Talk:Narryer Gneiss Terrane
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Is this really the oldest portion of the Earth's exposed, or surface crust? Maybe one of the oldest, but until more tests are done on a lot more of other zircon mineral content around the world, this seems like a hasty conclusion. --joseph 02:01, 16 February 2006 (UTC)
- The Acasta Gneiss of Canada might compete for the distinction, but it is splitting hairs as to what one considers "intact". The Manfred Complex, at 3730 +/- 30 Ma, isn't "intact" in any sense, and gets closest to the Acasta Gneisses 3800 Ma in age. Because it is an intrusion, it must have intruded a stable piece of the crust which existed before 3730 Ma. How old this crust is is arguable, but the best evidence we have is detrital zircons of 4.1 to 4.2 Ga. These are from rocks of the 3680 Ma metasedimentary package, which formed from the 4.1 to 4.2 Ga rocks, and formed upon a crust of those rocks.
- We have yet to map and identify a chunk of this 4.1-4.2 Ga rock, but it doesn't mean it is not there, by fairly solid inference. The best bet is to search for an intact contact of the Manfred Complex, or a low-strain contact, and sample it to see if there are zircons older than the Meeberrie and Dugel Gneisses in that contact. Alternatively, a xenolith within the Meeberrie and Dugel Gneisses would work, but the easiest remains the conglomerates.
- Being honest, too, the majority of data come from a very few decent outcrops. Most of the area is covered by thin soil and colluvium; perhaps one day some part of the gneiss belt will be sampled which will derive a 4.1 Ga age from an igneous rock, probably from a borehole. Alternatively, the margins of the Yarlarweelor Gneiss could prove fertile ground as they are better exposed, and contain some conglomerate lithologies. But again, the outcrop is a needle in the haystack proposition and, I can say from firsthand experience, it is not an easy area in which to work or map.
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