ISSN 1028�334X, Doklady Earth Sciences, 2013, Vol. 450, Part 2, pp. 587–591. © Pleiades Publishing, Ltd., 2013.Original Russian Text © M.L. Somin, L.M. Natapov, E.A. Belousova, A. Kroener, A.N. Konilov, V.A. Kamzolkin, 2013, published in Doklady Akademii Nauk, 2013, Vol. 450, No. 4,pp. 445–449.

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In the geological literature, such terms as “base�ment” and “cover” are commonly used. The term“basement” refers to a rock complex separated froman overlying complex of younger age (cover) by astructural�stratigraphic (often metamorphic as well)uncomformity. In the base of the basement, its erosionproducts can be found. This indicates that the base�ment and the cover are geologically connected. Ifthese structural�stratigraphic correlations are absent,the basement–cover system is debateable. Thus, acombination of infrastructure and suprastructure canbe confused with it, because there is tectonic displace�ment (caused by difference in rheological properties)between the parts of an initially united complex of dif�ferent metamorphisation (and therefore they differ instructural characteristics); but there are no structuraluncomformities here.

The type of combination in which rocks are meta�morphosed and deformed in substantially differentmanners as discussed in the present paper refers to thebase when the lower complex of highly metamor�phosed rocks and granitoids (crystallinicum) is coevalto the complex of overlying poorly altered deposits.This situation takes place when horizontal tectonicjustaposition of these complexes resembles the base�ment and cover combination. In this case, it is sug�gested to call crystallinicum a “pseudobasement” [1].The combination of the roots used here literally means“false basement” or “apparent basement” and has themeaning “what seems to be a basement, but is not.”

An example of a pseudobasement is the crystallini�cum that occurred at the base of the Paleozoic zonesection of Peredovoi Range, Greater Caucasus (Fig. 1).In the main Blyb salient of this zone above the crystal�linicum, which is metamorphozed in amphibolitefacies, one can find the volcanogenic–sedimentary(arc) strata of the Silurian, Devonian, and Lower Car�boniferous (Urup complex, hereinafter UC for brev�ity); note that these rocks are metamorphozed only togreenschists. In other salients (Beskes, Sakhrai,Dakh), UC rocks remained only in the form of smalltectonic wedges. The occurrence of a crystallinicumbeneath the poorly metamorphozed rocks of the Sil�urian–Lower Carboniferous are evidence of the pre�Middle Paleozoic (supposedly Proterozoic) age of thecrystallinicum for many authors [2, 4], who inter�preted this as the basement of the UC. In accordancewith another interpretation [5, 6], the crystallinicumand UC rocks are parts of a united zonal metamorphiccomplex.

Our observations, however, contradict both theseviewpoints. The metamorphism types of the crystal�linicum and UC are sharply different in terms of bothtemperature and pressure conditions, and the transi�tion zone between them is several tens of meters atmaximum. In the near�contact zone of the crystallini�cum, low�temperature blastomylonites can be found,while rocks with abundant relics of the initial structure(mostly effusives) occur almost immediately abovethem. It is important to note that the crystallinicum ismostly high�pressure complex. Within the limits of theBlyb salient, it includes conforming eclogite bodies(T = 680 ± 40°C, P = 16 ± 0.2 kbar [3, 7]); here, inparagneisses and orthogneisses that host eclogites, wefound high�sodium omphacites included into sulfideindicating a high pressure setting; in white micaswhich are paragenous to hornfelds and the phengitecontent is stably high, and this is pure phengite [3];alumina rocks turned into kyanite–K�feldspar–phengite–garnet gneisses; and hornfields in epidoteamphibolites also have features typical for high�pres�

A Pseudobasement in the Pre�Alpine Structure of the Peredovoi Range, North Caucasus

M. L. Somina, L. M. Natapovb, E. A. Belousovab, A. Kroenerc, A. N. Konilovc, and V. A. Kamzolkina

Presented by Academician Yu.M. Pushcharovskii June 9, 2012

Received June 22, 2012

DOI: 10.1134/S1028334X13060068

a Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russiab Gemoc Center, Department of Earth and Planetary Sciences, Macquire University, Sydney, Australiac Geological Institute, Johannes Gutenberg University of Mainz, Mainz, Germanyd Geological Institute, Russian Academy of Sciences,Moscow, Russiae�mail: somin@ifz.ru

GEOLOGY

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sure amphiboles [3]. Signs of high pressure (at least 8–9 kbar) are found in metamorphic rocks of the Dakh[8] and Sakhrai salients. In the latter case, high pres�sure is additionally indicated by the substantiallyphengite (up to 3.4 ph. un.) composition of whitemicas. The UC rocks, however, contain no signs ofhigh pressure.

