Precambrian domains in Lithuania: evidence of terrane tectonics

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 intermedia te, mostly metasedimentary granulites in the south-west and mafic metaigneo us granulites in the north-east. The former are interpreted as marine metap elites. while most of the mafic ones have been derived from island-are thol eiites. These rock complexes trend NW-SE and are marked by contrasting grav ity and magnetic anomalies. NE- and E-W-striking faults and shear zones com plicate the potential-field patterns. Sets of NW-trending anomalies also ex tend from Lithuania across the Baltic Sea to south-central Sweden and indic ate that the WLG complexes continue into the Baltic/Fermoscandian Shield. V oluminous anatectic granites alternate with the metapelites, whereas the ma fic granulites occur together with enderbites and charnockites. In the ELD, the main structures produce strong, NNE-SSW-oriented gravity an d magnetic anomalies which trend parallel to the Belarus-Baltic Granulite B elt (BBG) and other terranes situated still farther east. The ELD is compos ed of metasedimentary rocks interpreted as one-time graywackes, shales and dolomites accumulated in continental-margin are and shallow-water basinal e nvironments. Amphibolites and gabbros with MORB and IAT characteristics, an d voluminous granitoids are also present. The coexistence of juvenile malic rocks with continental-margin and shelf sediments suggests an oceanic back -arc setting. The two Lithuanian basement domains display contrasting metamorphic histori es that suggest separate developments before the eventual amalgamation. In the WLG, the metapelites indicate peak metamorphism at high temperatures (u p to 850-900 degreesC) and moderate pressures (8- 10 kbar). This was follow ed by cooling and reheating, and then an uplift event. Repeated magmatic un derplating accompanied the metamorphism. In the ELD, in contrast, the rocks have been subjected to comprehensive metamorphism under moderate, amphibol ite-facies conditions. That metamorphism, however, was not uniform througho ut. The metasediments in the cast have recorded pressures similar to those in the neighbouring BBG (7-8 kbar) but lower temperatures (650-680 degreesC ), while in the central and western parts of the ELD, metamorphism occurred at ca. 480-580 degreesC with pressures increasing from 3-4 kbar in the cen tre, to 6 kbar close to the western boundary. Reheating to 700 degreesC due to a ca. 1.5-Ga magmatic event is characteristic. The MLSZ, which separates the two Lithuanian basement domains from each oth er, is a N-S-oriented. ca. 30-50 km wide, westward-plunging crustal discont inuity marked by magnetic and gravity highs, mafic and felsic intrusions, a nd sheared rocks. Crustal thicknesses change from 42-44 km in the west to 5 0 km in the eastern side of the Zone, which also truncates a crustal low-ve locity layer characteristic of the WLG. The amalgamation of the WLG and ELD along the MLSZ occurred at ca. 1.71-1.66 Ga, after which time both domains were affected by the same post-kinematic, anorogenic magmatism ca. 1.58-1. 45 Ga ago. That event and related shearing were responsible for some ultima te refragmentation of the Lithuanian basement terranes. (C) 2001 Elsevier S cience B.V. All rights reserved.