New data on the age of eclogites from the Belomorian mobile belt at Gridino settlement area

Скублов, С. Г.; Astaf’ev, B. Yu.; Marin, Yu. B.; Березин, А. В.; Мельник, А. Е.; Presnyakov, S. L.

doi:10.1134/s1028334x11080290

Cited by 50 publications

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“…mately 1880 million years); the age was registered on the basis of zircon rims [12]. The U-Pb age of zircons from the pegmatite vein was slightly lower; these zir cons can be classified into two groups: (1) monotoni cally changed, dark gray in CL, having typically peg matite composition (high contents of Hf and HREE);…”

Section: The First Discovery Of Abnormal (Y+ree) Enriched Zircons In mentioning

confidence: 99%

The first discovery of abnormal (Y+REE)-enriched zircons in rocks of the Baltic shield

et al. 2011

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The content of rare elements in zircon may vary significantly, although it typically does not exceed cer tain threshold values, which have been ascertained empirically. The so called hydrothermal/metasomatic zircons are notable for the highest content of these ele ments, which are almost always present as isomorphic impurities [1]. While zircons enriched in Hf, U, and Th are common [2, etc.], considerable deviations in the Y and REE contents have been less frequent. The Y content typically lies within the range from 10 to 5000 ppm; the total REE content varies from 100 to 2500 ppm [3]. The maximum REE content (98 200 ppm) was registered in zircons from metasomatites of the unique Oklo uranium deposit (Gabon, Equatorial Africa) [4]. For the Baltic shield, an anomalously high content of REE was ascertained in zircons from metabasites of the Kontokki dike complex in the Kos tomuksha structure (29 800 ppm [5]) and rocks of the Panozero sanukitoid complex in Central Karelia (55 300 ppm in lamproites [6] and 68 800 ppm in monzogabbro [7]). REEs are concentrated in local domains and zones of variation of zircon grains, which were of contrasting dark color in the BSE images. A high Y content (up to 5 wt % of Y 2 O 3 ) was revealed in metamict zircons from the metasediments of the Dal radian complex in Scotland, which associate with xenotime and are formed upon intensive low temper ature fluid processing of rocks [8].The anomality of the composition of the zircons considered in this article was revealed by performing U-Pb dating (SHRIMP II, Center of Isotopic Research, All Russia Research Institute of Geology), which was accompanied by studying the distribution of rare and rare earth elements (Cameca IMS 4f, Yaroslavl' Branch of the Physical Technical Institute) according to the procedure described in [9]. After sev eral months the zircon content in the same domains was measured again on an ion microprobe; the results were in a close agreement, and no analytical errors were ascertained. The features of the structure and composition of zircons with respect to the major ele ments were studied on a JEOL JSM 6510LA scanning electron microscope with a JED 2200 energy disper sive spectrometer (Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences); the analysis results were standardized to 100%. The presence of inclusions of foreign mineral phases was verified on an electron microscope upon the maxi mum possible magnification by composition contrast (BSE regime). The correctness of determination of the microelemental composition on an ion microprobe (with a locality of 15-20 µm) was regulated by analyz ing on an electron microprobe (which was less sensi tive, but was characterized by a locality of 1-2 µm only). No foreign mineral phases were revealed in zir cons. When plotting the spectra of REE distribution, the compositions of minerals were normalized to the composition of C1 chondrite [10].Zircons of anomalous composition were found in a thin (up to 20 cm) pegmatite vein (sample 101) cross ing the e...

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Section: The First Discovery Of Abnormal (Y+ree) Enriched Zircons In mentioning

confidence: 99%

The first discovery of abnormal (Y+REE)-enriched zircons in rocks of the Baltic shield

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“…In this study we applied the LuHf garnet clinopyroxene geochronology, which are rock forming minerals of apo gabbro eclogites of the BMB. Consequently, the Lu-Hf datings of garnet provide unambiguous evidence for the age of meta morphism of the eclogite facies, provided that the blocking temperature was not exceeded (see discus sion below).We studied four eclogite samples from the Salma area (Samples 46 and 21) and Gridino (Samples 102 and 108), which were preliminarily dated using the U-Pb method by zircons providing evidence for the Svekofennian (~1.9 Ga) age of eclogites [3,4]. According to the specific geochemical characteristics, the studied grains or rims of zircons correspond to zir cons precisely of the eclogite facies [3,4].…”

