Crustal architecture of the Donets Basin: tectonic implications for diamond and mercury-antimony mineralization

TECTONOPHYSICS I

Tectonophysics268 (1996) 293-309

ELSEVIER

Crustal architecture of the Donets Basin: tectonic implications for diamond and mercury-antimony mineralization a*

H. de Boorder ' , A.J.J. van Beek a, A.H. Dijkstra a, L.S. Galetsky b, G. Koldewe a, B.S. Panov c a Utrecht University, Utrecht, Netherlands b Geoprognoz, Kiev, Ukraine c Technical University, Donetsk, Ukraine

Received 17 June 1995; accepted31 May 1996

Abstract Kimbedite-like rocks and minor diamond finds are reported in the Precambrian Ukrainian Shield south of the Donets Basin. Prolific mercury-antimony mineralization occurs in Carboniferous quartz arenites within the Basin. The tectonic setting is examined on the basis of recent data compilations and ongoing research in the Ukraine and Voronezh shield areas and the Pripyat-Dnieper-Donets palaeorift. In the Donets region, a straightforward analogy of any diamond district with the Archangelsk province is not likely in the absence of a Proterozoic shear comparable with the White Sea-Belomorian Mobile Belt. A deep-reaching, NNW-striking lithosphere lineament is identified here as the Kharkov-Donetsk lineament. It transects the rift between the Donets and Dnieper basins. The structures involved in this lineament have controlled Palaeozoic sedimentation and the extent of Late Permian inversion of the Donets basin. During the inversion, the lineament and associated deep-reaching longitudinal structures provided pathways for the migration of mineralizing fluids from deep levels in the lower crust and upper mantle. The intersection, in the Kharkov area, of this lineament with a northeasterly striking lithosphere root should focus diamond exploration towards the northern shoulders of the rift. The extreme attenuation of the crust beneath the Donets Basin, relative to the western basins of the rift, is associated with crustal detachment and subsidence during and possibly after inversion, concomitant with emplacement of asthenospheric materials at higher levels. Together with the continued subsidence in the western Donets Basin, during the Late Permian inversion, this invokes a tectonic setting for the Hg-Sb mineralization not unlike the orogenic-collapse-associated settings of Hg-Sb deposits in western Europe. Further investigation of the geodynamics of the Donets Basin would benefit from deep reflection seismics, petrogenetic studies of magmatic products and their xenoliths, and satellite remote sensing analysis. Keywords: Donets Basin; mantle lineament; inversion; diamond; mercury; antimony

1. Introduction The western part of the Donets Basin of the Pripyat-Dnieper-Donets (PDD) palaeorift is characterized by prolific mercury-antimony mineralization, * Corresponding author.

in addition to the occurrence of minor deposits of base metals. Gold and diamonds have been reported from recent fluviatile deposits, but accurate records are not available. Two drill holes reported to have found kimberlite-like rocks are located in an area where the Kharkov-Donetsk lineament intersects the southern shoulder of the rift (Figs. 1, 3 and 5).

0040-1951/96/$15.00 Copyright© 1996 Elsevier Science B.V. All rights reserved. PI1S0040-1951(96)00226-0

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50 °

48 °

:s Palaeorift )-

ament .=#10

Legend for the crustal columns sedimentary layer Vp < 6.4 km/s (upper crust) 6.4 < Vp < 6.8 km/s (middle crust) Vp > 6.8 km/s (lower crust) Vp = p-wave velocity The numbers in the columns represent the thickness in km.

~ [~

Outl=neof the East European Craton Area covered by figure

Fig. 1. Moho configuration and crustal layering of the southwestern part of the East European Craton, after Zaritsky (1992).

H. de Boorder et al./ Tectonophysics 268 (1996) 293-309

Several kimberlite districts are associated with relatively young rifts overlying Proterozoic mobile belts and deep-reaching transverse shears (e.g., White et al., 1995). Therefore an analysis was made of the setting of the Donets Basin, to verify earlier suggestions about the presence of a Proterozoic pre-Riphean mobile belt along the axis of the PDD palaeorift (Watson, 1977; Sinitsyn et al., 1992). Although the magnetic signatures of the Ukrainian Shield to the south, and the Voronezh Massif to the north of the rift, suggest a different position of these two parts of the Sarmatian segment, recent work by Shchipansky and Bogdanova (1996) indicates that the principal Precambrian structures are continuous across the rift. A major Proterozoic mobile belt along the PDD rift, which could also have acted as an earlier zone of weakness favouring the evolution of the late Palaeozoic rift, seems unlikely. In Europe west of the Tornquist-Teisseyre Suture Zone, or Trans European Suture Zone (TESZ), deposits of mercury and antimony occur in spatial and temporal association with the collapse of orogenic regions. This association is observed in the Alpine belts and also appears to occur in the Variscan and Cadomian belts (De Boorder and Westerhof, 1994; De Boorder et al., 1995). The setting of the prolific mercury-antimony mineralization of the western Donets Basin suggests a considerably different environment in view of the initially stable crust of the Sarmatian segment as compared to the unstable continental crust of late orogenic belts. However, Late Permian inversion of the Donets region, following the largely Carboniferous post-rift phase, appears to have been associated with extreme attenuation relative to the events in the Dnieper and Pripyat rift segments. The extreme attenuation in the Donets region can possibly be explained by detachment and subsidence of a major part of the crustal column during and after inversion, not unlike collapse of orogenic regions in Western Europe. 2. Tectonic precursors of the Pripyat-Dnieper-Donets rift Breakup of the continental crust is a highly variable process recognized to occur along pre-existing zones of weakness (Dunbar and Sawyer, 1989). Wilson (1993), with reference to Dunbar and Sawyer

