The nature of petrogenetic links between carbonatites and associated silicate rocks is a matter of discussion for several decades and still remains controversial among igneous petrologists. The ...Belaya Zima plutonic complex in southern Siberia is a typical intrusion, in which carbonatites are spatially and temporally associated with the ijolite series rocks and nepheline syenites. In this study we use whole-rock compositions and trace element characteristics of the major and accessory minerals for reconstructing the magmatic evolution of the complex and clarifying the origin of carbonatites. We conclude that the observed gradual increase of Zr, Nb and REE concentrations in the magma is consistent with the process of extensive fractional crystallization and gradual transition from silicate rocks to carbonatites, and inconsistent with the formation of carbonatites by liquid immiscibility. Magma differentiation by fractional crystallization continued during the carbonatite formation. The textural and analytical evidence indicates that the early calcitic carbonatites evolved to dolomitic and ankeritic carbonatites. In addition, maximum Nb and Zr concentrations occur in the calcitic carbonatite, whereas the REE content increases in the dolomitic and ankeritic facies.
•Сarbonatites are enriched in Zr, Nb and REE in comparison to alkaline silicate rocks.•Alkaline silicate rocks and carbonatites are related by simple fractional crystallization.•Fractional crystallization was continued during the carbonatite formation.
In this study, we present the results of U–Pb (ID-TIMS) geochronological studies of calcic garnet from the alkaline ultramafic complexes of Eastern Sayan province (eastern Siberia). New U–Pb ID–TIMS ...garnet ages obtained from different rocks of Bolshaya Tagna (632 ± 2 Ma) and Srednaya Zima intrusions (624 ± 5 Ma), as well as previously published garnet ages of the Belaya Zima complex (646 ± 6 Ma), allow us to constrain the timing and duration of episodes of alkaline ultramafic magmatism in Eastern Sayan province (619–651 Ma). Variations in the chemical compositions of rocks from three massifs indicate that the parental melts were separated from different magmatic chambers generated during the same episode of mantle melting. This study further highlights garnet U–Pb dating as a potentially robust, high-resolution geochronometer to constrain the evolution of the main pulse of alkaline ultramafic magmatism in the large magmatic provinces.
•The rocks crystallized from 18O- and D-depleted magma.•The rocks are products of fractional crystallization from a single parental magma.•Hydrothermal processes were responsible for the change in ...δ18O values.
The Belaya Zima alkaline–carbonatite complex in the East Sayan Mountains, Russia, is a multiphase concentric intrusion, which comprises diverse alkaline silicate rocks and carbonatites. Melteigites are the earliest products of crystallization and they are followed by ijolites–urtites, nepheline syenites, and finally, calcite, calcite–dolomite and ankerite carbonatites. In this paper, we present new geochronological data (Ar–Ar method), chemical and stable isotope (H, C and O) compositions of the main rock-forming minerals in carbonatites and the silicate rocks of the complex. The Ar–Ar dating of phlogopite from ankerite carbonatites is consistent with the age of syenite emplacement and implies that the silicate rocks and carbonatites are likely to be comagmatic. The evolutionary trends of pyroxene, fluorapatite, amphibole and phlogopite are consistent with fractional crystallization from a single parental magma. Primary Nb mineralization is represented by pyrochlore, whereas columbite-(Fe) is a replacement product of pyrochlore at the post magmatic, hydrothermal stage. At that stage, interaction of calcite carbonatites with hydrothermal fluids resulted in the formation of hydrothermal paragenesis comprising dolomite–ankerite, monazite-(Ce), ancylite-(Ce), minerals of burbankite group and Ca–REE fluorcarbonates. The hydrothermal processes were also responsible for cation leaching from pyrochlore, the replacements of phlogopite by tetraferriphlogopite and early Ca-rich pyroxene by aegirine. The stable isotope data suggest that the rocks crystallized from a 18O-depleted magma. Shifts toward higher δ18O values relative to the primary compositions are commonly observed in minerals from carbonatites, which is attributed to hydrothermal processes.
Yapısalcıların yaptıkları gibi, metinlerarasılığı yalnızca metinsel bir etki olarak görmek yeterli midir? Bir metin yalnızca başka metinlere gönderme yapan, anlamını onlara göre ortaya koyan kapalı ...bir dizge midir? Özellikle Pierre V. Zima'nın bir metin toplumbilimi bağlamında yaptığı kimi tanımlamalara bakılırsa yazınsal bir metin içerik olduğu kadar yapı düzleminde kapalı bir yapı olmakla sınırlı kalmamaktadır; aynı zamanda "açık bir yapı" olarak görülmelidir. Buna göre, açıklığı sağlamanın en etkili yolu metinlerarasılıktır. Ancak metinlerarasılık kapalı bir dizge anlayışına uygun olarak, yazınsallığın bir ölçütü olarak görülmesine karşın gitgide gerçeklikle ilişkileri içerisinde de tanımlanmıştır. Bir başka anlatımla, yazınsal bir metin aynı zamanda gerçeklikten beslenerek, göndergeselliğini dış gerçekliğe göndererek de somutlaştırmaktadır. Bu çalışmada metinlerarasılığın bildik tanımlamalarından ayrı olarak metnin dış gerçeklikle bağları irdelenerek göndergeselliğinin nasıl gerçekleştiği konusunda saptamalar yapmak, böylelikle metinlerarasılık kavramını salt metinsel bir etki olmakla kalmayıp aynı zamanda gerçeklikle nasıl ilişkilendirilebileceği üzerinde durulmaktadır.
Pyrochlore-group minerals are the main concentrators of niobium in carbonatites of the Belaya Zima alkaline pluton. Fluorcalciopyrochlore, kenopyrochlore and hydropyrochlore were identified in ...chemical composition. Their main characteristics are given: compositional variation, morphology, and zoning. During evolution from early calcite to late ankerite carbonatites, the UO
2
, TiO
2
, REE, and Y contents gradually increased. All carbonatite types are suggested to contain initial fluorcalciopyrochlore. However, in calcite–dolomite and ankerite carbonatites, it is partially or completely hydrated due to hydrothermal processes at the late stage of the pluton. This hydration resulted in the appearance of kenopyrochlore and hydropyrochlore due to removal of Ca, Na and F, and input of Ba, H
2
O, K, Si, Fe, and probably U and REE. At the last stage of the pluton, this hydrated pyrochlore was replaced by Fe-bearing columbite.
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