Age and composition of the lithospheric mantle beneath Jiande, Zhejiang Province: Implications for crust-mantle co-evolution of the South China Block

Abstract

Fresh mantle peridotite xenoliths entrained in the Eocene Jiande alkaline basalts from western Zhejiang Province, South China Block (SCB), offer an opportunity to elucidate the compositional and temporal relationships between the subcontinental lithospheric mantle (SCLM) and its overlying crust. The Jiande peridotites (20 lherzolites + one harzburgite) exhibit tightly correlated and relatively fertile whole-rock major element compositions (WR Mg#=89.0–90.8), indicating mild to moderate degrees (mostly ≤ 10 %) of mantle partial melting and limited metasomatism experienced by the Jiande SCLM. The above mantle processes are further underscored by: 1) the nearly unfractionated highly-siderophile-element (HSE) relative abundances ((Pd/Ir)_N = 0.81–1.37, normalized to primitive-upper-mantle values); 2) the robust positive correlations observed between ¹⁸⁷Os/¹⁸⁸Os (0.1196–0.1283) and ¹⁸⁷Re/¹⁸⁸Os ratios (0.112–0.451; R² = 0.79), and Al₂O₃ contents (1.5–3.8 wt%; R² = 0.85) across the majority of high-¹⁸⁷Os/¹⁸⁸Os samples. Both Re-Os (initial ¹⁸⁷Os/¹⁸⁸Os = 0.1172) and Al-Os isochrons (initial ¹⁸⁷Os/¹⁸⁸Os = 0.1181) also yield an age of ∼ 1.4 Ga for the predominant melt depletion event that shaped the formation of the Jiande SCLM, aligning well with the peak of two-stage depleted mantle Hf model ages of the SCB crustal rocks. This age-coupled crust-mantle differentiation event occurring in the Mesoproterozoic may have been controlled by tectonic-magmatic processes during the breakup of the Columbia supercontinent. In addition, two low-¹⁸⁷Os/¹⁸⁸Os (0.1108–0.1128) samples with ∼ 15–20 % melting degrees ((Pd/Ir)_N = 0.15–0.30) show the oldest Re-depleted Os model ages (2.2 and 2.4 Ga) among all the reported SCB mantle peridotites. These older SCLM materials may represent remnants linked to earlier crust-forming events that were incompletely reworked or replaced. Alternatively, they could represent mantle heterogeneity inherited by the newly formed lithospheric mantle during the Mesoproterozoic era.