Th-rich loparite from the Khibina alkaline complex, Kola Peninsula: isomorphism and paragenesis
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Abstract Th-rich (up to 18.4 wt% ThO 2 ) loparite occurs as an accessory phase in foyaite pegmatites at Mt. Eveslogchorr, Khibina complex, Russia. It is associated with aegirine, astrophyllite, eudialyte, lorenzenite, lamprophyllite, magnesio-arfvedsonite and gerasimovskite. Loparite crystals are zoned from niobian loparite (core) to niobian thorian and thorian niobian loparite (rim). Th-enrichment is accompanied by a decrease in Na, LREE , Sr and increase in A-site vacancies. The most Th-rich composition approaches (Na 0.39 LREE 0.19 Th 0.12 Ca 0.05 Sr 0.02 ) Σ0.77 (Ti 0.76 Nb 0.27 ) Σ1.03 O 3 . The mineral is partly or completely metamict and after annealing gives an X-ray diffraction powder pattern similar to that of synthetic NaLaTi 2 O 6 and naturally occurring loparite of different composition. For the Th-rich rim sample, the five strongest diffraction lines (Å) are: 2.72 (100) 110, 1.575 (60) 211, 1.925 (40), 1.368 (30) 220, 1.222 (20) 310; a = 3.867(2) Å. The X-ray diffraction patterns do not exhibit peak splitting or other diffraction lines typical of low-symmetry and ordered perovskite-type structures. Composition determinations, infrared transmission spectroscopy and X-ray diffractometry show that thorian loparite is partly replaced by betafite with LREE and Th as dominant A -site cations (‘ceriobetafite’). Some loparite samples also exhibit thin replacement mantles of belyankinite with high LREE 2 O 3 and ThO 2 contents. Both ‘ceriobetafite’ and belyankinite were formed due to metasomatic alteration of loparite.Keywords:
Aegirine
Pegmatite
Metamictization
Paragenesis
Nepheline
Metasomatism
Aegirine
Nepheline
Pegmatite
Sanidine
Nepheline syenite
Carbonatite
Diopside
Pyroxene
Oxonium ion
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This study introduces zircon U- Pb ages for the pegmatite veins from the Qinling Group( Complex)in the Northern Qinling Belt. It presents Cathodoluminescence( CL) images for the metamict zircons in the pegmatites from the North Qinling Terrane,and suggests that the structure of zircons in the pegmatites has changed after metamictization or recrystallization,and the CL images for zircons show black color with no zone belt. The LA-ICP-MS data of the metamict zircons for four pegmatite veins show obvious Pb loss with unconcentrated ages as a result of metamictization. One sample( QL1434-1) shows concordant age of 380 Ma( Late Eopaleozoic),which is considered as the formation age of the pegmatite vein.
Pegmatite
Metamictization
Recrystallization (geology)
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Metamict euxenite crystals from a pegmatite in Grenville rocks near St. Pierre de Wakefield, Quebec, gave the following ages: Pb 2O7 /Pb 2O6 1000 m.y., Pb 2O7 /U 235 710 m.y., Pb 2O6 /U 238 620 m.y., and Pb 208 / Th 232 550 m.y. Age of coeval microcline from this pegmatite by Rb/Sr is 1050+ or -50 m.y. Total Pb, U and Th analyses and Pb isotope analyses were made of different parts of the euxenite crystals and of the minerals enveloping them. Results of this work suggest that discordance in ages is due primarily to the following factors : 1) differential loss of U, relative to Th from the outer zones of the euxenite, 2) over-all loss of Pb from the euxenite with some evidence of greater loss of Pb 2O8 than of Pb 2O7 and Pb 2O6 . Experiments on driving off Pb from metamict uranothorite and crystalline thorianite, both of Grenville age, by heating under vacuum, showed that Pb from the metamict mineral is easily driven off but that from the crystalline mineral is much more difficult to move.
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Abstract Naujakasite (Na 6 (Fe,Mn)Al 4 Si 8 O 26 ) is a characteristic, rock-forming mineral of hyperagpaitic naujakasite lujavrite (arfvedsonite + naujakasite + steenstrupine + albite ± relict nepheline ± microcline ± ussingite) from the Ilímaussaq complex, South Greenland, and is only known from this complex. The qualitative stability relationships of naujakasite-bearing, low-variance liquidus mineral assemblages at near-solidus conditions in lujavritic magma can be modelled within the six-component system Si-Al-Fe-Na-O-H. The modelling suggests that an uncommon combination of elevated water activity and low oxygen fugacity is needed to stabilize arfvedsonite + naujakasite mineral assemblages in highly sodic liquids. These conditions are incompatible with the stable coexistence of aegirine and arfvedsonite. They are met in the arfvedsonite-bearing lujavrites of Ilímaussaq, but apparently not in rocks of similar composition in the Khibina and Lovozero complexes, Kola Peninsula, where aegirine- or aegirine-arfvedsonite-bearing mineral assemblages are characteristic. Liquid lines of descent leading to crystallization of naujakasite have to pass through the albite+nepheline+arfvedsonite liquidus field in the Si-Al-Fe-Na-O-H system, reaching this field at the high-Na side of the albite-nepheline-arfvedsonite plane.
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Nepheline
Peralkaline rock
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Paragenesis
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