295

(1989) and Wilson and Guiraud (1992), continues to support a general tendency for intra-continental rifts to follow weak zones in the lithosphere and to avoid the stronger cratons. In the case of the PDD palaeorift, the option of an underlying Proterozoic mobile belt (Watson, 1977; Sinitsyn et al., 1992) has been studied on the basis of recent regional data compilations (Makarova et al., 1974; Zaritsky, 1992; Galetsky, 1993). The results tend to support the existence of an older tectonic discontinuity underlying the rift since a distinct contrast across the rift is suggested by the magnetic signatures (Makarova et al., 1974). The verification of lateral movements in the Archaean and Proterozoic along a shear belt underlying the trend of the axis of the present rift, however, depends on the identification of marker units across the rift. Therefore, it is significant that, on the basis of detailed examinations of outcrops and drill cores, Shchipansky and Bogdanova (1996) conclude that the Archaean and Palaeoproterozoic lithostratigraphic units of the Voronezh Massif and those in the Ukrainian Shield are continuous. This continuity is part of the basis of the definition of the Sarmatian crustal segment by Bogdanova (1993) (see also Bogdanova et al., 1996). The principal tectonic units of Sarmatia can be traced across the Dnieper-Donets aulacogen. Several of the Archaean to Proterozoic tectonic boundaries have contributed to the segmentation of the late Palaeozoic PDD rift. Although a Riphean precursor rift has been suggested, and although crystalline blocks of the Ukrainian Shield adjacent to the rift reportedly show earlier counter-clockwise rotation (e.g., Chekunov et al., 1992), no direct evidence of a Riphean rift is available. An older zone of crustal or lithospheric weakness underlying the whole of the PDD rift therefore remains speculative. The problem, however, continues to merit attention in connection with the alleged diamond prospectivity of the rift and its shoulders. Proterozoic mobile belts are now beginning to be recognized as part of the tectonic framework of diamondiferous kimberlite (e.g., White et al., 1995), by analogy with the setting of the Archangelsk kimberlite province in northern Russia. The reported but unrecorded occurrence of diamonds in the Donets region (e.g., Chekunov and Naumenko, 1982) has fed speculations about the existence of kimberlite pipes.

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3. Thermal mantle anomaly, magmatism and transcrustal structure The distribution of the volcanic and plutonic rocks within the rift and on the adjacent crystalline massifs can, with caution (cf. Wilson, 1993), provide an estimate of the magnitude of the thermal anomaly in the underlying mantle. The extent of uplift of the surrounding crystalline shield may also provide an indication of the diameter of the thermal anomaly. Stephenson et al. (1993) estimate that the exposed basement to the northeast and to the southwest may signify a large basement uplift with a diameter >1000 km, bisected by the PDD rift. In addition, the distribution of the magmas richest in MgO (> 18%), and their centres of eruption and emplacement could help delineate deep-reaching, even transcrustal, channels of ascent and the principal elements of the tectonic framework. The volcanic and plutonic districts within the PDD rift appear primarily related to the northerly striking structures separating the principal segments of the rift. Locally, they spread out along the main boundary faults (Wilson, 1993; Wilson and Lyashkevich, 1996). The close association between known districts of igneous activity and major transverse structures, in preference to the longitudinal structures, supports the findings of Shchipansky and Bogdanova (1996). The transverse discontinuities seem to have controlled the deepest reaching faults and shear zones. The association with MgO-rich lavas shows that they reached about 100 km deep in the Dnieper Basin (M. Wilson, pers. commun., 1995). 4. The Donets Basin and the Pripyat-Dnieper-Donets (PDD) rift

