Plate tectonic theory is a cornerstone of earth science, and some studies have shown that some form of plate tectonics may have been quite large at least in the early Cambrian period (e.g., Paleoproterozoic) (Wan et al., 2020, Science Advance), but the similarities and differences between its tectonic style and modern plate tectonics remain highly controversial (Zhai and Peng, 2020, Science Bulletin). Therefore, it is necessary to identify and quantitatively characterize the tectonic processes that occurred in the earlier Cambrian period. The time scale (duration or rate) is one of the basic parameters of the construction process and is most closely related to the dynamic mechanism of the construction process. It can be said that accurately defining the time scale of the earlier Cambrian tectonic process is the key to further understanding the tectonic-thermal system of the time.
The metamorphic rate of metamorphism recorded by metamorphic rocks, such as peak duration and uplift-cooling rate, is the most direct reflection of heat source properties and the rate of the tectonic process. Based on the fact that isotope chronology is currently a widely used method for studying the rate of metamorphism, almost all of the world's reported cases of rapid metamorphism-tectonic processes come from the young orogenic belt of the Phanerozoic, but are nearly absent in the earlier Cambrian orogenic belts/tectonic belts (Viete and Lister, 2017, Journal of the Geological Society). What was the cause of the earlier Cambrian loss of rapid metamorphosis-tectonic processes? Is it a distortion caused by isotopic chronological uncertainty, a deviation in sample retention, or a true reflection of different tectonic systems? The key to answering these questions lies in how to more finely and accurately characterize the rate of metamorphosis-tectonic processes in the earlier Cambrian period.
Dr. Zou Yi, Postdoctoral Fellow of the Precambrian Discipline Group of the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, and Professor Chu Xu of the University of Toronto, Canada, took the lead in using the kinetic method of diffusion dating to explore the paleoproterozoic metamorphic-tectonic process, and carefully analyzed the P-T-thi evolution trajectory, peak duration, and uplift-cooling rate of the two types of hempites with the most structural indication significance in the paleoproterozoic, namely high-pressure argillacene granites and ultra-high temperature silica granites ( Figure 1-Figure 3).
The results showed that both types of granulites recorded metamorphic-tectonic processes lasting 1-5Myr, such as rapid tectonic uplift caused by collision extrusion (1.87Ga) or stretching (1.85Ga) (high-pressure slurry granulites; Fig. 1-2), and transient ultra-high temperature effects caused by local bedrock intrusion under stretching (1.85Ga ultra-high temperature slurry granulites; Fig. 3), and these transient metamorphic-tectonic processes were embedded in the background of long-term orogenic processes, controlling the mineral combinations currently observed The reaction structure and the mineral composition of the ring belt, however, such an ancient and rapid metamorphic-tectonic process time scale is less than the error of isotope chronology (Figure 4), and it is difficult or even impossible to identify by isotope chronology.
Fig. 1 (a-d) high-pressure servitite garnet elemental ring belt; (e) high-pressure slurry granulite metamorphic P-T-t trajectory, where orange values are the duration of metamorphic processes and 1.87Ga are zircon and monazite U-Pb ages; (f) diffusion models based on garnet boundary balance with matrix; (g-i) garnet multicomponent diffusion simulations and time scales of each metamorphic stage (optimal fit)
Figure 2 (a) P-T-t trace of high-pressure slurry granulite garnet element; (b) P-T-t trajectory of high-pressure slurry granulite metamorphic rock, where the orange value is the duration of the metamorphic process, of which 1.85Ga is the age of zircon and monazite U-Pb; (c) the multi-component diffusion simulation of garnetite and the corresponding diffusion ring band of each metamorphic stage, the duration corresponds to b; (d-e) Sensitivity test of isothermal decompression and isobaric cooling duration
Figure 3 (a) ultra-high temperature succinct granite garnet element ring belt; (b) ultra-high temperature succinct granite metamorphic P-T-t trajectory, where the orange value is the duration of each metamorphic stage, of which 1.85Ga is the age of zircon and monazite U-Pb; (c) multi-component diffusion simulation of garnet edge in contact with quartz/potassium feldspar and the corresponding duration; (d) The fe-Mg element diffusion simulation of the edge of garnet in contact with black mica and the corresponding cooling stage and duration
Fig. 4 There is no contradiction between the uplift-cooling rate of high-pressure slurry granulite and ultra-high temperature succulent granulite, the black line is the diffusion chronology result, the orange line and the number are isotopic chronology results, but isotope chronology cannot identify the rapid uplift-cooling process and the stage of the cooling process
The study provides the answer to the question of isotope chronology, which limits the error of isotope chronology to identify the earlier Cambrian rapid metamorphosis-tectonic processes, and diffusion chronology is currently an excellent way to address this deficiency. The study also pointed out that it is easy to artificially exaggerate the time scale of some important geological processes by directly using chronological data to calculate the time scale of the earlier Cambrian metamorphic-tectonic processes, without considering the external errors of the data or metamorphic kinetic processes; although there were rapid metamorphic-tectonic processes with a duration of 1-5Myr in paleoproterozoic, their uplift-cooling rate was still several times slower than that of ultra-high-pressure rocks in the Proteozoic orogenic belt, so even if there were plate tectonics in the Paleoproterozoic, Its tectonic style may differ from modern plate tectonics. The study also provides a petrologic idea for exploring the tectonic system of the earlier Cambrian period.
The results were published in the international journals journals journals of petrology, Journal of Petrology and Relations to Mineralogy and Petrology. The research is funded by the Postdoctoral Innovative Talent Program and the Youth Fund.
1. ZOU Yi, CHU Xu, LI Qiuli, ROSS Mitchell, ZHAI Mingguo, ZOU Xinyu, ZHAO Lei, WANG Yuquan, LIU Bo. Local rapid exhumation and fast cooling in a long-lived paleoproterozoic orogeny[J]. Journal of Petrology, 2020, 61(10): egaa091. DOI: 10.1093/petrology/egaa091.
2.邹屹, 李秋立, 初旭, 翟明国, Ross Mitchell, 赵磊, 周李岗, 王禹权, 刘博. Older orogens cooled slower: new constraints on Orosirian tectonics from garnet diffusion modeling of metamorphic timescales, Jiaobei terrain, North China Craton[J]. Contributions to Mineralogy and Petrology, 2021, 176(11): 1-24. DOI: 10.1007/s00410-021-01846-w.
Editor: Fu Shixu
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