Petrogenesis and metallogenic implications of Late Mesozoic intrusive rocks in the Tongling region, eastern China: a case study and perspective review

Large-scale Cu–Au mineralization is associated with Late Mesozoic intrusive rocks in the Tongling region of eastern China, which mainly comprise pyroxene monzodiorite, quartz monzodiorite, and granodiorite. To constrain the petrogenesis of the intrusive rocks and Cu–Au mineralization, detailed analyses of the geochronology, apatite <i>in situ</i> geochemistry, whole-rock geochemistry, and zircon Hf isotopic compositions were performed. Magmatic zircons from pyroxene monzodiorites, quartz monzodiorites, and granodiorites yield U–Pb ages of 136–149 Ma, 136–146 Ma, and 138–152 Ma, respectively, indicating that their formation ages are contemporaneous. Quartz monzodiorites and granodiorites (SiO₂ = 57.9–69.5 wt.%) are highly potassic calc-alkaline rocks with adakitic affinity and have low MgO and Y contents, low zircon ε_Hf(<i>t</i>) values (−11.7 to −39.0), high apatite Cl contents (&gt;0.2 wt.%), and log fO₂ values (−23.2 to −8.23), indicating that they may have formed when metasomatized mantle-derived magmas mixed with slab-derived magmas before undergoing crustal assimilation and fractional crystallization. Pyroxene monzodiorites (SiO₂ = 48.4–53.0 wt.%) are shoshonitic and record high MgO, P₂O₅, and Y contents, high zircon ε_Hf(<i>t</i>) values (1.55 to −7.87), high oxygen fugacity, low Nb and Ta contents, and low apatite Cl contents (mainly &lt;0.2 wt.%), suggesting that they were primarily derived from a metasomatized lithospheric mantle-derived magma that experienced the assimilation of lower crustal materials. The results indicate that the intrusive rocks and associated large-scale Cu–Au mineralization of the Tongling region resulted from the partial melting of the subducted oceanic slab in an oxidizing environment.

中国东部铜陵地区大规模铜金（Cu-Au）成矿作用与晚中生代侵入岩相关联，该类侵入岩主要包括辉石二长岩（pyroxene monzodiorite）、石英二长岩（quartz monzodiorite）以及花岗闪长岩（granodiorite）。为限定该侵入岩的岩石成因与铜金成矿机制，研究人员对其开展了年代学、磷灰石原位（in situ）地球化学、全岩地球化学以及锆石Hf同位素组成的详细分析。采自辉石二长岩、石英二长岩与花岗闪长岩的岩浆锆石U-Pb年龄分别为136~149 Ma、136~146 Ma与138~152 Ma，表明三者的形成时代近乎同步。石英二长岩与花岗闪长岩（SiO₂含量为57.9~69.5 wt.%）属于高钾钙碱性岩石，兼具埃达克质（adakitic）亲和性，具有低MgO与Y含量、低锆石ε_Hf(t)值（-11.7~-39.0）、高磷灰石Cl含量（>0.2 wt.%）以及log fO₂值（-23.2~-8.23），指示其形成于交代富集地幔源岩浆与板片源岩浆混合，并随后经历地壳同化混染与分离结晶作用的过程。辉石二长岩（SiO₂含量为48.4~53.0 wt.%）属于钾玄岩（shoshonitic）类岩石，表现出高MgO、P₂O₅与Y含量，高锆石ε_Hf(t)值（1.55~-7.87），高氧逸度，低Nb与Ta含量以及低磷灰石Cl含量（多<0.2 wt.%），表明其主要源自交代富集岩石圈地幔源岩浆，并经历了下地壳物质的同化混染作用。研究结果表明，铜陵地区的侵入岩及其伴生大规模铜金成矿作用，形成于氧化环境下俯冲洋壳的部分熔融过程。