lower crust layer is approximately 20 km thick and is underlain by a lens whose seismic characteristics are interpreted as a mixture of crust and mantle materials (II'chenko, 1992), and by the Moho. In the interpretation by II'chenko (1992) of DSS profile 10 across the Donets Basin, a cross-section emerges which shows a strongly attenuated upper crust layer on top of a thickened lower crust layer. The middle crust layer is missing below the rift, but can be identified again laterally away from the rift (Fig. 2A). Apparently, attenuation in the middle to lower crust was even stronger than at higher levels. The increased thickness of the lower crust layer underlying the Donets Basin, relative to adjacent regions to the southwest and the northeast, suggests that attenuation of the crust was accompanied or followed by emplacement of material from deeper levels. Reprocessing of this profile is in progress (D. O'Leary, R.A. Stephenson and T.V. II'chenko, pers. commun., 1995) and may result in a further detailed interpretation. At the subcontinental scale, the southern shoulder of the Donets Basin overlies the NE-striking axis of an elongated broad culmination of the asthenosphere at between 150 and 200 km depth (Fig. 3) with its apex beneath the Crimea at less than 100 km depth. This feature is based on the interpretation by Sollogub (1986) of heat flow, seismological, magneto-telluric and deep seismic sounding data. To the northwest, this culmination is bounded by a parallel depression, down to more than 250 km, in the top of the asthenosphere from Kishinev to Kirovograd and Kharkov (Sollogub, 1986). Although the intersection of these asthenospheric structures with the PDD rift places the Donets region in a special position, its significance for the processes within the rift is unclear beyond a spatial coincidence.

4.1. Broad characteristics

The region of the Donets Basin is characterized by a crustal column which is distinctly different from those in the Pripyat and Dnieper regions (Fig. 1). Based on seismic velocities, the sedimentary infill of the Donets Basin is now about 25 km thick and is immediately underlain by materials interpreted as lower crust (II'chenko, 1992). Here the upper and middle crust layers, present in the Pripyat and Dnieper segments, are lacking (Zaritsky, 1992). The

4.2. The Moho configuration and the Kharkov-Donetsk lineament

In the Pripyat and Dnieper regions, the Moho configuration closely follows the axis of the overlying rift (Fig. 1). From levels about 45 km depth below the adjacent shield areas it reaches a depth level of about 35 km beneath the rift. With the Moho at 45 km depth in the Donets Basin, this signifies a downward step of "-~10 km across the boundary

297

H. de Boorder et al./ Tectonophysics 268 (1996) 293-309

A. Present Seismic Structure

Legend

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40

mantle

Fig. 2. Section through crust of the Donets Basin. (A) Simplified DSS refraction seismic section #10 after II'chenko (]992). (B) Cartoon of thickeningby deformationon compressionduringinversionand subsequentdetachmentof lowerand part of middlecrust. (C) Cartoon of riftingby crustaldelamination.For locationsee inset of Fig. 7.

between the Dnieper segment in the west and the Donets segment in the east. This step, within the domain of the rift structure, is part of a much larger north-northwesterly striking structure in the Moho surface. As expressed by the Moho contours, a division has been proposed between the western and eastern parts of the southern East European Craton (e.g., Bogdanoff and Khain, 1981 sheet 17). Zaritsky (1992) suggests that it follows a 'lithosphere lineament' extending about 900 km from the Sea of Azov to the north-northwest (Figs. 1, 3 and 4). In this paper we call this structure the KharkovDonetsk lineament. The change in Moho depth from the Dnieper to the Donets segments was mentioned by Chekunov et al. (1992). These authors also indicate the presence of small-scale depressions of the Moho relative to the levels underneath the shield regions to the north and the south of the Donets Basin, and note that the Moho is generally depressed in-

stead of uplifted underneath the rift as in the western segments. 4.3. The sedimentary column

From the configuration of the surface of the crystalline basement (Zaritsky, 1992) there is also a northwesterly striking, prominent division between the Dnieper and Donets basins (Fig. 4). It coincides approximately with the trace of the KharkovDonetsk lineament and the trend of the Moho contours. Within the Donets Basin, the sediments reach a thickness of over 20 km. Available litho- and chrono-stratigraphical columns for the Donets Basin (e.g., Panov, 1976) suggest a maximum thickness of the sedimentary column of only 15 km. This is substantially less than the 20-25 km thickness suggested by the seismic data. The thickness of the sedimentary infill of the Donets basin as estimated

298

H. de Boorder et al./ Tectonophysics 268 (1996) 293-309

•Moscow

Mmsk

Kharkov-Donetsk lineament

";"

Warsaw

\~j Kiev

o+ Budapest

60 ~

#

m of Azov

f

•

Bucharest 200

~|

Black Sea

~"

Fig. 3. Contours on the lithosphere-asthenosphere boundary after Sollogub (1986) in relation to Pripyat-Dnieper-Donets rift and Kharkov-Donetsk lithosphere lineament after Zaritsky (1992). The extrapolated axis of the Kishinev-Kirovograd-Kharkov depression would intersect with the Kharkov-Donetsk lineament in the northern shoulder of the PDD in the Kharkov area.

from the seismic data is not necessarily the same as the stratigraphical thickness of the sediments. The question arises whether the difference could be due to thickening caused by tectonic compression during the inversion processes.

