Tonalite-trondjemite-granodiorite formation of the Archaean. Special features of composition and conditions of formation, Ukrainian Shield as an example

Tonalite-trondjemite-granodiorite formation (TTG) produces the main volume of acidic rocks of the continental crust. Similar rocks are never met later. Therefore the problems of their production are directly connected with the problem of the crust and mantle formation. The structure of the Archean TTG formation of granite-gneiss area of the Bug megablock and granite-grrenstone area of the Middle Dnieper megablock (MDMB) has been considered. Similar and different features have been found. The analysis of these data resulted in a conclusion that within the MDMB, West Periazovian and Khashchevate-Zavalie block of the Middle Bug area the events of formation of the Archean granite-greenstone area were similar, however these three blocks of the Ukrainian Shield demonstrate different levels of erosion damage reflected in PT-conditions of metamorphic transformations. The rocks of TTG formation are a part of complex structured stratum appeared as a result of impregnation (migmatization) by quartz-albite melt of the primary crust and/or of more ancient strata of predominantly basic composition. In the middle-lower crust a partial replacement of the primary crust occurred and in the upper one — the deposition of new portions of the melt on the earlier ones, piercement of granite masses and migmatization of volcanogenic stratum. During the Archean these events happened repeatedly, that resulted in partial replacement of the primary crust with plagiogranites. Modern notions have been considered on the processes of producing of TTG granite formation. It has been shown that according to thermal model distribution of temperatures in the crust does not cross the line of basalt water solidus. That is why the appearance of granite melts could not be the result of submergence to big depths (ultrametamorphism). Chronological and genetic relation with mantle melting, of which komatiites and spilites of green-stone structures were crystallized, assumed convective flows in the mantle. To explain the formation of tonalite and trondjemite melt a model of two-leveled crystallization differentiation of ultrabasic melt has been used. However appearance of primary basalt replacement in such a scale and assimilation of green-stone roots by granite melt are possible only in case of interaction of mantle fluids with the rocks of primary crust. An assumption has been made that the composition of some part of these fluids could be close to composition of granite (trondjemite). According to the author’s opinion such assumption confirms a hypothesis of V. Griffin and N. Pirson about formation of crystalline mantle on the border between the Archean and Proterozoic.

Lamprophyres and mineralization of the Koytash ore field (Southern Tien Shan)

Research subject. The Koytash ore field is located in the potentially productive Au, Ag, W, Mo, Ti, Fe, Cu, Pb, Zn and REE North Nuratau mineralisation zone of Tien Shan. The authors undertook a study of the composition of dikes breaking through the Paleozoic ore-bearing formations, as well as their petro- and ore-generating role in the formation of the Koytash-Ugat sulphide-rare-metal (W, Mo, Fe) specialised mineralisation. Materials and methods. A study of rock and mineral composition was performed at the Institute of Geology and Geophysics named after Kh.M. Abdullaev. The content of petrogenic and rare elements in rocks and sulphides was determined by ICP-MS using an ICPE-9000 mass-spec trometer in the Central Laboratory of the State Committee for Geology of the Republic of Uzbekistan. The chemical analysis of minerals was performed using a Jeol-8800Rh electronic microanalyser at the Institute of Geology and Geophysics named after Kh.M. Abdullaev. The micrographs of transparent sections were obtained using Nikon Optiphot 2 Pol and Polam R-311 microscopes. Results and conclusions. The conducted study showed that, in terms of their structure, the Koytash ore field dike formations can be regarded as lamprophyres. In terms of their chemical composition, these formations are mafic and intermediate rocks of the subalkaline series. It was found that the composition of lamprophyre dikes correlates with the size of the erosion section. Their melanocratic varieties are confined to the southern part of the intrusion (absolute elevations are 1000–1200 m), and leucocratic – to the northern (about 1900 m). This is assumed to be the result of crystallization differentiation of a single initial melt. The dikes of the Koytash ore field lamprophyres break through not only sulphide-rare-metal bodies of the Koytash-Ugat strip, but also skarn and carbonate rocks and, in turn, are broken through by quartz-polymetallic ore-bearing veins, which testifies to their inter-ore character.

