ISSN 1028334X, Doklady Earth Sciences, 2010, Vol. 434, Part 1, pp. 1222–1225. © Pleiades Publishing, Ltd., 2010. Original Russian Text © N.V. Koronovskii, L.I. Demina, M.S. Myshenkova, 2010, published in Doklady Akademii Nauk, 2010, Vol. 434, No. 2, pp. 224–227.

GEOCHEMISTRY

Fluidolite: A New Genetic Rock Type of the Elbrus Volcanic Region N. V. Koronovskii, L. I. Demina, and M. S. Myshenkova Presented by Academician D.Yu. Pushcharovskii April 28, 2010 Received April 28, 2010

DOI: 10.1134/S1028334X10090175

Fluidolite, a new genetic type of rock related to the taxon of the same rank as magmatic, sedimentary, and metamorphic rocks, according to the Petrographic Code [8], was distinguished on the northern side of the Elbrus Volcano for the first time. Currently fluidolites comprise rocks with the leading role of deep decom pression explosions of fluids in their formation. Such explosions result in fluid penetration into the enclos ing matter, escape or extraction of its individual ingre dients, and fixation of material transported by fluid in the new space that favors the formation of rocks and geological bodies with specific signs. The peculiarities of textures, structures, and mineral and chemical composition of these rocks are formed at the expense of fluid ability to transport suspension of fragments of deep rocks and minerals, fragments of melts, and products of their crystallization [8]. The study of fluid olites as products of fluid explosive processes in the Earth’s crust series is a new, intensely developing field in geology, because a wide range of natural resources including diamonds is related to them. Fluidolites are quite widely abundant in nature being associated not only with magmatic rocks, but sedimentary rocks as well [4]. Such rocks associating with magmatic complexes (tuffisite, boulder dykes, intrusive pyroclastites, ign imbrites, eruptive pseudoconglomerates, and others) currently are widely discussed. Some scientists sug gested to unite them into new classes [3] or rock sub types [2] emphasizing that scorching deepseated matter was transported to the Earth’s surface or upper crust not in liquid (magmatic melts providing the whole spectrum of magmatic rocks), but in fluidized state. In this case, the prevailing dispersed solid, as well as partially liquid material is in the mobile (pseudofluent) state at the expense of high pressure of gases. Since pressurized gases are capable of limited expansion, the dispersed material is “hung” in fluid flows. The speed of such material flow on the Earth’s

Geological Faculty, Moscow State University, Moscow, Russia email: [email protected]

surface (measured during the motion of fire ava lanche) varies from 100 to 200 km/h [7]. We distinguished three groups of fluidolites in the Elbrus volcanic region. The first group comprises lightgray rocks with homogenous massive structure composing a dome like body outcropped on the Tuzluk Mountain located to the north of Peredovoi Ridge on the Bechasyn Plateau of Lower and Middle Jurassic rocks, 1 km to the west of the Malka River. The zone of strongly deformed layers located directly on the strike of the Tuzluk Mountain is observed in the left flange of the river against the background of the monocline bedding of Jurassic deposits. Rocks contain numerous (which is not typical for magmatic rocks) splintered, rounded, and oval fragments, as well as balllike grains fractured inside of violet–pink quartz with a size of up to 5 mm and bigger, fragments of plagioclases of var ious basicity, microcline, biotite, granitoids, and biotite gneiss with sizes of a fraction to several cen timeters (Figs. 1, I; 2a, 2b). In addition to small fragments of the same minerals, the matrix con tains fragments of completely crystallized glass as spherolites and panicles. Fluidolites of the second group are outcropped on the steep northern side of the Elbrus in the area of the UlluMalienderku glacier, where they overly Late Paleozoic granite and form a vertical bench with a height of 10–15 m marking a small plateau. Its eastern end contains a body with vertical fluidal texture as a feeder for overlying rocks. Fluidolites of this group are represented by gray ignimbrite with welded tuff of black color with a width of up to 0.5 cm and a length of up to several centimeters (Fig. 1, II) containing rounded, oval, and splintered fragments of lavas (not known in this region) with various composition and texture and phenocrysts of opacitized, sometimes almost completely replaced biotite and plagioclase (Fig. 2b), as well as oval, flattened “fruitdrop” frag ments of quartz–feldspar rocks (Fig. 2d), fragments and balllike grains of pinkish quartz, plagioclase, and biotite plates. The fragments of fassaite, diopside, gar

