Ordovician island arc volcanic rocks in Halahu area of Qilian Mountain and their tectonic significance

Based on 1:50000 regional geological survey, the authors studied volcanic rocks of Hulahu area in Qilian Mountain in the aspects of field geology, petrology, geochronology and geochemistry. Zircon LA-ICP-MS U-Pb dating results show that the Halahu volcanic rocks were formed in Middle Ordovician (466.3 ±2.4Ma, MSWD=1.4). Geochemical data show that the Halahu volcanic rocks belong to low-K tholeiite series, and are characterized by enrichment of LILEs (Rb, Th, U, Ce, Zr and Nd) and LREEs and depletion of Sr, P, Ti and Ba. On the whole, a few samples have features similar to those of T-MORB, but most samples show affinities with island arc related to subduction. In addition, the Halahu volcanic rocks were associated with Late Ordovician arc granitoids. These features indicate that the island arc rocks were associated with Ordovician paleo-oceanic closure in Lajishan area.