Are Precambrian Charnockitoids of the Ukrainian Shield Comparable with Phanerozoic Island Arc Andesites?

Shniukova K.Y., Tomurko L.L.

M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of National Academy of Sciences of Ukraine, Kyiv, Ukraine

shniukova@igmof.gov.ua, tomurko@igmof.gov.ua

 

Within the Ukrainian Shield (US) no less than three stages of enderbite-charnockite formation are reliably dated, namely >3,0; 2,7-2,8 and 2,0 Ga. Charnockitoides of most ancient Archean stage are distributed in granulite-gneissic blocks, namely Dnister-Bug (Gayvoron complex) and Azov (Novopavlovka complex of the Orehovo-Pavlograd (OP) seam zone). The younger charnockitoides of Tokmak complex (Azov block) were formed at the border of Archean and Proterozoic stages (about 2,7 Ga). Enderbite-formation of the 2 Ga stage are exhibited in the Dnister-Bug block by Litin complex and in amphibollite-gneissic Ingul block by Tashlyk complex. All known intrusive charnockitoides of the US are refered to the same stage: Buki massif in Volyn block, Novoukrainsky massif in Ingul block (both are amphibollite-gneissic ones), Khlebodarovsky massif in Azov block. In the Precambrian, when the main masses of enderbites were generated, different geodynamical settings had been already probably existed. At least in granulite-gneissic regions, most characteristic feature of which is the andesitic direction of magmatism, continuous compression had been occuring. Ancient granulitic crust of the US is almost half composed by andesites (Lesnaya, 1988). Herein its similarity with the island-arc formations, where andesites embrace up to 60%. So, by the formal approach, enderbites may be considered as the plutonic analogues of calc-alkali andesites, which are characteristic for destructive geodynamic settings.

However, rather few members of the US enderbite-charnockite associations fall within the andesitic range of composition with 53-63% SiO₂. Thus, the majority of charnockitoides of both Dnister-Bug and Azov blocks are much more acidic, most ancient Gayvoron and Novopavlovka complexes belonging to low- and middle potassic series, and Tokmak complex to middle- and high potassic ones. In “andesitic” by composition metamorphogenous Gayvoron and rheomorfic Litin charnockitoides of the Middle Bug when increasing SiO₂ K₂O content goes down not up as in andesitic series, although their compositional changes follow the calc-alkali trend. In exclusively acidic metamorphogenous charnockitoides of OP seam zone trondhjemitic trend is exposed. Thereby, Archean metamorphogenous charnockitoides of the both granulite-gneissic blocks (Dnister-Bug and Azov) together with Proterozoic rheomorfic charnockitoides of the first one are not comparable with the Phanerozoic andesitic series.

Such comparison is possible only for Proterozoic (about 2 Ga) enderbite-charnockite associations of Ingul, Volyn and Azov blocks, composition of which lie within andesitic range. Enderbites of Ingul block differ chemically from analogous rocks of two other blocks. Both intrusive charnockitoides (Novoukrainsky massif) and metamorphogenous enderbites of Tashlyk complex (basics) fall into tholeiitic field on AFM diagram and correspond to spreading-zone basalts on Pearce diagram, while all other enderbites fall into calc-alkali field and correspond to orogenous (island arc and marginal continental) basalts along with the typical andesites. Charnockitoides of Ingul block (to the great extent intrusives of Novoukrainsky massif , less metamorphogenous Tashlyk complex) exhibit high values of FeO*/MgO and increase in K₂O with increasing SiO₂ that are characteristic for late- and postfolded enderbite-charnockite associations (Magmatic Rocks..., 1987). Consequently, an extensional conditions had been existed in Ingul block by the time of their formation, while in Volyn and Azov blocks compressional ones indicated by calc-alkali series still continued. Buki massif of Volyn block being more alkaline is comparable rather with sanukitoids than with andesites. I.B. Shcherbakov (2005) considered Ingul block to be a protoriftogenous system in which compressional conditions of the Paleoproterozoic had been inverted into extensional ones during the Mesoproterozoic. Tholeiitic character of Tashlyk complex is inherent only for its basic members, intermediate enderbites possessing calc-alkali features that can also testify to the reversion from extension to compression. In addition, calc-alkali enderbites of this complex possess unique high Al₂O₃ content (up to 21%) which is typical to early-folded charnockitoides (Magmatic Rocks..., 1987), that is those formed under compressional conditions. As a result, inconsistent petrochemical peculiarities of enderbites of the both amphibollite-gneissic blocks of US force us to refuse their comparison with andesites.

