ABSTRACT

The studied granitic gneiss bodies of the Golpayegan metamorphic complex, located in the central part of the Sanandaj Sirjan Zone (SaSZ), in wester n Iran. Zircon U-Pb dating of two samples shows that the crystallization of the protolith occurred at 557 ± 12 Ma in the Late Neoproterozoic (Ediacaran), broadly coeval to the Neoproterozoic-Early Palaeozoic basement in other parts of Iran. Geochemically, the protolith of the gneisses probably corresponds to differentiated I-type granites with subalkaline affinities in composition. The ratios of Y/Nb >1.2 reveal an affinity to Cordilleran I-type granites for the granitic gneisses. The high Th/U ratios (2.8 to 9), low Eu/Eu* (0.13 to 0.73), with, low contents of FeO (0.55 to 1.72 wt.%), MgO (0.07 to 0.53 wt.%) and MnO (0.01 to 0.04 wt.%) and the high ⁸⁷Sr/⁸⁶Sr_(i) ratios (0.70693 to 0.73557), negative ƐNd_(t) values (−4.4 to −1.7) and Nd model ages (T_DM2 = 1.35 to 1.56 Ga) suggest that the protolith may have been derived from partial melting of a pre-existing felsic crustal source (most likely differentiated granitoids). The new results reveal that the granitic source magma has been evolved in an active continental margin tectonic regime during the southward subduction of the Proto-Tethys ocean beneath the northern margin of Gondwana, like other coeval fragments dispersed in the entire Cadomian active continental margins. Also, the Cadomian crust widely extended in western Iran and it confirms these rocks have some clear affinity with Cadomian crust in the world.

摘要

研究的 Golpayegan 变质杂岩花岗片麻岩体位于伊朗西部的 Sanandaj Sirjan Zone (SaSZ) 中部。两个样品的锆石 U-Pb 年代测定表明，原岩的结晶发生在新元古代晚期（埃迪卡拉纪）的 557 ± 12 Ma，与伊朗其他地区的新元古代-早古生代基底大致同时发生。地球化学上，片麻岩的原岩可能对应于成分上具有亚碱性亲和性的分化I型花岗岩。Y/Nb >1.2 的比率表明花岗片麻岩与 Cordilleran I 型花岗岩具有亲和力。高 Th/U 比（2.8 到 9），低 Eu/Eu*（0.13 到 0.73），FeO（0.55 到 1.72 wt.%）、MgO（0.07 到 0.53 wt.%）和 MnO（ 0.01 至 0.04 wt.%) 和高⁸⁷ Sr/⁸⁶ Sr _(i)比率（0.70693 到 0.73557）、负 ƐNd _(t)值（-4.4 到 -1.7）和 Nd 模型年龄（T _DM2  = 1.35 到 1.56 Ga）表明原岩可能来自一个预先存在的长英质地壳来源（很可能是分化的花岗岩）。新结果表明，在原特提斯洋向南俯冲到冈瓦纳北部边缘下方期间，花岗质源岩浆已经在活跃的大陆边缘构造体系中演化，就像分散在整个卡多姆活跃大陆边缘的其他同时代碎片一样。此外，卡多姆地壳在伊朗西部广泛延伸，这证实了这些岩石与世界上的卡多​​姆地壳有一定的亲缘关系。