Saturday, April 16, 2016

Are the Oldest Known Supracrustal Rocks in the World in Labrador, Canada?

Occurrence and geochronology of the Eoarchean, ∼3.9 Ga, Iqaluk Gneiss in the Saglek Block, northern Labrador, Canada: Evidence for the oldest supracrustal rocks in the world


Shimojo et al


Understanding Earth’s early evolution requires decoding the cryptic geological record that is preserved in a few extremely rare terrains on Earth. The Saglek–Hebron Block in the North Atlantic Craton is one of the oldest terrains in the world. To better understand the Eoarchean history of the Saglek–Hebron Block, we undertook a comprehensive geological and geochronological investigation of this terrain. The Saglek–Hebron locality contains orthogneisses and supracrustal rocks, which are classified into two groups of the Eoarchean and Mesoarchean suites based on field associations defined by intrusion of the Mesoarchean mafic Saglek dykes. The Eoarchean suites contain the Nulliak supracrustal rocks and Uivak Gneisses. In this paper, we report cathodoluminescence (CL) observations and U–Pb ages of zircons from the Eoarchean orthogneisses to constrain the evolution of the Saglek–Hebron Block. We made detailed sketch maps (1:20) of critical outcrops to establish relative chronological relationships between lithologies. At least seven generations of mafic and felsic units were recognized in an outcrop of the St John’s Harbour South area. The first and second generations are mafic supracrustal rocks whereas the third to seventh generations are tonalitic gneisses, which are intruded into the supracrustal units. We study CL observation, geochemistry and U–Pb dating of zircons from the oldest and youngest tonalitic gneisses.

The CL observation of internal structures of the zircons showed they typically comprise three domains of core, mantle and rim. The cores have clear oscillatory zoning, whereas the mantles lack oscillatory zoning and exhibit dark CL emission and structureless CL images. The obvious correlation between the ages and chemical compositions of U contents and Th/U ratios indicates that the cores with low U contents and high Th/U ratios preserve the protolith age. The cores with clear oscillatory zoning, low U contents and high Th/U ratios of zircons from the oldest suite plot on a concordia line with ages ranging from 3953 to 3797 Ma. The oldest and average ages of the six oldest spots on the concordia line are 3953 ± 54 and 3920 ± 49 Ma, respectively. On the other hand, the average age of the four old spots of zircons from a younger generation of orthogneiss is 3869 ± 63 Ma. The geological and geochronological relationships between the third and seventh generation orthogneisses are consistent with each other, confirming an Eoarchean age of ca. 3.9 Ga for the oldest suite of the Uivak Gneisses. We name this oldest suite the Iqaluk Gneiss. As the protolith of the Iqaluk Gneiss was clearly intruded into the Nulliak supracrustal rocks, the Nulliak supracrustals must have a minimum age of ca. 3.9 Ga, indicating that they are the oldest supracrustal rocks on Earth. But, further studies should be necessary to obtain more precise age of the Iqaluk Gneiss and Nulliak supracrustal rocks.

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