Geochemical features of zircons from migmatites of the Nimnyr block,Aldan shield

This paper reports the results of chemical and structural study (electron microscopy and ion microprobe) of zircons from different-age generations of migmatite leucosomes in the basement rocks and Kurumkan Formation within the Nimnyr block, Aldan shield. The studied zircons show REE distribution pattern with a positive slope from LREE to HREE and positive Ce anomaly, which is typical of magmatic zircons, but have elevated LREE contents, which implies their crystallization from migmatite melt with subsequent fluid reworking. The transformations of the zircons were caused by fluid, which was separated during crystallization of the last LILE-enriched portions of the melt and inherited the geochemical features of the host rock—leucosome.     

Allanite-(Y) in areas of ongonite magmatism in the Far East: Isomorphism and petrogenetic implications

The research history of allanite-(Y) has been considered. The composition and genesis of this mineral is discussed with allowance for new findings in Li-F granites of the Russian Far East. Compositional anomalies in allanite-(Y) reflect the metastable character of its structure. Types of zoning and trends in the chemical substitution of major components are described. Two crystal chemical modes of allanite-(Y) formation and the two-step isomorphism of its components are suggested. The main tendencies in the chemical evolution of allanite-(Ce) and allanite-(Y) in granitic rocks of the Far East are pointed out. Allanite-(Y) is formed in the anomalous geochemical setting that characterizes crystallization of fluid-saturated subalkaline granitic melt and is regarded as an index mineral of rare-metal ongonite magmatism completing the Late Cretaceous Pacific Orogeny.     

Kirishites—high-carbonaceous hairlike fibers associated with volkhovites

Kirishites are highly carbonaceous hairlike fibers 30–100 μm in thickness and 3–30 mm long, which jut out as bunches on the surface of cinder and shungite fragments associated with volkhovites (Holocene tectitelike glasses corresponding to the rocks of kimberlite-lamproite-carbonatite series in composition). Kirishite fibers are zonal. Their inner (axial) zone is composed of high-nitrogen proteinlike compounds, whereas the outer zone is essentially carbonaceous, with a high content of organoelemental complexes (Si, Fe) and numerous micrometer-sized anomalies of major, volatile, trace, and ore elements. Longitudinal zoning is established in aposhungite kirishites: the consecutive change of maximum concentrations—K, Na, Cl, C, Mn → C, S, V, Ni, Cu, Zn → S, N, Ba, Te, Pb, Bi, Nd—is traced from the roots of fibers to their ends. It is suggested that as volkhovites were forming, fragments of cinder and shungite underwent partial melting. The highly carbonaceous compounds released due decompression and explosion were squeezed out from fragments and solidified as fibers during fall of fragments on the Earth’s surface.     

Allanite-(Y) and allanite-(Ce) paragenesis in tourmalinite of the Severnyi pluton,Chukchi Peninsula,and the relationship between yttrium and lanthanides in allanite

The morphology, optical properties, and composition of allanite-(Ce) and allanite-(Y) from raremetal tourmalinite of the Severnyi granitic pluton in the Chukchi Peninsula have been studied, as well as the composition and structure of host metasomatic rocks and the assemblage of rock-forming and accessory minerals. The hydrothermal origin of both allanite species and their stable combination have been established. Allanite-(Y) is partly replaced with allanite-(Ce), and metasomatic rims 2–10 μm wide enriched in LREE around allanite-(Y) have been identified. The degree of isomorphic Y+HREE substitution for LREE is estimated at 16%, on average; the maximum (Y+HREE)/LREE ratio does not exceed 0.25. It is assumed that the transition of allanite-(Y) into allanite-(Ce) might be caused by anincrease in acidity and decrease in aqueous fluid temperature at the late stage of the hydrothermal process.     

Rare earth and trace element distribution in zircons from miaskite lamproites of the Panozero complex,Central Karelia

Ion microprobe study of rare-earth and trace element composition of zircons from miaskite lamproites of the Panozero sanukitoid complex in Central Karelia made it possible to divide them into weakly altered and metasomatically altered zircons. The latter differ in elevated contents of LREE, Ca, Al, Fe, Ti, Sr, Ba, and Th, a flat REE distribution pattern, and a reduced Ce anomaly. Zircons show moderate to strong degrees of alteration, which are recorded in their geochemical features. The metasomatic alteration was caused by fluid related to the mantle alkaline magmatism. At the final stage, this fluid caused the crystallization of zircon domain with extremely high contents of REE (55000 ppm), Ti, Ba, Sr, and Th. The use of a Ti thermometer for unaltered zircons from the miaskite lamproites yields strongly underestimated temperatures, which require additional explanation or revision of the thermometer. For strongly altered zircons with sharply elevated Ti content, the use of this thermometer seems to be impossible.     

Geochemistry of metasomatic zircons from the Terskii greenstone belt

Local isotopic and geochemical studies of the zircons from metasomatites of the Terskii greenstone belt allowed us to determine two stages of metamorphism (2680 and 2025 My) and two stages of metasomatosis (2600 and 1800 My). Almost all the zircons were either metasomatic or affected to some degree by metasomatic processes caused by the enrichment of zircons in light rare earth elements, Th, U, Sr, and Ba, and flattening of the Ce anomaly.

     

Isotopic-geochemical study of zircons from metabasites of the Kontokki dike complex: Age of regional metamorphism in the Kostomuksha structure

Results of local isotopic-geochemical and chemical examination of zircons from metabasites of the Kontokki dike complex in the Kostomuksha structure, western Karelia, Russia, make it possible to interpret the concordant U-Pb zircon age of 2674 ± 13 Ma as the boundary between regional amphibolite-facies metamorphism and accompanying metasomatism. Zircons from the metabasites arte heterogeneous and consist of central parts with relict magmatic cores, metamorphic intermediate zones (which are pale in CL), and younger metasomatically altered zones (dark in BSE images), which development along boundaries between zones and lengthwise arrays of cracks permeable to fluids. The dark altered zones are characterized by high (for zircons) concentrations of LREE, MREE, Th, U, Ca, Sr, Ba, Fe, and Al. The REE distribution in the zircons was proved to be much less susceptible to overprinted metasomatic processes than the U-Pb system of the same zircons. Characteristics of the REE distribution in the zircons makes these zircons comparable with metasomatic zircons. Genetically, the metasomatic processes that affected the geochemistry of the zircons could be related to synmetamorphic granitoid intrusions (Bibikova et al., 2005).     

Assemblage of Fe–Mg–Al–Ti–Zn Oxides in Granulites of the Bunger Hills,East Antarctica: Evidence of Ultrahigh-Temperature Metamorphism

Geology of Ore Deposits - The composition and interrelations of oxides (minerals of the spinel supergroup, corundum, ilmenite, rutile) and silicates (garnet, sillimanite, orthopyroxene, cordierite)...