Geochemistry of the North Gondwana Paleozoic Araba and Naqus formation siliciclastics, Sinai: implications for provenance, paleoweathering, and tectonic setting

The Araba and Naqus formations in Ras Milan, Sinai, constitute part of the major continuous Paleozoic clastic belt stretching across northern Gondwana. They are sandstone-dominated successions, resting on Precambrian granitic basement, with lowermost thin basal polymictic conglomerate deposits (1–2 m), marking the unconformity. The Araba Formation section is composed mainly of varicoloured violet to reddish brown, fine to coarse-grained laminated and planner and overturned cross-bedded sandstones, while the overlying Naqus Formation is characterized by chaotically distributed poorly sorted quartz pebbles to cobbles within a moderately-sorted coarse- to medium-grained quartz arenite and by abundance and variety of large-scale tabular and trough cross bedding. Modal QFL classification of Araba and Naqus formation sandstones revealed that they are merely quartzose sandstones (quartz arenites). Geochemistry of the succession shows extremely wide compositional ranges for almost all major elements, but the Araba Formation is relatively enriched in SiO₂, TiO₂, Fe₂O₃*, Na₂O, P₂O₅, and depleted in Al₂O₃, MgO, CaO and K₂O relative to Naqus Formation. Only the Araba Formation samples are depleted in the transition elements Ni, Cr and Co and oppositely possess extreme enrichment in the HFSEs (i.e. Zr, Hf, Y, Nb, Ta, Th, U), whereas both formations show variable depletion in the LILE Rb, K and Ba, and excessive REE abundances relative to the PAAS and UCC, and LREE-enriched patterns with marked negative Eu anomalies. The provenance of the Naqus and Araba formations could be an earlier felsic magmatic source, which has undergone extensive sedimentary recycling. They possess high compositional maturity and were subjected to intensive to moderate weathering during humid climatic conditions. Passive margin tectonic setting is implied which denotes the derivation of Araba and Naqus formation siliciclastics from stable continental regions or plate interiors and deposition in intracratonic basins.

西奈半岛拉斯米兰（Ras Milan）地区的阿拉巴（Araba）组与纳库斯（Naqus）组，构成横贯北冈瓦纳大陆（northern Gondwana）的主要连续古生代碎屑岩带的一部分。二者为以砂岩为主的沉积序列，覆于前寒武纪花岗质基底（Precambrian granitic basement）之上，底部发育厚度1–2米的最薄层复成分砾岩（polymictic conglomerate）沉积，代表了不整合面（unconformity）。阿拉巴组剖面主要由杂色紫至红棕色、细至粗粒的纹层状、平板状及倒转交错层理砂岩组成；而上覆的纳库斯组则以基质为中等分选的中粗粒石英砂岩（quartz arenite）中杂乱分布分选差的石英砾至卵石为特征，同时发育大量多样的大型板状与槽状交错层理。对阿拉巴组与纳库斯组砂岩的QFL三角图解分类（Modal QFL classification）结果显示，二者均为纯石英质砂岩（quartzose sandstones）。该套地层的地球化学数据显示，几乎所有主量元素（major elements）的组成范围极宽，但相较于纳库斯组，阿拉巴组相对富集二氧化硅（SiO₂）、二氧化钛（TiO₂）、全铁（以Fe₂O₃计，Fe₂O₃*）、氧化钠（Na₂O）与五氧化二磷（P₂O₅），而亏损三氧化二铝（Al₂O₃）、氧化镁（MgO）、氧化钙（CaO）与氧化钾（K₂O）。仅阿拉巴组样品亏损过渡元素镍（Ni）、铬（Cr）与钴（Co），相反则极度富集高场强元素（HFSEs，High Field Strength Elements），即锆（Zr）、铪（Hf）、钇（Y）、铌（Nb）、钽（Ta）、钍（Th）与铀（U）；而两个地层均表现出大离子亲石元素（LILE，Large Ion Lithophile Elements）铷（Rb）、钾（K）与钡（Ba）的不同程度亏损，且相较于北美后太古宙澳大利亚页岩（PAAS，Post-Archean Australian Shale）与上地壳（UCC，Upper Continental Crust），稀土元素（REE）整体丰度偏高，轻稀土元素（LREE，Light Rare Earth Elements）富集且具显著的负铕异常（negative Eu anomalies）。纳库斯组与阿拉巴组的物源可能为早期长英质岩浆源（felsic magmatic source），且经历了广泛的沉积再旋回（sedimentary recycling）。二者具有较高的成分成熟度（compositional maturity），并在潮湿气候条件下经历了强至中等程度的风化作用。研究指示其形成于被动大陆边缘（passive margin）构造背景，意味着阿拉巴组与纳库斯组的硅质碎屑岩来源于稳定大陆区域或板块内部，沉积于克拉通内盆地（intracratonic basins）中。