Petrography and geochemistry of siliciclastic rocks of the Middle Eocene Gercus Formation, northern Iraq: Implications for provenance and tectonic setting
Petrography and geochemistry of siliciclastic rocks of the Middle Eocene Gercus Formation, northern Iraq: Implications for provenance and tectonic setting
复制标题
伊拉克北部中始新世 Gercus 组硅质碎屑岩的岩相学和地球化学：对物源和构造环境的影响
DOI：
10.1002/gj.3880
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发表时间：
2020
影响因子：
1.8
通讯作者：
F. Mohammad
中科院分区：
文献类型：
--
作者：
Y. Kettanah;J. Armstrong;F. Mohammad
红色中始新世Gercus组是一个广泛暴露的岩石单元，分布在伊拉克北部、土耳其东南部和伊朗西北部。硅屑岩（泥岩和砂岩）是该组的主要岩石，经岩石学和地球化学研究。Gercus组还含有一些石灰岩、石膏和砾岩层。砂岩有红白两种类型，以未成熟岩屑岩为主，由岩屑（85%）、石英（12%）和长石（3%）组成。岩石碎片主要由角状燧石和圆形石灰岩组成，少量的沉积、岩浆、火山和变质岩石表明它们的源岩接近。砂岩中碎屑颗粒数的物源判别图显示出循环造山带和碰撞造山带。岩石学、重矿物和主要和不可移动的微量元素（如La、Th、Sc、Zr、Hf和Ti）鉴别图表明，Gercus组起源于俯冲带之上的碰撞构造环境，是大洋岛弧（OIA）的一部分，其沉积物来源混合。包括伊拉克北部至东北部以及邻近的伊朗和土耳其的富含燧石和碳酸盐的地层和火成岩变质蛇绿岩复合体的再循环沉积物。研究结果还表明，Gercus组具有海相（湖泊和三角洲）和非海相（河流）的混合成因。红砂岩、白砂岩和泥岩在主要元素、微量元素和稀土元素（REE）含量上存在细微差异，并具有大致相似的后太古代澳大利亚页岩（PAAS）‐、上大陆地壳（UCC）‐和CI‐球粒陨石‐标准化分布模式，表明它们具有共同的起源。高场强元素（HFSE）、大离子亲石元素（LILE）和稀土元素（REE）在硅质碎屑岩中平均富集；相对于PAAS和UCC，部分岩石形成元素（RFE）和地幔形成元素（MRFE） （Mg、Ca、Cr、Co、Ni）富集，部分元素（Si、Al、Fe、Cu、Zn、Ga）富集；然而，与CI -球粒陨石相比，它们在Fe， Mn, Mg, Co, Ni, Cu, Zn, Ga， Na和P中缺乏，并且在所有其他RFE， MRFE， LILE， HFSE和REE中富集。这些岩石的REE含量很低（23 ~ 34 ppm），其lree相对于hree富集6倍。这些地球化学特征表明物源以基性/超基性岩为主。U/Th、V/Cr比值和自生U值显示了所研究的硅质碎屑岩的氧化沉积环境。
Red coloured Middle Eocene Gercus Formation is a widely exposed rock unit in northern Iraq, southeastern Turkey, and northwestern Iran. Siliciclastic rocks (mudstones and sandstones) are the predominant rocks of this formation which were studied petrographically and geochemically. The Gercus Formation also contains a few beds of limestone, gypsum, and conglomerate. The sandstones are of two types, red and white, mostly immature litharenites, consisting of lithics (85%), quartz (12%), and feldspars (3%). The rock fragments consist dominantly of angular chert and rounded limestone and smaller amounts of sedimentary, magmatic, volcanic, and metamorphic lithics indicating proximity of their source rocks. The provenance discrimination diagrams of the detrital grain counts in sandstones show recycled and collision orogen sources. Petrographic, heavy minerals and major and immobile trace element (e.g. La, Th, Sc, Zr, Hf, and Ti) discrimination diagrams indicated that the Gercus Formation was originated under a collision tectonic setting above a subduction zone as part of an Oceanic Island Arc (OIA), where its sediments were derived from mixed sources, including recycled sediments of chert‐ and carbonate‐rich formations and igneous‐metamorphic ophiolitic complexes of north to north‐east Iraq and adjacent Iranian and Turkish counterparts. The results also indicated a mixed marine (lacustrine and deltaic) and non‐marine (fluviatile) origin for the Gercus Formation. The red sandstones, white sandstones, and mudstones show minor differences in their major, trace, and rare‐earth element (REE) contents and have generally similar Post Archean Australian Shale (PAAS)‐, Upper Continental Crust (UCC)‐, and CI‐chondrite‐normalized distribution patterns indicating their common origin. The average of elements in the siliciclastic rocks is depleted in the high‐field‐strength elements (HFSE), the large‐ion lithophile elements (LILE), and REE; and enriched in some rock‐forming elements (RFE) and mantle rock‐forming elements (MRFE) (Mg, Ca, Cr, Co, Ni) and depleted in others (Si, Al, Fe, Cu, Zn, Ga) relative to PAAS and UCC; whereas, they are depleted in Fe, Mn, Mg, Co, Ni, Cu, Zn, Ga, Na, and P, and enriched in all other RFE, MRFE, LILE, HFSE, and REE compared to CI‐Chondrite. The REE content of these rocks is very low (23–34 ppm) and their LREEs are enriched six times relative to HREEs. These geochemical characteristics suggest a provenance dominated by basic/ultrabasic rocks. The U/Th, V/Cr ratios and the authigenic U values show an oxic depositional environment for the studied siliciclastic rocks.