Early Permian granitoids of the East Magnitogorsk zone (South- ern Urals): petrology, geochemistry, and geodynamic formation environment

The relevance of the problem. The Early Permian magmatism of the Southern Urals is poorly studied with the help of modern methods. The granitoid massifs of this age locally developed in the East Magnitogorsk zone contain important information about the geodynamic conditions of their formation. Clarification of this issue makes an important contribution to the understanding of the geodynamic development of the Urals. The nature of granitoids is still debatable. The connection with the massifs combined in the Balkan complex of gold-tungsten mineralization indicates the need for a comprehensive study. The purpose of the study is to determine the petrological and geochemical features of the rocks of the Balkan complex, to identify the mechanism of their petrogenesis and to establish the geodynamic conditions of their formation. Results. The petrological and geochemical study of the formations of the Balkan complex was carried out and their place in the typical taxonomy of granitoids was determined. Their belonging to the I-type is shown. Mineralogical and petrogeochemical methods were first studied for shonkinite xenoliths in granitoids. The mechanism of petrogenesis of rocks is proposed and the geodynamic setting of their formation is determined. It is shown that the monzonitemonzodiorite-quartz syenite-granosyenite-leucogranite series of rocks was formed as a result of crystallization differentiation of a single parental melting, and it was also concluded that the massifs of the complex are formed under conditions of early collision conditions with the important role of the subduction process. The mechanism of formation of the massifs of the complex is largely similar to mechanism for granitoids in other conflict areas, although it has its own specifics. Conclusions. 1). The Early Permian granitoids of the Balkan complex relates to type I. 2). All rocks of the complex, from monzonites to quartz syenites and leucogranites, including xenolith shonkinites, form a petrogenetic series formed as a result of crystallization differentiation of a single parent alkaline-gabbroic melting with increased water pressure. 3). The Balkan complex was formed in an early collisional setting under the action of deep subduction. 4). Transpression in the upper part of the crust induced formation of the massifs of the complex. 5). The Balkan complex is a kind of indicator of the growth of the newly formed crust as a result of collision and accretion processes.

Syenite formation after TTG gneiss: evidence from the Madiapala massif (Limpopo complex, South Africa) and experiment

