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

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, Al2O3, Yb, Re, and Th/Yb ratio) suggest that protoliths of the serpentinites originated as mantle res-idues. They underwent multiple episodes of melt depletion, leading to a highly refractory nature and high de-grees of partial melting (>18%). Moreover, subchondritic 187Re/188Os (0.0023-0.0399) and 187Os/188Os (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 (TRD), varying from 1.22 to 2.21 Ga, are analogues to their Re-Os model ages (TMA, 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.

Automated summary

The geochemical characteristics and Re-Os isotopes of serpentinites in the Qilian Orogenic Belt reveal their origin as mantle residues and multiple episodes of melt depletion, indicating two major periods of mantle melting during the Proterozoic.