4.4. Igneous activity In the Donets Basin, and particularly along its southern boundary fault system south of Donetsk, Nikolskij et al. (1973) and Panov (1978) have described mid- to Late Devonian alkaline ultramafic and alkali basaltic stocks, flows and breccias. These are cut by stocks and dykes of nepheline syenite affiliation of at most a Namurian age of --~330 Ma (Nikolskij et al., 1973). Late Devonian flows of tholeiitic basalts are associated with the deepreaching fault zones of the central and northern parts of the basin. Two other igneous complexes are mentioned by Nikolskij et al. (1973) to consist of stocks, flows and dykes of shonkinite, trachydolerite, monzonite porphyry, augite kersantite, camptonite, plagioporphyry and trachyliparite. These complexes

would have ages between 290 and 270 Ma and the lavas cover the coal measures of the C3z stage in the Late Carboniferous. South of Donetsk, an andesitic to trachyandesitic district occurs along the southern periphery of the Basin (Fig. 5). Isotopic age determinations indicate emplacement between 230 and 200 Ma. Finally, a northerly striking belt up to 20 km wide is known east of Donetsk, consisting of lamprophyre dykes in which ages of about 160 Ma have been measured (Nikolskij et al., 1973; Korcemagin, 1977). Unfortunately, geographical locations, chemical compositions and the sources of the isotope chronological analyses have not been reported in the available literature. Broadly speaking, the Devonian magmatism of the Donets Basin shows similarities with the magmatism of the rift's western basins. However, the "-~330 (Middle Carboniferous), the 290-270 (Permian) and the 230-200 (Triassic) Ma phases place the Donets Basin in a category of its own and suggest a complex interaction between mantle and crust, particularly at the time of the inversion of this part of the rift. The 160 Ma phase observed in the northerly striking lamprophyre

299

H. de Boorder et al./Tectonophysics 268 (1996) 293-309

500

48"

320

34°

360

380

40°

64 °

56 °

Marginal faults of the Pripyat-Dnieper-Donets Palaeorift

48"

Kharkov-Donetsk Lineament

C.,o

Contours of depth to basement (in km) r~

Outline of the East EuropeanCraton Area covered by figure

Fig. 4. Depth to basement of the southwestern part of the East European Craton, after Zaritsky (1992).

complex east of Donetsk suggests a relation with the opening of the Atlantic (P.A. Ziegler, pers. commun.). The Permo-Triassic and Late Jurassic magmatic rocks testify to repeated increase in heat flow in the Donets Basin which is lacking in the western basins.

4.5. Structure

Based on surface observations and mine surveys, the tectonic structures of the Donets Basin region (Fig. 5) are well documented (e.g., Popov, 1963). The folds in the central part are tight, to the ex-

300

H. de Boorder et al./ Tectonophysics 268 (1996) 293-309

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370

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400

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Permo-Triassicigneous complexes

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k i l l Approximate location of kimberlite-like rocks in drill holes

~50 km

Fig. 5. (A) Tectonic structures, Nikitovka mercury-antmony mining district and Permo-Triassic igneous fields in the Donets Basin, after Popov (1963). (B) Geometrical relationships of the structures in the Donets Basin, forming a dextral Riedel array across the rift as a whole.

tent that in the Main Anticline, along the centre of the rift, a steep axial thrust (the Central Fault) is present. Here they follow the general strike of the rift. In the marginal parts of the rift, the folds are open and often oblique to the boundary faults. The Central Fault may reach into the mantle, but evidence on its depth is debated (V.A. Privalov, pers. commun., 1993). In the northern part of the rift, a network of transcurrent faults and intervening enechelon folds has been interpreted as a dextral shear belt (De Boorder et al., 1995). In the southern part of the rift, the tectonic configurations appear more complex with a series of small graben-like structures that may point to collapse of the foot wall. Trans-

verse transcurrent faults with an east-northeasterly strike complicate the structural pattern to the east where the rift turns to its overall east-southeastern alignment. We relate this pattern to dextral transpression during the Late Permian inversion. Looking for extensional areas within the broad transpressional field, we infer at least three areas controlled by the intersection of the rift's boundary and central faults with the Kharkov-Donetsk lineament (Fig. 6). These areas are of potential significance for the migration of fluids from even very deep levels, in view of the inferred depth to which these structures may reach. The lineament itself definitely extends into the mantle, with the boundary faults, and possi-

H. de Boorder et al./ Tectonophysics 268 (1996) 293-309

301

I Compression N

Dnieper-Donets Basin Kharkov-Donetsk Lineament

Fig. 6. Releasingbends at the intersectionof the Dnieper-DouetsBasin with the Kharkov-Donetsklineamentduring Permian basin inversion. bly also the Central Fault, reaching at least to the Moho. Belous and Korolev (1973) distinguish four major blocks in the crystalline subsurface of the Donets Basin (Fig. 7). These are separated from each other by transverse transcurrent faults which do not displace the boundary faults of the rift. Belous and Korolev (1973) suggest that these faults have controlled rotation of the blocks during compression, and that they are accompanied by local extensional jogs. In the absence of more detailed data, the structural analysis can not be extended much further. However, on the basis of the distribution and thickness of the Permian sediments, Belous and Korolev (1973) conclude that whilst the Voroshilovgrad block in the east shows uplift, estimated in the order of 9 km, the Kharkov, Slavyansk and Konstantinovka blocks (the latter hosting the Kalmius-Torets and Bakmuth depressions) continued to subside during the Late Permian compression.

