Glikin A.E. Polymineral-Metasomatic Crystallogenesis

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182 5 Metasomatic Transformation of Aggregates

zone gives rise to the products, all of which are represented by translated monoc-

rystalline and polycrystalline automorphs (see Sect. 1.2) randomly divided from the

starting material. The examples include the hydrothermal processes of growing the

crystals of Y-Fe-garnet and Y-Fe-perovskite (Mill and Klevtsov 1966; Mikhailov

et al. 1973), anhydrous iron fluoride (Glikin et al. 1974), wolframite (Panov 1979),

polycrystalline aggregates of ornamental malachite and rosasite (Petrov et al.

1977a, b), lithium iodate and other water-soluble compounds (Kidyarov and

Mitnitskii 1977; Petrov et al. 1983) and so forth. It is highly probable that some part

of metasomatic veins is of the same nature.

The most complicated metasomatic transformation appears to be a polymineral

recrystallization (see Sect. 5.3), which is a series of matter redistribution processes

occurring under the action of oscillations and/or gradients of temperature, pressure,

and other intensive parameters. This type of recrystallization can manifest itself in

macroscopic and local phenomena of stratification–homogenization of aggregates

and extension–reduction of grains appearing as a result of the aggregate heteroge-

neity and accompanied by a strong influence of salting-in and salting-out processes.

The elementary processes of growth and dissolution during the recrystallization are

characterized by lack of spatiotemporal correlations; this allows to classify the

recrystallizations of this type as phenomena belonging to the fourth overelementary

class of metasomatic crystallogenesis (see Introduction).

So, both external effects and mechanisms of metasomatic processes occurring in

aggregates can be extensively variable. The metasomatic processes can belong to

different classes of metasomatic crystallogenesis depending upon their various

spatiotemporal correlations of elementary processes of growth and dissolution.

However, fragmentary experimental data present a poor basis for a unified system-

atic concept. We shall not go beyond a hypothesis of formation of rapakivi-type

structures in the course of metasomatic transformation of isomorphic–mixed crys-

tal aggregates and models of recrystallization of polymineral aggregates.

5.2 A Model of Formation of Rapakivi-Type Structures

Formation of rapakivi-type structures can be a result of replacement of a massif of

mixed crystals in a multicomponent solution. The model is suggested (Glikin 1990,

1991, 1995a, 2002) on the basis of comparison made between the main structural

features of various rapakivi granites (observed mainly in the facing materials and

partially in the so-called Gubanovskaya intrusion of the Vyborg massif and

described in the published literature) and morphological characteristics of synthetic

products of monocrystal replacement described in Chapters 1 and 3.

Rapakivi structures have been continuously studied for at least a century-and-a-

half, and still their origin remains a traditional subject for discussions (Delesse

1848; Chrustschoff 1894; Sederholm 1928; Levinson-Lessing and Vorob’eva 1929;

Escola 1938; Velikoslavinskii 1953; Sudovikov 1967; Sviridenko 1968; Levkovskii

1975; Velikoslavinskii et al. 1978; Shinkarev and Rundkvist 1986; Koval and

5.2 A Model of Formation of Rapakivi-Type Structures 183

Valasis 1989; Meyer 1989; Dempster et al. 1991; Collins 1994; Price et al. 1996;

and many others). The greatest attention is focused on rapakivi granites; however,

similar structures also occur among the other rocks, e.g., tonalites, diorites, gab-

bros, norites, and peridotites (Delesse 1848; Nockolds 1931; Carstens 1957;

Viluksela 1965; Leveson 1966; Van Diver and Maggetti 1975; Dahl and Palmer

1983; and others). Investigations have generally dealt with morphological and

chemical peculiarities of phenocrysts and well-marked textural features. The most

popular is magmatic genesis of rapakivi granites and similar rocks (see, e.g., review

by Rämö and Haapala 1995). However, the data obtained, which also include

experimental researches conducted under higher parameters (Seck 1971; Fenn

1977; Donaldson 1979; Lofgren 1980; Tingle 1981; Lofgren 1983; Grove et al.

