2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Geochemistry of nepheline syenites from the different complexes of Ukrainian Shield

Dubyna O.V., Amashukely Y.A.., Kryvdik S.G.

N.P. Semenenko Institute of geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine

kryvdik@ukr.net

Overwhelming majority of alkaline rock massifs from the Ukrainian Shield (USh) belong to two different age complexes: 1) alkaline-ultrabasic (2.0-2.1 Ga) and 2) gabbro-syenitic (1.7-1.8 and 0.4 Ga). Nepheline syenites are presented in massifs of both complexes.

 In the alkaline-ultrabasic complexes (Chernigivka, Proskurivka, Antonivka) the nepheline syenites (canadites, juvite, malignites and pulaskites) have a subordinate volume or are absent in general as it is observed in the small alkaline-ultrabasic intrusions (Gorodnica, Glumcha, Bolyarka) of North-Western part of USh. At the same time nepheline syenites (foyaites, pulaskites, mariupolites, malignites, juvites and phonolites) occupy the considerably greater volume in the gabbro-syenitic complexe are widely spread (Oktyabsky, Mala-Tersa, Pokrovo-Kyryevo) than in alkaline-ultrabasic one. Simultaneously in South-Kalchyk gabbro-syenitic massif only fayalite-hedenbergite syenites occur and are absent their nepheline varieties. These rocks on the whole are very similar to the fayalite-ferroavgite varieties of rapakivi granites. Therefore some investigators considered South-Kalchyk massif as a syenitic analog of the anorthosite rapakivi granite plutons. Nepheline syenites are absent in Davydky gabbro-syenitic massif which is located on the north-eastern border of Korosten anorthosite rapakivi granite pluton. Feldshpathoid syenite varieties are not revealed in Yastrebecky syenitic massif too (north-western part of USh).

Amongst the main petrochemical peculiarities of nepheline syenites from alkaline massifs of USh could be mentioned a predominance of alkali (up to 17% Na₂O+K₂O) in these rocks of gabbro-syenitic complexes while the similar rocks from alkaline-ultrabasic complexes alkali content don’t exceed 13%. The ratio Na₂O/K₂O in the nepheline syenites of alkali-ultrabasic complexes doesn’t change essentially (2.0-2.6). This ratio is some lower (on average 1.3) in the foyaites of Mala-Tersa massif, while in Pokrovo-Kyreevo massif is decreasing from malignite (1.3) to nepheline syenite (0.7). The greatest changing Na₂O/K₂O are typical for Oktyabrsky massif where this ratio is increasing from pulaskites (1.2) to eudialyte phonolite (2.5) and reach of the highest value is in magnetite foyaites (8.5) and mariupolites (up to 15).

Nepheline syenites from the mentioned massifs are distinguished by the agpaitic coefficient (K_a). In alkaline-ultrabasic complexes nepheline syenites belong to the miaskitic type (often K_a<0.90) whereas in gabbro-syenitic occur both the miaskitic (though agpaitic coefficient K_a ~1) and agpaitic varieties. Low or lower magnesity of rocks and rock-forming mafic minerals are the typical peculiarities of rocks from gabbro-syenitic complexes of USh, particularly the feldspathoid syenites. In the nepheline syenites of gabbro-syenitic of USh complexes the magnesity changes in the range 0-0.35 while in the similar rocks of alkaline-utrabasic complexes magnesity is rarely achieve only 0.2.

Among chemical distinctions of nepheline syenites the phosphorous content could be mentioned. The lowest its concentration (up to full disappearance) is the typical for last or final intrusive phases from gabbro-syenitic complexes (0-0.1% Р₂О₅). In the early pulaskites of Oktyabrsky massif some increasing P content (0.26-0.56% Р₂О₅) have been established. Increased magnesity is typical for these rocks in comparison to later nepheline syenites of this massif.

Enough different are nepheline syenites from alkaline complexes of USh by the rare and rare-earth elements concentration. High Sr and Ba concentrations in the nepheline and alkaline syenites from alkaline-ultrabasic complexes  is observing. The Sr content can already achieves of essential value (1.1-1.8% Sr) in ijolite-melteigites in Chernigivka massif. Alkaline syenites have more higher Sr concentration (on average 2299 ppm) in comparison to nepheline ones (1730 ppm) of this massif. High Sr concentration is also established in the nepheline syenites of Proskurivka and Antonivka massifs (accordingly 1811 and 2513 ppm). The Rb concentration is increasing from early mafic rocks to the latest nepheline and alkaline syenites.

