Vol 70, No 8 (2025)

The petrography, mineralogy and geochemistry of metamorphosed Permian–Triassic to Early Triassic rocks of the Anyui gabbro-dolerite complex, composing sills in metaterrigenous rocks of the Keperveem and Malyi Anyui uplifts of western Chukotka, were studied to determine the composition of the parental melt of these rocks and to assess the mobility of elements during their metamorphism. To solve these problems, the methods of petrological and geochemical modeling of melt crystallization were applied using the COMAGMAT version 3.72 program. It was established that the rocks (hypabyssal gabbros, gabbrodiorites, and diorites) are derivatives of a single parental melt formed in a large lower crustal magma chamber. These rocks are shown to have crystallized from intraplate continental tholeiitic basaltic parental melt that had a moderately differentiated composition with Mg# 52.1, corresponding to the Cpx–Pl cotectic and exhibited signals of crustal contamination. During regional metamorphism to the greenschist facies, the contents of a number of major, minor, and trace elements in the most of the studied rocks have been changed, with the estimated relative mobility of elements increasing as follows: Eu, V < Mn < Zn, U, Co < Cu, Pb < Sr < Fe, Ba, K, Rb < Ni < Cs < Mg < Ca, Na < Li. The elements immobile during metamorphism were Si, Al, Ti, P, REE (except Eu), Y, Sc, Nb, Ta, and probably also Zr, Hf, and Th (although the contents of the latter in some rocks may reflect the presence of xenogenic accessory minerals). The COMAGMAT program was applied to model the phase crystallization sequence established based on petrographic and mineralogical data on rocks, and the parameters of the compositions of the coexisting minerals during the fractionation stages of the parental melt before magnetite started to crystallize. The application of the petrological–geochemical modeling method in combination with data on the geochemistry and mineralogy of the gabbroids thus allows one to evaluate not only the compositions of the magmas and melts and their changes during fractionation but also an input/output of elements during metamorphism and the degree of their mobility.

The unique association of rare-earth minerals of the epidote group, as well as the evolution of lantanides and actinides, titanium and vanadium in their composition are described. Allanite-(Ce), often with a ferriallanite-(Ce) core, an outer zone of allanite-(Y) and a rim of REE epidote, forms pseudomorphs after chevkinite-(Ce) and perrierite-(Ce), as well as isolated crystals. They form intergrowths with biotite, developing near it in quartz gabbronorite-dolerites and gabbronorite-diorites of the island-arc mafic hypabyssal Pervomaysk-Ayudag complex within Mountain Crimea. Zonal allanite-(Ce) of similar composition, accompanied by REE epidote rims, is widespread in quartz diorites and plagiogranites of the same complex, where it is often developed in granophyric quartz-oligoclase intergrowths. Brown ferriallanite-(Ce) enriched in Ti (± Th) forms cores in brown allanite-(Ce) crystals enriched in Ti (± V) (up to 4.9 wt. % TiO₂). Allanite-(Ce) enriched in titanium (up to 3.5 wt. % TiO₂), which replaced ilmenite, is extremely rich in vanadium (up to 4.5 wt. % V₂O₃). Light colored low-titanium allanite-(Ce) has grown on titanium enriched allanite-(Ce). The distribution of lanthanides and yttrium in allanite-(Ce) and ferriallanite-(Ce) is: Ce > La > > Nd > Y > Pr > Sm ~ Dy ~ Gd > Er ~ Tb. The outer zones of allanite-Ce) crystals and rare-earth epidote are relatively enriched in Nd, whereas Nd > La in some instances. The ratio of yttrium and lanthanides in allanite-(Y) is specific: Y >> Ce ~ Nd ~ Dy ~ Er > La ~ Gd ~ Yb. Allanite-(Ce) of Crimean gabbroids is noticeably richer in La, Ti, V and poorer in Y, Sm, Gd in comparison with allanite-(Ce) of Crimean plagiogranitoids. According to (Fleischer, 1985), the distribution of lanthanides and yttrium in allanite-(Ce) of Crimean plagiogranitoids is close to the similar distribution in standard granites, differing in an increased proportion of Gd. The coloration causes and matter sources for magmatic Crimean allanite formation are considered. Allanite was partially replaced by monazite-(Ce) during the processes of regional metamorphism under the conditions of the prehnite-pumpellyite facies.

Distribution of dissolved molybdenum, tungsten, and vanadium was investigated in the northeastern part of the Black Sea down to a depth of 320 m. The depth of hydrogen sulfide appearance (the onset of the anaerobic zone) was about 165 m (at a potential density ~16.2 kg m^–3) in the studied region. Water samples representing dissolved (< 0.45 μm) species and dissolved plus labile particulate species of the elements were collected in July 2016 and 2017. The concentration of dissolved Mo increased with depth in the oxic zone, from 36 to 39 nmol/kg, and showed no difference from the sum of dissolved and particulate forms. In the anoxic, molybdenum decreased with the appearance of more than ~8 μM hydrogen sulfide reaching 3.3 nmol/kg at 320 m. The concentration of tungsten decreased from 160 pmol/kg at the surface to 113 pmol/kg at the redox interface (in the suboxic layer at depth 150m) in the presence of particulate manganese. As Mn oxyhydroxides dissolved in the hydrogen sulfide zone, W concentrations increased to 221 pmol/kg at the depth 180m, along with an increase in dissolved Mn. The distribution of W at the redox interface is controlled by the sorption properties of manganese oxide. Dissolved vanadium was depleted at a depth of 5 m and increased with depth in the oxic zone to 13 nmol/kg, with a decrease in the suboxic zone (down to 7.1 nmol/kg). In the anoxic zone, a maximum V concentration (up to 15.2 nmol/kg) was observed, coinciding with the maximum of dissolved Mn. The calculated balance of Mo and V in the Black Sea showed that about 1200 of Mo and 1200 of V are buried in sediments annually. As for tungsten, it is assumed its significant supply to the Black Sea in the form of suspended and colloidal matter from rivers, transformed in the process of suboxic diagenesis in sediments.