The Precambrian Geology of Lithuania

The crystalline crust of Lithuania is part of the bedrock of the East European Craton (EEC). It is covered by non-metamorphosed sediments, the thickness of which varies from 200 to 2300 m. The difficulty of access of the basement requires integrated research methods—potential fields, deep seismic sounding, drilling, and various geological and analytical methods. Lithuanian bedrock has been systematically investigated since the middle of twentieth century. The main results of these researches are presented in this volume—factual data, and intellectual products summarizing characteristics of rocks of main genetic types, their origin and age. The tectonic structure of crust, its evolution and correlation of rock formations, tectonic domains, and main stages of the geological history with adjacent platform and shield areas are analyzed. Mineral resources, deposits and occurrences of iron, copper, molybdenum, REE, thorium, apatite, marble and granite discovered in Lithuania are briefly mentioned. The results of this integrated research might serve as a methodological example of the investigation of platform areas.

This chapter presents information on the types, sources, and amount of material on the geology of the crystalline basement in Lithuania. Systematic investigations of the Precambrian crystalline basement in Lithuania started in the middle of the twentieth century. Since that time, the mapping of potential fields at various scales (up to 1:10,000) was carried out, the results of which were digitized, and various derivative maps compiled. The Sovetsk-Kochtla Jarve, Baltic Sea, EUROBRIDGE, POLONAISE, and PASSEQ DSS experiments provided information on the structure of the lithosphere. A total of 550 boreholes, drilled for mapping, exploration of mineral resources, and other purposes, supplied the drill cores, which are stored in the centralized drill core storage of the Geological Survey of Lithuania (GSL) and are available for examination. The drill core material has been investigated applying petrological, geochemical, and isotopic methods. Around 400 chemical analyses by ICP-MS methods have been produced, more than 150 age estimations having been made by the U–Pb (zircon, monazite, titanite, and apatite), Ar–Ar (hornblende, biotite), Sm–Nd (whole-rock), and Re–Os (molybdenite) methods. Summarizing these data, the geological maps of the surface of the crystalline basement were compiled.

A number of variants of structural subdivision of the crust in Lithuania have been proposed, some with contradictory meanings and different arguments. This chapter presents the subdivision of the crust, based on deep structural patterns (revealed by DSS experiments and potential field mapping), lithotectonic (first of all supracrustal) assemblages of a certain age (as reflected in the geological maps), geochemical data, and isotopic age estimations. The main tectonic domains—the West Lithuanian domain, the Lithuanian-Belarus Belt, Zarasai block—are briefly characterized. This model will be followed in the later chapters, while the ground for such subdivision will be presented in Chaps. 14 and 15. In this chapter, geological maps of the whole country and its particular areas, and basic potential field maps are presented.

Supracrustal rocks of the LBB represent the oldest rocks in the crystalline basement of Lithuania, which age is almost 1.9 Ga. The constituents of the supracrustal sequence are metasedimentary gneisses, identified by relics of psammitic texture primary layering, load casts, gravel clasts, detrital zircon; felsic metavolcanic gneisses with prominent porphyritic texture, amphibolite, and basic granulite—primary basalt and diabase, and rare dolomite marble of disputed origin. The geochemical patterns of rocks, suggest the formation of the supracrustal sequence in the subduction-related continental volcanic arc environment. The band of felsic porphyries which appear along the Varėna Iron Ore Zone is possibly the relic of such an arc. The clastic material derived from local acid and intermediate rocks, weakly weathered, implying rapid sedimentation. The basic metavolcanics are of tholeiitic series of normal alkalinity, moderately enriched in LREE. Normalized by NMORB and PM, they reveal a negative Nb anomaly. Felsic metavolcanics are more rare, appearing locally. They belong to the High-K-Calc-Alkaline series, enriched in LREE; formation took place in the subduction-related continental volcanic arcs tectonic environment.

