Gediminas Motuza

The Zemaiciu Naumiestis granitoids: new evidences for Mesoproterozoic magmatism in western Lithuania

Abstract We report new occurrences of ca. 1460 Ma magmatism in western Lithuania, where large volumes of granitoid melts were intruded into host supracrustal and charnockitoid rocks, and formed the Žemaičių Naumiestis pluton. Numerous granitoids of that age have previously been described from southern Sweden and from the Danish island of Bornholm, but none have been reported from the eastern side of the Baltic Sea, where the crystalline basement is covered by Phanerozoic sediments. Petrological studies demonstrate that the Žemaičių Naumiestis intrusion consists of quartz monzodiorites, monzogranites and syenogranites. The dominant ferromagnesian mineral is biotite. Rare clinopyroxene grains occur in the quartz monzodiorites. The rocks are fine to coarse-grained, often porphyritic. Chemically, the studied granitoids are dominantly alkali-calcic and shoshonitic, metaluminous to peraluminous, and ferroan to magnesian. The geochemical data indicate that the pluton consists of two rock suites, one representing monzodiorites and monzogranites, and other mostly syenogranites. The suites originated from slightly different sources. The rocks within the Žemaičių Naumiestis pluton are variably deformed and locally cataclased. Some are, however, rather massive. Two samples of monzogranite yielded ID-TIMS U-Pb zircon ages of 1462±8 (MSWD = 1.09) and 1459±3 Ma (MSWD = 0.28).

Study of Local Seismic Events in Lithuania and Adjacent Areas Using Data from the PASSEQ Experiment

The territory of Lithuania and adjacent areas of the East European Craton have always been considered a region of low seismicity. Two recent earthquakes with magnitudes of more than 5 in the Kaliningrad District (Russian Federation) on 21 September 2004 motivated re-evaluation of the seismic hazard in Lithuania and adjacent territories. A new opportunity to study seismicity in the region is provided by the PASSEQ (Pasive Seismic Experiment) project that aimed to study the lithosphere–asthenosphere structure around the Trans-European Suture Zone. Twenty-six seismic stations of the PASSEQ temporary seismic array were installed in the territory of Lithuania. The stations recorded a number of local and regional seismic events originating from Lithuania and adjacent areas. This data can be used to answer the question of whether there exist seismically active tectonic zones in Lithuania that could be potentially hazardous for critical industrial facilities. Therefore, the aim of this paper is to find any natural tectonic seismic events in Lithuania and to obtain more general view of seismicity in the region. In order to do this, we make a manual review of the continuous data recorded by the PASSEQ seismic stations in Lithuania. From the good quality data, we select and relocate 45 local seismic events using the well-known LocSAT and VELEST location algortithms. In order to discriminate between possible natural events, underwater explosions and on-shore blasts, we analyse spatial distribution of epicenters and temporal distribution of origin times and perform both visual analysis of waveforms and spectral analysis of recordings. We show that the relocated seismic events can be grouped into five clusters (groups) according to their epicenter coordinates and origin and that several seismic events might be of tectonic origin. We also show that several events from the off-shore region in the Baltic Sea (at the coasts of the Kaliningrad District of the Russian Federation) are non-volcanic tremors, although the origin of these tremor-type events is not clear.

Protomylonite evolution potentially revealed by the 3D depiction and fractal analysis of chemical data from a feldspar

Abstract An alkali feldspar megacryst from a protomylonite has been studied using laser ablation-ICP-mass spectrometry combined with cathodoluminescence imaging, Raman spectroscopy, and electron probe microanalysis. The aim was to determine the original (magmatic) geochemical pattern of the crystal and the changes introduced by protomylonitization. Digital concentration-distribution models, derivative gradient models, and fractal statistics, e.g., Hurst-exponent values are used in a novel way to reveal subtle changes in the trace-element composition of the feldspar. Formation of the crystal is reflected in a slightly chaotic trace-element (Ba, Sr, and Rb) distribution pattern that is more or less characterized by continuous development from a fairly homogeneous environment. Derivative gradient models demonstrate a microdomain pattern. Fractal statistics show that element behavior was changeable, with Ba and Sr always more persistent (continuing) and Rb always less persistent, with the latter showing a tendency to migrate. The variations in the Hurst exponent are, however, too large to be explained by magmatic differentiation alone. The observed element behavior may be explained by structural changes revealed by Raman spectroscopy and CL. In high-strain domains, T–O–T modes become stronger for Si–O–Al than Al–O–Al linkages. Increasing amounts of Al–O−–Al defects are demonstrated by cathodoluminescence. Both may result from small-distance diffusion creep, making the crystal geochemical pattern slightly patchy. In turn, the marginal part of the megacryst has a mosaic of randomly orientated, newly crystallized K-feldspars. The re-growth is confirmed by trace-element distribution patterns and fractal statistics which identify an abrupt change in the transformation environment. The novel set of tools used in this study reveals a complicated history of megacryst formation and transformation that otherwise would be difficult to unravel and decipher.