Modern style plate tectonics and island arcs are considered to have played an important role in the formation of the continental crust. The oldest zircons, however, seem to have formed before the formation of the continental crust. I have three related questions:
1.What was the mechanism of formation of these very very old zircon?
2. When and how did the embryonic continental crust start forming?
3. When was the present type plate tectonics initiated?
I think Rechard's answer is very pertinent and insightful. I just have a few short comments though. First, we have to define what is 'continental' crust. Most people agree that continental crust is characterized by a significant portion of felsic rocks. The oldest well-dated felsic rocks are the 4.02-4.03 Ga Acasta gneisses. There are some claims for pre-4.03 Ga rocks from NW Canada, but those rocks are mafic and the dating method used (Nd isotopes) has been disputed. The only reliable pre-4.03 Ga geological record is Hadean detrital zircons, most of which are reported from a meta-conglomerate layer from the Jack Hills, NW Australia, although sporadic Hadean zircon grains have been reported from other continents. From the internal zoning and trace element compoaitions, most people agree that these zircons crystallized from felsic melts, but the question is whether such felsic melts represent large granitoids plutons, and thus continental crust, or small melt patches in the dominently mafic crust (e.g. plagiogranite in ophiolite). This is highly debated at present and is extremly difficult, if not impossible, to tie down given the absence of the parent rocks of the Hadean zircons. Except for these, 3.6-3.9 Ga felsic rocks have been found in many Archean cratons, althought most of them are strongly modified by ductile deformation, high grade metamorphism and partial melting. The relatively well-preserved samples are dominated by TTG gneisses, which are generally thought to have derived from partial melting of hydrated mafic rocks, but whether this occurred in subduction-related or plume-related settings is also highly debated. So, in my opinion, it is premature to claim that all the oldest continental crust formed in non-subduction or stagnant lid setting.
Hi Qasim,
Reimink et al. (2016) discuss your question #2 using data from the Acasta Gneiss Complex in NW Canada. Their discussion also cites multiple works that cover similar topics. I hope this can be of some help.
Reimink, J.R., Chacko, T., Stern, R.A. and Heaman, L.M., 2016. The birth of a cratonic nucleus: lithogeochemical evolution of the 4.02–2.94 Ga Acasta Gneiss Complex. Precambrian Research, 281, pp.453-472.
DEar Richard Palin, Rongfeng Ge, and Eric Thiessen,
Thank you so much for your contributions. I am getting good lessons!
Regards,
Qasim
Dear Rongfeng GE,
For your kind information, I am copying a sentence from the Abstract of the paper of Sreenava et al. (2019), Geoscience Frontier. (http://creativecommons.org/
licenses/by-nc-nd/4.0/).
"Together with the recently reported Hadean, Eoarchean xenocrystic (up to 4.24 Ga) and modern detritus zircon grains from the Singhbhum craton, our results suggest that the Eoarchean detrital zircons represent crust generated by
recycling of Hadean felsic crust formed at ca. 4.3--4.2 Ga and ca. 3.95 Ga".
My knowledge on this topic is limited. However, I agree with most, that primary crustal (felsic rocks) material must have been generated due to magmatic differentiation.
Earth's Oldest Crust Dates to 4.4 Billion Years Ago. Zircon crystals from the Jack Hills of Australia, like the one above, reveal that continents arose just 160 million years after our solar system formed, much earlier than previously thought.
https://earthobservatory.sg/faq-on-earth-sciences/why-seafloor-so-recent-and-continental-crust-so-old
Eoarchean to Paleoproterozoic crustal evolution in the North China Craton: evidence from U-Pb and Hf-O isotopes of zircons from deep-crustal xenoliths
Article Eoarchean to Paleoproterozoic crustal evolution in the North...
Fusion temperature is the key. Zircon FT is 2715°C. Zircons are the first crystals to appear (of course, if after they got formed temperature became higher, they melted ... to form again with the cooling). Zircon is dense and with gravity the first crystals formed have drowned in the surrounding magma.
Fusion temperature of the most common mineral on earth, i.e., silica, is 1730°C (and it is light: it will float on the magma). Below this temperature, all sorts of crystals formed and aggregated to each others to form rocks.
Thanks a lot RM Palin.
You have a very important contribution. My compliments.
- Badges
- Science topic
- Similar topics
- Analytical Chemistry
- Column