胶西北上庄岩体岩石成因探讨——来自岩相学、地球化学、年代学与Sr-Nd同位素的制约

    Petrogenesis of the Shangzhuang pluton in northwestern Jiaodong: Evidence from petrography, geochemistry, geochronology, and Sr-Nd isotopic compositions

    • 摘要: 胶西北矿集区大规模金成矿作用与中生代岩浆活动有关, 其中尤以早白垩世岩浆作用关系最为密切. 对早白垩世上庄岩体进行了岩相学、地球化学、LA-MC-ICP-MS锆石年代学及Sr-Nd同位素研究. 结果表明, 上庄岩体稀土配分曲线与埃达克岩稀土曲线特征相似, 显示典型的埃达克岩地球化学特征. 岩体Sr-Nd同位素分析显示, 其87Sr/86Sr值为0.711 932, 143Nd/144Nd值为0.511 654, (87Sr/86Sr)i为0.511 577, εNd(t)值为-13.57, TDM2值为2 960 Ma. 根据LA-MC-ICP-MS锆石U-Pb年龄确定上庄岩体的形成年龄为128.7±0.29 Ma. 通过分析认为, 早白垩世早期太平洋板块俯冲后回撤, 幔源岩浆上涌提供持续热源, 导致下地壳铁镁质岩石脱水部分熔融形成中酸性岩浆, 与同期同条件下前寒武纪基底岩石熔融形成的酸性岩浆混合后, 沿扩张的走滑断裂上侵, 岩浆上升过程中受到上地壳玲珑期花岗岩的同化混染, 最终在合适位置形成上庄岩体.

       

      Abstract: The extensive gold mineralization in the Northwest Jiaodong metallogenic concentration area is genetically related with Mesozoic magmatism, particularly with the Early Cretaceous magmatic activity. This study presents an integrated analysis of the Early Cretaceous Shangzhuang pluton, involving petrography, whole-rock geochemistry, zircon U-Pb dating by LA-MC-ICP-MS, and Sr-Nd isotopic investigations. Results show that the rare earth element (REE) patterns of Shangzhuang pluton are similar to those of adakitic rocks, indicating its typical adakitic affinity. Sr-Nd isotopic analyses yield 87Sr/86Sr value of 0.711932, 143Nd/144Nd of 0.511654, initial (87Sr/86Sr)i of 0.511577, εNd(t) of -13.57, and a two-stage depleted mantle model age(TDM2) of 2 960 Ma. Zircon U-Pb dating constrains the emplacement age of the pluton to 128.7±0.29 Ma. Based on these data, it is proposed that during the early Early Cretaceous, the subduction and followed retracement of Pacific Plate induced upwelling of mantle-derived magmas, which provided a sustained heat source, triggering dehydration and partial melting of mafic lower crustal rocks to generate acidic-intermediate magmas. These magmas then mixed with contemporaneous acidic melts produced by partial melting of the Precambrian basement under similar thermal conditions. The hybrid magma then ascended along extensional strike-slip faults and underwent assimilation and contamination by upper crustal Linglong-type granites during its ascent. Ultimately, it was emplaced at favorable structural settings to form Shangzhuang pluton.

       

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