The specific features of the processes of structural–material and mineral–phase transformation of rocks, which take place against the background and with the participation of tectonic deformations, include several interrelated aspects: the influence of stress on the thermodynamic parameters of metamorphism [1] and its effect on the kinetics of syndeformation reactions, on their tribochemical nature, and on the rheology and geomechanical properties of rocks [2]. All these features are most vividly pronounced in the zones of localized deformations and faults, including the seismogenic ones, where such processes take place rapidly in a setting of elevated temperatures, pressures, stress, and dynamic slip rates and with the participation of the fluid component. Among the important aspects of this kind of transformations is the frictional melting of the geomaterial, which, as field observations and experiments have shown [3, 4], takes place only when the slip along the fault achieves a seismic slip velocity close to or exceeding 1 m/s, and the significant mechanical crushing of rocks. The end product of such transformations is tectonic pseudotachylytes (PSTs), an aphanitic, often vitrous material with varying proportions of mineral fragments and protolith microfragments, which occurs predominantly as injection veins, spatially and genetically associated with fault tectonites (mylonites, cataclasites, fault gouge, breccias). The issues of the genesis and formation settings of PSTs are a subject of study for specialists of various disciplines: geologists, petrologists, geochemists, seismologists, and rock mechanical engineers; i.e., they actually represent a problem for interdisciplinary research in geosciences.

While studying PSTs associated with paleoearthquake zones in rock complexes of very different ages and geostructural positions, we have already briefly covered some aspects of their genesis and morphostructural varieties, petrogenesis, and the geodynamic and geomechanical formation conditions in publications [5–8]. It was shown that, in addition to the intense mechanical crushing of the geomaterial, the most important part in their formation belongs to processes of the selective frictional melting of the mineral phases of the substrate associated with it. In this connection, a trend of PST research, which is separate but closely related to the others, is the geochemical study of the geomaterial melting products, including a number of issues concerning the redistribution of chemical elements during the ongoing phase transformations of the first kind. These issues, which constitute the objective and tasks of this publication, include the following aspects: which of the major, trace, and rare earth elements are involved in the melting processes, and which remain in restite or are removed from the slip zone; in what proportions are certain components redistributed among the protolith, the melt, and the restite and which of them are involved in melt recrystallization; is there a difference in the behavior of elements during the selective melting of a substrate of similar primary composition but different levels of metamorphism; and, finally, are there any significant differences between the compositions of PSTs near the generating fault surfaces (PSTGS) and in the injection places (cracks) of the displaced melt (PSTIC).

BRIEF CHARACTERISTICS OF PSTs IN THE NORTHERN LADOGA REGION: GEOLOGICAL POSITION, MINERALOGY, AND PETROCHEMICAL COMPOSITION

PSTs in the Ladoga region were first recognized, described, and dated by the authors earlier [5, 6] within virtually all zones (from greenschist to granulite) of the zonally metamorphosed Paleoproterozoic Ladoga complex of the Svecokarelides (1.89–1.75 Ga) in southeastern Fennoscandia. They formed in the metaterrigenous flyschoid substrate, predominantly of the initially arkosic type (the products of erosion and redeposition of the Archean granite–gneisses) at the postorogenic evolution stage of the pericratonic margin of the Karelian massif during the Early–Middle Riphean, simultaneously with the inception of the Pasha–Ladoga graben there [6], i.e., during the decline or at the very end of the tectono–thermal activity event in the region. A close spatial and genetic relationship between PSTs and the preceding cataclasites and blastocataclasites (BCT) after mica schists and gneisses of the Ladoga complex, which formed at the orogenic stage (1.79–1.75 Ga), has been revealed in virtually all areas of PST occurrence. In this connection, analytical studies at each PST sampling point covered a triad of samples, including (1) initial metapsammites (protolith), metamorphosed at various facies; (2) BCT that replace them and are partially reworked by overprinted greenschist diaphthoresis; (3)  PSTs proper, mainly as injection veins. The mineral phase compositions of the samples of the triad, with certain variations at each sampling site, were generally similar, to the extent that is possible during metamorphism under moderate andalusite-type pressures of the initially similar sequences of metapelites alternating with metapsammites. The most typical set of protolith minerals included, in addition to quartz, potassium and sodic feldspars, phyllosilicates (biotite, muscovite), ore minerals (magnetite, ilmenite, hematite, etc.), and accessory minerals (sphene, zircon, monazite, etc.). The stage of diaphthoresis was characterized by the most widely manifested chloritization of micas and hydrothermal alteration, expressed by the filling of cracks with quartz–chlorite–K-feldspar aggregate and the growth of porphyroblastic chlorite. PSTs were also locally affected by the subsequent hydrothermal impact represented by the reticular forms of predominantly chlorite aggregates, locally overprinted on their vitreous matrix. It should be specially noted in addition that the host rocks of PSTs within the granulite zone are represented by the plagio–orthoclase micaceous gneisses, in places with hypersthene relics, and have undergone overprinted granitization processes. As a result, they have either lost the flyschoid features of the parent strata, which are easily recognizable in lower temperature zones, or consisted initially of a gneissose intrusive rock. The characteristics of the specific features of frictional melting processes and the order of the selective melting of minerals (first muscovite, ~650°C; then biotite, ~850°C; in small volumes, plagioclase, ~1100–1200°C; and potassium feldspar and quartz as the minimum, ~1600°C) and subsequent glass crystallization in new mineral phases, taking into account the model estimates of the liquidus and solidus temperatures using the MELTS program, were presented in a special publication [8]. Here, we restrict ourselves merely to a brief description of the mineral parageneses of the triad elements in each of the three PST sampling points within the greenschist, amphibolite, and granulite zones of metamorphism (Fig. 1).

