Mineralogical and geochemical constraints on the origin of the ultramafic rocks from Wuwamen ophiolite on the southern margin of Middle Tianshan Mountains, Xinjiang

Abstract: The Wuwamen ophiolite is located on the southern margin of Middle Tianshan Mountains, Xinjiang. In this paper, petrology, mineralogy and geochemistry of the ultramafic rocks from Wuwamen ophiolite were studied to constrain their origin and tectonic setting. The ultramafic rocks from Wuwamen ophiolite are serpentinized lherzolites composed of olivine (Fo=89.1-90.6), orthopyroxene (Wo0.4-2.4En87.2~90.7Fs8.9-10.9; Mg#=89.0-91.0), clinopyroxene (Wo49.1-51.3En16.0~48.4Fs0.9-4.3; Mg#=90.2-92.1) and spinel (Mg#=71.8-77.5; Cr#=9.3-13.4). They are characterized by relatively lower content of MgO (37.74%-41.34%), and higher content of Al2O3 (2.58%-3.39%), CaO (2.23%-3.68%) and TiO2 (0.05%-0.11%). The ultramafic rocks are also depleted in rare earth elements (REE; total REE = 1.73×10?6-4.63×10?6) and incompatible elements (e.g., Rb=0.4×10?6-1.39×10?6, Zr=0.73×10?6-3.28×10?6, Hf=0.04×10?6-0.11×10?6), and enriched in compatible elements (e.g., Cr=2516×10?6-2793×10?6, Co=84.6×10?6-119×10?6, Ni=1641×10?6-2261×10?6). These data indicate that the ultramafic rocks from Wuwamen ophiolite are residues of mantle rocks after low degrees (ca. 5%-10%) of partial melting in a Mid-Ocean-Ridge (MOR) environment.