The early archaean Itsaq gneiss complex of Southern West Greenland: The importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution

摘要: Geochemical and isotopic studies of small volumes variably preserved≥ 3600 Ma rocks in gneiss complexes are crucial for documenting early Earth history. In the Itsaq Gneiss Complex Nuuk region, West Greenland, there is dispute whether granitic (sensu lato) orthogneisses dominating it mainly products a single ca. 3650 crust formation “super event,” or they formed several unrelated events between 3850 3560 Ma. Which these interpretations dates correct has major implications regarding what whole rock radiogenic record (Pb/Pb, Sm/Nd, Rb/Sr) reveals about continental terrestrial differentiation. There also debate some Greenland metasedimentary with 12C/13C data interpreted as evidence life are≥ only≥ old. Establishing age important debates concerning surficial environments origin life. Controversies have arisen because different approaches taken by workers, specifically respect to how much emphasis placed on field geology interpreting data. this paper, observations sampling from low strain zones, where geological context best preserved understood, closely integrated U-Pb zircon cathodoluminescence (CL) imagery zircons. This approach shows that most single-phase, well-preserved, meta-granitoid samples simple populations dominated oscillatory-zoned prismatic grains when their host magmas crystallized. On other hand, migmatites strongly deformed-banded gneisses more complex populations. The combined geochronology demonstrate 1) areas contain exposed during multiple magmatic/thermal not (as suggested Kamber Moorbath, 1998) granitoids containing abundant> zircons inherited cryptic, unexposed, older rocks; 2) abundant,≥ 3750 present, which locally well-preserved; 3) water-lain sediments reported showing C isotope were deposited Ma; 4) whole-rock Sm/Nd isochron fails distinguish any confidence 3800 rocks, 5) however, reinforces previous indications markedly depleted (≥ + 2.5 ϵNd) domains pre-3750 mantle.