Fig. 1.
figure 1

Schematic map of the metamorphic zoning of the Ladoga complex showing locations of the points of established and sampled PSTs [6]. (1) Karelian massif; (2) Ladoga complex; (3) Salma massif of rapakivi granites; (4–8) isograds of metamorphism: garnet, staurolite, sillimanite–muscovite, sillimanite–orthoclase, and hypersthene, respectively; (9) major faults and the Meieri thrust (M); (10) faults with established PSTs; (11) points of geochemical sampling of PSTs discussed in the article; (12) state border with Finland.

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PST veinlets in the low-temperature part of the section of the Ladoga complex (point LV-1355) are located in a weakly textured metasiltstone substrate (Figs. 2a, 2b) with the relics of psammitic structures, which, in terms of the level of metamorphic transformation, belongs to the biotite–chlorite subfacies of the greenschist facies and is composed predominantly of the biotite–chlorite–muscovite–albite–quartz mineral assemblage. At the same time, horizons enriched in carbon, in places with chlorite, actinolite, and carbonate, indicative of the contamination of the parent substrate with volcanosedimentary material from the nearby underlying sequence of the Upper Jatulian complex, are found in adjacent volumes. At this point only metapsammite was taken for study as a protolith for all PST varieties analyzed, because the flyschoid layering is poorly expressed in the outcrop directly and the proportion of metapelites is negligible. In addition to metapsammite (LV-1355/3), we tested BCTs in exocontact zones of PST veinlets (LV-1355/1-2 and LV-1355/5-2). The aphanitic substrate of the PSTs itself was analyzed separately (Figs. 2a, 2b), near the generating surface (PSTGS = LV-1355/1-1) and in the injection vein (crack) of the displaced molten material (PSTIC = LV-1355/5-1), accordingly. In addition, the bulk composition of PSTs was measured both together with protolith fragments and separately in the interstices between the fragments filled with the melt recrystallized by microlites.

Fig. 2.
figure 2

PST veinlets and associated BSTs in host rocks in zones: biotite–chlorite, points (a) LV-1355-1 and (b) LV-1355-5; sillimanite–muscovite, points (c) LV-1744-4 and (d) LV-1744-8; in the zone of granulite metamorphism, points (e) LV-1100-B-Gneiss and (f) LV-1100-B-Granite. Numbers indicate sampling areas on SEM, mentioned in the text and shown in the table of bulk compositions (Table 1). Gn, gneisses; Gr, granites; BCT, blastocataclasites; PSTGS, pseudotachylytes of generating surfaces; PSTIC, pseudotachylytes of injection cracks. (a), (c), and (d) are BSE images; (b), (e), and (f), optical microscopy.

