HADEAN TO NEOARCHEAN CRUSTAL EVOLUTION IN A SINGLE SAMPLE OF THE CEDAR LAKE GNEISS, WESTERN SUPERIOR PROVINCE, CANADA

We present the first U-Pb-Lu-Hf zircon isotope analyses using split-stream LA-MC-ICPMS for the granite-gneiss Winnipeg River terrane in the western Superior Province of Canada. The sample analysed is a homogeneous tonalite gneiss collected from the Cedar Lake dome, which comprises strongly deformed tonalite-trondhjemite-granodiorite (TTG) gneiss with mafic and banded amphibolite inclusions that was subsequently injected by TTG veins and structurally concordant granitoid sheets.

The zircon isotope systematics of the sample record over 1 Ga of geological history, involving the igneous formation of the TTG precursor that contained remnants of older crust in the form of xenocrystic zircon, followed by metamorphic overprinting events reflected by the growth of new zircon grains. Zircons in the sample have elongate, stubby and flattened sub-spherical morphologies. All are slightly rounded and with a polished appearance reflecting metamorphic overgrowths.

A small population of xenocrystic grains yielded a U-Pb age of ca. 3400 Ma and a single grain yielded an age of ca. 3500 Ma. Igneous zircon grains dominate the age spectrum with an average age of ca. 3224 Ma. Late metamorphic zircons define a minor population with an average age of ca. 2710 Ma. Hafnium isotope compositions of the xenocrystic zircons plot along an array that either follows an Eoarchean trend or a late-Hadean mafic source. The igneous zircons display an average εHf (3.25 Ga) = -3.9, consistent with a Neodymium crustal formation age of 3.67 Ga. The youngest cluster displays a distribution that mirrors the ca. 3224 Ma cluster with an average εHf (2.7 Ga) = -4.1.

Our results show that the Winnipeg River terrane records a history typical of many Archean cratons and that it contains basement rocks much older than previously recognized. Hf isotope signatures of the inherited zircons indicate that their protoliths ranged in age from Eoarchean (3.9 Ga) to late-Hadean (4.2 Ga), depending on their time-integrated Lu-Hf isotope evolution. In contrast, metamorphic zircons display pseudo-juvenile signatures in a closed system without mantle influence. Broad overlap in εHf space of the Cedar Lake gneiss with other granite-gneiss terranes in the Superior Province may suggest a shared Hadean-Paleoarchean geological history.