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Gn

Q

Q I

L F

L II

1 cm

III

Fig. 1. Fluidolite types of the Elbrus volcanic region. (I) Intrusive varieties with homogeneous structure and rounded gneiss frag ments (Tuzluk Mountain); (II) ignimbrite of the plateau with welded tuff and numerous lava fragments not known in sections of volcanogenic series of the region; (III) rocks of the feeder for ignimbrite. Here and in Fig. 2: Gn, gneiss; L, lava; F, welded tuff; Q, quartz; Micr, microcline; Pl, plagioclase; Bi, biotite; Fs, fassaite.

net, granitoids, granite gneiss, biotite and garnet– biotite gneiss, and twomica crystalline schist are con tained in lesser amounts (Fig. 2e). Welded tuff is com posed of glass with rare phenocrysts of hypersthenes, sometimes with a thin biotite rim, as well as corroded by glass tables and fragments of zoned plagioclase. Fragments of biotite plates are characterized by shift of individual parts along the cleavage (Fig. 2f). The texture of the matrix is similar to that in the previous group of rocks. DOKLADY EARTH SCIENCES

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The third group of fluidolites includes darkgray rocks without welded tuff and with taxitic structure demonstrating the alternation of massive and finely banded areas of fluidal structure (Fig. 1, III), which are oriented vertically in the outcrops in contrast to subhorizontal welded tuff in the second group of fluid olites. It is observed in the outcrop located at the base of the plateau on the northern side of the Elbrus that fluidolites of the third group are replaced by the rocks of the second group (ignimbrite) in a fan mode that provides evidence for their formation inside the feeder.

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Pl Pl Pl (a)

0.5 mm

(b)

0.5 mm

0.2 mm

(d)

0.5 mm

L Bi

F

(c)

Fs

F Bi

Q F Bi

(e)

0.5 mm

0.5 mm

(f)

Fig. 2. Fluidolite microtextures. (a) Fragment of balllike fractured quartz grain; (b) fragments of microcline, plagioclase, and biotite; (c) rounded fragments of old lavas; (d) fragments of fruitdrop fragment; (e) fassaite fragment; (f) shift of individual parts of biotite fragment along cleavage.

Rocks composing a dykelike body with a length of ~350 m intruding Permian deposits and located on the right bank in the Birdzhallysu River head are attrib uted to this group as well. Rocks of this group are char acterized by the same combination of rock and min eral fragments as ignimbrite.

Fluidolites of the second group were previously described as ignimbrite [5], and the first and the third groups, as rhyodacite tuff [1]. However, with an exter nal similarity of fluidolites to lithocrystalloclastic tuff varieties, the textural features, namely disintegration of mineral grains and lithoclasts of rocks occurring DOKLADY EARTH SCIENCES

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2.7 Ma, whereas the age of ignimbrite corresponds to the Early–Late Pleistocene. In our opinion, the geo logical sense of such datings is in the fact that they point to a xenogenous, but not magmatic character of most minerals of fluidolites, which is also confirmed by their petrographic study. The discovery of fluidolites in the Elbrus volcanic region supports the previously suggested model of col lision volcanism in the Caucasian region with the leading role of oxidation of deep transmagmatic fluids in magmatic petrogenesis [6].

Biotite/chondrite 300 200 100 50 30 20

Lavas

10 5 3 2

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Fluidolites

REFERENCES

1 0.5 0.3 La Ce

Sm Eu

Tb

Yb Lu

Fig. 3. REE distribution in fluidolites and acid lavas of the early stage of the Elbrus volcanic activity. Diagram was plotted by the data from [1]. Chondrite, after [10].

deep under the eruption centers, rounded and oval shape of fragments of both rocks and minerals result ing from tumbling (treated by gas–solid mixture), shift of individual parts of phyllosilicates, and other signs, we may relate them to fluidolites for sure. The anomalous chemical and isotopic composi tions are one of the main diagnostic signs of fluidolites [8], which is clearly reflected in rocks of the Elbrus volcanic region. For example, the graphs of REE dis tribution have an exotic appearance absolutely not typical for magmatic rocks, whereas the earliest lavas of Elbrus are characterized by REE spectra that are normal for acid rocks (Fig. 3). Quite contrasting isoto pic characteristics (87Rb/86Sr, 87Sr/86Sr) are estab lished for various components of early ignimbrite of this region including bulk samples. As this takes place, plagioclase and pyroxene demonstrate anomalously old, in the opinion of Chernyshev et al. [9], “geologi cally senseless” values of K–Ar age of 15.7 and

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