The crystallinicum and UC sharply differ in termsof lithology: the UC has no analogs of apogranitoidorthogneisses, micaceous schist and metaultrabasites,which are typical for crystallinicum; analogously, thecrystallinicum does not contain rocks that might beconsidered as conduits for basalts and rhyolites com�posing Devonian sections of the UC. Terrigenousrocks in these sections beneath the base of the LowerCarboniferous are absolutely free of erosion productsof sialic metamorphic rocks, which dominate in thecrystallinicum composition [9]. These productsappear in deposits younger than the Tournaisian.

Thus, we can conclude that the concept of gradualtransition from the crystallinicum to the UC green�stone rocks is erroneous, on the one hand, while theUC rocks cannot be considered as cover derivatives,on the other hand. Hence the question arises, is thecrystallinicum the real basement for the Middle Pale�

ozoic greenstone rocks? To answer this, let us considerisotope�geochronological data. To determine the ageof sedimentary and igneous protoliths of the crystal�linicum, we used U–Pb dating of zircons. Detritalgrains (n = 20) from phengite–garnet–hornfelds(metagraywacke) of the Blyb salient were analyzed onthe SHRIMP II instrument (at the Karpinskii All�Russian Research Institute of Geology (VSEGEI))and showed three groups of age values, with the young�est of them (n = 7) being 374 Ma, on average (LateDevonian). The Th/U ratio of these grains (0.3–0.8)indicates their initially igneous nature [10]. Eightgrains of detrital zircons (total n = 48) from garnet–phengite schists showed the youngest peak at about380 Ma (LA�ICP�MS, Gemoc Center, Sydney)(Figs. 2, 3). The rims of these zircon grains yielded agevalues of about 330 Ma indicating the time of meta�morphism. Igneous zircons from orthogneisses of thesame salient dated to 390–355 Ma (n = 6, SHRIMP II,VSEGEI) and 361.4 ± 3 Ma (LA�ICP�MS) (Fig. 4).Statistical representation makes this dating the mostvalid. Zircons from banded orthogneisses of theBeskes salient (exposed near Moshchevaya creek)yielded a value of 388 ± 4 Ma (SHRIMP, Perth).Metaaplite, crossing garnet amphibolites of the Dakh

10 km

40°

44°

Psheka R.

Psebay

Pseashkho Heights

Sochi

Solokhaup

2014

Bel

aya

R.

Big Laba R.Psou R.

Shakhe R.

Mal

aya

Big

Lab

a R

.

Belaya R.

BLDZh

SKh

DKh

E

1

6

2

7

3

8

4

9

5

10

N

Big Laba R.

BLBS

I

I I'

44°

I'BS

Nikitino

Krasnaya Polyana

Guzeripl’

Urup R

.

Mzy

mta R

.

Fig. 1. Geological scheme of the northwestern part of the Peredovoi Range. (1) Silurian–Lower Visean volcanogenic�sedimen�tary strata; (2) crystallinicum (metamorphic rocks and granitoids of the Peredovoi Range; in the section and in the map); (3)ultrabasites; (4) ophiolites; (5) Atsgara nappe; (6) crystallinicum of the Greater Caucasus Range; (7) post–Lower Visean depos�its; (8) transgression overlaying; (9) base of large nappes; (10) steep faults. Letters in circles denote salients of the crystallinicum:Blyb (BL), Beskes (BS), Dzhuga (DZh), Dakh (DKh), Sakhrai (SKh), Expeditsii Mts. (E).

La b

a R

.

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A PSEUDOBASEMENT IN THE PRE�ALPINE STRUCTURE 589

massif and metamorphosed with these rocks, wasdated by the classical TIMS method and showed anEarly Carboniferous age (353 ± 3 Ma [11]). Zirconsfrom metaplagiogranites (Solenovskii bridge, Blybsalient) demonstrated an age of 323 ± 5 Ma (TIMS,GEOKHI RAS).

