T198 Radiogenic Isotopes as Tracers of Geologic Processes: Dates, Rates, and Proxies (Posters)

Tuesday, 11 October 2022
2:00 PM - 6:00 PM
Exhibit Hall F

EARLY AND VARIABLE CONTRIBUTIONS FROM SUPRACRUSTAL ROCKS IN ARCHEAN MAGMAS FROM THE SAGLEK BLOCK (LABRADOR, CANADA)

Paper number: T198-2
Program: Technical Sessions
Time: 2:00 PM – 6:00 PM

Authors

Abstract

The Saglek-Hebron Complex (SHC, Labrador, Canada), is a polymetamorphic terrane preserving crustal rocks as old as ~3.9 Ga. Dominated by tonalite-trondhjemite-granodiorite (TTG) orthogneisses which host m- to km-scale enclaves of metasediments and mantle-derived rocks, the SHC records over a billion years of Archean magmatism. Zircon U-Pb data document six magmatic episodes at ca. 3.86 Ga, 3.74 Ga, 3.56 Ga, 3.33 Ga, 3.22 Ga and 2.75 Ga. The Hf isotopic compositions of zircon from these magmatic generations reveal a history of crustal re-working involving remelting of Eoarchean to Hadean mafic crust (Wasilewski et al., 2021).

When integrated with observations from radiogenic isotopes, oxygen isotopes in zircon can lend additional context for understanding early crustal histories, as δ18O from unaltered igneous domains can record the interaction of magmas and their protoliths with surface-derived material. This record, in turn, yields information about the tectonic conditions of early Earth. The limited δ18O data set for the SHC is dominated by mantle-like to slightly elevated δ18O values (~5-7 ‰), while a small number of high-δ18O values (+9 ‰) in metamorphic zircon are consistent with metamorphism at ~3.5 Ga involving mass transfer from mature supracrustal material (Vezinet et al., 2018; 2019).

Here, we present over 150 new δ18O analyses of igneous zircon for a suite of six samples, significantly expanding the temporal and lithologic diversity represented in the magmatic SHC zircon δ18O record. Our zircon δ18O dataset ranges from 5.0 to 8.8 ‰ and is spatially correlated with existing U-Pb and Lu-Hf information. We observe the highest δ18O values (7.9 – 8.8 ‰) in the 3.6 Ga TTG, which has zircon ƐHf interpreted to represent re-melting of a Hadean mafic crust (Wasilewski et al., 2021). These high δ18O values are bracketed by lower and more variable δ18O for the preceding 3.9-3.7 Ga (5.0 to 6.8 ‰) and subsequent 3.3-2.8 Ga (6.1 to 8.1 ‰) magmatic events, both of which have ƐHf suggestive of relatively more juvenile sources than the 3.6 Ga TTG. This zircon isotopic variability does not necessarily indicate the onset of widespread tectonic regime change, but rather suggests a unique regional magmatic event at ca. 3.6 Ga that involved a source with a higher relative contribution of supracrustal components.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-381494

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