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PSTs in the sillimanite–muscovite zone (point LV-1744) crosscut a sequence of the fractional flyschoid alternation of metapelites and metapsammites (Fig. 2c), represented by essentially micaceous, garnet-bearing gneisses (LV-1744–3A) and quartz–feldspar two-mica metapsammites (LV-1744–3B), respectively. In the same zone, in addition to the substrate of the PSTs themselves (Fig. 2d), which were sampled on the generating surface (LV-1744/8-1) and in the melt injection crack (LV-1744/4-1), the material of the blastocataclasites (BCTs) of the preceding fault formation stages, conjugate with them in the exocontact zone (LV-1744/4-2 and LV-1744/8-2), was also collected for analytical studies.

Within the granulite facies zone (Figs. 2e, 2f), numerous branched veinlets of the black glasslike matrix of PST (LV-1100-1) were revealed in the zone of fault-related BCTs (LV-1100-2), cross-cutting the coarse crystalline two-feldspar micaceous gneisses (LV-1100-3A), in some zones with veined segregations of coarse-grained granitoid material (LV1100-3B), which is close to subalkaline granites in terms of the total alkali/silica ratio.

The PST matrix itself at all observation points is composed, in addition to the small fragments of the protolith minerals (plagioclase, quartz, sometimes biotite), of a micro-sized aggregate of biotite microlites and dioctahedral white mica, crystallized from the molten material (Fig. 2d), as well as residual vitrified melt of quartz and/or plagioclase composition, in microlite interstices.

All noted differences in the initial composition and the degree of overprinted metamorphic alteration of the collected triad samples for each zone are reflected in the total bulk composition (Table 1) and in the position on the petrogenetic variation diagrams for metasedimentary rocks (Fig. 3a) and plutonic series (Fig. 3b) [9, 10]. The PSTs associated with the BCTs and their host rocks at point LV-1355 occupied a position closer to graywacke, whereas the distinctly flyschoid gneiss substrate at point LV-1744, together with the BCTs and PSTs, fell closer to the field of pelites (Fig. 3a). In contrast, the compositions of the rocks of the triad from the granulite zone (LV-1100) occupied a position in the field of granodiorites and granites (Fig. 3b), indicating in favor of the version of their initially apomagmatic origin.

Table 1. Bulk contents of major elements (wt %) in the PST matrix (yellow tint), in their host protolith (blue tint), and in BCT that replaces it (green tint)
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Fig. 3.
figure 3

Positions of PSTs and their host rocks on petrogenetic variation diagrams for (a) metasedimentary rocks and (b) plutonic series in coordinates R1 = 4Si – 11(Na + K) – 2(Fe + Ti); R2 = (6Ca + 2Mg + Al).

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MATERIALS AND METHODS

Bulk samples were analyzed for major, trace, and rare earth elements (REEs) by the method of inductively coupled plasma mass spectrometry (ICP-MS) on an ELAN-DRC-6100 quadrupole mass spectrometer according to the standard procedure adopted at the Karpinsky Russian Geological Research Institute, St. Petersburg (VSEGEI). The uncertainty of element content measurements does not exceed 5–10%. The lower limits of detection are 0.0002–0.01% for major elements and 0.005–0.01 ppm, for most trace elements and REEs. Part of the analytical data, the bulk compositions of protoliths, blastocataclasites, and certain sections of PST veinlets in particular, were obtained on a Tescan Mira LMS SEM with an Ultim Max 65 energodispersive (EDX) spectrometer at the Center for Collective Use of Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow (IPE RAS). It should be noted that the bulk composition of PSTs was studied in zones with fragments and that the composition of the melt proper was studied in the fragment-free matrix.

RESULTS OF ANALYTICAL STUDY OF THE GEOCHEMISTRY OF PSTs AND THEIR PROTOLITHS

The analysis of the bulk compositions of PSTs in comparison with their host rocks, as well as the comparison of the triad elements composition with each other at all three sampling points, showed a complicated pattern and oppositely directed trends in the behavior of certain major and trace elements and REEs in the course and as a result of the repeated activation of slips along faults, involving the selective melting of rock-forming minerals. In addition, as shown below, the established differences in composition between PSTGS and PSTIC and between PSTs with protolith fragments and without them, contributed to the variability of changes in their concentrations in the PST matrix proper.