<p>The Madiapala syenite massif is situated within the host Alldays TTG gneisses in the western part of the Central Zone(CZ) of the Limpopo Complex (South Africa). The age of the massif 2010.3±4.5 Ma corresponds to the period of Paleoproterozoic tectono-thermal event(D3/M3) in the CZ, which was characterized by fluid activity along regional and local shear-zones.</p><p>The model for the syenite rocks formation within the TTG gneisses was suggested in [1] on the basis of experiments on the interaction of a biotite-amphibole tonalite gneiss with H<sub>2</sub>O-CO<sub>2</sub>-(K,Na)Cl fluids at 750 and 800<sup>o</sup>C and 5.5 kbar. These experiments demonstrated that the leading factor for formation of the syenite assemblages in a tonalite gneiss is an increase of potassium activity in a fluid. Thus, the Madiapala syenites could be a product of the syenitization of the TTG gneisses. ICP-MS and ICP-AES for the syenite rocks, syenitized gneisses and host TTG gneisses reveal two varieties of syenite rocks in the massif (syenites and syeno-diorites), confirm the crustal source of the syenites and their close genetic relationship with the Alldays tonalite gneisses. The REE pattern for the syenite rocks indicate active crystallization differentiation within the syenite massif.</p><p>The earliest assemblage of the syenite rocks is K-feldspar + clinopyroxene + titanite ± apatite. The latter assemblage is albite+amphibole. In order to estimate the conditions for formation of the earliest assemblage, we constructed the P-T pseudosections for syenite assemblage and isopleths of Na and #Mg in clinopyroxene coexisting with K-feldspar and titanite using the PERPLE_X software. It showed that the earliest assemblage was formed in the temperature range 800-850<sup>o</sup>C and pressures between 6 and 9 kbar. The lg(a<sub>H2O</sub>) – lg(a<sub>K2O</sub>) pseudosections for the Alldays gneiss composition showed that the formation of the syenite assemblage proceeds via the increase of the K<sub>2</sub>O activity at constant P and T.</p><p>In order to reproduce the syenite mineral assemblage, experiments on the interaction of a biotite tonalite Alldays gneiss with a H<sub>2</sub>O-CO<sub>2</sub>-(K,Na)Cl fluid with variable salt concentrations were performed at 850<sup>o</sup>C and 6 kbar for 10 days using an internally heated gas pressure vessel. The starting materials were cylinder fragments of the Alldays gneiss and a mixture of oxalic acid with KCl and NaCl as a fluid.</p><p>Run products of experiments with KCl contain the assemblage of clinopyroxene + K-feldspar + titanite formed by reactions of Ti-bearing biotite with quartz and plagioclase, initiated by the alkali-bearing aqueous-carbonic fluid. At the run temperature, the assemblage coexists with a syenitic melt enriched in F, Cl and H<sub>2</sub>O, which was confirmed by Raman spectroscopy of studies of quenched glasses. Amphibole was formed only in the experiments with NaCl. Thus, the formation of amphiboles can be attributed to a later stage of the massif evolution, which was characterized by an increase in chemical potential of sodium. This result is consistent with the suggested model for the formation of the Madiapala syenite rocks.</p><p>This study is supported by RSCF project No18-17-00206</p><p>Literature<br>1. Safonov O. G., Aranovich L. Y. Alkali control of high-grade metamorphism and granitization//Geoscience Frontiers. 2014. Vol.5. pp.711-727.</p>

Calculation of changes in the Ta/Nb ratio in differentiates of granite melt based on experimental data

<p>A change in the Ta/Nb ratio in acid igneous rocks is related to crystallization differentiation processes. The genesis of rock-forming and accessory minerals, the formation of an aqueous fluid at the magmatic stage, or the separation of another liquid phase from a silicate melt through liquation can lead to a change in the Ta/Nb ratio and an increase in the contents of Ta and Nb in the residual melt. A calculation of the possible change in the Ta/Nb indicator ratio in the residual deeply differentiated granite melt is performed.</p><p>We used experimental data from various literature sources (T = 650–800 ºC, P = 1–2 kbar) on the solubility of columbite and tantalite in a silicate melt and on the distribution of Ta and Nb among a coexisting silicate melt, aqueous liquid, and aluminum fluoride melt. The Clarke values of these metals in acid rocks of the Earth’s crust were taken as the initial contents of Ta and Nb in the melt. The calculations were made using the mass balance method. It is shown that the separation of fluid in a closed magmatic system rock-forming minerals–silicate melt–water can lead to an approximately twice increase in Ta/Nb in the residual melt as compared to the initial Clarke value. In the system rock-forming minerals–silicate melt–alumino fluoride melt with the initial content of fluorine close to that in biotite granites, the Ta/Nb ratio in the residual melt can increase to ~1. Successive crystallization of minerals of the isomorphic columbite–tantalite series can lead to Ta/Nb > 2 in the residual melt. Crystallization of biotite causes a significant increase in Ta/Nb but significantly prevents the accumulation of these metals in the residual silicate melt.</p>

Forces generating crystallization differentiation, and the evolution of the melt composition on the example of plagioclase