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“…We studied four eclogite samples from the Salma area (Samples 46 and 21) and Gridino (Samples 102 and 108), which were preliminarily dated using the U-Pb method by zircons providing evidence for the Svekofennian (~1.9 Ga) age of eclogites [3,4]. According to the specific geochemical characteristics, the studied grains or rims of zircons correspond to zir cons precisely of the eclogite facies [3,4].…”

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First Lu-Hf garnet ages of eclogites from the Belomorian Mobile Belt (Baltic Shield, Russia)

et al. 2012

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Eclogites from the north eastern border of the Bel morian Mobile Belt (BMB) have not only been used for regional geodynamic reconstructions [1], but also as first direct evidence for plate tectonic processes in the Mesoarchean [2]. M.V. Mints and coworkers (2010) dated zircons from eclogites of the Salma area related to the so called Belomorian Meso-Neoar chean eclogite province and interpreted the age of 2.87 Ga obtained for some zircons as the age of meta morphism of the eclogite facies [2]. However, rocks of the Salma area undoubtedly underwent eclogite meta morphism with an age of ~1.9 Ga in the Paleoprotero zoic [2][3][4]; for this reason, there is a question of whether or not zircon datings of 2.87 Ga correspond to the age of Mesoarchean eclogite metamorphism [1, 2], or whether this is the age of the magmatic protolith (gabbro) of eclogites [3,4], or if it represents a low P metamorphic event. In this study we applied the LuHf garnet clinopyroxene geochronology, which are rock forming minerals of apo gabbro eclogites of the BMB. Consequently, the Lu-Hf datings of garnet provide unambiguous evidence for the age of meta morphism of the eclogite facies, provided that the blocking temperature was not exceeded (see discus sion below).We studied four eclogite samples from the Salma area (Samples 46 and 21) and Gridino (Samples 102 and 108), which were preliminarily dated using the U-Pb method by zircons providing evidence for the Svekofennian (~1.9 Ga) age of eclogites [3,4]. According to the specific geochemical characteristics, the studied grains or rims of zircons correspond to zir cons precisely of the eclogite facies [3,4]. Eclogitized gabbro (Samples 46 and 102) are the most abundant eclogite type in the BMB. These are homogeneous fine granular bimineral rocks mainly composed of omphacite partially replaced by decompression sym plectites of plagioclase and clinopyroxene with a very low portion of the jadeitic end member and numerous garnet porphyroblasts. Eclogitized ultrabasic rocks (Samples 21 and 108) form coarse granular garnetclinopyroxene bodies (with a thickness up to 20 cm) and aggregates in the central parts of metaultrabasite bodies transformed mainly into secondary tremo lite-actinolite schist (Sample 21) or amphibolite (Sample 108) along the perimeter.Mineral monofractions were sorted according to standard methods using heavy liquids at the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, and finally manually separated under a binocular microscope. For each sample three garnet monofractions were independently analyzed (Table 1). Both whole rock powders and mineral sepa rates were dissolved using a selective digestion proce dure, aiming to dissolve the major silicate phases, while leaving behind refractory phases, such as rutile and zircon [5]. This approach circumvents a potential age bias due to zircon and other mineral inclusions [6] and frequently results in high precision Lu-Hf ages (e.g., [5,7,8]). These authors also provide detailed methodological sections, ...

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“…Further detailed geochronological studies of these eclogite facies rocks (including samples from the same previously dated eclogite body of Stolbikha Island [1]) and zircon REE analyses revealed that thẽ 2.7 Ga age can be assigned to the magmatic zircon core, whereas the age of eclogitization obtained on zircon rims is Svecofennian (~1.9 Ga) [2]. These results support a Svecofennian age for eclogitization restricted to ductile shear zones in the Gridino area [3], which contain rootless bodies of eclogitized metabasites and Paleoproterozoic dykes.…”

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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)

et al. 2012

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New data on the age of eclogites from the Belomorian mobile belt at Gridino settlement area

Cited by 50 publications

References 7 publications

The first discovery of abnormal (Y+REE)-enriched zircons in rocks of the Baltic shield

The first discovery of abnormal (Y+REE)-enriched zircons in rocks of the Baltic shield

First Lu-Hf garnet ages of eclogites from the Belomorian Mobile Belt (Baltic Shield, Russia)

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)

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