The highest Sr and Ba concentration have been established in the slightly differentiated varieties of nepheline syenites from gabbro-syenite of USh complexes. This tendency is mostly displayed in Oktyabrsky massif. So the highest Sr (1140 ppm) and Ba (3187 ppm) contents are fixed in the nepheline syenites (pulaskites) with relatively increased magnesity of this massif. At the same time in the nepheline and alkaline syenites from late intrusives (mariupolites, phonolites) the concentration of these elements is progressively decreasing, and their contents are 15-52 (Sr) and 18-41 (Ba) ppm.

Accumulation of Zr, Nb, Y and REE occurs from more early varieties of nepheline syenites (pulaskites) to the most differentiated ones (mariupolites, phonolites) of Oktyabrsky massif. The complex (zircon, pyrochlore) Mazurivka deposit is related to mariupolite of this massif. So in the pulaskites the average Zr and Nb concentrations are 352 and 120 ppm and quite low Y (37 pmm) and REE (143 ppm) while in the mariupolites the average Zr and Nb are 4878 and 1550 ppm, and Y (211 ppm) and REE (594 ppm) are increasing. Analogically the increasing incoherent elements content is observed in the essentially syenitic complexes of South Greenland (Ilimaussaq, Motzfeldt, Tugtutôq).

Summarize a geochemical data on gabbro-syenitic of USh complexes could be mentioned that decreasing of P, Sr, Ba contents and increasing of incoherent elements also occur in the latest differentiates in the Yastrebecky and South Kalchyk massifs (in which feldshpathoid syenites are absent). Decreasing Sr and Ba and increasing incoherent elements concentrations in syenites of gabbro-syenitic complexes is the result of crystalline differentiation and intensive feldspar fractionation. It is also confirmed (besides the peculiarities of chemical and mineral composition variations of these rocks) by the chondrite-normalizing REE patterns from a rocks and minerals. So the significant negative Eu anomalous  are on the REE patterns from the dike microfoyaites and phonolites (Eu/Eu* 0.45 and 0.35 correspondingly) of Oktyabrsky massif that are considered as the latest rocks. The range of Eu/Eu* is 0.74-0.76 in apatites from the earliest endocontact nepheline syenites (pulaskites) while this ratio is decreasing to 0.61 in apatite from the syenites of central part of this massif.

Average trace elements concentrations (ppm) in the nepheline syenites of USh

In nepheline syenites of Proskurivka and Antonivka massifs concentration of Zr (53 and 45 ppm correspondingly) and Nb (30 and 16 ppm) are anomaly low as for such rock types. The decreasing of Zr, REE and Y concentrations from the most melanocratic to leucocratic rocks are observed in both massifs. In this direction Nb concentration is decreasing in Antonivka massif whereas this tendency is opposite in Proskurivka massif. Canadites of Chernigivka massif have a significant Zr (1180 ppm) and Nb (376 ppm) concentrations in contrast to the nepheline syenites from previous massifs. Such contrast of incoherent elements distribution from the nepheline syenites of alkaline-ultrabasic of USh complexes we explain by different geodynamic conditions of their forming: riftogenic – for Chernigivka; and pressure (or collision) – for Proskurivka and Antonivka massifs.

The LREE prevail (La/Yb_N=14-38) in the chondrite-normalizing REE patterns from the nepheline syenites of alkaline-ultrabasic complexes. In general La/Yb_N ratio is similar in the nepheline syenites from different complexes although in some canadites of Chernigivka massif predominance of LREE can be more significant (La/Yb_N=142). Furthermore lacking or negligible negative Eu anomalous on the REE patterns are typical for these rocks while in some of them slightly positive anomalous (Eu/Eu*=0.98-1.12) are observed. Similar regularity is observed in Ilmenogorsk-Vishnevogorsk miaskites, nepheline syenites of Khibiny and Ozernaya Varaka.

Insignificant negative Eu anomalous are also observed in the apatites from nepheline syenites (and other rocks too) of alkaline-ultrabasic of USh complexes. Probably these peculiarities of REE and some incoherent elements distribution in the nepheline syenites have been explained by different rock-forming processes – liquation and crystallization differentiation for alkaline-ultrabasic and gabbo-syenitic complexes accordingly. Negligible changing of magnesity in rocks and mafic rock-forming minerals of alkaline-ultrabasic complexes, absence of incoherent element enrichment in the nepheline and alkaline syenites (some depletion of these elements is observed in comparison to alkaline melano- or mesocratic rocks of these massifs), lack of significant negative Eu anomalous both in the nepheline syenites and ijolite-jacupirangites are evidences of liquation processes of these complexes formation in favor of this  assumption.

Consequently the different geodynamic conditions and petrogenetical model of their formation caused a contrast geochemical peculiarities of similar rocks from various alkaline complexes of USh.

This study was financially supported by the project of Ukrainian NAS №1-F/2011 in collaboration with RFBR.