Prolonged Slab-derived Silicate and Carbonate Metasomatism of a Cratonic Mantle Wedge (Maowu Ultramafic body, China)
Authors: Zhao Yi , Zheng Jian-Ping , Xiong Qing 
Published: September 2021

Mass transfer between crust and mantle at continental margins is a crucial process in shaping mantle heterogeneity and material cycling in deep Earth. Mass transfer is usually archived by the slab-derived and/or asthenosphere-derived component, which metasomatize a cratonic mantle wedge. Clarifying the origin and temporal sequences of these records is a fundamental aim yet is poorly resolved. Comprehensive petrography, whole-rock and mineral compositions [including platinum group elements (PGE)], zircon U–Pb ages, trace elements, and Hf isotopes of the Maowu ultramafic body (comprising harzburgites and garnet orthopyroxenites) in the Dabie orogen (Central China) are presented in this study to decode the complex metasomatic histories beneath the southern North China Craton (NCC). The ultramafic body represents a typical cratonic mantle-wedge fragment with an equilibration pressure of 4·0 ± 1·0 GPa and temperature of 750 ± 50 °C. The harzburgites have high Mg# values (up to 92) and Ni contents (2537–2892 ppm), low Al2O3 (0·26–0·76 wt%) and CaO (0·05–0·32 wt%) contents, and high olivine Fo values (91–93), supporting the origin from a cratonic mantle wedge with partial melting extents of ~20–25 %. Garnet orthopyroxenites occurring as veins in the harzburgites have a wide range of Mg# values (83–91), high Ni contents (963–2353 ppm), and significant enrichment in light rare earth elements and large ion lithophile elements. They show PGE contents and patterns similar to those of the host harzburgites. These characteristics, combined with geochemical modeling, suggest that the garnet orthopyroxenites were produced through the reaction of the harzburgites with silicate melts derived from the continental crust. Three types of zircons were identified in the Maowu ultramafic body. The group-I zircons with U–Pb ages ranging from ~1927 to ~465 Ma were separated from the harzburgites. They are characterized by oscillatory zones, high heavy rare earth element (HREE) contents, a wide range of εHf(t) values (from −24·9 to +21·0), and felsic inclusions (e.g. quartz and plagioclase). They are interpreted as records of the intense silicate-melt metasomatism triggered by the addition of crust-derived silicate sediments (containing Proterozoic detrital zircons) into the NCC mantle wedge. This metasomatism formed the garnet orthopyroxenite veins in the southern NCC mantle wedge. The group-II zircons have dark cores and bright rims and yield a range of U–Pb ages from 421 ± 8 to 256 ± 6 Ma in both the harzburgites and garnet orthopyroxenites. They contain high Th and U contents (up to 5000 ppm) and display steep HREE patterns, negative Eu anomalies, and positive εHf(t) values (+0·6 to +8·2). Carbonates were identified as inclusions in the zircons and as pods or veinlets in the matrix of the harzburgites. These features may reflect a long-term and pervasive metasomatism by oxidized carbonate fluids in the garnet orthopyroxenite-veined mantle wedge, coeval with the secular subduction of the Tethyan slab that carried mainly sedimentary carbonates. The group-III zircons from both the harzburgites and garnet orthopyroxenites show concordant U–Pb ages of 227 ± 6 Ma. They are interpreted to record the Triassic continental collision between the Yangtze Craton and NCC. Collectively, this study provides geochemical, mineralogical, and petrological evidence that the periphery of a cratonic mantle with prolonged fluxing and various metasomatism can archive the complex history along a convergent margin that evolved from oceanic subduction to continental collision.