NASA Astrobiology Institute (NAI)

Geology of the Nuvvuagittuq Supracrustal Belt

June 21, 2008 - July 30, 2008

N 58 degress, 16.853 minutes W 77 degrees, 43.614 minutes

This research project was to carry out fundamental geological investigations (mapping, sampling, geochronology, protolith assignment) of a previously unknown ancient volcano-sedimentary terrane in northern Québec. In 2001, ca. 3.8 Ga rocks of the ca. 8 km2 Nuvvuagittuq supracrustal belt (NSB) were discovered during a geologic transect by the Québec Geologic Survey; they are the oldest sediments so far recognized in Canada, and it is oldest terrane of its kind discovered in nearly 40 years. As yet, very little is known about these rocks. Our reconnaissance high-resolution (1:50) mapping, geochemical analyses of selected samples, and ion microprobe U-Pb zircon geochronology of transecting granitoid gneisses in an NSB supracrustal enclave at Porpoise Cove confirmed a minimum age of 3.75 Ga for a diverse lithological assemblage. Identified protoliths include sediments such as banded iron-formations (“BIFs”) and conglomerates. Our preliminary results show that the Inukjuak rocks are as old as the well known 3.77-3.81 Ga rocks from the Isua supracrustal belt in West Greenland. They preserve isotopic fingerprints of early Fe- and S-cycling, and were brought to mid-amphibolite facies metamorphic conditions. To make progress we need to know the aerial extent of these rocks, the diversity of the lithotypes present, geologic history, state of preservation, and petrogenetic relationship (if any) with other documented ancient terranes such as the West Greenland rocks.

Nature and timing of the Archean-Proterozoic transition

May 10, 2008 - May 18, 2008

S 20 degrees, 56.433 minutes E 119 degrees, 50.424

At present, the Archean-Proterozoic boundary is defined at the convenient round number age of 2500 Ma, but this age has no real significance in terms of global geodynamics and has nothing to do with stratigraphic principles. It represents an average age of major (but not global) changes in Earth evolution, but there are examples of all types of geology present before this age that are also present after and thus it is not clear what the current boundary really separates. There are only a few places on Earth that host a complete stratigraphic section across the Archean-Proterozoic boundary, and arguably the best of these is in Western Australia, within the Mount Bruce Supergroup. Previously, it was thought that the base of the Mount Bruce Supergroup (Fortescue Group) represented the Archean-Proterozoic transition, as it lies unconformably on granite-greenstone basement. However, it was discovered that this major break was much older (2.78 Ga) than equivalent transitions elsewhere (2.45 Ga in North America) and thus the change from granite-greenstone geology to platform sedimentation was found to be an unsuitable candidate for a major timescale boundary, since it is diachronous. Another problem is that an unconformity can not be used to define a timescale boundary – it has to be within a continuous stratigraphic succession. Another suggestion is that the boundary be placed at the base of the first major iron formation unit within the Hamersley Group (Marra Mamba Iron Formation), which may be appropriate. However, banded iron formations were deposited before this time (e.g. Murchison domain BIFs in the Yilgarn), so that it is not clear that the lowest Hamersley BIF is significant on a global scale. Also, the age of this unit is 2630 Ma and this time for an Archean-Proterozoic boundary would exclude some classical granite-greenstone terranes. A third suggestion, put forth in informal discussions by myself and others, is that the global rise in atmospheric oxygen could be used to defined the Archean-Proterozoic boundary, as this was a key development in Earth history that allowed for the rise of more complex life forms. Much work has already been done on this problem, which has shown that the rise in atmospheric O2 was through gradual accumulation in the atmosphere and oceans. Molybdenum isotopes, for example, show that the rise in O2 commenced at 2.7 Ga, which is the same age as the oldest evidence for cyanobacteria in the form of stromatolite fossils and biomolecular evidence (both from Western Australia). A major indicator of significant atmospheric O2 is the disappearance of the S-MIF signature, which can be readily measured in a variety of rocks. Research has found that this signature disappears somewhere around 2.3-2.4 Ga, but it has not been precisely dated. A problem with using this method to define the Archean-Proterozoic boundary (although perhaps not absolute) is that, once again, it is not geological stratigraphic component, but an isotopic one, and it is not something that one can see in the rock record. An alternative possibility for the Archean-Proterozoic boundary may be the transition between BIF deposition in the Hamersley Group and clastic sedimentation in the Turee Creek Group. Previous mapping by the Geological Survey has shown that this transition is gradual and reflects a change from predominantly chemical sedimentation below (BIF and unusual shales) and clastic sedimentation above (siltstones and sandstones, with clasts of feldspar). It appears that this change in rock types may have resulted from a change in weathering regime, reflecting changes in atmospheric composition, with a change from predominantly chemical weathering under conditions of relatively high pCO2 (CO2 + H2O = carbonic acid) during deposition of the Hamersley Group, to mechanical weathering in the Turee Creek Group under conditions of generally cooler and more oxygenated conditions. Indeed, the latter supposition of a more oxygenated atmosphere during deposition of the Turee Creek Group is supported by the onset of a global glaciation event at c. 2.4-2.3 Ga, which is recorded in the Turee Creek Group by the Meteor Bore Member. The disappearance of the S-MIF signature occurs within this period of global glaciation (1st snowball Earth?). We now have the samples and have initiated analyses of these rocks.