4.6. Mineralization The Donets segment of the rift is endowed with a variety of metal deposits, amongst which mercury and antimony are of particular significance. Gold and diamond are known to occur, but no accurate records are sofar available. Cinnabar, mercury and stibnite occur together in Carboniferous quartz-sandstone in the major Nikitovka mining district in the Gorlovka area (Figs. 5 and 7). Showings of these minerals are reported to occur in many other locations in the Donets Basin and in the Dnieper and Pripyat basins to the west (Belous and Korolev, 1973; Panov, 1976; Chekunov and Naumenko, 1982; Dvornikov, 1992). Base metals occur in uneconomic deposits associated with salt domes in the northwestern part of the Donets Basin. In the Nikitovka district, the ore deposits occur in veins following faults in the hinge of the Main Anticline and in associated superimposed en-echelon

302

H. de Boorder et al./Tectonophysics 268 (1996) 293-309

Kharkov

~r~a ~

• seismic section # 10

Kiev~,~ Kharkov/ 200 km

Donetsk~-

m

Kharkov-DonetskLineament ~, B - Bakhmut depression KT- Kalmius-Toretsdepression

oio ¢~ Nikitovka

Donetsk N

Schak~

Fig. 7. Crystalline basement blocks in the Donets Basin, after Belous and Korolev (1973). In inset, location of DSS section in Fig. 2.

folds (Korcemagin, 1977). Locally, the mineralization spreads out along bedding planes with a gangue of quartz, carbonate and clay minerals, particularly dickite. Dispersion haloes of mercury are widespread in coal seams and bituminous shales near the sulphide deposits (Dvornikov, 1992). Very low grade showings and mercury anomalies in coal seams are known across the Slavyansk, Konstantinovka and Voroshilovgrad blocks. They show control by the major fault structures (Belous and Korolev, 1973; Dvornikov, 1992). The age of the mineralization is uncertain. However, the present deposits are at most of (?Late) Permian age in view of their setting in inversion-related folds and fractures. The spatial and temporal association in the Donets Basin should not preclude an equally prolific mineralization below the Mesozoic cover in the Pripyat and Dnieper basins. However, a direct association with the inversion processes would, in view of their relative insignificance in these western basins, render these basins less prone to metalliferous mineralization. A remobilization from deposits associated with the De-

vonian alkaline mafic and ultramafic magmatism has not thus far been substantiated. Diamond has been found in alluvial deposits, and kimberlite-like rocks have been reported from two drill holes in the southern shoulder of the Donets Basin (Chekunov and Naumenko, 1982). They are suggested to be associated with the alkali-ultrabasic to alkali basaltic magmatism of Devonian age (Chekunov and Naumenko, 1982). In the absence of accurate records little significance can be attached to these findings. However, the locality of the drillhole site (Fig. 5) within the zone underlain by the Kharkov-Donetsk lineament lends credibility of these reports. An important aspect of current models concerning the occurrence of diamonds is in the early formation of depressions of the lithosphere-asthenosphere boundary (LAB) (Haggerty, 1986; Boyd and Gurney, 1986; Mitchell, 1991; Helmstaedt and Gurney, 1995), their preservation at these depths, and their sampling by later kimberlitic magmas. In this respect, the extrapolation to the northeast of the de-

H. de Boorder et al./ Tectonophysics 268 (1996) 293-309

pression in the lithosphere-asthenosphere boundary along the line Kishinev-Kirovograd-Kharkov (Sollogub, 1986) focuses attention more on the northern shoulder of the PDD rift in the Kharkov area than on the southern shoulder near Donetsk (Fig. 3). Other N-S-directed tectonic discontinuities underlying the rift further west, known to have acted as magma channels, are of comparable importance, depending on their interaction with the Kishinev-KirovogradKharkov LAB depression. It is of significant interest that the high-pressure mineral moissanite (SIC) has recently been found in Devonian volcanics of the Dnieper Basin by Z.M. Lyashkevich (M. Wilson, pers. commun.).

