Metamorphism of metachert from the Southern Alps, New Zealand. A thermodynamic forward modelling study

<p>Scattered, scarce occurrences of garnet- and quartz-rich metamorphic rock, probably derived from Mn- and Fe-rich chert, occur within metamorphosed greywacke sequences worldwide. The metamorphism of such garnetiferous metacherts has not previously been investigated using modern thermodynamic forward modelling techniques due to the lack of appropriate, internally-consistent activity-composition (a–x) models for Mn-bearing minerals. The present study applies thermodynamic forward modelling using the recently-proposed a–x models of White et al. (2014) to investigate the metamorphism of garnetiferous metachert samples from the Southern Alps, New Zealand. Pressure-temperature (P–T) pseudosections are used in combination with results from petrography, element composition mapping using micro X-ray fluorescence (µXRF) and scanning electron microscope (SEM) methods, and garnet composition data from analytical transects by electron probe microanalysis (EPMA), to study metachert metamorphism. All the samples are compositionally layered, so the possibility exists that an input bulk rock composition might not match the effective bulk composition at the site of garnet growth. If a mineral assemblage stability field in a calculated P–T pseudosection matched the mineral assemblage in the rock, this was taken as an initial indication of a permissible input bulk rock composition. In that case, refined constraints on the P–T conditions were sought by comparing calculated and measured garnet compositions. The studied rocks include samples that are carbonate-bearing, which require consideration of the effects of fluid composition in mixed H₂O–CO₂ fluids, as well as a sample in which the garnet is strongly zoned, texturally-complex, and inferred to be of polymetamorphic origin. The effects of element fractionation by that garnet were investigated by recalculating the P–T pseudosection using a new bulk rock composition with the garnet core content removed. In none of the samples did the calculated and observed composition isopleths for the garnet cores match, suggesting that initial garnet nucleation in these Mn-rich rocks was locally controlled. For most samples in which the calculated and observed mineral assemblages matched, successful estimates of the peak metamorphic conditions were obtained. A garnet chert (A12E) from the mylonite zone of the Alpine Fault at Vine Creek, near Hokitika, gave a tight intersection of composition isopleths, indicating peak metamorphic conditions of 510 °C/5.5 kbar, after recalculation to correct for element fractionation by the strongly-zoned garnet. This tight, modern constraint is within error of previously-reported results from traditional geothermobarometry (420–600 °C/5.9–13 kbar) and Raman spectroscopy of carbonaceous material (RSCM T = 556 °C) from nearby sites. A peak metamorphic estimate of 520–550 °C/7–10 kbar was obtained from a dolomite-bearing sample from the garnet zone near Fox Glacier (J34), in good comparison with published temperatures from Raman spectroscopy of carbonaceous material in nearby metagreywacke samples (526–546 °C). The prograde metamorphic P–T path was probably steep, based on growth of the garnet core at ~475535 °C/5–9 kbar. The successful results for these garnet chert samples show that the new a-x models for Mn-bearing minerals extend the range of rock types that are amenable to pseudosection modelling. Results obtained in this study also serve to highlight several possible concerns: a) garnet nucleation and initial growth in very Mn-rich rocks may be subject to local compositional or kinetic controls; b) bulk rock compositions may not always mimic the effective bulk composition; c) the existing a–x models for Mn-bearing minerals and white micas may need refining; and d) some rocks may simply be ill-suited to thermodynamic forward modelling. Items a) and b) may be indicated by the common observation of a mismatch between predicted and measured garnet composition isopleths for garnet cores, and by a mismatch between garnet composition isopleths and the appropriate mineral assemblage field for sample AMS01, from the mylonite zone, Hari Hari, Southern Alps. For item c) every P–T pseudosection calculated using the new a–x models for Mn-bearing minerals showed garnet stable to very low temperatures below 300 °C. In addition, the P–T pseudosection for an oligoclase-zone metachporphyroblasts of Fe-Ti oxides (magnetitert (Sample J36) from Hari Mare stream, Franz Josef - Fox Glacier, indicated that the white mica margarite should be present instead of plagioclase (oligoclase), for a rock in which oligoclase is present and margarite is absent, a problem previously noted elsewhere. Item d) is exemplified by a very garnet-rich ferruginous metachert sample (J35, garnet zone, headwater region, Moeraki River, South Westland) which proved impossible to model successfully due to its complex mineral growth and deformation history. This sample contained multiple generations of carbonate with differing compositions, amphibole (not incorporated for modelling with the new a–x models for Mn-bearing minerals), large e associated with smaller, possibly later-formed ilmenite), and the garnet bands were offset by late deformation. The garnetiferous metachert samples studied here preserve in their textures and compositions clues to their growth mechanism and metamorphic history. The textures in at least two of the samples are consistent with the diffusion controlled nucleation and growth model for garnet. This research has successfully used state of the art thermodynamic modelling techniques in combination with the latest internally consistent a-x models on Mn-rich metachert, for the first time, extracting P–T conditions of the metamorphism of garnetiferous metachert from the Southern Alps.</p>

