Abstract
Late OrdovicianâEarly Silurian intermediate igneous rocks have been detected in the Dunhuang Block, NW China. These igneous rocks help to constrain the tectonic evolution of the Dunhuang Block and the southern Central Asian Orogenic Belt in the early Paleozoic. We carried out zircon UâPb dating, whole-rock geochemistry and zircon Hf isotope analyses for the early Paleozoic gneissic diorites from the Sanweishan and northern Huangshuihou areas in the Dunhuang Block. The zircon UâPb geochronology shows that the emplacement ages of the gneissic dioritic rocks were c. 443â440 Ma. The dioritic rocks show varying SiO 2 (48.1â63.1 wt%) and MgO (1.87â3.52 wt%) contents, with high Mg# (46â52) values and negative Eu anomalies (Eu/Eu* = 0.62â0.94). Zircons in the gneissic diorites from the northern Huangshuigou and Sanweishan areas yield variable Δ Hf ( t ) values of â4.4 to +10.4 and â8.6 to â6.2, respectively, coupled with low Rb/Sr (0.04â0.34), Rb/Ba (0.06â0.31) and Al 2 O 3 /(MgO + FeO T ) (0.72â1.47) ratios, indicating that they were predominately generated by disequilibrium melting of the juvenile crust and mixed with ancient crustal and minor mantle melts. In conjunction with previously published data, we propose that the early Paleozoic gneissic dioritic rocks were formed in a continental margin arc setting related to the southward subduction of the Palaeo-Asian Ocean. Our findings suggest that the Dunhuang Block was modified and reactivated by the Paleozoic orogenic events related to the evolution of the Palaeo-Asian Ocean, resulting in extensive Paleozoic magmatismâmetamorphism over the whole Dunhuang area. Supplementary material: Analytical methods, U-Pb geochronological data, zircon trace element data, zircon Lu-Hf isotopic compositions, and geochemical data for analysed samples, and additional figures are available at https://doi.org/10.6084/m9.figshare.c.6174601
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