中文摘要：

四方台杂岩体是沿北秦岭商-丹带出露的最大的基性-超基性杂岩体，出露面积约40km^2，由超基性-基性-中性岩石组成，侵入到丹凤群中。该岩体样品SiO2含量在45％～54％之间，在AFM图中则显示出钙碱性系列和拉斑系列共存的特征。与普通幔源岩相比，全岩富集稀土元素，尤其富集轻稀土元素，具有明显的Rb、Th、Nb、P、Zr负异常和Ba、Pb、K、Sm正异常。辉长岩是该岩体的主体，灰黑色，中粒，地球化学特征显示其经历壳-幔物质交换。与典型岛弧辉长岩相比，具有Rb、Th负异常，结合特殊的岩石组合和紧邻商-丹断裂的地质特征，认为四方台镁铁-超镁铁质杂岩体可能是沿商-丹带发生俯冲板块断离或者山根拆沉，扰动软流圈，发生下地壳部分熔融和壳-幔物质交换形成的。普遍的Eu正异常表明成岩过程中受到斜长石堆晶作用的影响。

英文摘要：

The Sifangtai complex is the largest mafic-ultramafic intrusion along the Shang-Dan fault zone in the North Qinling orogenic belt. It covers an area of -40 km^2, consists mainly of ultramafites, pyroxenite and minor intermediate rocks, and intruded into Danfeng Group. The SiO2 contents of the Sifangtai complex range from 45% to 54%. These rocks belong to two series, tholeiitic and calc-alkaline series. Compared to common mantle rocks, they have high REE concentrations, especially LREE, with negative Rb, Th, Nb, P and Zr anomalies and positive Ba, Pb, K and Sm anomalies. Gabbro is the main component of the complex, with grey-black in color and medium granularity. Its geochemical characteristics indicate that it is likely to be a product of crust-mantle interaction. In comparison with typical arc-related gabbros, it has negative Rb and Th anomalies. Considering that it is located near the Shang-Dan zone, we propose that the formation of the Sifangtai mafic-ultramafic complex was due to the break-off of the subducted slabs or the delamination of the mountain root along the Shang-Dan＇fault zone, which disturbed the asthenosphere, resulting in partial melting of lower crust as well as mantle-crust interaction. The Sifangtai complex was affected by plagioclase accumulation as indicated by positive Eu anomaly.