Metamorphism of metachert from the Southern Alps, New Zealand. A thermodynamic forward modelling study

<p>Scattered, scarce occurrences of garnet- and quartz-rich metamorphic rock, probably derived from Mn- and Fe-rich chert, occur within metamorphosed greywacke sequences worldwide. The metamorphism of such garnetiferous metacherts has not previously been investigated using modern thermodynamic forward modelling techniques due to the lack of appropriate, internally-consistent activity-composition (a–x) models for Mn-bearing minerals. The present study applies thermodynamic forward modelling using the recently-proposed a–x models of White et al. (2014) to investigate the metamorphism of garnetiferous metachert samples from the Southern Alps, New Zealand. Pressure-temperature (P–T) pseudosections are used in combination with results from petrography, element composition mapping using micro X-ray fluorescence (µXRF) and scanning electron microscope (SEM) methods, and garnet composition data from analytical transects by electron probe microanalysis (EPMA), to study metachert metamorphism. All the samples are compositionally layered, so the possibility exists that an input bulk rock composition might not match the effective bulk composition at the site of garnet growth. If a mineral assemblage stability field in a calculated P–T pseudosection matched the mineral assemblage in the rock, this was taken as an initial indication of a permissible input bulk rock composition. In that case, refined constraints on the P–T conditions were sought by comparing calculated and measured garnet compositions. The studied rocks include samples that are carbonate-bearing, which require consideration of the effects of fluid composition in mixed H₂O–CO₂ fluids, as well as a sample in which the garnet is strongly zoned, texturally-complex, and inferred to be of polymetamorphic origin. The effects of element fractionation by that garnet were investigated by recalculating the P–T pseudosection using a new bulk rock composition with the garnet core content removed. In none of the samples did the calculated and observed composition isopleths for the garnet cores match, suggesting that initial garnet nucleation in these Mn-rich rocks was locally controlled. For most samples in which the calculated and observed mineral assemblages matched, successful estimates of the peak metamorphic conditions were obtained. A garnet chert (A12E) from the mylonite zone of the Alpine Fault at Vine Creek, near Hokitika, gave a tight intersection of composition isopleths, indicating peak metamorphic conditions of 510 °C/5.5 kbar, after recalculation to correct for element fractionation by the strongly-zoned garnet. This tight, modern constraint is within error of previously-reported results from traditional geothermobarometry (420–600 °C/5.9–13 kbar) and Raman spectroscopy of carbonaceous material (RSCM T = 556 °C) from nearby sites. A peak metamorphic estimate of 520–550 °C/7–10 kbar was obtained from a dolomite-bearing sample from the garnet zone near Fox Glacier (J34), in good comparison with published temperatures from Raman spectroscopy of carbonaceous material in nearby metagreywacke samples (526–546 °C). The prograde metamorphic P–T path was probably steep, based on growth of the garnet core at ~475535 °C/5–9 kbar. The successful results for these garnet chert samples show that the new a-x models for Mn-bearing minerals extend the range of rock types that are amenable to pseudosection modelling. Results obtained in this study also serve to highlight several possible concerns: a) garnet nucleation and initial growth in very Mn-rich rocks may be subject to local compositional or kinetic controls; b) bulk rock compositions may not always mimic the effective bulk composition; c) the existing a–x models for Mn-bearing minerals and white micas may need refining; and d) some rocks may simply be ill-suited to thermodynamic forward modelling. Items a) and b) may be indicated by the common observation of a mismatch between predicted and measured garnet composition isopleths for garnet cores, and by a mismatch between garnet composition isopleths and the appropriate mineral assemblage field for sample AMS01, from the mylonite zone, Hari Hari, Southern Alps. For item c) every P–T pseudosection calculated using the new a–x models for Mn-bearing minerals showed garnet stable to very low temperatures below 300 °C. In addition, the P–T pseudosection for an oligoclase-zone metachporphyroblasts of Fe-Ti oxides (magnetitert (Sample J36) from Hari Mare stream, Franz Josef - Fox Glacier, indicated that the white mica margarite should be present instead of plagioclase (oligoclase), for a rock in which oligoclase is present and margarite is absent, a problem previously noted elsewhere. Item d) is exemplified by a very garnet-rich ferruginous metachert sample (J35, garnet zone, headwater region, Moeraki River, South Westland) which proved impossible to model successfully due to its complex mineral growth and deformation history. This sample contained multiple generations of carbonate with differing compositions, amphibole (not incorporated for modelling with the new a–x models for Mn-bearing minerals), large e associated with smaller, possibly later-formed ilmenite), and the garnet bands were offset by late deformation. The garnetiferous metachert samples studied here preserve in their textures and compositions clues to their growth mechanism and metamorphic history. The textures in at least two of the samples are consistent with the diffusion controlled nucleation and growth model for garnet. This research has successfully used state of the art thermodynamic modelling techniques in combination with the latest internally consistent a-x models on Mn-rich metachert, for the first time, extracting P–T conditions of the metamorphism of garnetiferous metachert from the Southern Alps.</p>

