Palaeozoic diorites from the south‐western Dunhuang terrane, NW China: Constraints on tectonic evolution of southernmost CAOB

Abstract

The petrology, geochemistry, zircon U-Pb geochronology, and Hf isotopes of Duobagou diorite and Karatashtage diorite were studied to establish the evolution of Palaeozoic orogeny in the Dunhuang area in Western China. The weighted mean 206 Pb/ 238 U age of 448 ± 2 Ma in zircon grains from Duobagou diorite indicated an origin in the late Ordovician Period; age-corrected ε Hf (t) values of −0.98 to +4.46 correspond to a two-stage zircon Hf model age (T DM2 ) of 1,145 Ma to 1,477 Ma. Karatashtage diorite has a weighted mean 206 Pb/ 238 U age of 436 ± 3 Ma, indicating an origin in the early Silurian Period; age-corrected ε Hf (t) values of −1.13 to +7.96 correspond to a two-stage zircon Hf model age (T DM2 ) of 718 Ma to 1,301 Ma. These findings indicate that Duobagou and Karatashtage diorites were derived from ancient crust. Duobagou diorite has high levels of Al 2 O 3 (15.76%~17.64%), MgO (2.18%~5.44%), Mg # (40~51), Na 2 O (2.78%~4.19%), Sr (466~1,160 ppm), and Ba (523~892 ppm), low levels of K 2 O (1.29%~3.13%) and TiO 2 (0.64%~1.0%), variable levels of Cr (73.4~253 ppm) and Ni (42.2~98.8 ppm), and depleted HREEs. Karatashtage diorite is characterized by high levels of Al 2 O 3 (16.33%~17.59%), MgO (4.15%~7.10%), Mg # (50~59), Na 2 O (3.39%~4.41%), Cr (73.4~253 ppm), Ni (42.2~98.8 ppm), Sr (466~1,160 ppm), and Ba (523~892 ppm), low levels of K 2 O (1.29%~3.13%) and TiO 2 (0.64%~1.0%), and depleted HREEs. The Duobagou pluton and the Karatashtage pluton display arc signa-tures, suggesting that they were formed by the partial melting of a subducted slab under garnet-amphibolite facies in an active continental margin, and an interaction between melt and mantle materials during emplacement. Subduction of the oceanic plate in the Dunhuang terrane occurred during the Early Silurian Period (435-430 Ma), which is close to that of the Beishan orogenesis but later than the formation of the Hongliugou-Lapeiquan ophiolitic belt. We suggest that the Dunhuang terrane underwent an orogenic process related to the Palaeo-Asian Ocean during the Early Palaeozoic Era, and that it belongs to the Central Asian Orogenic Belt (CAOB).

Duobagou Permian–Triassic granites from the Dunhuang orogenic belt, NW China: implications for the tectonic evolution of the southernmost Central Asian Orogenic Belt

“…4), which represents the emplacement age of the pluton. In addition, six 206 Pb-238 U zircon ages at 411-448 Ma indicate that these are also inherited zircons, probably from early Palaeozoic magmatic rocks that have been reported in past research (Wang et al 2016b;Zhu et al 2019).…”

“…4), which could represent the intrusion age of this sample. Three ages (432-435 Ma) from the zircon cores indicate that these are inherited zircons, probably from Silurian magmatic rocks that have been reported in past research (Wang et al 2016b;Zhao et al 2017;Zhu et al 2019).…”

“…1a). The Dunhuang region, as a Palaeozoic orogenic belt and part of the CAOB, represents a vital component of the southernmost CAOB within the Tianshan-Beishan-Mongolia-Xing'anling orogen (Yue et al 2001;Xiao et al 2010;Zhao et al 2016;Wang et al 2018;Zhu et al 2019). Meanwhile, the Beishan orogenic belt located to the north side of the Dunhuang region is also a part of the southernmost CAOB.…”

“…The Dunhuang region is widely covered by Cenozoic sediments and exposes Precambrian tonalite-trondhjemite-granodiorite (TTG) gneisses, potassic granites and supracrustal rocks (Dunhuang Complex), and Silurian-Carboniferous magmatic and metamorphic complexes (Mei et al 1997;Zhang et al 2013;Zong et al 2013;He et al 2014;Wang et al 2016a;Zhao et al 2017Zhao et al , 2019Zhu et al 2019) (Fig. 1b).…”

“…The Dunhuang region has long been regarded as the Precambrian basement of the Tarim Craton (TC) or the North China Craton (NCC). However, recent studies have demonstrated that the Dunhuang region is part of the CAOB and shares a similar geological history with the Beishan orogeny (Zhao et al 2016(Zhao et al , 2019Feng et al 2018;Zhu et al 2019;Xu et al 2019). In the southern CAOB, the Beishan belt underwent a complex tectonic evolution and crustal growth during the Palaeozoic Era, and the widespread late early Palaeozoic granitic rocks in the southern Beishan orogeny constrain the timing of the collisional orogenic event (Zhang et al 2011;Li et al 2012;He et al 2014;Zhao et al 2017).…”