1984; Wark and Stimac 1992; and others), show that not all the rapakivi characteristic

features can be accounted for by magmatic origin, and at least a part of them can

be attributed to action of metasomatic processes (Velikoslavinskii 1953; Sudovikov

1967; Levkovskii 1975; Glikin 1990, 1991, 1995a, 2002).

The tasks set forth in the present monograph do not include opening a discussion

of rapakivi granite genesis from the points of view of conventional comparison of

magmatic and metasomatic interpretations; here the problem is considered from

positions involving metasomatism.

Experimental data (Glikin and Sinai 1983, 1988, 1991, 2004; Kryuchkova

et al. 2002; Glikin et al. 2003) show phenomena of metasomatic replacements of

monocrystals, which are similar to the basic morphologic features of rapakivi.

Hence, we consider rapakivi granites as products of metasomatic transformation

of magmatogene feldspar rocks. The primary rocks are supposed to have had a

heterogeneous giant-grained structure, which comprised crystals reaching

10–20 cm in sizes. Taking into account a petrogenic type of rapakivi granites

(Velikoslavinskii et al. 1978; Koval and Valasis 1989; and others), the primary

rocks are likely to be anorthosites. The entire or almost entire rapakivi mass can

be considered as a totality of innumerous interpenetrated and separated polymin-

eral (mainly quartz-feldspar) pseudomorphs and automorphs after feldspar crys-

tals. Distinctions in the forms of replacement products result from a significant

dispersion of isomorphic component ratios in the initial crystals and the influence

of compounds belonging to the same isomorphic line and non-isomorphic com-

pounds present in the forming solution. Similar structures can also appear as a

result of metasomatic transformations of amphibolites, pyroxenites, and other

monomineral rocks characterized by variable contents of isomorphic components

in individuals.

Further, attention will be paid to some structural peculiarities of the granites,

which can have a great importance for our conclusions.

The most important and enigmatic rapakivi elements are considered to be feld-

spar phenocrysts, which are most explicitly described in published literature. It is

to be reminded that phenocrysts can have spheroid, faceted, and partially faceted

form (Figs. 5.1–5.3). As a rule, they are separated from each other by the medium-

grained and coarse-grained quartz-feldspar mass, where spheroid and faceted phe-

nocrysts can be co-located, or, in some rare cases, interlock with each other.

184 5 Metasomatic Transformation of Aggregates

Fig. 5.1

A typical fragment of a rapakivi structure. A sample from collection of Petrography

Department of St. Petersburg State University: (1) spherical phenocryst containing concentric

solid inclusions and an intermittent external zone; (2) combined phenocryst including a spherical

superficial zone containing solid inclusions; (3) a polymineral faceted-blurred case-shaped

pseudomorph

Fig. 5.2 A mass of rapakivi granite. Adjoining faceted and spherical feldspar phenocrysts:

Framed is a spherical phenocryst surrounded by a circular polycrystalline rim

Spheroid phenocrysts have rough surfaces and are usually zoned (Fig. 5.1) as a

result of heterogeneous distribution of mineral inclusions and isomorphic components

in individuals. In general, spheroid phenocrysts are characterized by concentric

location of the zones, a uniform zone thickness, localization of inclusions in the

core parts of the crystals, and absence of internal growth pyramids (sectors). If a

phenocryst is sufficiently saturated with inclusions, its central part acquires a poly-

crystalline structure. The external zones can be intermittent. Quartz inclusions often

have bizarre vermiform shapes (Fig. 5.4).

Faceted phenocrysts show a typical zonal–sectorial texture determined, as a rule,

by compositional changes. At the same time, the central zone frequently does not

coincide with the crystal geometrical center, and thicknesses of various zones can

vary along their contours.

According to our classification, combined phenocrysts include partially faceted

phenocrysts, faceted phenocrysts containing spheroid zones in their central parts,

and spheroid phenocrysts comprising polygonal zones.

Structural heterogeneities of the main granite mass comprise “granite ovoids”

(Velikoslavinskii 1953) or “orbiculars” (Sudovikov 1967), which are spheroid

ensembles of feldspar crystals having diameters up to 10–15 cm; sometimes they

have cyclic-concentric forms (Figs. 5.5 and 5.6). As a rule, feldspar crystals

forming the orbiculars are isolated composing “chain-like orbiculars” (Fig. 5.6).