Crystallization differentiation processes in the melt volume are investigated for albite-anorthite continuous solid solution series. It has shown that crystallization differentiation occurs in the isothermal melt volume due to hydrodynamic instability of the melt/solid particles system. The time of particle settling in a 10 cm thick melt layer is estimated for different particle sizes. In terrestrial conditions, the existence of large melt volumes with long lifetime is possible in the case of a long-lived heat source of high thermal power. This source is a mantle thermochemical plume with a mushroom-shaped head. The particle settling time is estimated for the melt layer thickness, i. e. plume head thickness equal to 10 km. A calculation technique is presented for composition of the melt remaining after settling of plagioclase particles. The results of calculations of changes in the melt composition due to crystallization differentiation at a temperature T = 1410 °C and a pressure P = 6,3 kbar are presented. For a melt whose composition corresponds to N 47,5 (weight percentage of anorthite is 47,5 %), the oxide content in the settled plagioclase, the composition of the melt in its intercrystalline spaces, and the residual melt composition are calculated. At constant temperature, the crystallization differentiation of the melt whose composition corresponds to plagioclase leads to the compositional changes in the initial melt. Calculations of the melt composition have shown that the melt is depleted in anorthite component owing to settling of plagioclase particles. The composition of plagioclase therewith shifts to the liquidus line, reaching its limit on this line

. Influence of crystallization differentiation on the composition of the residual melt for plagioclase under different p-t conditions

Crystallization differentiation processes in the melt volume for albite-anorthite solid solution series have been studied. For the albite-anorthite system, the change in the melt composition due to crystallization differentiation is calculated for pressure values P = 6,3 kbar and 1 bar and temperature T = 1410 °C, 1350 and 1300 °C. A calculation technique is presented for composition of the melt remaining after settling of plagioclase particles. The residual melt compositions have been calculated for different initial melt compositions and different P-T parameters. The change in composition due to crystallization differentiation of the melt is the difference in the percentage composition for each oxide on the liquidus line and the initial melt composition. The dimensionless ratios (similarity criteria) for the initial melt composition and the change of the oxide content , , , have been obtained. The change of each oxide percentage is calculated in weight percents and in the dimensionless form (as values of above-mentioned similarity criteria). The initial melt is depleted in different components. The depletion is due to settling of plagioclase particles and melt volume reduction. The latter is the sum of the solid particles and the melt volumes in the intercrystalline spaces of the settled particles’ layer. It is shown that the processes of crystallization differentiation are the sum total of hydrodynamic (geodynamic) and petrological processes. These processes can be studied using the methods of similarity theory. The compositional change in the melt due to crystallization differentiation can be represented in the form of an analytical relationship between the petrological similarity criteria

Conditions of accumulation and fractionation of zirconium and hafnium in alkaline-carbonatite systems

The patterns of the distribution and fractionation of strategic metals (Zr, Hf) in the Kugda intrusion (Polar Siberia) have been studied. The contents of these elements significantly exceed their concentrations in other rocks (Zr 246 ppm, Hf 7.4 ppm). A significant increase in Zr and Hf from early rocks (olivinite and melilite rocks) to later differentiation products, syenites with up to 570 ppm of Zr and 16 ppm of Hf, has been revealed. During the evolution of the Kuga magmatic system, notable fractionation of Zr and Hf occurred. The Zr/Hf ratios in the dike rock, similar in composition to the primary Kugda Massif magma, and the early intrusions are fairly close to that of chondrite (Zr/Hf = 37 [1]), while in the latest phases this ratio increases by almost 5-fold. Our study showed that the distribution coefficient of Hf (Kd = 0.58) in alkaline pyroxenes is noticeably higher than that of Zr (Kd = 0.40). Consequently, fractionation of this mineral leads to an increase in the Zr/Hf ratio in the residual liquids. Another mineral concentrating up to 400 ppm of Zr and up to 1520 ppm of Hf is perovskite, which has a very wide crystallization field in the rocks of the Kugda Massif, especially in the earliest olivinite. The data obtained showed that the Zr/Hf ratio in the perovskite of olivinite varies between 2327, that is, noticeably below both the chondritic and the primary magma values. Early crystallization of perovskite is the main reason for increasing the Zr/Hf ratio in melilitolites (up to 54). Thus, the main process of forming the Kugda Massif was continuous crystallization differentiation, accompanied by a noticeable fractionation of rock-forming and accessory minerals (pyroxene and perovskite).

