Geochemical particularities subalkaline rocks in the South Kalchyk massif

Bezsmolova N.V.

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

bezsmolovan@meta.ua

South Kalchyk massif located in the East Azov area is unique in petrological, mineralogical and geochemical aspects. In literature this massif (area about 250 km²) is often called as syenitic one. At present, it has no identical analogues though similar massifs or occurrence of gabbroids and syenites are in Greenland and also within the Ukrainian Shield.

Syenites and quartz syenites are prevailing rocks (65% of the massif), granites make 25-30% [1]. Minor development have basic rocks and their differentiates: gabbroids, ferrogabbroids, ore (apatite, ilmenite, titanomagnetite) and highly enriched in iron mafites and ultramafites, and andesine anorthosites. The basic rocks and their differentiates (mafites and ultramafites) have insignificant development. But these rocks are of great petrogenetic importance becaus features of their composition allow to consider that they are genetically connected with syenites. It makes the basis to belong [4] the South Kalchyk massif  to gabbro-sienite complex, on the one hand, and to compare it with anortosite-rapakivigranitic plutons, - on the another one. There are all reasons to consider that syenites of the South Kalchyk massif as differenciates of  the basic (basalt) magma.

 The uniqueness of South Kalchyk massif are as following: 1. almost all of rocks (differentiates) including gabroides and ultaromafites are highly enriched in iron; 2. intermediate monzonite-syenite dyferentsiates are intensively enriched in Zr, TR and Y. [5, 6, 2, 4]. Among syenites of the massif fayalite-hedenbergite and two-fiedspar varieties are predominat.

Almost all rocks of the South Kalchyk massif, beginning gabbroidеs and mafites, belong to highly differentiated varieties. For early rocks (gabbroids and mafites) intensive enrichment in iron, titan and phosphorus, partly zirconium and the rare earths and depletion by chrome and nickel are characteristic. The next monzosyenites (a series monzonites- syenites-quartz syenites) are strongly enriched in zirconium, rare earths and yttrium at moderate enrichment in niobium. In syenties of the South Kalchyk massif content of Zr, TR and Y reaches (or even exceeds) the same concentration, of these elements in agpaitic feldspathoid syenties of Lovozero and Ilimaussaq, which are the most rich by these rare metals rocks. Finally the differentiation of such melts leads to formation of ores deposits, rich in these rare metals of nonconventional type (Azov, Yastrebetsky). However high concentrations of these metals accumulate only in syenites and monzonite-syenites. In more late granites, quartz syenite and granosyenite their content essentially decreases. Judging by two probe, in biotit-amphibolic quartz syenites content of Nb slightly increases (to 175 ppm). Sometimes high concentration of Nb (to 900 ppm) is fixed in ore alkali feldspathic syenites in the Azov deposit too. However in this deposit more late quartz biotitic syenites (sometimes their composition reaches granite) are considerably depleted in Zr, TR and Y in comparison with syenites. But contents of Nb in them does not decrease. The main concentrator Nb^- is ilmenite. According to J.J.Jurku, etc., (1973) [7] this ilmenite contains 0,85 % Nb2O5.

On the general background of such differentiation in this syenitic melt decreas of Ti, P, Sr and Ba occur. If in syenitic of the South Kalchyk massif the average contents of Ba makes nearby 900 ppm, in ore taxitic syenites of the Azov deposit - only 140 ppm; in the same way contents of Sr directly varies 180-200 and 24 ppm accordingly. Even more contrast image of distribution of these elements is in syenite of the Yastrebetsky massif: in endocontact (less differentiated) facies syenites the content of Ba is nearby 520 ppm, and in the layered series 20-50 ppm; accordingly Sr behaves 30-40 and < 15 ppm [3].

In the next granitic and quartz syenits similar processes of distribution of the mentioned elements are possible. However as it was mentioned above Zr, TR and Y have been extracted mainly by the previous syenitic  melt. At the same time Ba and Sr in the primordial content of intrusive granitoid phase was similar or a few than in syenites [1]. In later differentiate (Kamyanymohyly, Perga granites) their contents sharply go down (till 10-15 ppm) at substantial growth of Nb and at moderate or raised consistence of Zr, Y.

It is obvious that such way of behavior of the mentioned incompatible elements сan be better explained by the mechanism of crystallized differentiations with intensive feldspathic fractionation. Thanks to that we have a high decrease of Sr and Ba in final differentiates of separate intrusive phases and also strong negative europiums anomalies in pattern of the rare earth elements in syenits and granites as well as inconsidered in South Kalchyk massif and Azov deposit and other similar massifs and rocks (Yastrebetskiy syenites, Kamyany Mohyly and Perga granites) too. Considerable negative europium anomaly (0,41 ppm) manifest in apatites of gabbroids of a considered massifs.

It can be assumed that the South Kalchyk massif is prospective for revelation of other similar of Azov deposit rare metal occurrences. In our opinion unusual or unique features of the syenites and partially gabbroids of South Kalchyk massif are caused by abyssal conditions of their crystallization (by reduced oxygen fugacity) and by Fenner or similar to it trend of fraction crystallization.

 

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