Evolution of the Tugtutôq Central Complex, South Greenland: A high-level, rift-axial, late-Gardar centre
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Peralkaline rock
The Ediacaran peralkaline granites, which were emplaced during the post-collisional tectonic extensional stage, have a limited occurrence in the northern tip of the Nubian Shield. In this contribution, we present new mineralogical and geochemical data of Mount El-Sibai granites from the Central Eastern Desert of Egypt. The aim is to discuss their crystallization condition, tectonic setting, and petrogenesis as well as the magmatic evolution of their associated mineralization. Mount El-Sibai consists of alkali-feldspar granites (AFGs) as a main rock unit with scattered and small occurrences of alkali-amphibole granites (AAGs) at the periphery. The AAG contain columbite, nioboaeschynite, zircon and thorite as important rare metal-bearing minerals. Geochemically, both of AFG and AAG exhibit a highly evolved nature with a typical peralkaline composition (A/CNK = 0.82–0.97) and formed in within-plate anorogenic setting associated with crustal extension and/or rifting. They are enriched in some LILEs (Rb, K, and Th) and HFSEs (Ta, Pb, Zr, and Y), but strongly depleted in Ba, Sr, P and Ti with pronounced negative Eu anomalies (Eu/Eu* = 0.07–0.34), consistent with an A-type granite geochemical signature. The calculated TZrn (774–878 °C) temperatures indicate that the magma was significantly hot, promoting the saturation of zircon. The texture and chemistry of minerals suggest that they were crystallized directly from a granitic magma and were later subject to late- to post-magmatic fluids. Both granitic types were most likely generated through partial melting of a juvenile crustal source followed by magmatic fractionation. The lithospheric delamination is the main mechanism which causes uplifting of the asthenospheric melts and hence provides enough heat for crustal melting. The produced parent magma was subjected to prolonged fractional crystallization to produce the different types of Mount El-Sibai granites at different shallow crustal levels. During magma fractionation, the post-magmatic fluids (especially fluorine) contribute significantly to the formation of rare metal mineralization within Mount El-Sibai granites.
Peralkaline rock
Petrogenesis
Amphibole
Fractional crystallization (geology)
Hornblende
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The Kharitonovo pluton is located in the central part of the large Mongolian-Transbaikalian province of Late Paleozoic alkaline syenites and granites. The province stretches for a distance of almost 2000 km and contains over 350 plutons. The Kharitonovo pluton occupies an area of $$230 km.^{2}$$ It is made up of A-type granitoids forming two successive alkaline and peralkaline syenite-granite series; syenites largely predominate over granites. Rocks of both series are characterized by abundant mesoperthitic feldspar (about 90 vol % in syenites, more than 60% in granites) while plagioclase is almost absent. In the peralkaline series mafic minerals are riebeckite-arfvedsonite, kataphorite and aegirine with secondary annitic biotite. In the alkaline series edenite and iron-rich biotite are the major mafic minerals. In the alkaline series coeval mafic rocks commonly occur in the form of synplutonic composite dikes; they indicate at least two stages of mafic magma injection into the silicic magma chamber. Major and trace element data are used to test various petrogenetic models for the origin of the syenites and granites. Of the alternatives considered, it is most likely that each series resulted from crystal fractionation of syenite magma. Mass balance calculations suggest that in the early alkaline series, crystal fractionation was probably combined with mixing of felsic and mafic magmas, a conclusion also supported by field evidence. The peralkaline series syenite parental magma could have been produced by partial melting (about 20%) of the earlier alkaline syenites. In this respect alkaline syenites can be regarded as parental rocks for the whole pluton. We suggest that the alkaline syenite magma originated via two possible petrogenetic schemes: (1) partial melting of deeply buried crustal rocks or (2) crystal fractionation of hybrid melt produced by mixing of subalkaline basaltic magma (80%) with about 20% of silicic lower crustal melt. In either case some additional input of potassium and possibly other incompatible elements is required in order to achieve the observed composition.
Peralkaline rock
Felsic
Silicic
Hornblende
Igneous differentiation
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Peralkaline rock
Amphibole
Trachyte
Pegmatite
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Abstract. The REE-mineralized alkaline granites in Egypt are divided into the following three classes: (1) Mesozoic, anorogenic nepheline syenite ring complexes with REE amounting up to 1.3 %, particularly in their fenitized parts (e.g. Abu Khruq), (2) an orogenic peralkaline syenite-granite, composed of i) Zr, Nb, - REE, and Th-enriched peralkaline granite-syenite complexes with REE amounting up to 0.5 % (e.g. Um Hibal, Tarbite North and South, Gharib, and Zarget Naam) and ii) Y, Th, HREE, and P-enriched post-Cretaceous peralkaline complexes that intrude the Phanerozoic rocks of the Southwestern Desert with REE amounting up to 2 % (e.g. Gara El Hamra), and (3) upper Proterozoic, post-orogenic siderophyllite alkali feldspar granite with REE amounting up to 0.8 %, particularly in their apical miarolitic pegmatites and albitized zones (e.g. Kadabora-Abu Dob and Um Naggat). Special attention is given to the Abu Khruq and Gara El Hamra granitic bodies.
