The westernmost Late Miocene–Pliocene volcanic activity in the Vardar zone (North Macedonia)

Late Miocene to Pleistocene volcanism within the Vardar zone (North Macedonia) covers a large area, has a wide range in composition, and is largely connected to the tectonic evolution of the South Balkan extensional system, the northern part of the Aegean extensional regime. The onset of the scattered potassic to ultrapotassic volcanism south from the Scutari-Peć transverse zone occurred at ca. 8.0 Ma based on this study. Here, we focused on three volcanic centers located on deep structures or thrust faults along the western part of the Vardar zone, for which there is none to very little geochronological and geochemical data available. Pakoševo and Debrište localities are represented as small remnants of lava flows cropping out at the southern edge of Skopje basin and at the western edge of Tikveš basin, respectively. Šumovit Greben center is considered as part of the Kožuf-Voras volcanic system, and it is located on its westernmost side, at the southern edge of Mariovo basin, which is largely composed of volcaniclastic sediments. We present new eruption ages applying the unspiked Cassignol-Gillot K–Ar technique on groundmass, as well as petrological and geochemical data, supplemented with Sr and Nd isotopes to complement and better understand the Neogene-Pleistocene volcanism in the region. Eruption ages on these rocks interlayered between sedimentary formations allow to better constrain the evolution of those sedimentary basins. Rocks from the three volcanic centers belong to the high-K calc-alkaline–shoshonitic series based on their elevated K content. The oldest center amongst these three localities, as well as other Late Miocene centers within the region, is the trachyandesitic Debrište, which formed at ca. 8.0 Ma, and exhibits the highest Nd and lowest Sr isotopic ratios (0.512441–0.512535 and 0.706759–0.706753, respectively). The basaltic trachyandesite Pakoševo center formed at ca. 3.8 Ma and its Nd and Sr isotopic ratios (0.512260 and 0.709593, respectively) bear the strongest signature of crustal contamination. The rhyolitic Šumovit Greben center is a composite volcanic structure formed at ca. 3.0–2.7 Ma. Its youngest eruption unit has a slightly higher Nd and lower Sr isotopic ratios (0.512382 and 0.709208, respectively) representing a magma with a lesser extent of crustal assimilation than the other samples from this center. The overall trend through time in the Sr and Nd isotopic ratios of the Late Miocene to Pleistocene mafic volcanic centers in the region implies an increasing rate of metasomatism of the lithospheric mantle.

Preliminary geochemical characterization of gas manifestations in North Macedonia

<p>Like most of the Balkan Peninsula, North Macedonia is a geodynamically active area. As such it has many hydrothermal features and gas manifestations. Until now, no systematic study about the geochemical characterization of the geogenic gases was made before in this country. In August 2019, 24 gas samples were collected in the study area. All, except one collected at Duvalo (soil gas), are gases bubbling or dissolved in thermomineral waters (temperatures from 12 to 66 °C). They were analysed in the laboratory for their chemical (He, Ne, Ar, O<sub>2</sub> , N<sub>2</sub> , H<sub>2</sub> , H<sub>2</sub>S, CH<sub>4</sub> and CO<sub>2</sub>) and isotopic composition (δ<sup>13</sup>C-CO<sub>2</sub>, δ<sup>13</sup>C-CH<sub>4</sub>, δ<sup>2</sup>H-CH<sub>4</sub> and R/R<sub>A</sub>). Most of the gases have CO<sub>2</sub> as the main component (400-998,000 ppm) while the remaining are enriched in N<sub>2</sub> (1300-950,000 ppm). Helium ranges from 0.3 to 2560 ppm while CH<sub>4</sub> from 1.6 to 20,200 ppm. R/R<sub>A</sub> and <sup>4</sup>He/<sup>20</sup>Ne ratios indicate a generally low atmospheric contamination, a prevailing crustal contribution and mantle contributions between 1 and 20% considering a MORB endmember. The highest mantle contributions are found in the SE part of the country very close to the sites that show the highest R/R<sub>A</sub> values in continental Greece [1]. This area is characterised by extensional tectonics and Plio-Pleistocene volcanism. A quite high mantle contribution (about 15%) is also found in two manifestations in the NW part of the country along a main normal fault system. With the exception of the sample of Smokvica, which has very low CO<sub>2</sub> (1400 ppm) and δ<sup>13</sup>C-CO<sub>2</sub> (-15.7 ‰ V-PDB), all free gases show a relatively narrow range in δ<sup>13</sup>C-CO<sub>2</sub> values (-4.6 to +1.0 ‰ V-PDB) indicating the mixing between a mantle and a carbonate rock source. The isotope composition allows us to assign the CH<sub>4</sub> origin to three sources. The largest group can be attributed to a hydrothermal origin (δ<sup>13</sup>C-CH<sub>4</sub> around -20 ‰ V-PDB and δ<sup>2</sup>H-CH<sub>4</sub> around -100‰). Three samples collected in the SW part of the country have a thermogenic origin (δ<sup>13</sup>C-CH<sub>4</sub> around -35 ‰ V-PDB and δ<sup>2</sup>H-CH<sub>4</sub> around -160‰ V-SMOW). Finally, one sample (Smokvica) with the highest values (δ<sup>13</sup>C-CH<sub>4</sub> -7.2 ‰ V-PDB and δ<sup>2</sup>H-CH<sub>4</sub> -80‰ V-SMOW) may be attributed to abiotic processes in a continental serpentinization environment or to methane oxidation.</p><p>This research was funded by the DCO Grant n. 10881-TDB “Improving the estimation of tectonic carbon flux”, GINOP-2.3.2-15-2016-00009 ‘ICER’ project and PO FSE Sicilia 2014 – 2020 (CUP: G77B17000200009).</p><p>References:</p><p>[1] Daskalopoulou et al., 2018 – Chemical Geology, 479, 286-301</p>