The extended area of the Bt-Cpx-Fs-Qtz-Kfs-Pl gneisses was mapped in southwest Lithuania. The prominent porphyritic texture, formed by euhedral, zonal plagioclase, and in places also ferrosillite phenocrysts, and the chemical composition corresponding to andesite-dacite indicate the volcanic origin of rocks. The sequence is designated as the Sūduva suite, and the area of its distribution is called the Sūduva volcanic field. The rocks are of normal alkalinity, but, based on the potassium content, they belong to the High-K-Calc-Alkaline series. The chondrite normalized REE abundance diagram shows a moderate content of REE and enrichment in LREE. The negative Eu anomaly is weak. A weak negative Nb anomaly on the primitive mantle normalized spider diagrams, and geochemical patterns of rocks demonstrate formation in the environment of the subduction-related continental volcanic arcs. The three estimations of age (U–Pb zircon) are very close (1.842–1.844 Ga) and indicate a short but intensive episode of volcanism. The Sūduva volcanic field is located in the area of the mid-Lithuanian suture zone between the WLD and the LBB, presumably discordantly overlying both domains, and might be the remnant of the continental volcanic arc.

The supracrustal sequence of the WLD consists of metapsammitic and semi-pelitic rocks, and subordinated felsic volcanics. The remarkable feature is the absence of basic metavolcanics. The provenance rocks of WLD supracrustals are felsic volcanic rocks and less basic volcanics. Some amount of detritus originated from the older passive continental margin. The formation of sequence took place in the environment of oceanic volcanic arcs. The period of formation, indicated by age of the youngest detrital zircons and the age of oldest intrusions, is 1.88–1.85 Ga. The WLD supracrustal sequence might be correlated with the rocks of the Västervik basin in Sweden, based on lithological association, geochemical peculiarities, and age. The essential difference is a larger amount of clastic material derived from the passive continental margin (quartzite, arkosic sandstones). Essential differences and regularities have been noticed when comparing metasedimentary supracrustals of the Lithuanian part of the LBB and the WLD. WLD sediments contain essentially more Fe, Mg, Ti, Sc, and other elements, representing mafic source rocks.

In the LBB, the oldest recognizable manifestation of magmatism is the anatectic melting of the oldest supracrustal rocks (see Chap. 4), producing neosome of granitic and charnockitic composition, which was collected in bodies of various sizes, often with transitional boundaries. The age of these bodies is around 1.89‒1.87 Ga. The later stage of intrusive magmatism is marked by the Randamonys suite, to which gabbro, diorite, granodiorite, and tonalite are attributed, composing Randamonys pluton and presumably some other intrusions in Southern Lithuania. Gabbro demonstrates an affinity to the tholeiitic series, while the more acid and felsic rocks form the calc-alkaline trend. The rocks attributed to this suite in places are sheared, and presumably represent the magmatism of the collisional period. The age of rocks varies from 1.85 to 1.79 Ga. In the Zarasai block, the rocks in all boreholes are strongly sheared. This complicates the reconstruction of their protolith and the comparison with counterparts from other domains. The age of sheared granites is 1.802‒1.788 Ga.

The Varėna suite is spatial and plausibly genetic association of ultramafic rocks, composed of olivine, orthopyroxene, clinopyroxene, magnetite, dolomite and apatite, substituted in different degrees by serpentine, amphiboles, phlogopite. These rocks form complex bodies, which occupy few square kilometres on the surface of the crystalline basement. The host rocks are amphibolite, metapsammitic gneisses, and porphyriticy metadacite of elevated sodic alkalinity and signs of metasomatic alteration. Sixteen such bodies fixed, in the area of 30x10 km, extended in the N-S direction. This area is named the Varėna iron ore zone (VIOZ) because the magnetite rocks are ore of industrial grade and quantity. In VIOZ the numerous anomalous abundances of LREE and Th are fixed both in the Varėna suite and surrounding rocks. The origin of rocks is disputed, should it be given metasomatic (skarn type formation) or intrusive origin. Bodies, which occupie the area square kilometers on the surface of the crysalline basement. Such bodies appear.

