PRECAMBRIAN DOMAINS IN LITHUANIA: EVIDENCE OF TERRANE TECTONICS

Abstract

The West Lithuanian Granulite (WLG) and East Lithuanian domains (ELD) form the Proterozoic basement of Lithuania and can be distinguished on the basis of differing structural patterns, lithologies, and evolutionary histories. They are juxtaposed along the Mid-Lithuanian Suture Zone (MLSZ).In the WLG, the main lithotectonic complexes comprise felsic and intermediate, mostly metasedimentary granulites in the south-west and mafic metaigneous granulites in the north-east. The former are interpreted as marine metapelites, while most of the mafic ones have been derived from island-arc tholeiites. These rock complexes trend NW–SE and are marked by contrasting gravity and magnetic anomalies. NE- and E–W-striking faults and shear zones complicate the potential-field patterns. Sets of NW-trending anomalies also extend from Lithuania across the Baltic Sea to south-central Sweden and indicate that the WLG complexes continue into the Baltic/Fennoscandian Shield. Voluminous anatectic granites alternate with the metapelites, whereas the mafic granulites occur together with enderbites and charnockites.In the ELD, the main structures produce strong, NNE–SSW-oriented gravity and magnetic anomalies which trend parallel to the Belarus-Baltic Granulite Belt (BBG) and other terranes situated still farther east. The ELD is composed of metasedimentary rocks interpreted as one-time graywackes, shales and dolomites accumulated in continental-margin arc and shallow-water basinal environments. Amphibolites and gabbros with MORB and IAT characteristics, and voluminous granitoids are also present. The coexistence of juvenile mafic rocks with continental-margin and shelf sediments suggests an oceanic back-arc setting.The two Lithuanian basement domains display contrasting metamorphic histories that suggest separate developments before the eventual amalgamation. In the WLG, the metapelites indicate peak metamorphism at high temperatures (up to 850–900°C) and moderate pressures (8–10kbar). This was followed by cooling and reheating, and then an uplift event. Repeated magmatic underplating accompanied the metamorphism. In the ELD, in contrast, the rocks have been subjected to comprehensive metamorphism under moderate, amphibolite-facies conditions. That metamorphism, however, was not uniform throughout. The metasediments in the east have recorded pressures similar to those in the neighbouring BBG (7–8kbar) but lower temperatures (650–680°C), while in the central and western parts of the ELD, metamorphism occurred at ca. 480–580°C with pressures increasing from 3–4kbar in the centre, to 6kbar close to the western boundary. Reheating to 700°C due to a ca. 1.5-Ga magmatic event is characteristic.The MLSZ, which separates the two Lithuanian basement domains from each other, is a N–S-oriented, ca. 30–50km wide, westward-plunging crustal discontinuity marked by magnetic and gravity highs, mafic and felsic intrusions, and sheared rocks. Crustal thicknesses change from 42–44km in the west to 50km in the eastern side of the Zone, which also truncates a crustal low-velocity layer characteristic of the WLG. The amalgamation of the WLG and ELD along the MLSZ occurred at ca. 1.71–1.66Ga, after which time both domains were affected by the same post-kinematic, anorogenic magmatism ca. 1.58–1.45Ga ago. That event and related shearing were responsible for some ultimate refragmentation of the Lithuanian basement terranes.