Proterozoic mantle melting recorded by the Re-Os isotopic systematics of ophiolites from the Qilian Orogenic Belt, northwestern China

Abstract

• The Minhe serpentinites are mantle residues of multi-episode partial melting. • Fluid/melt metasomatism exerted a limited impact on the Re-Os systematics. • Re-Os ages provide temporal constraints on Proterozoic melt extraction. Re-Os isotopes of the mantle-derived ultramafic rocks can provide temporal constraints on the melt extraction of ancient mantle. Here, new geochemistry and Re-Os isotopes of serpentinites from the Minhe ophiolite in the Qilian Orogenic Belt (QOB) are presented. Their geochemical compositions (e.g., abundances of heavy rare earth elements, Al 2 O 3 , Yb, Re, and Th/Yb ratio) suggest that protoliths of the serpentinites originated as mantle residues. They underwent multiple episodes of melt depletion, leading to a highly refractory nature and high degrees of partial melting (> 18%). Moreover, subchondritic 187 Re/ 188 Os (0.0023 – 0.0399) and 187 Os/ 188 Os (0.1135 – 0.1210) ratios of the serpentinites, and decoupling of the Re-Os systematics and metasomatic proxies (e.g., LOI, Sr, La, and Th/Yb), demonstrate that fluid/melt metasomatism exerted a limited impact on their Re-Os isotopic systematics. Their Re-depletion ages (T RD ), varying from 1.22 – 2.21 Ga, are analogues to their Re-Os model ages (T MA , 1.29 – 2.31 Ga). Therefore, Re-Os isotopic ages recorded by ophiolites in QOB indicate that at least two major melt-extraction episodes occurred in the lithospheric mantle during Proterozoic (1.2 – 1.6 Ga and 2.0 – 2.2 Ga). Notably, the 1.2 – 1.6 Ga mantle melt-depletion event may have a genetic affinity with the 1.1 – 1.7 Ga crustal growth. Mantle rocks from an ophiolite have a great potential in tracking ancient geological events, such as Precambrian mantle melting.