Zircon-bearing metasomatized peridotite from early Paleozoic Tongbai Orogen sub-arc mantle trapped between the North China and Yangtze cratons

The zircon-bearing Liushuzhuang Peridotite in the Tongbai Orogen provides insights into the composition and evolution of Paleozoic sub-arc lithospheric mantle between the North China and Yangtze cratons. This orogenic peridotite is dominated by clinopyroxene-free spinel facies harzburgite to dunite. The refractory nature is indicated by high olivine Mg# (up to 92.4), elevated spinel Cr# (74–87), and low orthopyroxene Al 2 O 3 (mostly < 1.0 wt.%). The occurrence of zircon, amphibole, phlogopite, apatite, and LREE-enriched orthopyroxene, with Raman probe analyses showing amphibole and phlogopite inclusions in zircon, testifies to extensive fluid-driven metasomatism of the mantle at ca. 420 Ma (U–Pb zircon). The high spinel TiO 2 contents (0.4–1.4 wt.%), the Cl-enriched apatite, and the hydrous secondary phases indicate that the metasomatic agent was volatile- (CO 2 + H 2 O) and TiO 2 -rich. The elevated oxygen fugacity (∆log f O 2 (FMQ) > 2.5) coupled with high spinel Cr# of peridotites suggest that they formed within highly oxidizing supra-subduction zone environment. In-situ 87 Sr/ 86 Sr i amphibole and apatite data indicate that the metasomatic fluids were slightly to moderately radiogenic (0.7032–0.7090) and therefore likely partially derived from the subducted Shangdan oceanic crust that separated the North China and Yangtze cratons. The very low 176 Lu/ 177 Hf (mostly < 0.0002) of metasomatic zircon and similar ε Hf (t) to Tongbai Orogen arc-related magmas implies that zircon grew during modification of sub-arc mantle. Juxtaposition with the host gneisses occurred after 410 Ma and before U–Pb closure during apatite cooling at ~ 340 Ma. The Liushuzhuang Peridotite therefore records intense metasomatism of the Paleozoic sub-arc mantle wedge prior to collision of the North China and Yangtze cratons.