Petrology of metamorphic rocks from the Atbashy complex, Southern Tien-Shan, Kyrgyzstan

Postcollisional granites of the Kokshaal Segment of South Tien Shan comprise about 20 postkinematic intrusions composed of biotite-amphibole granites, biotite granites and Li-F topaz-bearing leucogranites. The granites were emplaced coevally with tholeiitic mafic rocks and alkaline syenites. Geochemically the granites are classified as A-type and characterized by elevated Fe/(Fe+Mg) and K2O/Na2O values and high concentrations of Na2O+K2O, Rb, HFSE. On the discrimination diagrams Y-Nb and Rb-(Y+Nb) compositions of the granites plot into the field of intra-plate granites. On a regional scale, the compositional variations of the Kokshaal granites can be explained by fractionation of potassium feldspar and amphibole. The granites were probably derived from the crustal protoliths represented by Precambrian metamorphic rocks of the Tarim microcontinent. Crystallization ages of the Kokshaal granites, established utilizing U-Pb zircon dating, fall in the relatively narrow range between 280 and 295 Ma corresponding to the early Permian, which is in agreement with ages of postcollisional granites elsewhere in Tien Shan. Genesis of granites was related to trans-crustal shear zones.

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TEM/AEM characterization of fine-grained clay minerals in very-low-grade rocks: Evaluation of contamination by empa involving celadonite family minerals

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100165938 ◽

1996 ◽

Vol 54 ◽

pp. 694-695

Author(s):

Gejing Li ◽

D. R. Peacor ◽

D. S. Coombs ◽

Y. Kawachi

Keyword(s):

Electron Microscopy ◽

Volcanic Rocks ◽

Analytical Electron Microscopy ◽

Metamorphic Rocks ◽

New Mineral ◽

Chemical Compositions ◽

Low Grade ◽

Fine Grained ◽

Depth Analysis ◽

Mineral Aggregates

Recent advances in transmission electron microscopy (TEM) and analytical electron microscopy (AEM) have led to many new insights into the structural and chemical characteristics of very finegrained, optically homogeneous mineral aggregates in sedimentary and very low-grade metamorphic rocks. Chemical compositions obtained by electron microprobe analysis (EMPA) on such materials have been shown by TEM/AEM to result from beam overlap on contaminant phases on a scale below resolution of EMPA, which in turn can lead to errors in interpretation and determination of formation conditions. Here we present an in-depth analysis of the relation between AEM and EMPA data, which leads also to the definition of new mineral phases, and demonstrate the resolution power of AEM relative to EMPA in investigations of very fine-grained mineral aggregates in sedimentary and very low-grade metamorphic rocks.Celadonite, having end-member composition KMgFe3+Si4O10(OH)2, and with minor substitution of Fe2+ for Mg and Al for Fe3+ on octahedral sites, is a fine-grained mica widespread in volcanic rocks and volcaniclastic sediments which have undergone low-temperature alteration in the oceanic crust and in burial metamorphic sequences.

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