2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Zircon from alkali granite pegmatites of the Kola region:

mineralogy, geochemistry and U-Pb geochronology

Lyalina L.M., Zozulya D.R., Bayanova T.B., Selivanova E.A., Savchenko Ye.E.

Geological Institute, Kola Science Centre, Apatity, Russia, lialina@geoksc.apatity.ru

The remarkable feature of the Kola region in the Baltic Shield is the vast occurrence of the Neoarchean (2.61-2.67 Ga) alkali granite – syenite intrusions and related numerous rare-metal Zr-Nb-Y-REE deposits. The last ones are represented mainly by mineralized granites and nepheline syenites, interior pegmatites, silexites, country rock pegmatites, amazonite rand-pegmatites. From geological structure the interior pegmatites are the earliest postmagmatic bodies, related to alkali granites. In the paper the detailed mineralogical, geochemical and U-Pb isotope studies are presented for zircon from the interior pegmatite body “Zirkonovoye” from the Belye tundry alkali granite massif.

Zircon is main constituent of the rare-element mineralization in pegmatites and bears the valuable genetic information. Having the overall high contents on the pegmatite zircon becomes the main rock-forming mineral in some parts of pegmatite body. Four genetic types of zircon are distinguished from the combined morphological, structural and physical properties of the mineral. The relationship character between different zircon genetic types allows assigning the mineral to different chronological stages of pegmatite formation, i.e. pegmatitic itself, pneumatolytic-pegmatitic, and hydrothermal (Fig. 1).

Fig. 1. Sequence of formation for different zircon generations from the pegmatite body “Zirkonovoye”, Belye tundry alkali granite massif.

Zircon-I is represented by crystals of chocolate-brown color. The crystals are characterized by simple forms: {110}+{111}±narrow {100} for prismatic; {111}±{110}±{100} for dipyramidic crystals. The change of habits indicates on fluctuation of temperature and pH conditions. Induction hatching on the crystal faces for dipyramidic crystals is observed that indicates on close space during crystallization. The internal structure of the “chocolate” type zircon is characterized by zoning of two types – primary one (the crystal growth was during rhythmically changed conditions) and secondary one (the growth of rims on zircon-II).

Zircon-II is represented by cream-colored crystals and intergrown segments in grains of different zircon generations. The habits forms of zircon II are the same as from zircon I, but the faces of the zircon-II crystals are characterized by unevenness likely related to simultaneous (eutectic) growth of the marginal zones of “cream” zircon and quartz and formation of globular morphology of intergrowths. Inner inhomogeneity (zoning, sectoriality) was not observed in the zircon-II, but the phase inhomogeneity presents as the abundant mineral inclusions. Inclusions are represented by quartz, halena, fergusonite, apatite group, thorite, biotite, thorianite, xenotim, and monazite. They are of irregular form and homogenously distributed throughout the grains of zircon-II. Rarely the healing of cracks by quartz and cumulus distribution of thorite, thorianite, fergusonite and halena are observed. The presence of fluid inclusions is also not excluded as the polished faces of zircon-II bear the numerous imperfections. Such inner structure of zircon-II suggests its rapid growth under the high fluid saturation, during which the zircon as a sponge absorbs the different phases (Zircon, 2003), and this assumption is confirmed by experimental studies (Krasnova, Petrov, 1997). The above mentioned intergrowths of zircon-I and zircon-II present the rims of cream-colored zircon-II of different thickness and idiomorphic rate on the zircon-I. Noteworthy, that zircon-II also forms intergrowths with later zircon generations III and IV, thereby suggesting the repeated crystallization of the cream-colored zircon-II. But the main portion of zircon-II was crystallized during the second stage after fracturing and partial dissolution of chocolate-colored zircon-I (Fig. 1).

Zircon-III is represented by tiny (<1mm) idiomorphic prismatic and dipyramidic crystals with brilliant luster. The all crystals are immersed into fine-grained quartz aggregates. Zircon-III is characterized by distinct inner inhomogeneity – thin rhythmic zoning and sectoriality. The zoning pattern is the same one as for hydrothermal zircon, and sectoriality pattern indicates on the ununiform growth in moving solution that is confirmed by asymmetric structure of the crystals.

The latest zircon-IV is represented by pink-colored tiny (<1mm) xenomorphic grains and rim zones around the yellow-colored zircon-III. Only the rhythmic zoning is revealed for this type.

Fig. 2. Diagram ZrO₂-HfO₂ for zircon of different genetic types (I, II, III) from the pegmatite body “Zirkonovoye”. Group IV – magmatic zircon from Belye tundry and Zapadnokeivsky alkali granite massifs.

Fig. 3. Chondrite normalized REE pattern for the “yellow” type zircon from the pegmatite body “Zirkonovoye”, Belye tundry alkali granite massif.

From the ZrO₂/HfO₂ ratio the zircon composition is changed from earliest genetic types (average 22.15) to latest (average 17.33), that is in the accordance with crystallization sequence of mineral formation (Fig. 2). The trace element study of zircon by the local technique revealed that the mineral is selectively concentrates LREE (except of La) and inherits the geochemical specific of alkali granite magma (high Y content, negative Eu anomaly). The positive Ce-anomaly indicates on oxidized conditions of mineral crystallization (Fig. 3). The calculated crystallization temperature for zircon is very high and closed to alkali granite magma minimum temperature. The U-Pb isotope dating of pegmatitic zircon (2656±5 Ma) shows its temporal closeness to the time of granite formation (2654±6 Ma), that confirms the assignment of interior pegmatite to the earliest post-magmatic event of alkali granite complex.