S. G. Skublov

General relations in the trace-element composition of zircons from eclogites with implications for the age of eclogites in the belomorian mobile belt

Analysis of currently available data (877 individual high-precision zircon analyses) on the composition of zircons from eclogite complexes worldwide reveals general relations in the zircon composition: an anomalous decrease in the Th concentration (no higher than 3 ppm on average) and the Th/U ratio (0.33 on average), a significant decrease in the concentrations of all REE (to 22 ppm) and particularly LREE (<2 ppm), and relatively low concentrations of Y (34 ppm), U (100 ppm), and P (41 ppm) at an elevated Hf concentration (11 400 ppm on average). The REE patterns of eclogitic zircons are noted for pronounced flat HREE patterns, poorly pronounced (if any) negative Eu anomalies, strongly reduced positive Ce anomalies (Ce/Ce* = 11 on average), and U-shaped configurations of LREE patterns up to the development of negative Nd anomalies. The relations detected in the distribution of trace elements and REE in eclogitic zircons are of universal nature and occur irrespective of the rock type (metabasites, metaultrabasites, or gneisses) and the metamorphic pressure (eclogites of high and ultrahigh pressure). The application of the aforementioned criteria makes it possible to reliable distinguish eclogitic zircons from those of magmatic or metamorphic genesis (not related to high-pressure metamorphism). Eclogites in the Belomorian Mobile Belt (in the Salma and Gridino areas) were determined to contain zircons in metagabbro eclogites; the cores of these zircons have an age of 2.8–2.9 Ga and are of magmatic genesis, whereas their outer metamorphic zones have an age of 1.9 Ga and a trace-element composition typical of eclogitic zircons. Hence, the Belomorian Mobile Belt was affected only by single (Svecofennian, at ∼1.9 Ga) episode of eclogite metamorphism of Archean rocks.

New U-Pb and Sm-Nd ages and P-T estimates for eclogitization in the Fe-rich gabbro dyke in Gridino area (Belomorian Mobile Belt)

New data from isotope geochronology (U-Pb, Sm-Nd) petrological study provide evidence of the Svecofennian age (∼1.9 Ga) for eclogitization in the Fe-gabbro dyke inductile shear zones of Gridinrea. P-T estimates of eclogitization were computed using the THERIAK/DOMINO software. A close timing relationship between dyke magmatism and eclogitization is inferred.

Geochronology, mineral inclusions, and geochemistry of zircons in eclogitized gabbronorites in the Gridino area, Belomorian province

A comprehensive (mineralogical, geochronological, and geochemical) study of zircons from an eclogitized gabbronorite dike was carried out in order to identify reliable indicators (mineralogical and geochronological) of the genesis of zircons in their various populations and, correspondingly, the age of certain geological events (magmatic crystallization of the gabbroids, their eclogitization, and overprinted retrograde metamorphism). The three populations of zircons separated from two rock samples comprised xenogenic, magmatic (“gabbroic”), and metamorphic zircons, with the latter found exclusively in the sample of retrograded eclogitized gabbroids. The zircons of group I are xenogenic and have a Meso- to Neoarchean age. Mineral inclusions in them (quartz, apatite, biotite, and chlorite) are atypical of gabbroids, and the geochemistry of these zircons is very diverse. The zircons of group II contain mineral inclusions of ortho- and clinopyroxene and are distinguished for their very high U, Th, Pb, and REE concentrations and Th/U ratios. These zircons were formed during the late magmatic crystallization of the gabbroids at temperatures of 1150–1160°C, and their U-Pb age of 2389 ± 25 Ma corresponds to this process. The eclogite mineral assemblages crystallized shortly after the magmatic process, as follows from the fact that the marginal portions of the prismatic zircons contain clinopyroxene inclusions with elevated contents of the jadeite end member. The zircons of group III contain rare amphibole and biotite inclusions and have low Ti, Y, and REE concentrations, low Th/U ratios, high Hf concentrations, contain more HREE than LREE, and have an U-Pb age of 1911 ± 9.5 Ma, which corresponds to the age of the overprinted amphibolite-facies metamorphism.

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).

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.

First data on chemical composition of zircon from lithium-fluorine granite of the Severnyi pluton, Chukchi Peninsula

The chemical composition of zircons from lithium-fluorine granite of the Severny pluton in the Chaun region, Chukchi Peninsula, has been studied. The magmatic origin of zircons has been established. Elevated LREE, Li, and Nb contents; moderate enrichment in Hf; and deep Eu anomalies are characteristic of zircons from rare-metal granite. The contents and distribution of chemical elements in studied zircons differ from those of biotite granite. Zircons from lithium-fluorine granite of the Severny pluton also differ from the worldwide analogs in moderate Hf, Y, and REE concentrations and extremely low contents of U, Th, and other incompatible elements.

U-Pb Age and Geochemistry of Zircons from Xenoliths of the V. Grib Kimberlitic Pipe, Arkhangelsk Diamond Province

595 Deep seated xenoliths transported by kimberlites are the main reliable source of information on the composition and age of the lower crust and lithos pheric mantle for the northern margin of the East European Platform [1]. The high productive Middle Paleozoic V. Grib pipe in the Arkhangelsk diamond province is distinguished by the high concentration and variability of xenoliths with prevalence of mantle peridotites and eclogites, as well as lower crustal gran ulites with a total portion of ~80% from all xenoliths from this pipe [2].