Variability of major element contents in the Protolith–BCT–PST series. The results of the analysis of major element contents in host rocks during their blastocataclasis and subsequent melting at all three sampling points are presented in Table 1. The style of variations of all major elements, which is fairly similar or of the same type, irrespective of their absolute content values in the three varieties of the triad under consideration, has attracted attention. Worthy of notice as the dominant trend is the general increase in the basicity of the PST substrate relative to both BCT and the protolith, considering the steady decrease in the silica content in the series of the given triad (from 75% in granite and 65–70% in gneisses, through 62–67% in BCT, to 53–58% in PST with protolith fragments). Moreover, some measurements of the recrystallized PST matrix between visible quartz and plagioclase fragments suggest that the composition of this oxide approaches the composition of the ultrabasic melt (rows 2, 7, 9, and 21–23 in Table 1). Changes in the contents of other major elements, even with low variations of the absolute values, also demonstrate certain tendencies, which are most obvious in the binary diagrams of oxides at point LV-1744 (Fig. 4), which show oppositely directed trends of increase or decrease in their concentrations in the Protolith (metapsammites and metapelites)–BCT–PST series.

Fig. 4.
figure 4

Binary diagrams for oxides of major elements in the “Protolith–BCT–PST” triad at point LV-1744. r, calculated values of correlation coefficients for the pairs of oxides discussed.

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It can be seen that PSTs in all oxide pairs occupy extreme positions in the lines of these trends and that their compositions approach the compositions of metapelites, moving quite far away from the composition of a metapsammite. BCTs occupy an intermediate position between the protolith and PST. Judging from the established trends, the Al, Mg, and K contents in the melt matrix distinctly increase; the iron content increases as compared with the original metapsammite, but decreases slightly in comparison with the metapelite, which can be due to the significant melting of biotite and the fact that the fault in this place affected predominantly metapsammites but just barely affected metapelites proper (the proportion of the latter in the outcrop is three times lower than that of the metapsammites). In contrast, the Ca and Na concentrations notably decrease.

It can be assumed that the enrichment in these elements is due to their supply from muscovite and biotite, whereas the deficiency in calcium and sodium, as well as the opposite orientation of the trends of the two alkaline elements, potassium and sodium, is due to the insignificant involvement of plagioclase in melting as compared with micas. It is quite likely that this tendency could have emerged as far back as at the stage of blastocataclasis, when the plagioclases of the matrix of the protolith underwent sericitization, and was later enhanced by selective partial melting processes. Taking the aforementioned general tendency for silica into account, all these changes are quite consistent with the succession mentioned above and the degree of involvement in the selective frictional melting of the major rock-forming minerals of the rocks under consideration: muscovite in full measure; a significant part of biotite; plagioclase in part; and, least of all, quartz [8].

Against this general background, one more interesting aspect of the redistribution of major elements during frictional melting deserves attention. It is expressed in the dissimilarity of melt compositions near the generating surfaces (PSTGS) and in its injection veins (cracks) (PSTIC). Comparison of the bulk compositions of these two varieties of PST shows certain differences between them (Table 1): the former occupy extreme position in trends with increasing aluminum and magnesium contents; the iron content in them increases slightly, but it decreases in the latter variety; the proportion of potassium increases only in the displaced melt but remains unchanged on the generating surface, and the sodium concentration practically does not change in both PST modifications. All of the noted features concern the migration of elements in the melt and indicate either the relative mobility of some and the inertness of the others or their belonging to different portions of the melt, periodically formed in the fault with somewhat different basicity values, which deeply affects the mobility (viscosity) and the range of displacement in fault zone [8]. In order to evaluate and visualize this effect, variations in the melt composition within injection cracks and near the bounding rectilinear PST generation zones were studied in more detail.

At point LV-1355-5 the PST compositions in the low-temperature zone were measured (separately with and without protolith fragments) at the tip of the wedge-shaped crack and at its base in the generating surface zone (Fig. 2b, rows 4, 6–9 in Table 1). Comparison between their compositions shows a noticeable increase in the basicity of the melt emplaced into the crack relative to the content of SiO2 in BCTs and in individual bands of the generation zone. At the same time, the latter, on the whole, is characterized by noticeably higher contents of alumina and sodium and iron oxides, which are probably more inert in the melt and accumulate in the generation band.