Additionally, solid feldspar shells with thickness up to 1–2 cm and containing

the granite mass inside and also surrounded with it can also occur (“shell orbicu-

lars,” Fig. 5.5). Such shells (solid and chain-like) can encompass spheroid phe-

nocrysts (Figs. 5.2 and 5.3). The shells surrounding the phenocrysts can be

polygonal (Fig. 5.3). There are also observed locked polygonal crystal chains,

which can be reliably defined as faceted-blurred case-like pseudomorphs (Figs.

5.1 and 5.3).

Structural-morphological characteristics of spheroid phenocrysts apparently form

a continuous series. In general, the mentioned heterogeneities of the rapakivi structure

5.2 A Model of Formation of Rapakivi-Type Structures 185

Fig. 5.3

A fragment of a rapakivi structure containing a spherical phenocryst surrounded by a

faceted polycrystalline rim (Photo by Prof. V. A. Popov)

186 5 Metasomatic Transformation of Aggregates

Fig. 5.5

“Shell orbiculars” (According to data provided by Velikoslavinskii, 1953)

Fig. 5.4 A feldspar crystal comprising monocrystalline quartz inclusions having bizarre shapes

(According to data provided by Velikoslavinskii, 1953)

and found correlations can be presented in the form of a diagram (Table 5.1)

constructed on the basis of the replacement product classification (see Table 1.2).

Metasomatic developement after anorthosite of rapakivi is in a good agreement

with some indications of metasomatic contacts between these rocks (Koval and

Valasis 1989); however, the authors consider that anorthosites are developed after

rapakivi and not the other way around.

Close location of faceted (growing) and spheroid (dissolving) crystals is an evi-

dence of metasomatic replacement, as this process consists in growth of crystals

due to dissolution of the other (see Chapters 1 and 4). Dissimilarities in behavior of

adjoining feldspar crystals can be accounted for by differences in their isomorphic

compositions. Additional evidence is separation of phenocrysts: this phenomenon

supports the above assumption about their being relics in a bulk of metasomatite

mass. In some rare cases of formation of intergrown (or, probably, twinned) phen-

ocrysts, resorption attributes on the intergrowth borders are observed.

A scheme of formation of a rapakivi structure, which unites the elements

described above, is represented in Fig. 5.7.

5.2 A Model of Formation of Rapakivi-Type Structures 187

Fig. 5.6

“Chain orbiculars” (According to data provided by Sudovikov, 1967)

Table 5.1 Systematization of the main heterogeneities of the rapakivi structure with references

to the classification of replacement products of monocrystals (Table 1.3–1)

Note: Figures in the circles are index figures. Division of monocrystalline formations into those

containing inclusions and those, which do not contain any inclusions, is an example of introduc-

tion of additional taxons discussed in Sect. 1.5.

188 5 Metasomatic Transformation of Aggregates

As mentioned above, structural heterogeneities of rapakivi fall into classification

of products of monocrystal replacement occupying several positions of this classi-

fication (compare Tables 1.2 and 5.1). In Table 5.1 all the phenocrysts are classified

as monocrystalline replacement products, while the shell-like and orbicular forms,

including polygonal ones, are considered to be polycrystalline products. With all

this, according to the definitions given in Sect. 1.2, the faceted phenocrysts are

considered as faceted pseudomorphs, the spheroid phenocrysts are defined as local-

ized automorphs, and combined phenocrysts together with polygonal orbiculars are

determined as blurred pseudomorphs.

Among these formations, only the shell-like and chain-like orbiculars (indices 11

and 12 in Table 5.1) can be reliably interpreted as the products of metasomatic

replacement. Their well-known analogs are the Lisegang rings, which positively have

a metasomatic origin; a double concentric orbicular shown in Fig. 5.6 supports the

analogy. Among the replacement products obtained by us, the circular (in cross sec-

tion) and bag-shaped localized automorphs (reactions Ia/6, III/3, 45, 47, 48 in Table

1.1; Figs. 1.10 and 1.11) can serve as analogs. The mechanism of their formation has

been already discussed (Glikin and Sinai 1991; Glikin 1996a, b; and Sect. 1.4.3 of the

present monograph), and reported data contain no other alternative models.