4.7. Summary and implications relative to the significance of the Kharkov-Donetsk lineament and the special nature of the Donets Basin Comparing the Donets Basin with the Dnieper and Pripyat basins, several major differences are noted. These include: (1) the architecture of the crust, which in the Donets Basin appears extremely attenuated and modified (Figs. 1 and 2A); (2) the position and configuration of the Moho, with a complex step from 35 to 45 km from the Dnieper Basin to the Donets Basin, and with depressions rather than uplifts underneath the Donets segment (Fig. 1); (3) the excessive thickness of the sedimentary infill of the Donets Basin, relative to the thickness derived from the stratigraphy (Fig. 4); (4) the igneous activity, with one additional phase of nepheline syenite affnifies in the Namurian, and two phases of more saturated magmatism in the Permian and the Triassic, and implied concomitant increase in heat flow, in the Donets Basin (Fig. 5); (5) the notable inversion of the Donets Basin, which to the west appears to be expressed only by an unconformity at the base of the Triassic; (6) the prolific mercury and antimony mineralization in the Donets Basin. In two of these aspects, Moho configuration and depth to crystalline basement, the Kharkov-Donetsk lineament is directly involved (Figs. 1 and 4). Concerning the other aspects, the lineament constitutes a major divide between the Dnieper and the Donets

303

basins. The age of this lineament is uncertain. However, it may well be of Proterozoic origin in view of its strike and its apparent continuation into the Kursk-Belgorod structure on the Voronezh Massif. To the northwest, it converges with other northerly striking major tectonic discontinuities of Proterozoic age in the Sarmatian segment towards the Moscow Basin (Bogdanova et al., 1996). Additional independent indications to its significance are: (1) the vitrinite reflections in the Dnieper Basin, measured at 5 km depth, are highest (>2.76) to the southsoutheast of Kharkov, near Izyum (Ulmishek et al., 1994); (2) the subcrustal extension factor/~ in the Dnieper Basin is, according to model calculations by Van Wees et al. (1996), highest (--~10) in the southeastern part of the basin near and over the KharkovDonetsk lineament; this value is compatible with magma generation at depths down to 200 km (M. Wilson, pers. commun.). The Donets Basin, as part of the PDD palaeotift, is of a special nature because it is also a recognized part of the Palaeozoic Scythian Fold Belt to the south. It had a direct connection with the Urals Foredeep during the Carboniferous (Nikishin et al. (1996), through the Karpinski Swell and the PreCaspian Basin (Sobornov, 1995). A further eastward connection (Fig. 8) is known with the Mangyschlak rift and the southern Tien Shan dislocations (Karpinski, 1883; Panov, 1976). As the western limit of the Donbas and of the western branch of the Pre-Caspian Basin, the Kharkov-Donetsk lineament controlled a major barrier between these and the Devonian Dnieper and Pripyat structures. A relation between the Donets Basin and the above structures to the east appears to be at least as, if not more, important than the relations with the Dnieper and Pripyat basins. In this context it is of interest to note (Sobornov, 1995) that inversion in the Karpinski Swell, after comparable rift and post-tift phases in the Devonian and Carboniferous, respectively, took first place only in the Late Triassic-Early Jurassic, The Karpinski Swell was affected by a second phase of inversion during the Pliocene due to compression in the Caucasus orogenic belt. In the PDD system, Palaeozoic rifting proceeded from the southeast to the northwest (Chekunov et al., 1992), while the (first) inversion appears to have moved from the Donets Basin to the southeast.

H. de Boorder et al./Tectonophysics 268 (1996) 293-309

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~

Caledonian and-f ~".'~....Tscharsk ,~%...,..,~: Variscan belts

, [."~i"::~':':~:~" 80

7oi"1

~, ~"

\ "N~rchangels~,,,,

""~ :q, £ ~"~J/~

---rien Shan 70

Kharkov-Donets lineament

60

60

Caspian Sea

BlackSea

Carpathians/" 10

50

20

30

~ 40 40

mercuryprovinces 50

30

Fig. 8. Distribution of mercury provinces in the former Soviet Union within belt in which Caledonian and Variscan orogenic belts were effected by Mesozoic and Tertiary deformation (after Kusnezov, 1971). Structural trends between Urals and Siberian Craton after Matte (1995).

In connection with metalliferous mineralization, it is also of interest to note that most of the mercury deposits of western and central Asia (Fig. 8) occur

within a belt skirting the southern offshoots of the Urals and the southern peripheries of the Siberian cratons (Karpinski, 1883; Kusnezov, 1971; Panov,

H. de Boorder et aL / Tectonophysics 268 (1996) 293-309

1976). In this context, the Donets Basin could be viewed as the western offshoot of an eastern geodynamic domain rather than as just another segment of the PDD palaeorift. The Pripyat-Dnieper part of the system would then represent only an almost trivial breakthrough of this domain onto the Sarmatian segment. This might explain the apparent absence of Proterozoic precursor structures of the present rift. At the same time, the Kharkov-Donetsk lineament would stand out as a very major structure, and the more western transverse structures, like the Krivoy Rog-Krementschuk belt, would have acted as subsequent obstructing ramps during the propagation of the rift to the northwest. A discussion of this very long zone of dislocations in Asia is beyond the scope of this paper. However, further keys to the geodynamic evolution of the PDD rift are likely to be found to the east.