The history of magmatism in the WLD and the LBB is essentially different, both in terms of its amount, petrological association, tectonic factors, and chronology. In the WLD, as compared to the LBB, magmatism was much more intensive, intrusive bodies occupying larger areas, including the Baltic offshore. Plutons of the Kuršiai suite composed of charnockitic and granitic rocks intruded between 1.86 and 1.82 Ga in the transitional period from orogenic to cratonic conditions. The Kuršiai suite reveals an affinity to the Askersund intrusive series in Sweden, which is regarded as the earliest phase of the Transscandinavian Igneous Belt (TIB 0). The crystallization of the Kuršiai charnockitic batholith took place in PT conditions, corresponding to a depth of ~30 km, which suggests the existence of a continental-type crust, extended from the WLD up to the western borders of Sweden. Coeval intrusions of granite with biotite, garnet, cordierite are also attributed to the Kuršiai suite, the plutons of which are located mainly along the eastern border of the WLD. In the WLD, basic igneous rocks are practically absent. Small bodies of basic granulites are fixed in two boreholes and gabbro is fixed in one. The rest of the cases are various types of basic inclusions in charnockitic bodies or the melanosome of migmatites after pelitic rocks.

Cratonic composite batholiths and smaller bodies are fixed in both the WLD and the LBB. Along the northern border of the WLD, the Riga AMCG batholith is located. In the LBB, a single body of granite of the same age was found, but more such bodies probably exist. Another case of AMCG magmatism is a belt of intrusions, located along the northern border of Poland (the Mazury suite) and its extension in Lithuania, which are named the Veisiejai and Kabeliai suites. The Mazury suite embraces composite anorthosite, gabbro, charnockitic, and granitic intrusions. The geochemical patterns of Mazury and Veisiejai granite are similar and are characteristic of the A-type. The geochemistry of the Kabeliai pluton is different. These are more alkaline and contain fewer elements, characteristic of mafic rocks (Fe, Ti, Mg, V, Co, and Sc), but contain more U, Th, and REE, and reveal their positive correlation with potassium. Both suites have been formed in the same tectonic environment, but the formation of the Mazury-Veisiejai suite was caused by underplating of the basic magma and its differentiation, while the composition of Kabeliai granite was influenced by the anatectic melting of metasedimentary and metavolcanic rocks. The magma source determined some metallogenic features—abundant anomalies of REE, Th, and Mo. The age of the Mazury suite is estimated at 1.52‒1.48 Ga, while that of Kabeliai granite is estimated at 1.505 Ga.

The earliest known manifestations of the cratonic magmatism in the WLD are granitic plutons in the Baltic offshore, the age of which is 1744 and 1764 Ma, attributed to the TIB 1. In the Telšiai Shear Zone, there are fixed veins and stocks of granite and granodiorite the age of which is 1.64, 1.613, 1.46, and 0.354 Ga. The Riga AMCG batholith intruded at 1.58 Ga. Voluminous granitic magmatism took place around 1.450 Ga, forming Žemaičių Naumiestis (ŽN) pluton, which is composed of granite and quartz monzonite, the Pilsotas pluton of clinopyroxene granodiorite, and the number of smaller bodies. Comparison of the ŽN and Kuršiai rocks showed enrichment of ŽN granite in U, Th, and K. The heat flow generated by this granite might explain the formation of the West Lithuanian geothermal anomaly, the most intensive area of which coincides with the ŽN batholith. In the WLD, basalt or diabase veins of a few millimeters to a few meters thick have been fixed, in places cutting the non-metamorphosed sediments and pyroclastic deposits, of the Veiviržėnai suite, the age of which is regarded Mesoproterozoic to Neoproterozoic.

The youngest intrusive rocks in the WLD are Carboniferous granite veins (in the Telšiai Shear Zone), diabase sills (offshore), and carbonatite-alkaline intrusions (northeast Poland). As compared to the LBB, magmatism in the WLD was much more intensive and long-lasting.