Phase equilibria constraints on crystallization differentiation: insights into the petrogenesis of the normally zoned Buddusò Pluton in north-central Sardinia

The Buddusò Pluton in NE Sardinia (Italy) is a normally zoned intrusion composed of three units with chemical composition ranging from hornblende-bearing tonalites (SiO2∼ 65 wt%) to leucocratic monzogranites (SiO2∼ 76 wt%). Zircon crystals in the pluton are dated at 292.2 ± 0.7 Ma and have εHf values ranging from −4 to −8, with no systematic differences observed between the units. The pluton, which is isotopically homogeneous at the whole-rock scale in terms of Sr and Nd isotopes, shows textural evidence indicating local crystal–melt segregation. In this paper, we have implemented a novel approach based on path-dependent phase-equilibria modelling to test the hypothesis that the internal chemical variability of the pluton was generated by crystallization differentiation of a homogeneous parental magma. Our modelling indicates that this hypothesis is valid if the mechanism by which this occurs is compaction in a rheologically locked crystal-rich magma and if the separation occurs at 0.3 GPa from a tonalitic magma with water content >2 wt%. Finally, a subset of the magmatic enclaves in the pluton are considered to be autoliths, formed by the disruption of the compacted crystal mush and interaction between these cumulates and the felsic melt.

FEATURES OF MELTING IN THE THERMOCHEMICAL PLUME CONDUIT AND HEAT AND MASS TRANSFER DURING CRYSTALLIZATION DIFFERENTIATION OF BASALTIC MELT IN A MUSHROOM-SHAPED PLUME HEAD

The number Ka=N/N1is used to evaluate the thermal power of a plume;Nis the thermal power transferred from the plume base to its conduit, andN1is the thermal power transferred from the plume conduit into the surrounding mantle. At the relative thermal power 1.9<Ka<10, after eruption of the melt from the plume conduit to the surface, melting occurs in the crustal block above the plume roof, resulting in the formation of a mushroom-shaped head of the plume. A thermochemical plume originates at the core-mantle boundary and ascends (melts up) to the surface. Based on laboratory and theoretical modeling data, we present the flow structure of melt in the conduit and the head of the thermochemical plume. The features of melting in the plume conduit are elucidated on the basis of the phase diagram of the CaO-MgO-Al2O3-SiO2model system. The two upper convection cells of the plume conduit relate to the region of basic and ultrabasic compositions. Our study shows that melting in these cells proceeds according to monovariant equilibria of eutectic type L=Cpx+Opx+An+Sp and L=Fo+An+Cpx+Opx. In case of the CaO–MgO–Al2O3–SiO2–Na2O system, crystallization differentiation proceeds as separation of plagioclase crystals. Separation of plagioclase crystals enriched in anorthite component leads to enrichment of the residual melt in silica and alkaline components. Assuming the initial basaltic melt, we calculated the compositional changes in the melt, which are powered by the heat and mass transfer processes in the mushroom-shaped plume head. The calculations were performed in two stages: (1) after settling of refractory minerals; (2) after settling of plagioclase in the melt resulting from the first stage. In the second stage, the melt contains 88.5 % of plagioclase component. The calculations were performed for melt temperatureTmelt=1410 °C and pressureP=2.6 kbar and 6.3 kbar. The calculated weight contents of oxides, the normative compositions for solid phase, and the oxide content and normative composition for the residual melt were tabulated. The SiO2content in the residual melt amounts to 59.6–62.3 % and corresponds to the crustal SiO2content.