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 of variably preserved greater than or equal to 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 late) 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 C-12/C-13 data interpreted as evidence for life are greater than or equal to 3850 Ma or only greater than or equal to 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 held 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 trot las 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, greater than or equal to 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 (greater than or equal to + 2.5 epsilon(Nd)) domains in the pre-3750 Ma mantle. Copyright (C) 2000 Elsevier Science Ltd. [References: 82]