Early Paleozoic intermediate igneous rocks in the Dunhuang Block, southern Central Asian Orogenic Belt: geochronology, petrogenesis, and implications for tectonic evolution

Late Ordovician-Early 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 U-Pb 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 U-Pb geochronology shows that the emplacement ages of the gneissic dioritic rocks were c. 443-440Ma. The dioritic rocks show varying SiO2 (48.1-63.1wt%) and MgO (1.87-3.52wt%) contents, with high Mg# (46-52) values and negative Eu anomalies (Eu/Eu*=0.62-0.94). Zircons in the gneissic diorites from the northern Huangshuigou and Sanweishan areas yield variable epsilon(Hf)(t) values of -4.4 to +10.4 and -8.6 to -6.2, respectively, coupled with low Rb/Sr (0.04-0.34), Rb/Ba (0.06-0.31) and Al2O3/(MgO+FeOT) (0.72-1.47) 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 magmatism-metamorphism over the whole Dunhuang area.