Late Cambrian tonalite-trondhjemite association in the eastern segment of North Qilian suture zone: petrogenesis and geodynamic implications

The late Cambrian tonalite-trondhjemite association in the eastern segment of North Qilian suture zone (E. NQSZ) is important for understanding of the sub-arc magmatic process and tectonic evolution. Here we report data for feldspar-quartz porphyry and fine-grained trondhjemite with abundant tonalite enclave/interlayer from the Leigongshan (LGS) pluton, and medium-grained tonalite from the Beilinggou (BLG) pluton. Zircon U‒Pb ages suggest emplacement of the plutons at ca. 506‒481 Ma, associated with early subduction of the North Qilian Ocean. The LGS tonalite enclave/interlayers and trondhjemite hosts share many characteristics in common, including the same mineralogy and texture, similar mineral chemistry, whole-rock trace element ratio and Sr‒Nd isotopic compositions. The tonalite enclave/interlayers probably represent quenched blobs/layers of an evolved, tonalitic magma branch resulting from periodic injection of magma into the magma chamber. All the LGS-BLG tonalite-trondhjemites display low abundances of incompatible trace elements (e.g., K, Rb, Th, Zr and REE) and depleted Sr‒Nd‒Hf isotopes (⁸⁷Sr/⁸⁶Sr = 0.707490‒0.707840; ε_Nd(<i>t</i>) = +4.0‒4.3; zircon ε_Hf(<i>t</i>) = +4.0‒13.1), indicating little/no contribution of the continental crust. Geochronological and geochemical consistency between the coeval arc sodium-rich volcanic rocks and studied tonalite-trondhjemites further suggests crystal fractionation of a common basaltic parental magma. The parental magma was generated in continental-arc rifting environments due to slab rollback in the early stage of subduction.

北祁连缝合带东段（E. NQSZ）的晚寒武世英云闪长岩-奥长花岗岩组合（tonalite-trondhjemite association），对于理解弧下岩浆作用与构造演化具有重要意义。本文报道了采自雷公山（LGS）岩体的长石石英斑岩、含丰富英云闪长岩包体/夹层的细粒奥长花岗岩，以及北岭沟（BLG）岩体的中粒英云闪长岩的相关数据。锆石U-Pb年代学结果显示，两岩体的侵位时代约为506~481 Ma，与北祁连洋早期俯冲作用相关。雷公山岩体中的英云闪长岩包体/夹层与寄主奥长花岗岩具有诸多共性，包括一致的矿物学与结构特征、相似的矿物化学组成、全岩微量元素比值以及Sr-Nd同位素组成。英云闪长岩包体/夹层可能代表了演化后的英云闪长质岩浆分支经周期性注入岩浆房后淬冷形成的团块/层体。所有雷公山-北岭沟英云闪长岩-奥长花岗岩均表现出低不相容微量元素（如K、Rb、Th、Zr及稀土元素（REE））含量，以及亏损的Sr-Nd-Hf同位素组成（⁸⁷Sr/⁸⁶Sr=0.707490~0.707840；ε_Nd(t)=+4.0~+4.3；锆石ε_Hf(t)=+4.0~+13.1），表明其源区几乎未受到大陆地壳物质的贡献。同期富钠弧火山岩与本次研究的英云闪长岩-奥长花岗岩之间的年代学与地球化学一致性，进一步指示其源自同一玄武质母岩浆的晶体分异作用。该母岩浆形成于俯冲早期因板片回撤引发的陆弧裂谷环境。