2011

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Chemistry of biotite from granitoids of the Ukrainian shield as an indicator of their alkalinity and differentiation degree

Konoval N.M., Krivdik S.G.

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

Nasa246@ukr.net, kryvdik@ukr.net

Biotite as femic mineral is present in almost all types of granitoids — from their most aluminous to oversaturated with alkalis agpaitic species, including their ultrametamorfogenic anatectic (granulite and amphibolite facies) and anorogenic complexes. Most often, biotite is the only one femic mineral of granitoids and it concentrates, besides iron and magnesium, a significant or major part of such trace elements as Ti, Nb, Ta, Sc, Sn.

The Ukrainian shield (USh) is characterized by a very wide variety of granitoids. Most of them have been formed as a result of ultrametamorphism and anatexis in the amphibolite facies (tonalites, trondhjemite, plagiogranites, biotite and biotite-amphibole K-Na-granites), although the granitoids of the granulite facies (enderbites and charnockites) are also essential (in the most eroded geoblocks of USh they occupy 20-50% area). As part of the granulite facies granitoids also belong to so-called intrusive charnokitoids.

Anorogenic granitoids has smaller area of development, but these rocks play an important role in the metallogeny of USh. These anorogenic granitoids are part of anorthosite-rapakivi-granitic plutons (Korosten and Korsun-Novomirgorod), gabbro-syenite (South Kalchyk) or granosyenite-granite (Kalmius-Elanchyk) massifs. The most differentiated granites belong to alkaline-feldspars or alkaline (with aegirine and riebeckite) varieties.

Biotites from granitoids of granulite facies are charatterized by moderate magnesity (35-55%) and aluminia (Fig.1) with a high titanium (to 5-6% TiO₂) content (Fig. 2).. It is likely that biotites of intrusive charnokitoids become less aluminous with increasing iron content, i.e. there is some analogy of charnokitoids with alkaline granites. Reduction of alumina in these biotites enriched in iron occurs even in the presence of such aluminous minerals as essentially almandine garnet.

Biotite from granites of the most common amphibolite facies are characterized by moderate magnesity (30-55%) and alumina (Fig. 1), but slightly lower concentration of titanium (2-4% TiO₂) (Fig. 2). There are distinguished granites of the real amphibolite and transition to the granulite facies (high amphibolite). In the last biotites titanium content increases slightly and, as it were, the aluminum content is lowered.

The most diverse are biotites from anorogenic granites. By the increase of differention degree, in these rocks amount of alkali feldspars (relative to plagioclase) increases and the iron content in biotite becomes higher up to the formation of extremely enriched in iron varieties of them. Thus there is such an evolution of chemistry of biotites: in alkaline granites (Perga type) biotites become very low-alumina (annite), in some alkaline-feldspar so-called plumasitic granites (Kamyani Mogyly type), biotite, vice versa, become a high-alumina (siderophyllite) (Fig. 1). Interestingly, in biotites of alkaline granites with decreasing aluminum, there is some increase in titanium content (up to 3,4% TiO₂). Although, as we know, the increase in titanium content in the biotites is usually due to high temperature (as is typical for biotites of charnokitoids). Obviously, increasing alkalinity of mineral formation medium also contributes to entering of titanium in biotite (and other femic minerals — Ti-augite, kaersutite, Ti-aegirine, Ti-richterite). Probably in low-alumina biotites in wich formula (Si + Al) < 4 titanium occupis tetrahedral positions. This is explained by the absence of reverse (tetraferribiotitic) absorption in such low-alumina biotites.

Further development of the magmatic chambers, in which have been formed alkaline-feldspar and plumasitic granites, leads to the crystallization of lithium-bearing and lithium micas, as is the case in Perga, Katerynivka and Kamyani Mogily massifs. Increased lithium also occurs in some biotites from granites of amphibolite facies. It is expected that these granites are associated with lithiumbearing pegmatites with spodumene and petalite.

Consequently, the biotites in granitoid systems (primarily anorogenic type) on the final stages of their evolution are enriched with iron, partially titanium and divided into low-alumina (annite) and high-alumina (siderophyllite) branches.

Figure 1. Minal mica composition of USh granities.

Figure 2. Content of TiO₂ versus MgO in biotites of USh granities.

The granities from wich biotites have been analized: 1. – K-Na granities of Kirovograd, Zhytomir, Anadolsck, Mokra Moskovka, Tokiv granities; 2. – Plagiogranities of sura, Obitochna granities; 3. – Charnockites and enderbite of Gaivoron, Berdichiv, Tokmak granities; 4. – Intrusive charnockites of Khlibodarivka, Nova Ukrainka and Buky complekxes; 5. – Alkaline granities of Perga; 6. – Subalkaline alkalifelspar granities of Kamyani Mogily and Katerynivka complexes.