Geochemical characteristics and zircon U-Pb age of gabbros in the Zhaqiaohe ophiolite mélange, and its limitation on the ocean ridge environment

This paper is the result of geological survey engineering.

Objective Zhaqiaohe ophiolite is located at the junction of the Middle Qilian and South Qilian tectonic belt. The ophiolite in the belt is composed of serpentinite, serpentinized pyroxene peridotite, gabbro, diabase, block bedded altered basalt, a small amount of pillow basalt and lenticular silicite. It is the west extension of the Lachishan ophiolite melange belt in Qinghai Province.

Methods In this paper, the study on the chronology and petrogeochemistry of gabbro in Zhaqiaohe ophiolite has been carried out.

Results According to the characteristic judgment of the main chemical components of the gabbro, such as SiO₂, Na₂O, K₂O, (Fe₂O₃ + FeO) and MgO, the gabbro is tholeiitic series, belonging to tholeiite. According to the REE distribution pattern, there is no Eu anomaly or weak negative anomaly, indicating that the rock experienced the separation and crystallization of plagioclase in the melting process, and the REE distribution curve is approximately flat; the trace elements have obvious Ta and Sr positive anomaly characteristics, indicating that gabbro in ophiolite in this area has similar characteristics with E-MORB. The results of LA-ICP-MS zircon U-Pb dating show that the gabbro crystallization age is (524.0±2.5) Ma.

Conclusions It is suggested that these gabbros originated from the initial slowness without large-scale melting. It is suggested that the Zhaqiaohe ophiolite may have formed in the oceanic ridge environment during the initial cracking stage of the paleo ocean basin.