Hydrothermally-altered mafic crust as source for early Earth TTG: Pb/Hf/O isotope and trace element evidence in zircon from TTG of the Eoarchean Saglek Block, N. Labrador

Abstract

The North Atlantic craton hosts extensive exposures of Eoarchean crust, spread through areas of Western Greenland and Northern Labrador (Canada). Of these two areas, the crust of the Saglek Block of Northern Labrador has received far less attention from the scrutiny of modern analytical methods than its better documented Western Greenland equivalent, the Itsaq Gneiss Complex. Here, we present the first coupled trace element and U–Pb/Hf/O isotope dataset for zircon from an early TTG component of the Saglek Block.The combination of textural, elemental and isotopic in-situ analyses enables selection of the least disturbed zircon domains. From these it is demonstrated that the oldest felsic remnants exposed in the Saglek Block were emplaced 3.86 ± 0.01 billion yr (Ga) ago through partial melting of basaltic protoliths. The Hf isotope signature of the oldest zircon domains from the Saglek Block TTG indicates derivation from sources that did not undergo substantial Lu/Hf fractionation, resulting in initial Hf isotope compositions that are chondritic within uncertainty. The oxygen isotope ratios of the least disturbed zircon portions vary from 5.38 ± 0.16‰ to 6.64 ± 0.19‰ and document the interaction of the TTG protoliths with Earth's early hydrosphere at low temperature (≤150–200 °C) prior partial melting in the Eoarchean. The results support TTG production in the Eoarchean from variably hydrated basaltic protoliths.