Geochronology, provenance, and tectonic setting of the meta-sedimentary rocks from the North Shahrekord metamorphic Complex, Iran

The meta-sedimentary rocks from the North Shahrekord metamorphic Complex (NSMC) comprise micaschists, paragneisses, quartz-feldspathic schists, and marbles. Detrital zircons from the paragneisses yield ages of ca. 2811–507 Ma with most of the dated grains belonging to the Late Neoproterozoic to Cambrian, suggesting a maximum depositional age of the sedimentary protolith during the Late Ediacaran to the Early Cambrian times (543 ± 12 Ma). In contrast, detrital zircons from the quartz-feldspathic schists are mostly dominantly 692–577 Ma and a few at ca. 2369–703 Ma, suggesting that the maximum depositional age of 577 ± 9 Ma, Late Neoproterozoic. The age distribution of detrital zircons suggests that their main provenance was located within the Arabian-Nubian Shield and adjacent terranes. Rutiles from a paragneiss give a U-Pb lower-intercept age of 181 ± 3 Ma, indicating the occurrence of a high-pressure metamorphic event during the Early Jurassic. The trace element contents of the two rock units show enrichment in LREE relative to HREE and negative Eu anomalies, indicating upper continental crustal sources. The geochemical evidence (e.g. low to moderate Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration, high Index of Compositional Variability (PIA), and low Th/U) suggests that the quartz-feldspathic schists and paragneisses originated from an immature, and immature to less mature, respectively, intermediate-felsic igneous source and their protolith experienced a simple sedimentary recycling history with relatively weak to moderate chemical weathering. Geochemical and petrographic data also suggest that the precursor to the quartz-feldspathic schists and paragneisses were deposited in a continental island arc and a back-arc basin setting, respectively. Our new data, including depositional age, provenance, and geochemical evidence, provide perspectives on palaeogeographic affinities (provenance of sediments with different weathering and recycling materials), and paleotectonic reconstructions (from arc to back-arc) of the Iranian basement, exposed in the Sanandaj-Sirjan Zone, during Ediacaran.

北沙赫尔科德变质杂岩（North Shahrekord metamorphic Complex, NSMC）中的变沉积岩包括云母片岩、副片麻岩、石英-长石片岩以及大理岩。对副片麻岩中的碎屑锆石定年结果显示，其年龄范围约为2811~507 Ma，绝大多数测年颗粒的形成时代集中于新元古代晚期至寒武纪，指示该沉积原岩的最大沉积年龄处于埃迪卡拉纪晚期至早寒武世（543±12 Ma）。与之相反，石英-长石片岩中的碎屑锆石年龄主要集中于692~577 Ma，少量年龄介于2369~703 Ma，表明其最大沉积年龄为577±9 Ma，时代为新元古代晚期。碎屑锆石的年龄分布特征显示，其主要物源区位于阿拉伯-努比亚地盾及邻近地体内部。对一块副片麻岩中金红石的定年得到U-Pb下交点年龄为181±3 Ma，指示早侏罗世时期发生过高压变质事件。两种岩石单元的微量元素含量均表现为轻稀土元素（LREE）相对于重稀土元素（HREE）富集，且具有负铕异常，表明其物源来自上陆壳。地球化学证据（例如低至中等的化学风化指数（Chemical Index of Alteration, CIA）与斜长石风化指数、较高的成分变异指数（PIA），以及较低的Th/U比值）显示，石英-长石片岩与副片麻岩分别源自未成熟与未成熟至弱成熟的中长英质火成物源，且其原岩经历了以偏弱至中等化学风化作用为特征的简单沉积再循环历史。地球化学与岩石学数据同时表明，石英-长石片岩与副片麻岩的原岩分别沉积于大陆岛弧与弧后盆地环境。本研究获得的新数据（包括沉积年龄、物源特征与地球化学证据），为萨南达季-西尔詹带出露的伊朗基底在埃迪卡拉纪时期的古地理亲缘性（即经历不同风化与沉积旋回的沉积物物源）以及古构造重建（从岛弧到弧后盆地的演化）提供了新的认识。