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Earth's first continental crust is formed by Archaean and mainly consisted of tonalite–trondhjemite–granodiorite with a small amount of diorites (DTTGs), which has an essential role in probing early crust–mantle dynamic regime and in understanding the formation mechanism of continental crust. Here, we present zircon U‒Pb dating and Lu‒Hf isotopes, whole-rock geochemistry, and petrography on DTTGs rocks in the Dunhuang Block. Three episodes of DTTGs were emplaced circa 2.67 Ga, 2.60 Ga, and 2.50 Ga. The circa 2.67 Ga TTGs exhibit high SiO2 contents (68.14–71.70 wt%), low MgO contents (0.65–1.31 wt%), and high ratios of (La/Yb)N (146 on average), with their enriched Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t) = -5.77–0.53], indicating origination from partial melting of amphibolites at thickened lower crust. In contrast, the circa 2.60 Ga transitional TTGs exhibit relatively high MgO contents (2.80–3.39 wt%), flat REE (Rare earth element) patterns with moderate ratios of (La/Yb)N (20.49 on average), and dispersed Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t)= −3.99–3.08]. Accordingly, circa 2.60 Ga transitional TTGs melts were produced by partial melting of the shallower crust induced by mantle-derived magma upwelling. The circa 2.50 Ga diorites exhibit low SiO2 (55.72–59.11 wt%) but high MgO (3.51–4.52 wt%) contents with positive Nd-Hf isotopes [ƐHf (t) = -0.16–4.17 and ƐNd (t) = 2.00–4.45], suggesting that they originated from partial melting of mantle wedges metasomatized by fluid from subduction slabs. Combined with the detailed petrogenetic studies and crustal thickness variation, we conclude that the complex crust–mantle interaction may be an essential reason for the Neoarchaean diversity of DTTGs from the Dunhuang Block, which experienced prolonged arc accretion before Neoarchaean, followed by delamination between 2.67 and 2.60 Ga and subsequently transitioned to subduction. [ABSTRACT FROM AUTHOR]

The Amdo microcontinent which separates the Qiangtang terrane to the north and the Lhasa terrane to the south is a key terrane for reconstructing the tectonic evolution of Central Tibet. We report the new finding of retrograde eclogites within the Amdo microcontinent in this study. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile and quartz and underwent a four‐stage metamorphic evolution, including a peak eclogite facies stage (M1) at ~20–24 kbar and 580–620°C, followed by an HP granulite facies decompression stage (M2) at ~13–15 kbar and 750–780°C, a subsequent MP‐UHT granulite facies heating stage (M3) at 8–10 kbar and >840°C and a final amphibolite facies retrogression (M4) at 5.3–6.0 kbar and 560–580°C. The eclogites exhibit rare earth element distribution patterns and trace element abundances similar to those of N‐MORB and arc‐related volcanics, with depleted whole‐rock εNd(t) values of 3.4 to 4.2, and are inferred to have formed in a back‐arc basin tectonic setting. Zircon and rutile U–Pb dating yields a protolith age of 226 ± 5 Ma, a peak eclogite facies metamorphic age of 190 ± 1 Ma, an HP granulite facies metamorphic age of 179 ± 1 Ma and an amphibolite facies retrograde age of 172 ± 1 Ma. The clockwise P–T–t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back arc basin was subducted to depths of ~80 km. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back‐arc basin and the absence of arc magmatic rocks in the northern Amdo microcontinent. [ABSTRACT FROM AUTHOR]

Tonalitic–trondhjemitic–granodiorite (TTG)-like leucosomes, which are formed from anatectic melts, record crustal growth and evolution and thus provide constraints on the process of anatexis. However, potential Mg isotope disequilibrium during crustal anatexis makes interpreting the genesis of anatectic melts challenging, thereby potentially hindering the application of the Mg isotope system in investigations of crustal evolution and crustal anatexis. A key factor in Mg isotope fractionation during anatexis is the behaviour of garnet, which becomes preferentially enriched in light Mg isotopes compared with other Mg-hosting minerals during partial melting processes. In this study, a combination of petrography, whole-rock geochemistry, and mineral microanalysis reveals that garnets in leucosomes from the Dulan area of the North Qaidam orogen, were formed by dehydration melting of hornblende. Garnet-bearing leucosomes yield low δ26Mg values (−0.70‰ to −0.33‰) compared with garnet-free leucosomes (−0.19‰ to −0.16‰) and global arc rocks (−0.35‰ to 0.06‰). The different Mg isotopic compositions are attributed to the effect of peritectic garnets on the Mg isotope system in anatectic melts. Quantitative modelling further indicates that the lower Mg isotopic values of garnet-bearing leucosomes require the incorporation of ∼5% peritectic garnets into anatectic melts during magma ascent. Our results demonstrate the influence of peritectic garnets on the Mg stable isotope system and highlight that this influence on the Mg isotopic compositions of peritectic minerals during crustal anatexis must be considered in interpretations of crustal evolution using the Mg isotope system. • The Grt-bearing TTG-like leucosomes show low δ26Mg values than Grt-free ones. • The Grt-bearing leucosomes could be explained by incongruent melting and garnet entrainment. • Garnet entrainment is first-order factor of δ26Mg values of TTG-like melts. [ABSTRACT FROM AUTHOR]