Mineralogy and geochemistry of cumulates from the Hongguleleng ophiolitic mélange, western Junggar, Xinjiang: Implications for the origin and tectonic setting

Abstract

The Hongguleleng ophiolitic mélange is located in western Junggar, Xinjiang, which has relatively complete rock assemblages. The well-developed cumulate facies is an important feature and can provide important clues for studying the genesis and tectonic setting of the ophiolitic mélange. The Hongguleleng ophiolitic mélange can be classified into metamorphicperidotite, ultramafic-mafic cumulates and volcanic rocks from bottom to top. The cumulates include ultramafic cumulates and mafic cumulates. The ultramafic cumulates are mainly composed of cumulate peridotite and dunite, which are the major lithofacies hosting chromite. The mafic cumulates consist of troctolite and gabbro, which are the end-member or transition types with olivine-mafic plagioclase-clinopyroxene. The results show that olivine in the cumulates is chrysolite (Fo = 83–88), the clinopyroxene is diopside (En = 44–59) and the plagioclase is bytownite (An = 70–87). Mafic cumulates have similar distribution patterns of rare earth elements (REEs) and trace elements, such as obvious positive Eu anomalies that indicate the accumulation of plagioclase. The εNd(t) values of the Hongguleleng cumulates range from 3.47 to 6.11, which indicates that their source was mid-ocean ridge basalt (MORB). According to the chemical compositions of chromian spinel, we calculated that the parent magma melt contained 12.95–16.25 wt% Al2O3, 0.59–1.90 wt% TiO2 and 1.43–3.96 FeO/MgO, which is basically equivalent to the composition of MORB. Petrological and geochemical characteristics show that the diagenetic process of the Hongguleleng cumulates was mainly controlled by crystallization differentiation. The cumulates may have formed in a plate tectonic mid-ocean ridge spreading environment. The cumulate chromitites were formed through fractional crystallization of MORB melts.