A similar comparison was made in a melt injection crack in the PST veinlets of the granulite zone, point LV-1100-B (Fig. 2f, rows 17, 19, 21–23 in Table 1), where even a purely visual comparison with the aid of optical microscopy of the melt matrices at the tip of the crack and along both sides of it (domain I), as well as at its base, which is part of the zone of the generating surface (domain II), demonstrates a noticeable difference in hue and composition between them. The first domain is filled with turbid brownish material with an insignificant amount of small quartz and plagioclase fragments, whereas the second domain is filled with a dark brown, almost black aphanitic substrate with abundant large and small fragments of the protolith. A comparison of the measured compositions shows a significant difference between the two portions of the melt in these domains: the substrate in the zone of the generating surface (area 3 in Fig. 2f and row 19 in Table 1) contains silica in the range of 60–65%, whereas the filling of the first domain, as a whole, is characterized by a much lower SiO2 content (43–45%), sometimes down to 35–38% SiO2 (area 6); i.e., it is characterized by an ultrabasic composition. The PST matrix in the generation zone is more magnesian and aluminous, and it concentrates noticeably more potassium and iron in itself.

Relying on these examples, it can be assumed that an enhanced capability to move and be injected is possessed by the more basic melt precisely due to its lower viscosity, whereas the more acidic melt, generated on displacement surfaces, remains within the fault band. Also possible, however, is another version, that the earlier crack filling portion of the melt, which formed from the most low-melting micaceous phases, is blocked by a later generation, in which a certain portion of plagioclase and quartz was additionally involved in partial melting in the course of sliding, which led to an increase in the silica concentration, and this melt, being more viscous, remained in the slip zone. At the same time, one cannot rule out the third version, that the variability of the compositions of various portions of the melt in its generation zone can be due to the heterogeneity of the protolith itself, crosscut by the fault. One more cause of the variability of PST compositions from one place to another is the development of banding in their matrix (Fig. 2b), the formation mechanisms of which were partially reviewed by the authors in another publication [8]. Here we merely state that the significant variations of major element contents, which have been established in these bands (rows 6–9 and 19–23 in Table 1), allow for melt differentiation during its cooling as well as a nonsynchronous supply of portions of the melt from different sources as possible reasons. The latter explanation is reflected in their differences both in the structure and in the matrix composition (Fig. 2d), which also testify to the insignificant mixing of the nonsynchronous portions of the melt.

Variability of trace element and REE concentrations in the Protolith–BCT–PST triad. The analytical results of measuring the contents of these elements in the varieties of the triad are given in Tables 2 and 3. The general character of the distribution of REEs alone in the source rocks and their cataclastic and melt-related derivatives, at first glance, corresponds to the isochemical version of changes, with the same type of content variations at all three sampling points, with the exception of the granite vein protolith LV-1100-3B (Fig. 5). However, a detailed examination of all data obtained indicates a more complex pattern with differently oriented changes in the concentrations of various groups and individual elements.

Table 2. Concentrations (ppm) of trace elements and REEs in triads of the studied samples (Protolith–BCT–PST) from three sampling points
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Table 3. Concentrations (ppm) of rare-earth elements (REEs) in triads of studied samples (Protolith–BCT–PST) from three sampling points
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Fig. 5.
figure 5

REE distribution spider plot normalized to CI chondrite [11] for samples in the “Protolith–BCT–PST” triad from three sampling points.

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Taking into account the above-mentioned two-stage order of rock transformations in the fault zones under consideration (low-temperature blastocataclasis → frictional melting), the variations in content were estimated separately in the Protolith–BCT and BCT–PST pairs. The changes in the mass (%) of each element were calculated by analogy with the approach [12] in the following way: [(BCT – Protolith)/Protolith] × 100; [(PST – BCT)/BCT] × 100.

The stage of blastocataclasis in the Protolith–BCT pair. As is obvious from the diagrams (Fig. 6), somewhat different tendencies of changes in the content of trace elements are manifested in various groups of these elements at the stage of mechanical destruction and partial recrystallization of minerals in faults, usually accompanied by the hydrothermal alteration of the substrate (chloritization, sericitization, and albitization). Some lithophile elements (Be, Ba, and Sc) in the low-temperature zone of the chlorite subfacies (LV-1355) tend toward a decrease in their concentrations; i.e., they are apparently removed from the fault zone by fluids, whereas others (Sr, Rb, and Cs) accumulate in the altered minerals. One can also note the elevated Rb, Ce, and Y concentrations, characteristic of biotites [13] in association with muscovite. In higher temperature zones (LV-1744, LV-1100), some of the elements change places: the enrichment group consists of Ba, Rb, and Sc, whereas the content of Sr decreases. A similar sharply differentiated pattern is also observed in the group of high field strength elements (HFSE), which are usually considered as resistant to the effect of hydrothermal processes. As regards REEs, they demonstrate different tendencies in the low-temperature and high-temperature zones of regional metamorphism. The enrichment in LREEs and a directional decrease in HREE concentrations take place in the first case, and the directly opposite trend of changes in their contents is manifested in the other two. Such chemical anomalies, as proved by experiments and natural examples, can be due to continuous interaction between the fluid and the rock at temperatures higher than 350°C [14]. In addition, this probably can also be due to differences in ambient temperatures in the range of 330–460°C, which we determined with the aid of chlorite thermometry at the depth levels where the discussed diaphthoric alteration took place at the three sampling points under consideration. It can be assumed that the mobilization activity of the elements changes in accordance with changes in temperature.