The chain-like polygonal orbiculars provide with almost the same information

(indices 5 and 6 in Table 5.1). Reliability of describing these orbiculars as the

blurred automorphs and also reliability of their identification are not high as they

normally are not too distinctly developed. Also, any chain-like orbiculars can be

visually identified only in a few particular cases due to their overlapping with the

Fig. 5.7 A suggested scheme of formation of a rapakivi structure after anorthosite substrate (the

relative locations in the initial and secondary structures are numbered equally)

Rapakivi

automorphs. Sometimes, in private discussions, the origin of the orbiculars has

been explained by specific conditions of nucleation of new individuals created by

a diffusion field induced at the growth front of a crystal growing in magma (index

5 in Table 5.1) or they appear as a result of skeletal growth (index 6 in Table 5.1).

These alternative views seem lame; a polygonal shape of diffusion fields surround-

ing the crystal has not been proved yet; at present what is known is a polygonality

of thermal fields for specific conditions of crystallization from a melt (see, e.g.,

Klapper 1994), and the type of skeletal growth discussed also has not been proved

either experimentally or theoretically.

So, we consider the orbicular crystal ensembles as the key evidence of metaso-

matic genesis of rapakivi.

In the present monograph spheroid phenocrysts are considered to be the products

of isomorphic monocrystal replacement, which had proceeded with a partial vol-

ume-deficit of feldspar, similar to that observed in the model reactions Ib/27, 29, 31,

35 (Tables 1.1 and 1.2). Roundish forms are also typical for the model pseudo-

morphs of this type (Figs. 1.5b and 1.6d). There are two reasons for their formation.

One of them is a significant volume-deficit, which induces decomposition of the

peripheral part undergoing replacement into several isolated fragments; a simulation

analog of this process is reaction Ib/35 (Fig. 1.6d). Another reason is a usual dissolu-

tion. It can precede the replacement, if the solution is undersaturated in the begin-

ning of the process, or accompany it, if the temperature raises simultaneously with

the replacement or the process is accompanied by an additional salting-in reaction.

We consider solid concentric inclusions in rounded phenocrysts to be secondary

formations implanted in the course of replacement process. Implanting the fluid

inclusions, typical for volume-deficit replacements, is discussed in detail in Sect. 3.2;

the inclusions have been observed not only in water-soluble model systems (Figs.

1.6a–d), but also in feldspar, which was subjected to an autoclave treatment

(Tsuchiyama 1985; Wark and Stimac 1992; Johannes et al. 1994). It was demon-

strated (see Sect. 4.3) that fluid inclusions in systems containing isomorphic com-

pounds and an additional non-isomorphic compound can be partially or entirely filled

with the non-isomorphic compound in crystal form (Figs. 1.6e–j), which gives rise to

poikilitic crystals (Glikin and Sinai 2004). The vermiform quartz inclusions in the

phenocrysts (Velikoslavinskii 1953) are morphologically similar to the fluid inclu-

sions observed in the model crystals; a similar structure has been demonstrated in

myrmekite formations, the genesis of which is considered by some authors as metaso-

matic (Tikhonenkov 1961; Collins 1988; Garcia et al. 1996). Another evidence of

secondary, hydrothermal origin of quartz in these inclusions can be derived from the

fact that temperature of quartz crystallization did not exceed 570°C (Koval and

Valasis, 1989), which was proved by the absence of wavy extinction (Fig. 5.4), typical

for magmatic quartz, having undergone the α–β-transition.

