Middle Cambrian granites in the Dunhuang Block (NW China) mark the early subduction of the southernmost Paleo-Asian Ocean

Abstract

The formation and evolution of the Central Asian Orogenic Belt (CAOB) were closely associated with the opening and closure of the so-called “Paleo-Asian Ocean” (PAO). The subduction of the southern PAO is expected to have resulted in widespread Paleozoic arc magmatism in the southernmost CAOB. However, the precise location and timing of such subduction remain uncertain. The Dunhuang Block (DHB) in the southernmost CAOB is thought to have recorded such subduction. In this contribution, we report the results of the zircon U–Pb ages, Hf isotopes, and whole-rock geochemical data for the Lianghu, Dapokouzi, and Xiaowan plutons in the northeastern DHB, NW China. Zircon U–Pb dating reveals that these plutons formed at ca. 510 Ma (the Middle Cambrian) and may represent the oldest arc-related magmatism in the DHB. Geochemically, the Dapokouzi pluton is characterized by high SiO2 (70.7–71.2 wt%), Na2O (5.11–5.26 wt%), Al2O3 (15.5–15.6 wt%), and Sr (730–733 ppm) contents, high Sr/Y (380–398) ratios, moderate Mg# (49–50) values, low MgO (0.61 wt%), K2O (2.00–2.01 wt%) and Y (1.84–1.93 ppm) contents, and weakly positive Eu (1.39–1.44) anomalies, indicating geochemical affinities of adakitic rocks. They have positive εHf(t) values (+11.2 to +13.9) close to the depleted mantle line and young two-stage model ages (TDM2 = 571–742 Ma). All the geochemical characteristics show that the Dapokouzi pluton was derived from the remelting of juvenile crust with addition of minor metasomatized mantle-derived materials and a residue phase dominated by garnet-bearing amphibolite. Similarly, the Lianghu and Xiaowan plutons have high SiO2 (70.2–74.4 wt%) and Al2O3 (12.8–14.3 wt%), low MgO (0.61–1.01 wt%) with Mg# values of 35–43 and K2O (0.43–1.55 wt%) contents. Compared to the Dapokouzi pluton, they exhibit low Sr (121–155 ppm) and Y (15.3–23.1 ppm) contents, low Sr/Y (5.25–9.54) ratios and Cr-Ni-Co concentrations, and pronouncedly positive εHf(t) values (+11.0 to +14.7), indicating that the magmas originated from the remelting of juvenile lower crust followed by significant fractional crystallization at relatively shallow crustal depths. Data from this and previous studies convincingly suggest the DHB was located in a thickened continental arc setting during the early Paleozoic, probably related to the early subduction of the PAO lithosphere. Crustal thickening, as indicated by the geochemical signature of the Dapokouzi pluton, was most likely associated with the underplating of mantle-derived mafic magmas during the slab subduction. This study therefore highlights that the subduction of the southern PAO might have initiated prior to 510 Ma, possibly dating back to the early Cambrian.