Fig. 6.
figure 6

Diagrams of changes in the relative contents of elements at the stage of mechanical crushing in a fault (in the “Protolith–BCT” pair). (a) LV-1355; (b) LV-1744; (c) LV-1100. Groups of elements: LIL, lithophile; LREEs and HREEs, light and heavy rare-earth elements; HFS, high field strength elements; HSEs, highly siderophile elements; SCEs, strongly chalcophile elements.

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The stage of frictional melting in BCT–PST pairs. By considering the changes in the elemental composition of the metaterrigenous rocks of the Ladoga complex at the stage of synorogenic diaphthoresis (blastocataclasis), it is now possible to estimate the variability associated with the redistribution of elements during the partial frictional melting at a later stage of the extension collapse in the orogen. It should be noted here that the pattern of the variability of the elemental composition of PST caused by this process can be additionally complicated from one place to another due to the highly localized character of melting in the slip zone in the initially heterogeneous (at microlevel) substrate and due to the poor miscibility of the melts that form in the course of the multistage frictional melting process [8]. In addition, the comparative analysis of the compositions of the melt matrix on the PST generating surfaces and in displaced melt injection areas (Fig. 7), as well as the differences in trace element and REE concentrations revealed by this analysis, expand the range of factors that affect their redistribution in the seismic slip zone.

Fig. 7.
figure 7

Diagrams of changes in element concentrations during frictional melting in the “BCT–PST” pair, on generating surface (PSTGS) and in the injection crack (PSTIC). (a), (b) LV-1355; (c), (d) LV-1744; (e) LV-1100 (PST/BCT); (f) LV-1100 (PST/Granite).

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In the greenschist zone, the melt matrix near the generating surface (Fig. 7a), in addition to the more than twofold enrichment in potassium mentioned above, is slightly enriched in Ba, Rb, and Cs, but somewhat depleted in Na, Ca, Be, and Sr. Eu forms a distinct negative anomaly, apparently inherited from the metaterrigenous rocks of the protolith. Because the elements of the first group are usually concentrated in muscovite and biotite, while those of the second group are predominantly concentrated in plagioclases, this conclusively testifies to the obviously active transition of micaceous phases into the melt and to the limited melting of plagioclases. A somewhat different situation is recorded in the zones of melt injection (Fig. 7b): the trend of an increase in concentrations arises starting from the lithophile elements and then extending to the LREE spectrum with a peak of the elevated relative Eu concentration, after which all HREE contents decrease invariably. The latter circumstance is quite consistent with the empirically established tendency toward a decrease in HREE concentrations during the joint magmatic crystallization of biotite and muscovite [13]. The noted downward trend continues further in the series of high field strength elements, culminating in a sharp deficiency in the elements of the siderophile group. These tendencies are probably due to the different mobility of various elements in the melt.

The zones of higher temperatures also display their inherent general tendencies as well as certain differences in the behavior of elements in the PST generation bands and in melt injection zones. Of the general trends in these two types of PST localization, the emergence of a peak of elevated relative Eu concentrations; an increase in the proportions of Cs, Cr, and Y; and a very significant accumulation of Co and chalcophile elements attract attention in the first turn. The characteristic trends of changes in the concentrations in the general REE series are also detectable: a weak but regular decrease in contents from light to heavy lanthanides (Figs. 7c, 7e) is discernible in both zones (sillimanite–muscovite and granulite) near the generating surfaces. In contrast, injection cracks display a distinctly opposite enrichment trend (Figs. 7d, 7f), which is typical of biotites from metaintrusive rocks, which are characterized by elevated Y and HREE contents (from the middle of the series to Lu) against the fairly low Rb concentrations [13]. It is quite likely that the enrichment in europium, as well as the increase in the concentrations of the highly mobile chalcophile and siderophile elements, is actually due to the processes of the hydrothermal reworking of the substrate by reproduced fluids, overprinted on PSTs and recorded at microlevel, by analogy with the processes at the late stages of the Svecofennian tectogenesis, which we described in the Ladoga region [8].

