关键词: 大岔大坂, 辉长岩, 矿物学, 单斜辉石, 弧后盆地环境

Abstract:

Gabbro is a major component of oceanic crust and is of great significance for understanding the magma processes beneath different types of spreading ridges and their corresponding relationships with tectonic settings. This study systematically investigates the petrology, mineralogical and geochemistry of the gabbro bodies in the Dachadaban ophiolite in the North Qilian Orogen. Petrographic results show that these medium-fine-grained gabbros are mainly composed of plagioclase (around 60 vol.%), clinopyroxene (around 35 vol.%) and orthopyroxene (around 5 vol.%). Geochemical characteristics inidcate that the gabbros exhibit high-Mg features (MgO=8.90%-11.33 wt.%, Mg^#=76-80), with low-Ti (TiO₂=0.36%-0.49 wt.%) and low- Si (SiO₂=47.10%-47.94 wt.%) contents, while their CaO content is comparable to that of mid-ocean ridge basalt (MORB). Furthermore, these gabbros display extremely low rare earth element (REE) abundance with pronounced light REE (LREE) depletion, along with depletion in high field strength elements (HFSEs; e.g., Zr, Hf, Ti) and enrichment in large ion lithophile elements (LILEs). The overall composition of these entire rocks is similar to that of the back-arc basins. In terms of mineral composition, clinopyroxenes belong to diopside (Mg^#=78-80) with a distribution pattern of light rare earth element depletion, indicative of the differentiation products of MORB-type magma. The calculation of mineral equilibrium temperature and pressure yields that the crystallization temperature of gabbro is 1200-1246℃ , the pressure is 5.6-8.9 kbar, and the corresponding depth is 15-29 km, indicating characteristics of a deep magma chamber. Together with the previous results, the study suggests that the Dachadaban gabbro was formed in a back-arc basin setting.

Key words: Dachadaban, gabbro, mineralogy clinopyroxene, back-arc basin