Alakit and Daldyn kimberlite fields,Siberia,Russia:Two types of mantle sub-terranes beneath central Yakutia?

Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle.Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields.In the Alakit field.Cr-diopsides are alkaline,and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions,while in the Daldyn field harzburgitic pyropes are frequent.The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions,while in the Daldyn field they show varying compositions and often continuous Pressure-Temperature(P-T) ranges with increasing Fe~# with decreasing pressures.In Alakit,Crpargasites to richterites were found in all pipes,while in Daldyn,pargasites are rare Dalnyaya and Zarnitsa pipes.Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements(LREE)and stronger REE-slopes,and enrichment in light Rare Earth Elements(LREE),sometimes Th-U,and small troughs in Nb-Ta-Zr.In the Daldyn field,the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances,while those from sheared and refertilized peridotites have smooth patterns.Garnets from Alakit show HREE minima,but those from Daldyn often have a trough at Yand high U and Pb.PTX/O2 diagrams from both regions show similarities,suggesting similar layering and structures.The degree of metasomatism is often higher for pipes which show dispersion in P-Fe~# trends for garnets.In the mantle beneath Udachnaya and Aykhal,pipes show 6-7 linear arrays of P-Fe~# in the lower part of the mantle section at 7.5-3.0 GPa,probably reflecting primary subduction horizons.Beneath the Sytykanskaya pipe,there are several horizons with opposite inclinations which reflect metasomatic processes.The high dispersion of the P—Fe~# trend indicating widespread metasomatism is associated with decreased diamond grades.Possible explanation of the differences in mineralogy and geochemistry of the mantle sections may relate to their tectonic positions during growth of the lithospheric keel.Enrichment in volatiles and alkalis possibly corresponds to interaction with subduction-related fluids and melts in the craton margins.Incorporation of island arc peridotites from an eroded arc is a possible scenario.     

Permian age of the Burpala alkaline pluton, Northern Transbaikalia: Geodynamic implications

This paper presents the U-Pb zircon age of pulaskite of the main phase (294 ± 1 Ma) and the rare metal syenite (283 ± 8 Ma) of the Burpala alkaline pluton. The geochronological data show that it was formed in the Early Permian. By age, it is comparable with the Synnyr pluton of the Synnyr rift zone, alkaline granitic rocks and bimodal volcanic associations of the Uda-Vitim rift zone, and carbonatites of the Saizhen rift zone of the Central Asian foldbelt. These intraplate igneous complexes were formed almost simultaneously with crustal granitic rocks of the Angara-Vitim batholite. All of this gives ground to suppose that the origination of their parental melts is a result of the influence of the mantle hot spot or mantle plume on the lithosphere that led to extensive crustal anatexis.     

Petrology and composition of rare-metal alkaline rocks in the South Gobi Desert,Mongolia

Earlier, a belt of alkali-granite plutons and a carbonatite province were discovered in the South Gobi Desert, Mongolia. The Lugingol pluton of pseudoleucitic syenites with carbonatites was assigned to the alkali-granite belt. However, new dating showed that it is 40 Myr younger than the Khan-Bogdo pluton and a large fault separates it from the alkali-granite belt. In the same part of the South Gobi Desert, a dike series of alkaline K-shonkinites with a rare-metal carbonatite vein was found by V.I. Kovalenko west of the Lugingol pluton, near Mt. Baruun Hasar Uula, and a dike series of alkali and nepheline syenites was found by us northeast of the Lugingol pluton. These data give grounds to distinguish an intrusive complex of K-alkaline shonkinites and leucitic syenites with Late Paleozoic REE-bearing carbonatites. Thus, three alkaline-rock complexes of different ages are distinguished in the South Gobi Desert. We present refined geological maps of these complexes. The plutons of all three complexes are deposits of trace elements (REE, Nb, Zr, Y, P). The chemical composition of the silicate rocks of the complex, rare-metal agpaitic pegmatites, and carbonatite and apatite rare-metal ores was considered in detail. Shonkinites from Mt. Baruun Hasar Uula and the Mountain Pass mine (United States) and their carbonatites, along with the Lugingol carbonatites, belong to a single association of K-alkaline rocks and carbonatites, as evidenced by their identical chemical, mineral, and geochemical rare-metal compositions. Rare-earth element patterns and spidergrams show similarities and differences between the rare-metal rocks of three complexes as well as paragenetic differences between their rare-metal minerals. A rare process is described—the amorphization of rare-metal minerals, related to their high-temperature crystallization in a medium with abnormal silica contents of the Khan-Bogdo pegmatites. The parental magmas of the alkali-carbonatite complexes were generated from the EM-2 contaminated mantle that had undergone recycling, whereas the parental magmas of the Khan-Bogdo agpaitic alkali granites were produced from depleted mantle.     

Chemical composition and geochemical characteristics of the Koksharovka alkaline ultrabasic massif with carbonatites,primorye

New geochemical data are discussed on the magmatic complexes of the Koksharovka alkaline ultrabasic massif of Late Jurassic age obtained by the ICP-MS method. Based on the first results on rare earth geochemistry of carbonatites and associating pyroxenites and geological observations, the magmatic origin of the Koksharovka carbonatites was substantiated, and the problems of formation of accompanying igneous rocks were considered.     

The First Precise Data on the Age of Charoite Mineralization (Eastern Siberia,Russia)

Charoite is a unique mineral and a rock of the same name. It is known from the only deposit in the world at the contact with Early Cretaceous syenite and other alkaline rocks of the Malyi Murun massif. The data on ⁴⁰Ar/³⁹Ar dating of tinaksite, tokkoite, and frankamenite associated with charoite are reported in this paper. All these minerals, except for frankamenite, have shown clear plateau ages overlapping with each other within the analytical errors. The weighted mean of the plateau ages of tinaksite, tokkoite, and microcline is 135.86 ± 0.26 Ma. Considering that these minerals are syngenetic to charoite, this age is the time of charoite crystallization. Frankamenite with an older age (137.55 ± 0.46 Ma) may reflect the polychronous crystallization of the charoite association.     

Structure,age, and ore potential of the Burpala rare-metal alkaline massif,northern Baikal region

The Burpala alkaline massif is a unique geological object. More than 50 Zr, Nb, Ti, Th, Be, and REE minerals have been identified in rare-metal syenite of this massif. Their contents often reach tens of percent, and concentrations of rare elements in rocks are as high as 3.6% REE, 4% Zr, 0.5% Y, 0.5% Nb, 0.5% Th, and 0.1% U. Geological and geochemical data show that all rocks in the Burpala massif are derivatives of alkaline magma initially enriched in rare elements. These rocks vary in composition from shonkinite, melanocratic syenite, nepheline and alkali syenites to alaskite and alkali granite. The extreme products of magma fractionation are rare-metal pegmatites, apatite-fluorite rocks, and carbonatites. The primary melts were related to the enriched EM-2 mantle source. The U-Pb zircon ages of pulaskite (main intrusive phase) and rare-metal syenite (vein phase) are estimated at 294 ± 1 and 283 ± 8 Ma, respectively. The massif was formed as a result of impact of the mantle plume on the active continental margin of the Siberian paleocontinent.