九州大学学術情報リポジトリ
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Petrogenesis of the Metacarbonatite Rocks from Amesmessa Area (In Ouzzal Terrane), Hoggar
Shield, Algeria
シェールバル, ムラド
http://hdl.handle.net/2324/2236219
出版情報：九州大学, 2018, 博士（工学）, 課程博士 バージョン：
権利関係：
（様式２）
氏 名 ： シェールバル ムラド
論文題名 ：
Petrogenesis of the Metacarbonatite Rocks from Amesmessa Area (In Ouzzal
Terrane), Hoggar Shield, Algeria (アルジェリア、ホガール楯状地アメスメサ地域
におけるメタカーボナタイトの岩石学的研究)区 分 ： 甲
論 文 内 容 の 要 旨
REEs (Rare Earth Elements) are generally considered to be the most critical of elements, indispensable for many high-tech applications such as smart-phones and electric vehicles. Currently, carbonatites are the main source of REE due to their high REE grade, especially that of L(Light)REE. However, it is challenging to identify carbonatite rocks in metamorphic context. Indeed, the difficulty of distinguishing between metamorphosed carbonatites and metamorphosed limestones (or dolomites) in the field can lead to the misidentification of carbonatites in metamorphic belts. In this respect, geochemical features of rocks, Sr and REE contents are distinctive to distinguish probable carbonate protolith, whether these rocks were carbonatite, or in fact limestone converted to marble. This thesis presents new findings on REE mineralization at the Amesmessa carbonatite, located in the In Ouzzal Terrane, western Hoggar, Algeria. This area is recognized as Archean rocks, composed of two units dated to 3.5–2.7 Ga. This study focuses on sampled metacarbonates from the Amesmessa area, employing field relations and a geochemical approach on whole rock, complemented with petrological and mineralogical methods in order to constrain petrogenesis. The petrogenesis of the Amesmessa metacarbonate rocks, which have previously been interpreted as marbles of Archean age, are reconsidered in the present study.
The results of this study are presented in this dissertation and consist of six chapters:
Chapter I states the significance of this study, aims and objectives, gives an outline of the chapters content and explains the methodology.
Chapter II reviews the classification, composition, occurrence and genesis of carbonatites; the properties and uses of REE; different types of REE deposition; and the current state of knowledge of the transport and enrichment of the REE from the mantle to ore deposits.
Chapter III is a brief review of the literature that relates to Tuareg Shield and its different subdivisions.
After providing an initial regional overview and a short geological history of the Hoggar Craton, this chapter explores the geological setting and patterns of In Ouzzal terrane in greater detail. Finally, the main features of the Amesmessa area are summarized.
Chapter IV describes field evidences assembled during field work and presents interpretations for the Amesmessa metacarbonate to illustrate relationships between Amesmessa metacarbonates, faults and country rocks. The Amesmessa occurrence, comprising carbonatite, syenite, pyroxenite, anorthosite and rhyolite, intrudes clearly charnockites and the granulite basement. Syenites and pyroxenites represent the major portions of the complex, with pyroxenites occurring at the core and syenites forming a near sigmoid structure around the carbonatite body. Anorthosite exposures are seen at several places in the pyroxenite body. Small boulders of carbonatite, probably representing stocks or plugs, are spread throughout pyroxenitic and syenitic bodies. The carbonatites also occur as small dykes varying in width between 40 cm and 1m; veins (fracture fillings that are 5–20 cm in width), and lenses up to 3 m long. Outcrop evidences obtained from the spatial and cross-cutting relations are decisive criteria for establishing the origin of the Amesmessa metacarbonate. This study suggests that
the Amesmessa metacarbonates are an intrusive phase, based on (i) the spatial association of ultramafic, syenite and carbonate rocks, an ellipsoidal shape surrounded by granulitic rocks, (ii) simple cross-cutting relationships and structural relations from field observations, (iii) lensoid and dike-shaped bodies of metacarbonate rock, (iv) contacts between metacarbonate and inclusions exhibit signs of reaction, suggesting fenitization. In two outcrops of the ultramafic rocks, the one enclosing carbonate rocks displayed severe hydrothermal alteration, which was possibly caused by the intrusion of carbonate melt, (v) abundant mafic to ultramafic xenoliths of different sizes, which can be rounded or angular in shape within the metacarbonate rock, enveloped during the development and solidification of metacarbonate rock. The enclaves are always of meta-plutonics (country rocks: granulite, charnockites and mafic–ultramafic rocks), and not of metasedimentary origin. Samples were collected from all the rock units described above. The chapter also describes petrographic examination and mineral identification in thin sections. The metacarbonate rocks consist of calcite, biotite, clinopyroxene, hornblende, olivine and apatite. The Amesmessa carbonatite shows abundant evidence of deformation. It has been deformed and metamorphosed which is most obvious in carbonate microstructures and in phlogopite. Calcite is present in all thin sections of the metacarbonate samples. It forms granoblastic aggregates with coarse-grained texture exhibiting twinning and polygonal mosaic textures, most certainly recrystallized. Initial response to stress in metacarbonate rocks includes bent and segmented thin mechanical twinning lamellae in calcite overprinted by thick straight lamellae produced probably during compression. With increasing strain, twin lamellae grow thicker, develop bent or lenticular shapes.
Phlogopite also exhibit bending, indicating their transport in a crystal mush.
Chapter V discusses the whole-rock composition of Amesmessa metacarbonate and surrounding rocks based on Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and X-ray Fluorescence (XRF) analysis. The results are graphed, and compared to other worldwide occurrences. The data presented illustrate that the Amesmessa metacarbonates have many of the petrologic and geochemical characteristics of carbonatites, such as elevated LREE, Sr, Ba and Y content. Their chondrite-normalized REE compositions show similar patterns with typical carbonatites. The average total REE concentration in the metacarbonate is up to 5,700 ppm. However the associated rocks; syenite and pyroxenite contain only 60 and 12 ppm, respectively. Sr concentration is also high (>1,390 ppm) which is normally high in carbonatitic rocks and low in marbles. In this work, the highest levels of light lanthanides (up to 7,775 ppm) were detected in samples with abundant calcite and apatite. A comparative study has been done to compare the obtained data with such from well-known metamorphosed carbonatites together with marbles from different localities. The results reveal an overlap of compositions and provide distinctive features for either a sedimentary or igneous origin. On the basis of available chemical analyses and outcrop evidence, it becomes clear that the Amesmessa metacarbonate rocks consist of metamorphosed carbonatite.
Finally, Chapter VI is a discussion based on the previous 5 chapters. It discusses the effects of metamorphism and summarizes the results and relates them to the aims and hypotheses of the thesis. The implication from this study are various and of relevance for the igneous and metamorphic petrology of carbonatites, and for wider settings in the entire Hoggar region.The thesis concludes with suggestions for further work that would extend knowledge and understanding of metamorphosed metacarbonatites. This is a major finding because carbonatites have been described so far only in the northern part of the In Ouzzal (Ihaouhaouene).
The Amesmessa carbonatites and those of Ihaouhaouene may correspond to the same magmatic event which occurred during the Eburnean magmatic activity.