One acceptable block remains, namely Azov. Similarity to andesites becomes apparent in enderbite magmatism of Azov block from the very end of Archean: first in Tokmak complex, then in Khlebodarovsky massif. Perhaps it indirectly confirms Kalyaev’s and Glevassky’s opinion (1984) who believed Azov block to behave as an active continental margin of Andian type during the Proterozoic. In charnockites of Tokmak complex K₂O content increases with increasing SiO₂ and calc-alkali trend is exhibited, but for lack of basic rocks regularities are poorly displayed. Intrusive enderbites of Khlebodarovsky massif are most similar to andesites by all petrochemical and geochemical features.

When searching the analogues of enderbites throughout the later geological history we were guided by their major mineralogical peculiarity, namely availability of hypersthene. Hypersthene-containing andesites are known among andesites of calc-alkali series of modern island arcs, particularly in the Kurilo-Kamchatskaya island arc. Bipyroxene andesites are widespread here among middle- and high potassic varieties, volcano Ebeko (island Paramushir) being reputed a standard. Majority of US Proterozoic charnockitoides refers to middle potassic series, too. Generally, the higher water content in the melt the more potassic andesites are (Frolova et al., 1989). Fractional crystallization of basaltic magma leads to the formation of primitive andesitic magmas, to which bipyroxene andesites correspond, and water contents in the system not exceed 1-3 wt% by the range of pressure from 1,5 to 10 kbar and temperature 1050-1150º С (Kadik et al.) at that. Besides island arcs bipyroxene andesites are defined among the late orogenic volcano-plutonic associations of fold belts of various age and location: caledonides of Britain, hercynides of South Mongolia and Kazakhstan, mesozoides of Okhotsko-Chukotsky and Sikhote-Alinsky belts, as well as in modern Andian-type active continental margines and collizion zones of Himalayan type (South Ands, Central Anatolia) (Magmatic Rocks..., 1987).

In whole chemical composition of hypersthene-containing middle-potassic andesites of fold belts and island arcs is very similar to those of some US magmatic enderbites. Distinctions between them are the next. 1. Enderbites, as a rule, are more ferriferous, that results in higher ferriferousity of orthopyroxene (in enderbites Fs >40-60, in andesites <40). Charnockitoides of Novoukrainsky massif contain even eulite. On the contrary, in andesites sometimes low-ferriferous orthopyroxenes are present (enstatite and bronzite). 2. In enderbites clinopyroxene is represented by salite or subcalcic augite, while in andesites mainly by high-calcic augite, at times salite, rarely pigeonite-augite. 3. Na₂O content is higher in enderbites, affecting more acid character of plagioclase (in enderbites An _25-30, in andesites An _40-70). 4. Bipyroxene andesites being sometimes even more high-potassic than enderbites never contain K-feldspar. Higher K₂O content in andesites are explaned by the fact that alkali redistribution within water-saturated andesitic calc-alkali magma occurs under the influence of fluid phase, as it was proved for Ebeko volcano (Frolova et al., 1989). 5. Andesites possess higher Rb content. 6. Nb, LREE, Ba and Sr abundances are little higher in Proterozoic enderbites as compared with bipyroxene andesites (for last two elements it is probably due to a somewhat higher calcium content in enderbites). 7. Finally, andesites do not contain quartz by the same silicity. Generally, the presence of more than 5% of quartz in the rocks with SiO₂ content less than 57%, as it takes place in enderbites, is forbidden for young and modern volcanism. So, different, first of all from the mineralogical point of view, rocks were forming in the Precambrian and in the Phanerozoic by the similar or even the same initial chemical composition of magma.

Andesitic magmatism in the Phanerozoic is connected with the subduction processes, but both subduction environments and characteristic for them low-temperature magmatic regimes being manifested later had been absent in the Early Precambrian earth’s crust (Sharkov, 1983). Early Precambrian magmatic melts had been relatively dry, while andesitic magma is more water-rich. Therefore in the ancient mobile zones high-temperature dry melts of acid-intermediate composition had been originated instead of relatively low-temperature water-saturated andesitic series. The first were regarded (Sharkov, 1983) as a specific Early Precambrian precursor of andesitic calc-alkali magmatism of later mature earth’s crust. We would like to emphasize the resemblances not distinctions between ancient and young magmatism: in spite of dryness, the Precambrian enderbitic magmas overcome orthopyroxene barrier, and primitive andesitic magmas can be not so water-saturated. Now that even on the ancientmost shields researchers concede the existence of plate tectonics elements and in the younger regions ever more variety of regimes of geodynamical settings are revealing, the task of comparison of the products of ancient and Phanerozoic magmatism assumes ever greater importance.

 

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