Unique Clinkers and Paralavas from a New Nyalga Combustion Metamorphic Complex in Central Mongolia: Mineralogy, Geochemistry, and Genesis
| Autor: | N. S. Karmanov, Alexei V. Ivanov, I. S. Peretyazhko, E. A. Khromova, E. A. Savina |
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| Rok vydání: | 2018 |
| Předmět: |
Olivine
Mineral 010504 meteorology & atmospheric sciences Geochemistry Partial melting Mineralogy Hyalophane Melilite engineering.material 010502 geochemistry & geophysics Feldspar 01 natural sciences chemistry.chemical_compound chemistry Geochemistry and Petrology Mineral redox buffer visual_art Nepheline engineering visual_art.visual_art_medium Geology 0105 earth and related environmental sciences |
| Zdroj: | Petrology. 26:181-211 |
| ISSN: | 1556-2085 0869-5911 |
| DOI: | 10.1134/s0869591118020054 |
| Popis: | The paper presents mineralogical and geochemical data on clinkers and paralavas and on conditions under which they were formed at the Nyalga combustion metamorphic complex, which was recently discovered in Central Mongolia. Mineral and phase assemblages of the CM rocks do not have analogues in the world. The clinkers contain pyrogenically modified mudstone relics, acid silicate glass, partly molten quartz and feldspar grains, and newly formed indialite microlites (phenocrysts) with a ferroindialite marginal zone. In the paralava melts, spinel microlites with broadly varying Fe concentrations and anorthite–bytownite were the first to crystallize, and were followed by phenocrysts of Al-clinopyroxene ± melilite and Mg–Fe olivine. The next minerals to crystallize were Ca-fayalite, kirschsteinite, pyrrhotite, minerals of the rhonite–kuratite series, K–Ba feldspars (celsian, hyalophane, and Ba-orthoclase, Fe3+-hercynite ± (native iron, wustite, Al-magnetite, and fresnoite), nepheline ± (kalsilite), and later calcite, siderite, barite, celestine, and gypsum. The paralavas contain rare minerals of the rhonite–kuratite series, a new end-member of the rhonite subgroup Ca4Fe 8 2+ Fe 4 3+ O4 [Si8Al4O36], a tobermorite-like mineral Ca5Si5(Al,Fe)(OH)O16 · 5H2O, and high- Ba F-rich mica (K,Ba)(Mg,Fe)3(Al,Si)4O10F2. The paralavas host quenched relics of microemulsions of immiscible residual silicate melts with broadly varying Si, Al, Fe, Ca, K, Ba, and Sr concentrations, sulfide and calcitic melts, and water-rich silicate–iron ± (Mn) fluid media. The clinkers were formed less than 2 Ma ago in various parts of the Choir–Nyalga basin by melting Early Cretaceous mudstones with bulk composition varies from dacitic to andesitic. The pyrogenic transformations of the mudstones were nearly isochemical, except only for volatile components. The CM melt rocks of basaltic andesitic composition were formed via melting carbonate–silicate sediments at temperatures above 1450°C. The Ca- and Fe-enriched and silicaundersaturated paralavas crystallized near the surface at temperatures higher than 900–1100°C and oxygen fugacity $$f_{O_2 }$$ between the IW and QFM buffers. In local melting domains of the carbonate–silicate sedimentary rocks and in isolations of the residual melts among the paralava matrix the fluid pressure was higher than the atmospheric one. The bulk composition, mineral and phase assemblages of CM rocks of the Nyalga complex are very diverse (dacitic, andesitic, basaltic andesitic, basaltic, and silica-undersaturated mafic) because the melts crystallized under unequilibrated conditions and were derived by the complete or partial melting of clayey and carbonate–silicate sediments during natural coal fires. |
| Databáze: | OpenAIRE |
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