Among the known composition of the continental crust, the early Precambrian basement accounts for more than 75% of the total crust. These early Precambrian basement bodies are composed of a set of rock combinations represented by tonalite, trondhjemite and granodiorite.

Among the known continental crustal compositions, the early Cambrian basement accounted for more than 75% of the total crust. These early Precambrian basement bodies are composed of a set of rock combinations represented by tonalite, trondhjemite and granodiorite (granodiorite). Jiang Boming et al. (1981) called this rock combination the "TTG rock series" based on its initials. The TTG rock series occupies more than 80% of the exposed area of ​​the craton's metamorphic basement and is a key geological record of the formation and evolution of the continental crust in the Archaean . Studying the evolution of continental crust during the Archaean period is largely about studying the origin and evolution of the TTG rock series. Although there are still many disagreements in the academic community about the source rock composition of TTG and the geodynamic background it indicates, it is generally believed that based on its trace element characteristics, most TTG is the product of partial melting of hydrous mafic rocks under relatively high pressure. . But in fact, for most medium-acidic rocks, their trace element composition is largely determined by accessory minerals, and the evolution of magma is an extremely complex process. Many studies have shown that magma The participation of various accessory minerals during the evolution process greatly affects the trace element geochemical characteristics and even isotopic characteristics of the melt. Traditional methods such as Sm-Nd/Lu-Hf isotope analysis, trace element geochemical simulation, and phase balance simulation do not consider the complex behavior of accessory minerals in the magma evolution process, so in a sense, the results are not reliable.

Considering that most rocks contain very little accessory minerals themselves, and most accessory minerals have extremely low Fe content, the presence of accessory minerals usually does not affect the Fe isotope characteristics of magma. In addition, different rock-forming minerals usually have different Fe isotope fractionation coefficients. Therefore, non-traditional stable isotopes such as Fe isotopes provide new tools for constraining the source area characteristics and evolution processes of types of granites such as TTG.

Figure 1 The TTGs studied in this study all belong to the so-called "high-pressure" type, but the δ⁵⁶Fe values ​​of most TTGs are consistent with the evolutionary trend of iron isotope fractionation in the process of low-pressure amphibole fractionation. A few TTGs have lower δ⁵⁶Fe characteristics. , there are 4 stable isotopes of

Fe formed under high pressure, namely ⁵⁴Fe (5.84%)? ⁵⁶Fe(91.67%)? ⁵⁷Fe (2.12%) and ⁵⁸Fe (0.28%). The main factors controlling Fe isotope fractionation of different minerals in high-temperature magma systems are valence state and coordination number. Theoretical predictions and practical observations indicate that heavy isotopes tend to concentrate in mineral phases with lower coordination numbers and higher electrovalence. Since Fe³⁺ usually has a lower coordination number and stronger Fe-O bond strength than Fe²⁺, under equilibrium conditions, Fe³⁺-rich minerals in nature generally have heavier properties than Fe²⁺-rich minerals. Fe isotope characteristics, such as: garnet is relatively enriched in Fe²⁺ and has a lighter δ⁵⁶Fe characteristic. Therefore, during the magma differentiation process dominated by garnet, the δ⁵⁶Fe of the residual magma usually has an increasing trend; magnetite is relatively enriched in Fe³⁺ and has heavier δ⁵⁶Fe characteristics. Therefore, in the magnetite-dominated magma differentiation stage, the δ⁵⁶Fe of the residual magma usually has a decreasing trend. The Fe³⁺ content and δ⁵⁶Fe value of hornblende are between those of garnet and magnetite.

Following the above ideas, Associate Researcher Liu Peng of the State Key Laboratory of Lithosphere Evolution of the Institute of Geology and Geophysics, Chinese Academy of Sciences, and his collaborators Researcher Guo Jinghui, Researcher Zhai Mingguo, Researcher Ross Mitchell, etc., together with Professor Wang Zaicong of China University of Geosciences (Wuhan), Fe isotope studies were conducted on the TTG rock series in the Jidong region of the North China Craton. The research team reported a set of TTG gneiss series formed by a hornblende-dominated crystallization differentiation process in this area in 2019, providing clear evidence for the low-pressure separation crystallization origin of TTG for the first time (Liou and Guo, 2019) , but there are still some trace element characteristics of TTG indicating that garnet is involved in its formation process, which is contrary to field geological evidence.The Fe isotope study shows that the δ⁵⁶Fe value of most TTG is relatively high, which is similar to that of the surrounding rock diorite , which is consistent with the evolution trend of iron isotope fractionation during the differentiation process of angular amphibole under low pressure. The δ⁵⁶Fe values ​​of a few TTGs are significantly lower than the average value of mid-ocean ridge basalts, so they cannot be the products of partial melting of basalts such as E-MORB and OIB under high pressure. In short, most of the TTG gneisses developed in this area belong to high-pressure TTG according to the traditional definition, but Fe isotope evidence shows that their formation process did not involve the participation of high-pressure mineral phases such as garnet (Fig. 1). Therefore, So-called "high-pressure" TTG can be formed completely at low pressure. Previous research by

believed that approximately 80% of Archean TTG gneiss belongs to high-pressure and medium-pressure types, which means that there will be a large amount of garnet remnants during the formation of 80% of TTG. If this conclusion is correct, then the Archean TTG should have a systematically higher δ⁵⁶Fe value than the continental crust that was mainly composed of granite during the Phanerozoic. However, statistical results show that the Fe isotope characteristics of Archean TTG and Phanerozoic rocks are no different (Fig. 2), which may indicate that the formation process of Archean continental crust is consistent with today. However, this comparison is based on limited TTG Fe isotope data, and more Fe isotope research on TTG is still needed in the future.

Figure 2 The Fe isotope characteristics of Archean TTG obtained based on limited data are not much different from those of Phanerozoic rocks.

The research results were published in the international academic journal Earth and Planetary Science Letters (Liu Peng, Wang Zaicong, R. N. Mitchell, L. S. Doucet, Li Ming, Guo Jinghui, Zhai Mingguo. Fe isotopic evidence that “high pressure” TTGs formed at low pressure[J]. Earth and Planetary Science Letters, 2022, 592: 117645. DOI: 10.1016/j.epsl.2022.117645).

Editor: Fu Shixu

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