HIGH-P ECLOGITE IN THE EASTERN TRANS-HUDSON OROGEN, CANADA: MODERN-DAY COOL THERMAL GRADIENTS IN THE PALEOPROTEROZOIC

The Trans-Hudson Orogen (THO) is a middle Paleoproterozoic continental collisional belt that has been proposed to represent an ancient analogue to the Cenozoic Himalayan Orogen. A notable divergence in this correlation has been the absence of high-pressure rocks in the THO compared with the Himalaya, and it has been debated whether this reflects a secular tectonic change – with the requisite cool thermal gradient precluded by warmer ambient mantle temperatures during the Paleoproterozic – or a preservation phenomenon.

In this contribution we document a new discovery of eclogite from the THO, which fills in the high-pressure gap in the comparative record between the two orogens. The eclogite was sampled from a mafic boudin associated with metavolcaniclastic schist and hosted by Archean basement orthogneiss of the lower-plate Superior craton. The THO eclogite was characterised by the assemblage garnet­–omphacite–kyanite–rutile–biotite–zoisite at high pressure, prior to retrograde symplectite growth. The metavolcanic schist contains evidence of an early kyanite-grade history, prior to retrograde growth of lower-pressure plagioclase–gedrite–cordierite coronas around garnet.

Phase equilibria modelling of eclogite and schist reveals a multi-stage, approximately isothermal, decompression history from ~27 to 7 kbar at 700 °C. This pressure­–temperature (P–T) history overlaps within error with derived P–T paths for the type Himalayan Tso Morari eclogite locality. U–Pb dating of monazite in the metavolcanic schist indicates that it experienced prograde metamorphism at ca. 1830 Ma, which provides a maximum age for the timing of peak eclogite-facies metamorphism in the THO. Notably this age coincides with the onset of terminal Superior–Churchill collision in the eastern segment of the orogen, which is analogous with the relative timing of eclogite-facies metamorphism in the Himalaya.

Overall, the exceptional structural, thermobarometric and temporal similarities of the THO eclogite with eclogitic units in the Himalaya suggest that modern-day plate tectonic processes are applicable to at least the middle Paleoproterozoic. Analysis of global trends in metamorphic thermal gradients through time indicates that the THO eclogite extends the rock record of cool thermal gradients (<350°C/GPa) by over 1 Ga.