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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

, : 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. Geochimica et Cosmochimica Acta 64(17): 3035-3060

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


DOI: 10.1016/s0016-7037(99)00431-7

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