The East Junggar is an important part of the Central Asian Orogenic Belt (CAOB). Using in situ zircon dating and Hf isotopic analysis by LA-ICP-MS and MC-ICP-MS, respectively, a detrital zircon of 4040 Ma age was found in sedimentary sequences from the Aermantai ophiolitic mélange, East Junggar. This is the oldest age record in the East Junggar terrane, and also marks the first zircon locality in the CAOB with an age older than 4.0 Ga, which is attributed to the Hadean crust. The 4040 Ma detrital zircon has an ε Hf (t) value of-5.2 and a two-stage Hf modal age of 4474 Ma, suggesting the presence of very old (Hadean) crustal material in the source area. Beside peak ages of 446 Ma, we found four age groups of 3.6-3.1 Ga, 2.53-2.37 Ga, 1.14-0.89 Ga and 0.47-0.42 Ga from 141 effective measuring points. The age of 426±4 Ma for the five youngest detrital zircons defines the lower limit of the deposition time of sedimentary sequencess in the Aermantai ophiolitic mélange. The 0.47-0.42 Ga zircons exhibit 176 Hf/ 177 Hf ratios of 0.282156 to 0.282850, corresponding to variable ε Hf (t) values from-9.3 to 12.0 and Hf model ages from 2011 to 646 Ma. These characteristics are similar to those of the early Paleozoic igneous and gneissic zircons from the Altai, but significantly different from those of the East Junggar. Based on the material structures of felspathic greywacke, the morphology, internal texture and age distributions of dated detrital zircons, in combination with a study of the regional geological data, it is suggested that the sedimentary sequences in the Aermantai ophiolitic mélange was deposited in the Late Silurian, with the main provenance from the Altai Orogen in the north. This indicates that the early Paleozoic ocean represented by the Aermantai ophiolitic mélange was readily closed during the Late Silurian, and the northern edge of the East Junggar terrane was accreted to the Altai Orogen. The joint of them then served as a marginal orogen in the southern edge of the Siberia Paleocontinent.

The Sanjiang Indosinian orogen, located in the eastern part of the Paleo-Tethys tectonic domain, is a critical region to study the Paleo-Tethyan Ocean evolution. Middle Permian–Late Triassic magmatic rocks are widespread in the Deqin–Weixi–Madeng area of southwestern (SW) Sanjiang Indosinian orogen, yet their petrogenesis and tectonic setting remain disputed. In this study, LA-ICP-MS zircon U-Pb age and Hf isotopes, and whole-rock elemental and Sr-Nd isotope geochemistry of Madeng dacite were studied. The Madeng dacite was dated at ca. 241.7 and 243.4 Ma. The samples had high Al2O3 (12.91 to 14.39 wt.%) but low MgO (0.62 to 1.76 wt.%) contents, and were alkali-rich (Na2O + K2O = 6.97 to 8.66 wt.%) with A/CNK > 1.1, strongly resembling peraluminous S-type granites. The rocks were enriched in Rb, K, Th, U and LREE, but depleted in Ba, Sr, Nb, Ta, P and Ti, and showed obvious negative Eu anomalies, suggesting fractionation of Ti-bearing minerals (e.g., rutile and ilmenite) and plagioclase. The dacite had an initial 87Sr/86Sr value of 0.705698 to 0.710277, and negative εNd(t) (−11.28 to −10.64) and εHf(t) (−13.99 to −8.60), indicating a continental meta-sedimentary source. Their average Nb/Ta (12.24) and Th/U (4.65) were also consistent with continental crust. According to the lithological assemblage and geochemical features, we propose that the Deqin–Weixi–Madeng area intermediate-felsic magmatism was generated in a subduction-related tectonic setting.