Recognition of a 1.85 Ga oceanic rifting environment in southeastern Sweden

<p>The Fröderyd Group constitutes a deformed volcanic sequence, which together with the 1834 Ma Bäckaby tonalites occurs as a xenolith, within the 1793-1769 Ma TIB 1b unit of the Transscandinavian Igneous Belt (TIB) in southern Sweden. The Bäckaby tonalites, together with coarse-grained clastic metasedimentary sequences of the Vetlanda Group, belong to the Oskarshamn-Jönköping Belt (OJB; Mansfeld et al., 1996). In turn, the Fröderyd Group was considered to be an older, probably Svecofennian, unit by Sundblad et al. (1997).</p><p>The Fröderyd Group is composed of ca. 80% mafic and ca. 20% felsic volcanic rocks, with subordinate carbonate units. Mafic rocks are represented by tholeiitic basalts and spilitized pillow lavas with MORB affinity.</p><p>In this study, a sample from a metamorphosed rhyolite, belonging to the Fröderyd Group, was dated at 1849.5±9.8 Ga U-Pb zircon age (LA-ICPMS). This age is significantly younger than the Svecofennian crust, which was formed from 1.92 to 1.88 Ga. Instead, it is coeval with the oldest TIB granitoid generation (TIB 0), which intruded into the southwestern margin of the Svecofennian Domain, but the Fröderyd Group is still the oldest crustal component southwest of the Svecofennian Domain.</p><p>Geochronological, petrographical studies and field observations have shown that the southern margin of the Svecofennian Domain was affected by ductile deformation shortly after the intrusion of the 1.85 Ga TIB granites (Stephens and Andersson, 2005). This took place during an intra- or back-arc rifting above a subduction boundary in a retreating mode and caused formation of augen gneisses and emplacement of 1847 Ga dykes into the TIB 0 granitoids. Rifting was followed by a collision of the rifted slab with the Svecofennian crust which is evidenced from emplacement of pegmatitic leucosomes during 1.83-1.82 Ga into the 1.85 Ga orthogneisses.</p><p>It is interpreted, that the Fröderyd Group was formed within an oceanic rifting environment, collided with the rifted Svecofennian slab and later amalgamated onto the Svecofennian Domain. The proposed geological evolution includes two deformation events during the period of ca. 1.85-1.82 Ga, which is in accordance with Röshoff (1975). Furthermore, it is evident that the Fröderyd Group was formed as a separate unit outside the Svecofennian Domain, although they have a common geological history.      </p><p>References</p><p>Mansfeld, J., 1996. Geological, geochemical and geochronological evidence for a new Palaeoproterozoic terrane in southeastern Sweden. Precambrian Res. 77, 91–103.</p><p>Röshoff, K., 1975. Some aspects of the Precambrian in south-eastern Sweden in the light of a detailed geological study of the Lake Nömmen area. Geologiska Föreningens i Stockholm Förhandlingar 97, 368–378.</p><p>Stephens, M.B. and Andersson, J., 2015. Migmatization related to mafic underplating and intra- or back-arc spreading above a subduction boundary in a 2.0–1.8 Ga accretionary orogen. Sweden. Precambrian Res. 264, 235–257.</p><p>Sundblad, K., Mansfeld, J. and Särkinen, M., 1997. Palaeoproterozoic rifting and formation of sulphide deposits along the southwestern margin of the Svecofennian Domain, southern Sweden. Precambrian Res. 182, 1–12.</p>

Last deglaciation in the central Balkan Peninsula: geochronological evidence from the Jablanica and Jakupica Mts (North Macedonia)

<p>Several studies applied numerical age determination methods to examine glacial phases of the central Balkan Peninsula. However, the resulting conflicting datasets require further discussion. This study provides <sup>10</sup>Be Cosmic Ray Exposure (CRE) ages of a succession of glacial landforms in the Jablanica and Jakupica Mts (North Macedonia), aiming at a better understanding of Late Pleistocene glacier development in the area.</p><p>In the Jablanica Mt. (~41.25° N; Crn Kamen, 2257 m a.s.l.) six glacial stages were identified (Temovski et al., 2018). The CRE ages of five glacial stages (from the second oldest to the youngest) range from 16.8<sup>+0.8</sup>/<sub>-0.5</sub> ka to 13.0<sup>+0.4</sup>/<sub>-0.9</sub> ka. Accordingly, the most extensive glaciation in the Jablanica Mt. occurred before ~17 ka (Ruszkiczay et al., 2020).</p><p>Based on the accumulation area balance ratios (AABR) of the reconstructed glaciers, their mean equilibrium line altitudes (ELAs) were estimated. The average ELA of the glaciers was 1792±18 m a.s.l. during the largest ice extent, and 2096±18 m a.s.l. during the last phase of the deglaciation.</p><p>Independent reconstructions of key climatic drivers of glaciological mass balance suggest that glacial re-advances during the deglaciation in the Jablanica Mt. were associated to cool summer temperatures before ~15 ka. The last glacial stillstand may result from a modest drop in summer temperature coupled with increased winter snow accumulation. In the study area no geomorphological evidence for glacier advance after ~13.0<sup>+0.4</sup>/<sub>-0.9</sub> ka could be found. Relying on independent climate proxies we propose that (i) the last glacier advance occurred no later than ~13 ka, and (ii) the glaciers were withdrawing during the Younger Dryas when low temperatures were combined with dry winters.</p><p>In the Jakupica Mt. (~41.7° N, Solunska Glava, 2540 m a.s.l.) a large plateau glacier was reconstructed. The study area comprised six eastward facing, formerly glaciated valleys. Cirque floor elevations range from ~2180 m a.s.l. at Salakova Valley, to between ~2115 and ~2210 m a.s.l. on the carbonate plateau. The lowest mapped moraines are descending down to 1550-1700 m a.s.l. Due to the large plateau ice and the complicated system of confluences, glacier reconstructions using semi-automated GIS tools are problematic. Four to six deglaciation phases were reconstructed, and a preliminary estimation of the ELAs based on the maximum elevation of the lowermost lateral moraines leads to ELA values of 1800±50 m a.s.l. for the most extended phase. Multiple CRE ages for the subsequent glacial stages are also being acquired for Jakupica Mts.</p><p>This research was supported by the NKFIH FK124807 and GINOP-2.3.2-15-2016-00009 projects, by the INSU/CNRS and the ANR through the program “EQUIPEX Investissement d’Avenir” and IRD and by the Radiate Transnational Access 19001688-ST.</p><p>Ruszkiczay-Rüdiger Zs., Kern Z, Temovski M, Madarász B, Milevski I, Braucher R, ASTER Team (2020) Last deglaciation in the central Balkan Peninsula: Geochronological evidence from Jablanica Mt (North Macedonia). Geomorphology 351: 106985</p><p>Temovski M, Madarász B, Kern Z, Milevski I, Ruszkiczay-Rüdiger Zs. (2018) Glacial geomorphology and preliminary glacier reconstruction in the Jablanica Mountain, Macedonia, Central Balkan Peninsula. Geosciences 8(7): 270</p>