Few different disjunctive systems appear in particular crustal domains. In the LBB, the most significant of these are the northeast-southwest striking faults, which run parallel to the MLSZ, separating blocks of the accretionary prism, which were presumably formed in the orogenic period. Along these faults, rocks suffered both ductile and brittle deformation, and obvious rocks are mylonites, cataclasites of various grades, and pseudotahylyte. The system of orthogonal, east-west, and north-south striking faults is younger. Along these faults, brittle deformations are observed. In the WLD, the main fault system is striking from northwest to southeast Baltic offshore, while in the coastal area, it turns to east and further to the northeast (Klaipėda sigmoid). This system is represented by a 12 km-wide Telšiai Shear Zone, which might be linked to the Loftahammarn-Linköping deformation zone in Sweden. In all boreholes drilled in the Zarasai block, rocks appeared to be strongly mylonitized. The, position of shear zones in Zarasai block is predominantly east-west or northwest-southeast.

The knowledge of the deep structure of the crust and the upper mantle is based on the results of numerous DSS experiments carried out in Lithuania and the adjacent areas. Interpreting these data and combining with gravity data, the number of seismic velocities and the density models of the crust and the uppermost mantle were created. These show the essential difference in the deep structure of the main domains. In the WLD, the crust is thinner (40‒45 km), homogeneous, with a thick upper and middle crust and a thin lower crust. A dome of presumably lower density and hotter rocks is detected in the mantle. In the LBB, the crust is thicker (50‒55 km) and is divided into blocks separated by faults. The transitional change of the crustal structure takes place along the Mid-Lithuanian Suture Zone (MLSZ), at a distance of 30‒50 km. The borders of tectonic unit mainly coincide with the shear zones, obscured in places by intrusions.

The isotopic age estimations in Lithuania are comparatively numerous and acquired by various methods. A general overview of isotopic dating reveals the chronology of the events behind the formation of the crystalline crust of Lithuania and adjacent areas. The supracrustal sequence of the crystalline crust of Lithuania consists of four stratigraphic units, all in the realms of the Orosirian period of the Paleoproterozoic era: Shchuchin formation (1.89‒1.88 Ga), Ašva formation (1.882‒1.86 Ga), Sūduva volcanic suite (1.846‒1.845 Ga), basic metavolcanics of Zarasai block (> 1.800 Ga). The history of magmatism is different in different tectonic units. The first stage of magmatism manifested in both domains occurring at 1.85‒1.79 Ga. It is the most voluminous magmatic pulse represented in the WLD by intrusions of the Kuršiai suite and in the LBB by intrusions of the Randamonys suite, with anatectic granite and felsic volcanics of the Sūduva suite overlying the suture zones of both domains. This stage of magmatism is related to the penecontemporaneous orogenic processes. In both domains, intrusions of Mesoproterozoic cratonic granites took place. In general, in the WLD, the magmatic events are much more numerous, mainly more voluminous, and continued longer (up to the Carboniferous or even the Permian). In the WLD, magmatism was mainly acid or intermediate, while in the LBB, many phases—both volcanic and plutonic were basic or even ultrabasic.

The structure and evolution of the continental crust in the Circum-Baltic area is described following the model of primary formation of microcontinents and accretion to their margins the oceanic volcanic arcs, which is regarded as orogenies. The main process of formation of the WLD was accretion to the southern margin of Bergslagen microcontinent the oceanic volcanic arcs, in the course of the movement of the Svecofennian oceanic plate to the north-northeast. This proces is regarded as Svecobaltic orogeny. The formation of the LBB was influenced mainly by movement to the northwest of the Lithuanian-Belarus microcontinent and its collision with the WLD. This collision we propose to consider as different Litbelian orogeny. Nearly coeval orogenic processes intensified in WLD magmatism, strong reworking of its crust and delamination of melting residue. These processes caused the formation of a mantle dome, a stronger heat flow, a deeper erosion of the crust, and more intensive and long-lasting magmatism. The sketch of tectonic structure of Circum-Baltic area reflects the main stages of the formation of the crust, in Lithuania.