Petrogenesis and geodynamic model for (Ta, Nb)-fertilized Nuweibi albite granite, Egyptian Nubian Shield: Juvenile crust-mantle mixing and metasomatic enhancement
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Petrogenesis
Metasomatism
Mylonite
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Tourmaline
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As shown in the accompanying paper by Tuttle and Bowen, two kinds of albite exist. They are distinguished by optical properties and differences in X-ray powder patterns. The generally known "normal" albite (called "low" albite in this paper) is one type, the other (here called "high" albite) can be obtained by heat treatment of "low" albite or by direct synthesis. This investigation deals with some of the "low-high" albite relations. X-ray investigations combined with optical measurements have been carried out. The optical measurements are similar to those reported by Tuttle and Bowen. Structural differences as revealed by the X-ray photographs are discussed. The lattice of "low" albite differs from that of "high" albite in that (010)(100) changes from 90°30' to 88°15' and (010)(001) from 86°20' to 86°0'. This lattice difference is accompanied by changes in intensities of the X-ray reflections. The structural differences, as far as they are expressed as intensity differences of the X-ray reflections, are of the same order of magnitude as the differences in microcline and orthoclase.
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Geochemical Aspects of Na-Metasomatism in Sargaz Granitic Intrusion (South of Kerman Province, Iran)
The Sargaz granitic intrusion has been emplaced in Sargaz ophiolitic suite, south-east of Sanandaj-Sirjan metamorphic zone, south of Kerman province. The central part of the intrusive body contains pinkish coarse-grained granite, but the fractured northern part, neighboring Chah-Mazraeh fault, has been subjected to pervasive Na-metasomatism and related subsolidus reactions. In the northern altered rocks, the primary magmatic textures have been changed into a new generation of albite along with chlorite, epidote and sericite. Petrographically, in Sargaz altered rocks, albite occurs as overgrowth, crack-filling, vug-filling and interstitial forms. The first form has been replaced the primary plagioclase, and/or alkali feldspars by a coupled dissolution-reprecipitation mechanism, while, the other forms have been crystallized from Na-rich alkali fluids during Na-metasomatism. In Sargaz unaltered granites, primary feldspars contain oligoclase (An23.8-An10.6) and K-feldspar (kf70-kf95.9), while, metasomatic feldspars are entirely albite (An8.4-An0.3) without any chemical zonation. Na-metasomatism in these rocks resulted obvious mass changes in rock composition, as the altered rock are enriched in Na, La, Y, Yb, Hf and Th and depleted in K, Fe, Mg, Ca, Sr, Co and Zn. Si, P, Rb, Ti, Al and Zr possibly acted as immobile elements during Na-metasomatism. Evidences in Sargaz intrusion show that alkali Na-rich fluids caused Na-metasomatism as dissolution of primary quartz and then, crystallization of albite. Microcracks facilitated infiltration of fluids. During the metasomatism, enough quartz grains were dissolved, thereby releasing silica for the formation of different forms of new albites, thus, the role of quartz dissolution, is more important than those expected earlier.
Metasomatism
Alkali feldspar
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Pegmatite
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Abstract Mylonites are strongly deformed rocks which generally occur within thrust zones in many erogenic belts. In regard to their genesis, opinions are divided on the issues of the relative importance of mechanical and chemical processes and the type of deformation involved. A study of the mylonites derived from trondheimite in a part of the south-western Norwegian Caledonides and from basement granite in the eastern Himalayas reveals that shearing strain dominates in the initial stage of deformation, while ductile mylonitic microstructures are developed during a later flattening stage. Chemical mobility plays an important role in mylonite genesis.
Mylonite
Shearing (physics)
Flattening
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