Considering that we obtained only the bulk compositions of the PST matrix, which consists of the recrystallized and residual portions of the melt, an unambiguous assessment of trace element and REE distribution between them is difficult. However, in view of their experimentally established fractionation between biotite and the water-saturated granite melt [15] at melting parameters close to those established for PSTs in the Ladoga region [8], it is possible to touch upon the aspect of compatible or incompatible behavior of some elements in a hypothetical form for the purpose of the given situation of frictional melting. Starting from the experimentally obtained distribution coefficients [15, Table 2], one should expect that Ba and Rb from the lithophile group as well as Nb and Ta from the high field strength group could enter the biotite structure as compatible elements (Kd > 1). Nb is more easily incorporated into biotite than Ta, and this accounts for the appearance of high Nb/Ta ratios during the partial melting of biotite-rich rocks [16]. In addition, it can be assumed that, in the case of melt recrystallization and biotite precipitation as microlites, Nb will concentrate predominantly in biotite and Ta, in the restite melt. Because Nb and Ta have an affinity for Ti, their high concentrations can be encountered in Ti-bearing minerals (ilmenite, titanite, etc.), which we recorded in the PST matrix. As regards REEs, the elements of the central part of their series (MREEs) with the minimum values of distribution coefficients (Gd, Tb, and Dy) are least likely in micas, whereas LREEs and HREEs, as less incompatible, can concentrate in biotite in the K position (LREEs) or in the octahedral position of Mg and Fe (HREEs) [15]. It can be added to this that the PST matrix, recrystallized by biotite microlites, can also contain a whole series of highly compatible, due to the very high distribution coefficients in the biotite/melt system, transition metals, high concentrations of which were recorded in our cases (Kd: 17 for Ti; 35, for V; 47, for Co; 174, for Ni; and 5.8, for Zn) [15].

The elements incompatible in micas (Kd < 1), Sr, Ce, Cs, and Eu, most likely concentrated in the residual melt, which was composed of quartz in some places and plagioclase in others. The latter element (Eu) could be localized in zones of residual plagioclase glass, creating, in most cases, the peaks of elevated concentrations relative to the protolith. The incompatibility of these elements is indirectly confirmed by their comparison with Yb as one of the most pronounced incompatible elements in melts of basic composition. The manifestation of a positive linear correlation with this element in the varieties of the triad (Figs. 8a, 8b) may indicate the manifestation of incompatible properties of these elements in the rocks under consideration.

Fig. 8.
figure 8

Correlation of the contents of (a) Sr–Ce, (b) Cs–Eu, and (c) ytterbium and total LOI/Eu and (d) LOI/ K2O + MgO + Fe2O3, in the varieties of the triad of all three sampling points.

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DISCUSSION AND CONCLUSIONS

The data presented, apart from the main task of filling the factual content of the geochemical aspects of the transformation of rock material during frictional melting in a fault zone, allow one, to varying degrees, to highlight some of the issues raised in the introductory part and that concern the mobility, redistribution, and fractionation of major elements, trace elements, and REEs during the formation of seismogenic PSTs after metaterrigenous rocks. It should be emphasized that the process of preparing the substrate for melting and the respective changes in the elemental composition commenced at the previous phases of the formation of fault-related blastocataclasites at the stage of the post-tectogenic orogeny, when noticeable variations in the contents of certain components took place along with the mechanical grinding of the matrix, accompanied by its hydrothermal reworking. At the same time, the elements of various groups (lithophile, REEs, high field strength, etc.) display different tendencies in three different temperature zones, most probably associated with the continuous interaction of fluids and rocks, as well as their different mobility depending on the ambient temperature.

