar) define segments of steep PT-paths and isothermal metamorphic (field) gradients, suggesting high heat flow through the lower crust. The presence of syn-metamorphic mafic dykes and granites further suggest heat input (including from the mantle) during exhumation. The rates of cooling and exhumation in the Manicouagan Imbricate Zone were modelled using all the available PT-data and U-Pb ages for zircon, monazite, titanite and rutile. The results indicate that the early stages of exhumation were charecterized by near isothermal conditions. Uplift rates were relatively fast (>1 mm/yr) consistent with tectonic exhumation. Rates subsequently slowed, probably during thrust emplacement of the highest structural levels (the Boundary zone) over the rest of the Tshenukutish terrane. Differences between titanite and rutile cooling ages suggest that late-stage cooling and exhumation was slow, consistent with other high-P terranes. The PTt-evolution of the Tshenukutish terrane is compatible with models for tectonic exhumation by extrusion during northwest thrusting, with extension on top of the pile. Evidence for high heat flow and rapid uplift during the initial exhumation in the Manicouagan Imbricate Zone supports current models which suggest that asthenospheric upwelling immediately followed extensional collapse of the Grenville orogen.

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