Peralkaline rock
Nepheline syenite
Pegmatite
Trachyte
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The Kharitonovo pluton is located in the central part of the large Mongolian-Transbaikalian province of Late Paleozoic alkaline syenites and granites. The province stretches for a distance of almost 2000 km and contains over 350 plutons. The Kharitonovo pluton occupies an area of $$230 km.^{2}$$ It is made up of A-type granitoids forming two successive alkaline and peralkaline syenite-granite series; syenites largely predominate over granites. Rocks of both series are characterized by abundant mesoperthitic feldspar (about 90 vol % in syenites, more than 60% in granites) while plagioclase is almost absent. In the peralkaline series mafic minerals are riebeckite-arfvedsonite, kataphorite and aegirine with secondary annitic biotite. In the alkaline series edenite and iron-rich biotite are the major mafic minerals. In the alkaline series coeval mafic rocks commonly occur in the form of synplutonic composite dikes; they indicate at least two stages of mafic magma injection into the silicic magma chamber. Major and trace element data are used to test various petrogenetic models for the origin of the syenites and granites. Of the alternatives considered, it is most likely that each series resulted from crystal fractionation of syenite magma. Mass balance calculations suggest that in the early alkaline series, crystal fractionation was probably combined with mixing of felsic and mafic magmas, a conclusion also supported by field evidence. The peralkaline series syenite parental magma could have been produced by partial melting (about 20%) of the earlier alkaline syenites. In this respect alkaline syenites can be regarded as parental rocks for the whole pluton. We suggest that the alkaline syenite magma originated via two possible petrogenetic schemes: (1) partial melting of deeply buried crustal rocks or (2) crystal fractionation of hybrid melt produced by mixing of subalkaline basaltic magma (80%) with about 20% of silicic lower crustal melt. In either case some additional input of potassium and possibly other incompatible elements is required in order to achieve the observed composition.
Peralkaline rock
Felsic
Hornblende
Silicic
Igneous differentiation
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Peralkaline rock
Baddeleyite
Ultramafic rock
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Porphyritic
Peralkaline rock
Petrogenesis
Fractional crystallization (geology)
Adakite
Igneous differentiation
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Peralkaline rock
Cassiterite
Porphyritic
Greisen
Columbite
Aegirine
Fluorite
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Peralkaline rock
Fractional crystallization (geology)
Aegirine
Amphibole
Allanite
Incompatible element
Pegmatite
Carbonatite
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In Mongolia, rare earth element (REE) mineralization of economic significance is related either to the Mesozoic carbonatites or to the Paleozoic peralkaline granitoid rocks. Carbonatites occur as part of alkaline silicate-carbonatite complexes, which are composed mainly of nepheline syenites and equivalent volcanic rocks. The complexes were emplaced in the Gobi-Tien Shan rift zone in southern Mongolia where carbonatites usually form dikes, plugs or intruded into brecciated rocks. In mineralized carbonatites, REE occur mainly as fluorocarbonates (bastnäsite, synchysite, parisite) and apatite. Apatite is also present in the carbonatite-hosted apatite-magnetite (mostly altered to hematite) bodies. Alkaline silicate rocks and carbonatites show common geochemical features such as enrichment of light REE but relative depletion of Ti, Zr, Nb, Ta and Hf and similar Sr and Nd isotopic characteristics suggesting the involvement of the heterogeneous lithospheric mantle in the formation of both carbonatites and associated silicate rocks. Hydrothermal fluids of magmatic origin played an important role in the genesis of the carbonatite-hosted REE deposits. The REE mineralization associated with peralkaline felsic rocks (peralkaline granites, syenites and pegmatites) mainly occurs in Mongolian Altai in northwestern Mongolia. The mineralization is largely hosted in accessory minerals (mainly elpidite, monazite, xenotime, fluorocarbonates), which can reach percentage levels in mineralized zones. These rocks are the results of protracted fractional crystallization of the magma that led to an enrichment of REE, especially in the late stages of magma evolution. The primary magmatic mineralization was overprinted (remobilized and enriched) by late magmatic to hydrothermal fluids. The mineralization associated with peralkaline granitic rocks also contains significant concentrations of Zr, Nb, Th and U. There are promising occurrences of both types of rare earth mineralization in Mongolia and at present, three of them have already established significant economic potential. They are mineralization related to Mesozoic Mushgai Khudag and Khotgor carbonatites in southern Mongolia and to the Devonian Khalzan Buregtei peralkaline granites in northwestern Mongolia.
Carbonatite
Peralkaline rock
Nepheline syenite
Rare-earth element
Petrogenesis
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