As we can see, the obtained data, despite certaindifferences in age values (due to both different tech�niques used and different material for analysis), indi�

cate the Devonian–Carboniferous age of the initialigneous rocks of the crystallinicum and the Devonianage of its sedimentary protoliths. Based on the zirconanalysis, the age of regional metamorphism is betweenthe Late Devonian and the Early Carboniferous. Theresults of K–Ar dating of micas generally do not con�tradict this: muscovite from the pegmatite dike thatcrosses eclogites is dated at 334 Ma (K = 7.7%); mus�covite and biotite from orthogneiss (Sakhrai Riverarea) date to 338 and 334 Ma, respectively [9]. SomeK–Ar dates and Lu–Hf ages of eclogites [7] aregrouped around 322 Ma. These values probably reflectthe age of cooling of the minerals below the thresholdtemperatures; the known [3, 10] older dates could becaused by an excessive share of argon, typical for high�pressure metamorphism settings.

In general, the age of crystallinicum rocks in thementioned salients of Peredovoi Range correspond tothe age of the UC poorly altered strata (~425–340 Ma),and sometimes to an even younger age. This indicatesthe complete tectonic fitting of these complexes dur�ing horizontal tectonic motions, and the crystallini�cum is not the basement of arc series but their tectonicunderlayer, or pseudobasement. Hence, salients of thecrystallinicum form a big tectonic window in the UCallochthonous masses and upperlying pre�UpperPaleozoic rocks. Their root zone seems to be locatedin the southern part of the Greater Caucasus Rangezone, where the Laba metamorphic complex, compa�rable to the UC in both lithology and age, is located[10]. Therefore, the crystalline rocks in the northernElbrus subzone of the Greater Caucasus Range com�pose the tectonic window.

8

03000

U�Pb�age, Ma600

7

6

5

4

3

2

1

26002200180014001000200

~470 Ma

~545 Ma

~642 Ma ~2540 Ma

~2060 Ma~2660 Ma

Sample no. 08�13n = 48

~350 Ma

Relative probability

Fig. 2. Histogram of the U–Pb age values (LA�ICP�MS)for the detrital zircons from sample no. 08�13 collected inmetapsammites of the Blyb metamorphic complex, BigLaba River area; n means the number of dated grains.

C 536

R 338

08�13�3350 µm 50 µm

R 349

C 1162

08�13�36

08�13�49

446

08�13�42

50 µm 50 µm

473

Fig. 3. Cathode luminescence images of some grains from sample no. 08�13; digits near grains mean ages in Ma; C, cores;R, metamorphic rims; circles, dating craters.

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Exposures of Devonian arc strata are known in theeastern part of the Peredovoi Range as well. There isevidence of the crystalline pseudobasement beneaththem [1]. However, in this place it is represented byLate Paleozoic (age of metamorphism) low�pressurecrystalline schists and S�granites, which are both typ�ical of the Elbrus subzone. Thus, the pseudobasement

of the Peredovoi Range is heterogeneous and alsoformed during tectonic coupling of complexes of dif�ferent origin.

Complexes of the pseudobasement can be found inother fold zones as well. For example, the exposures ofEarly Carboniferous (age of metamorphism) high�pressure granulites of the Bohemian massif in Central

5

200 520

Number of grains207Pb/235U

4804404003603202800

240U−Pb�age, Ma

4

3

2

1

Inheritedpopulations

Average = 361.4± 3.4 (0.95%)MSWD = 1.6, concordance

probability = 0.086

Pb loss

~223 Mametamorphic

event ?

0.15 0.750.650.550.450.350.25

0.08

Rim

Core

Core

Rim

206Pb/238U

0.06

0.04

0.02

200

300

400

500

Fig. 4. The results of U–Pb (LA�ICP�MS) dating of zircons from sample no. 0�40�1, orthogneisses of the Blyb complex, area ofBig Blyb River mouth. The upper panel is the concordia diagram; in the lower panel, histogram for ages of these zircons.

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A PSEUDOBASEMENT IN THE PRE�ALPINE STRUCTURE 591

Europe [13] and Paleocene metamorphic rocks in thesouthern part of Sredinnyi Range, Kamchatka [12],can be referred to this type, because their ages areyounger than the age of the overlying allochthonousrocks.

The existence of a pseudobasement suggests thatthe relationships between rock complexes in the deepcrust are much more complicated than is usuallybelieved, because a significant role in these relation�ships is played by horizontal displacement structures.The metamorphic rocks that are found when drillingor hypothesized during geophysical explorationbeneath poorly metamorphozed strata should beinterpreted as a basement only after special geologicaland geochronological studies.

ACKNOWLEDGMENTS

This work was supported by the Russian Founda�tion for Basic Research (project no. 10–05–00036a).

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