LA-ICP-MS zircon U-Pb dating and geochemical characteristics of gabbro in Qingshuiquan, east section of East Kunlun, and its tectonic significance

Located in central Eastern Kunlun suture zone on the northern side of Qingshuiquan ophiolite, the Qingshuiquan gabbro consists mainly of hornblende gabbros. The LA-ICP-MS zircon U-Pb age is 452.1±5Ma, suggesting the formation of gabbro in Late Ordovician. The gabbro samples have 43.89%~45.99% SiO₂, 1.30%~2.32% Na₂O, 0.68%~1.39% K₂O, and 5.72%~6.54% MgO, with low P₂O₅ (0.26%~0.40%), TiO₂(0.74%~0.95%) and Mg^#(53~54). The geochemical characteristics indicate that the gabbro belongs to tholeiitic rocks; REE patterns show that the rocks are enriched in LREE and depleted in HREE; the trace elements are characterized by enrichment of LILE (Cs,Rb,Ba,especially positive anomaly of Ba) and obvious depletion of HFSE (Nb,Ta,Ti,Zr and Hf). Tectonic discrimination diagrams indicate that the gabbro formed in an island arc setting of active continental margin. Combined with regional studies, the authors hold that Qingshuiquan gabbro rocks might have formed at the initial stage of the northward subduction of Central Eastern Kunlun back-arc limited ocean, represented by the Qingshuiquan ophiolite.