Origin of Peculiar Horizons from the Nuvvuagittuq Greenstone Belt: Testing the Conglomerate Hypothesis
Abstract
Earth's earliest rock record is fragmentary, and often distorted by metamorphic changes suffered under great temperatures and pressures. Recent work may, however, have opened the door to more direct examination of the earliest Earth. Measurements of Nd-142 from rocks of the Nuvvuagittuq Greenstone Belt (NGB), N. Quebec yield an apparent 4.28 Ga isochron (O'Neil et al., 2008). These are potentially the oldest rocks on Earth and understanding the protoliths of the rocks of the NGB may help constrain Earth's earliest surface environment. Peculiar horizons of quartz-rich rocks within the NGB have been hypothesized to represent metamorphosed conglomerates. In order to evaluate this hypothesis, we apply a series of consistency tests aimed at observations ranging from the field to microscale. First, we made high-resolution (10m grid) maps of the purported conglomerate horizons, and interpreted the map pattern of these horizons for their conformity with sedimentary contacts. The horizons are continuous throughout the mapped area, and do not cross cut any other lithologies, which is necessary, but not sufficient, evidence for a sedimentary origin. Second, we examined the mineralogy and three-dimensional geometry of the potential clasts in large polished blocks. The potential clasts fall into just two different types: the dominant type is made up primarily of coarse grains of quartz; the second type is distinctly subordinate and is largely made up of fine-grained equigranular quartz and relict plagioclase with minor amounts of biotite. The dominant type can occur as lozenge-shaped pods up to 5 cm thick, while the subordinate type more commonly occurs as thin (<2 cm thick) undulose layers. Neither the mineralogies nor the geometries of the potential clasts offer certain indication of a sedimentary origin, though they are potentially consistent with one. Third, we are comparing the identity and chemistry of trace minerals within the potential clasts to that of the surrounding matrix. Preliminary results suggest that the matrix hosts many zircon, monazite and xenotime neoblasts as well as a variety of sulfide minerals, while most of the trace minerals in the dominant type of potential clasts are euhedral sulfides, with rare chromite-rich regions. Trace zircon and monazite are more common in the subordinate type of potential clasts, with zircon occasionally exhibiting that show a concentric but truncated zonation. And finally, we are also measuring sulfur isotope compositions of sulfides from the potential clasts and from the matrix. The key principle linking these two consistency tests is the presence or absence of mineralogical, mineral chemical, and isotopic heterogeneity.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSM.U21D..07K
- Keywords:
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- 5225 Early environment of Earth