5. Tectono-thermal setting of mercury and antimony mineralizatiqn In Europe, west of the Trans European Suture Zone (TESZ), the deposits of mercury and antimony are found in spatial and temporal association with the collapse of orogenic regions. This association is observed in the Alpine belts and also occurs in the Variscan and Cadomian belts (De Boorder and Westerhof, 1994; De Boorder et al., 1995). There, the underlying mechanisms involve the gravitational instability and subsequent detachment of the orogenic root and/or subducted slab, the consequent emplacement of the asthenospheric mantle at higher levels, increased heat flow and fluid activity, together with often bi-modal magmatism (White et al., 1992). The question arises if the setting of the prolific mercury and antimony mineralization of the western Donets Basin differs from that of the western European deposits. The region of the Donets Basin is distinguished by a crustal architecture in which layers with seismic velocities indicating the presence of upper crust and middle crust materials, appear to be strongly reduced or completely missing (Figs. 1 and 2A). A column of 25 km with seismic characteristics of sedimentary units (Zaritsky, 1992) overlies a 20-km column of seismically defined lower crust material underlain by the Moho. Thus, the western Donets Basin is

305

anomalous compared to the Pripyat and Dnieper segments. Also, in these two segments, the Moho occurs at depths of about 35 krn which is about 10 km higher than the Moho underlying the adjacent shield areas laterally away from the rift. In the Donets Basin the Moho is located at about 45 km depth, which is at the same general level as the Moho beneath shield areas to the north and south (Fig. 1). The apparent local absence of seismically defined upper and middle crust layers can be viewed as an indication of detachment (Fig. 2B) following delamination (Fig. 2C) of a substantial part of the crust. This indication is reinforced by II'chenko (1992, pers. commun., 1993) interpretation of the NE-SW seismic profile #10 across the Donets region, in which both the upper, middle and lower crustal layers are shown to wedge out along the line of section. These wedges may indicate the existence of listric faults and shearzones. The exposed crystalline shield adjacent to the south of the Donets Basin has fully retained its Precambrian signatures. In the absence of any indication of late Palaeozoic metamorphic core complexes at the present surface, and the deep crust reflecting the recognized regional layering almost immediately away from the rift domain, the extreme attenuation is likely to have been resolved practically within the crustal segment underlying the rift itself. This implies that the apparently missing crustal volumes may have mostly subsided (Fig. 2B). The Middle to Late Devonian mafic to ultramafic magmatism of the Donets Basin is similar to the Devonian magmatism of the Pripyat-Dnieper parts of the rift. It can be related to the initiating mantle plume and associated magma ponding at the base of the crust. In this, the Donets Basin would not have been very different from the western segments. In the Donets Basin, however, the additional Permian alkaline mafic and Triassic trachy-andesitic phases of magmatism, which reflect a highly anomalous thermal state of the lower crust and upper mantle between 270 and 240 Ma, are indicative of intense interaction between mantle and crust during and following inversion. During this period, heat flow within the Basin is very likely to have increased, resulting in enhanced circulation of potentially mineralizing brines. The weakened (Permo-Carboniferous) crust of the Donets region, in itself already unstable, was

306

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then caught up in the compressional field of the Variscan orogeny, manifested by the belt underlying the present Scythian Platform to the south, to which it had already been connected during an earlier stage (Nikishin et al., 1996). The western Dnieper and Pripyat regions, from which it was separated by the Kharkov-Donetsk lineament structure as emphasized above, did not have this earlier connection. The compression of the Donets segment probably led to crustal thickening by the beginning of the Late Permian. Fragments of eclogite have been recovered from drill holes in the northern shoulder of the PDD rift, south of Kharkov (V. Kolosovskaya, pers. commun.). Eclogitic lithosphere (N.I. Pavlenkova, pers. commun., 1995), resulting from transformation of gabbroic materials at the base of the crust, could have acted as a dense sinker which pulled material of the lower lithosphere into the asthenosphere (cf. Kr6ner, 1979). Thus, detachment of dense eclogitic lithosphere from the overlying lighter crust would have allowed asthenospheric material to fill the gap between the sinking lithospheric slab and the crust, facilitating further crustal attenuation. Continued downward pull may even have contributed to compression involved in the inversion mechanism. Together, these options affirmatively answer the question if part of a (thickened?), gravitationally instable crust could have detached and sunk into the asthenospheric mantle, by analogy to the detachment of an orogenic root during orogenic collapse (Dewey, 1988; White et al., 1992). This suggestion is reinforced by the reported continued subsidence of the Kharkov and Slavyansk blocks, and probably also the Konstantinovka block (Belous and Korolev, 1973), after the onset of compression at the end of the Early Permian. Chekunov and Naumenko (1982) mention small mercury occurrences in the Dnieper graben. They also draw attention to the widespread and substantial mercury mineralization in the Donbas. In their view these are indications of high mantle activity and 'mercury breathing', following earlier ideas of Russian authors who considered mercury mineralization to stem directly from the mantle (e.g., Kusnezov, 1971; Smirnov et al., 1972; Nikolskij et al., 1973; Smirnov et al., 1976). Although we tend to arrive at a similar conclusion on the basis of structural and geodynamic indications (De Boorder and White,