Concentricity of zones formed by inclusions in spheroid phenocrysts corre-

sponds to a volume-deficit replacement. It reflects a diffusion nature of the replace-

ment, as diffusion of components in the solutions captured within the implanting

inclusions is a limiting stage of the process. Alternative concepts assuming the

capture of solid inclusions to occur during the growth process cannot explain

5.2 A Model of Formation of Rapakivi-Type Structures 189

Table 5.2 Differentiation of polymineral mixtures into layers in the course of their

recrystallization

Amount of the separated component (wt%)

Description of the experiment

In the initial

mixture

(P, %)

Separated

(L, % of P)

Remaining in

the aggregate

(R)

Content in the

aggregate (M)

NaNO

–K

Separated is

NaNO

, 125 tests, 42°C, fraction

0.25–0.5 mm

0.0

100.0

20.0

30.0

35.0

40 1.3 96.8 39.2

45 8.8 80.4 41.9

50 20.0 60.0 37.5

55 25.0 54.5 40.0

60 40.0 33.3 33.3

70 51.3 26.7 38.4

80 68.8 14.0 34.8

90 88.8 6.9 38.3

95 91.3 3.9 42.5

Ba(NO

)

–BaCl

·2H

O Separated is

Ba(NO

)

, 43 tests, 42°C, fraction

0.25–0.5 mm

0.0

1.4

8.8

100.0

93.0

70.7

10.0

18.9

23.2

40 22.3 44.2 22.8

50 36.0 28.0 21.9

60 48.8 18.7 21.9

70 61.0 12.9 23.1

42°C, fraction 0.5–1.0 mm 10 0.0 100.0 10.0

20 0.8 96.0 19.4

30 9.8 67.3 22.3

40 24.1 39.8 20.9

50 36.6 26.8 21.1

60 49.1 18.2 21.4

70 63.5 9.3 17.9

80 74.8 6.5 20.6

90 88.1 2.1 16.0

21°C, fraction 0.25–0.5 mm 10 0.0 100.0 10.0

20 0.0 100.0 20.0

30 5.2 82.7 26.2

40 14.7 63.0 29.6

50 20.3 59.4 37.3

60 45.5 24.2 26.6

70 62.2 11.1 20.6

80 76.1 4.9 16.3

90 87.7 2.6 18.7

21°C, fraction 0.5–1.0 mm 10 0.0 100.0 10.0

20 0.0 100.0 20.0

30 6.4 78.7 25.2

40 13.2 67.0 30.9

50 27.6 44.8 30.9

(continued)

Table 5.2 (continued)

Amount of the separated component (wt%)

Description of the experiment

In the initial

mixture

(P, %)

Separated

(L, % of P)

Remaining in

the aggregate

(R)

Content in the

aggregate (M)

60 43.2 28.0 29.6

70 60.5 13.6 24.1

80 72.3 9.6 27.8

90 84.2 6.4 36.7

KAl(SO

)

·12H

O–NiSO

·7H

Separated is KAl(SO

)

·12H

O, 25

tests, 42°C, fraction 0.02 mm

2.0

20.0

48.5

86.7

50.0

19.2

13.3

25.0

22.7

42°C, fraction 0.1 mm 15 6.3 58.0 9.3

40 25.0 37.5 20.0

60 54.5 9.3 12.3

85 80.0 5.9 24.2

NaCl–K

Separated is NaCl 40

tests, 42°C, fraction 0.25–0.5 mm

0.0

100.0

10.0

20.0

30 3.2 89.3 27.5

40 6.3 84.2 36.0

50 7.9 84.2 45.5

60 9.1 84.8 56.0

70 10.8 84.6 67.0

80 20.7 74.1 75.0

90 28.0 68.9 86.0

42°C, fraction 0.5–1.0 mm 10 0.0 100.0 10.0

20 1.9 90.5 18.5

30 4.9 83.7 26.1

40 11.7 70.8 32.0

50 17.5 65.0 39.5

60 31.2 48.0 42.0

70 44.0 37.1 46.5

80 69.8 12.8 34.0

90 82.3 8.6 43.5

21°C, fraction 0.25–0.5 mm 10 0.0 100.0 10.0

20 0.0 100.0 20.0

30 1.4 95.3 29.0

40 1.7 95.8 38.0

50 2.5 95.0 49.0

60 2.8 95.3 59.0

70 3.3 95.4 69.0

80 3.1 96.1 79.5

21°C, fraction 0.5–1.0 mm 10 0.0 100.0 10.0

20 0.0 100.0 20.0

30 0.0 100.0 30.0

40 2.7 93.0 38.5

50 2.1 95.8 49.0

60 11.3 81.2 55.0

(continued)