Speaking about the variability of the compositions and the redistribution of elements at the stage of PST formation, first of all, the directional increase in basicity during the transition of the protolith into the melt should be noted as a general tendency, as well as its partial and selective melting in the form of the facilitated melting of micas (biotite, muscovite) and the low degree of involvement of plagioclase and, least of all, quartz in this process. This is reflected in the enrichment of the melt, first of all, in the major elements, trace elements, and REEs that are characteristic of these phyllosilicates, and in its depletion of the corresponding elements of the refractory phases. A significant contribution to the uneven concentration of elements in the PST matrix is made by the directional succession of melt recrystallization, which we revealed [8]: first the microlites of biotite are conceived, then, the white dioctahedral micas, whereas the residual melt of sometimes plagioclase, sometimes quartz, composition concentrates in vitrified form in the interstices between the microlites. Each of these phases includes an individual set and concentration of elements. Of the most obvious trends, one can once more pay attention to the change in Eu behavior during the transition from the low-temperature zone (negative anomaly) to higher temperature zones with the peaks of its relatively high contents there. Its concentration can be guided by two factors: transportation in the fluid assimilated in the melt (LOI) or the effect on the melt of the regenerated fluids invading the fault zone during the slip. This has its own logical explanation. Due to the ability of europium to create complex compounds easily with the hydroxyl groups of fluids as well as its affinity for iron and chalcophile elements, its accumulation in the fault zone quite distinctly correlates with the content of LOI in PST (Fig. 8c) and with an increase in the concentration of these elements, which are highly mobile in the fluid phase (Table 2). Speaking about the role of the fluid component, it should be noted that one of its sources in the fault zone, where PST is formed, could be the processes of dehydration of hydrous minerals, which is associated with coseismic frictional heating. The released fluids not only contribute to frictional melting (dehydration melting), but also partially invade the melt, interacting with part of the dissolved elements, or interact with the protolith and, probably, with the vitrified melt. As an example, we can cite the fact of a linear correlation between the content of LOI and the K2O, MgO, Fe2O3 (Fig. 8d), and Rb concentrations in the PST melt, which is consistent with the high degree of involvement of biotite, the carrier of these elements, in melting.

Another significant feature is associated with the markedly different character of accumulation or removal of individual elements in the two PST localization types, along the generating surfaces and in melt injection zones. Their most outstanding feature is the difference in the LREE and HREE concentration trends (Table 3): the first type of localization (PSTGS) in this series displays a downward trend, whereas the second type (PSTIC) displays a reverse trend of their enrichment (Figs. 7c–7f). The deficiency of light REEs may indicate a low involvement of plagioclase in the partial melting process, and an increase in heavy REE concentrations is quite consistent with the preferential biotite crystallization in the melt. Among the general trends, one can also note very high La/Lu ratios in all PST matrices relative to the protolith, which is particularly pronounced in the granulite facies zone, apparently indicating the effect of ambient temperature on the degree of melt differentiation. Finally, taking into account our estimates of the solidus and liquidus temperatures of PST melts [8], we may state that the established features and behavior trends of the major elements, trace elements, and REEs during frictional melting in seismic slip zones correspond to the situation of partial, selective melting and are controlled by the lowest melting points of individual rock-forming minerals.

In conclusion, it should be emphasized that the data presented on the redistribution of elements in the course of frictional melting predominantly concerned the major rock-forming minerals of the discussed metaterrigenous rocks (micas, plagioclases, potassium feldspars, and quartz). However, a substantial contribution to this redistribution process was undoubtedly also made by the accessory phases (apatite, monazite, ilmenite, sphene, zircon, etc.), the concentrators of many trace elements and REEs. Evaluation of the role of these auxiliary phases in the discussed processes of the formation of PST compositions is a separate and still poorly studied problem that requires special studies. Our preliminary results testify to the possibly varying effect of individual phases involved in melting and to the changes in the LREE and HREE concentrations. For instance, the samples from the zone of greenschist metamorphism (LV-1355) are characterized by the distinct predominance of apatite (which is virtually absent in the PST matrix) among the accessory minerals in the host substrate, which attests to its transition to the melt. This precisely may account for the aforementioned increase in the concentrations of HREEs, of which apatite is considered as the carrier [17]. Further analytical studies in this direction will allow us to evaluate the contribution of other accessories to the redistribution of trace elements and REEs during the formation of PST in various metamorphic zones of the Ladoga region, taking into account the respective changes in the compositions of rock-forming mineral phases, as well as the temperatures of the surrounding substrate during the formation of PST.