1992; De Boorder and Westerhof, 1994; De Boorder et al., 1995), we consider that the processes involved in the release of mercury, particularly in large tonnages, from a mantle environment, are largely speculative. Therefore, we prefer not to consider mercury mineralization a priori to indicate high mantle activity, but to view it rather as a consequence of particular, though largely unknown, mantle processes. As far as we have been able to establish, the only published indications for a direct mantle source of mercury mineralization have been reported from Almad6n in central Spain. Here, mercury is locally found in close association with explosive alkaline basaltic volcanics (Hernfindez, 1984; Ortega Girones, 1986; Borrero Dominguez and Higueras Higueras, 1991; De Boorder and Westerhof, 1994; Higueras Higueras, 1995) derived from the mantle. The origin of the metals is unclear. This is because we do not yet understand the chemistry and physics of mercury and antimony release from the mantle in the absence of well-defined source areas. Comparing the major mineralization of the Nikitovka district of the Donets Basin with the apparently minor, unrecorded deposits reported by Chekunov and Nanmenko (1982) in the western segments of the PDD rift, it appears that Moho uplift, even to the extent of 5 to 10 km, and the physico-chemical processes involved, is in itself not enough to produce appreciable metal concentrations at high crustal levels. Apparently, substantial crustal detachment is required together with implied uplift of the asthenospheric mantle. There are no grounds to assume that the detached crustal material itself could be the source of the metals. Therefore, sourcing may have to be sought in the reactions involved in the interaction between substantial volumes of detached crustal material and the asthenospheric mantle. In addition, major pathways, like the Kharkov-Donetsk mantle lineament and its intersections with the longitudinal fault zones of the rift, may provide the primary requirements for emplacement at high crustal levels. The PDD rift, and particularly its Donets segment, is an important area for the study of the relations between tectonics, magma generation, interaction of crust, lithospheric and asthenospheric mantle and the release of metals from the mantle. Additional high-resolution deep reflection seismics is required. The analysis of satellite remote sensing data of the regions

H. de Boorder et al./ Tectonophysics 268 (1996) 293-309

307

Acknowledgements

adjacent to the rift, including the Kharkov-Donetsk zone, will help trace more accurately the principal elements of the tectonic framework on the basis of neotectonic reactivation. A thorough review of available petrochemical data, together with new analyses of all accessible magmatic products and xenoliths of deep mantle materials, is of very high priority.

Constructive reviews by Svetlana V. Bogdanova, Herb Helmstaedt, David J. Mossman and Peter Bankwitz are very much appreciated. A.J.J.v.B., A.H.D. and G.K. gratefully acknowledge financial support by the Molengraaff Fund.

6. Conclusions

References

Major Proterozoic transverse structures across the PDD rift have exerted a prominent influence on the evolution of its segments. These may have to be seen as ramps across which the rift propagated from east to west. The Kharkov-Donetsk lineament, primarily defined by Moho contours, controls the divide between the Donets and Dnieper basins. This lineament is ill understood but would represent an exceptional tectonic discontinuity. Its age is unknown. Since it fits into the pattern of the northerly striking major structures of the Sarmatian segment, however, it may be of Early Proterozoic origin. The area where drill holes are reported to have found kimberlite-like rocks to the south of Donetsk is located over the intersection of the southern rift shoulder and the K h a r k o v - D o n e t s k lineament thus lending credibility to these reports. Here, the lithosphere-asthenosphere boundary is at an estimated depth of about 150 km. The region of the northern shoulder of the Donets rift basin is underlain by the axis of a depression in the lithosphere-asthenosphere boundary >250 km deep. An intersection with the Kharkov-Donetsk lineament would give the Kharkov area a higher diamond prospectivity. Relative to the Dnieper and Pripyat basins, the Donets Basin is distinguished by three additional phases of magmatism, namely, in the Early Carboniferous, the Permian and Triassic. Permian inversion of the Donets Basin was accompanied by detachment of a substantial part of the continental crust at the same time as subsidence continued in the western part of the Basin. The detached part of the lithosphere was most probably replaced by materials from the asthenosphere. The setting of the prolific m e r c u r y antimony mineralization of the western Donets Basin is therefore not unlike the orogenic collapse setting of most mercury and antimony deposits to the west of the Trans European Suture Zone.

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