Petrographic and Geochemical Studies of the Host Rock of the Chaghoo Copper Orebody in the Southwest of Karaj

The study area is located 5 km southwest of Mahdasht city in Karaj on the Urmia-Dokhtar magmatic arc. In this area, Eocene volcanic and pyroclastic rocks are observed including basaltic andesite lavas, andesite, Trachyandesiticand trachyte lavas, tuff, and ignimbrite, along with plutonic rocks. There are two spectra of basic and acidic for the rocks in the area, of which basic rocks are chemically calc-alkaline in nature.Among the signs of subduction rocks in the area are enrichment in the Ta, Nb, and Ti lavas, as well as the anomaly of the HFSE index relative to the LILE of incompatible elements content. The geochemical and petrogenetic studies indicate the origin of the area’s plutonic rocks and the role of differential crystallization accompanied by the crustal rocks-contamination and digestion of magma in the evolution of the magma forming these rocks. This magma has been originated from the low-grade partial melting of an enriched mantle origin beneath the continental lithosphere with the lherzolite garnet composition at a depth of 100 to 110 km in a post-collision tensile environment. Investigating the fluids involved in the region, the homogenization temperature with the temperature of copper veins formation is between 120 to 306 ° C, with the salinity percentage varying between 6.45 to 15.96% of sodium chloride weight. Accordingly, this metamorphic hydrothermal orebodyis located in the mesothermal category. The presence of sub-faults, joints, and cracks in the host rock has provided a low-pressure environment for a proper place for copper mineralizationas veins.

High-pressure and high-temperature single-crystal X-ray diffraction of complex garnet solid solutions up to 16 GPa and 823 K

P–V–T equations of state (EoS) of synthetic garnet solid solutions with ternary grossular–almandine–pyrope compositions relevant to the Earth’s upper mantle have been determined in order to examine whether garnet properties can be accurately interpolated from those of the end-members. Volumes have been measured as a function of pressure using single-crystal X-ray diffraction measurements performed inside a diamond anvil cell. Isothermal bulk moduli and first pressure derivatives were obtained by fitting the P–V data using a third-order Birch–Murnaghan equation of state. Two nominally eclogitic garnets (Prp47Alm19Grs31And3 and Prp53Alm19Grs18And3Sps7) were found to have isothermal bulk moduli (KT0) and pressure derivatives (K′T0) of 170(3) GPa, 4.1 (4) and 173 (2) GPa, 3.8 (5), respectively. KT0 and K′T0 for an almandine-rich garnet (Prp26Alm63Grs6And5) were found to be 175 (3) GPa and 3.7 (7), respectively. High-temperature compression experiments at 703 K and 823 K were carried out on sample Prp47Alm19Grs31And3, resulting in the high-temperature EoS term (∂KT/∂T)P = − 0.025 (6) and a thermal expansion (α0) of 2.86 (4) × 10−5 K−1. The results imply that the bulk moduli of aluminous garnet solid solutions stable at upper mantle conditions can be deduced from the properties of the end-members with minimal uncertainty. We show that the difference in the bulk sound velocity determined for a multicomponent eclogitic garnet composition and obtained for the same composition from the end-member properties is better than 0.5% for pressures and temperatures corresponding to Earth’s upper mantle.

FEATURES OF GARNET AND CLINOPYROXENE IN DIAMONDIFEROUS ECLOGITES FROM THE UDACHNAYA KIMBERLITE PIPE, YAKUTIA: METASOMATOSIS EVIDENCE

Mineralogy of diamondiferous eclogite xenolites showing metasomatosis evidence from the Udachnaya kimberlite pipe is discussed. The paper also reviews features of diamonds they contain, compositions of primary garnets and omphacites as well as alteration of structural and species compositions of original garnets and clinopyroxenes during metasomatosis. Based on pyrope structure update, two-phase garnet composition is suggested, which is mostly represented by complex pyrope associated with Ca-pyrope. In all samples, primary omphacite is replaced by another clinopyroxene variety depleted in Na2O, which is typical of partial melting products. Geothermometry results suggested that the eclogites formed within a temperature range of 1,000–1,2000 °C. Based on diamond morphology, data on total N content in diamonds and its aggregation, multiple stages of diamond formation in eclogites and the most probable growth of later diamond generations impacted by metasomatizing mantle fluids containing carbon are postulated. It is suggested that certain diamond formation stages probably had a time gap of several hundred million years.

Garnet-amphibole miaskites of the Ilmenogorsky miaskite massif (Southern Urals): Mineralogy and geochemistry

Research subject. This paper presents original findings about textural-structural, mineralogical, petrological, and geochemical features of the garnet-amphibole miaskites (firstes) of the Ilmenogorsky miaskite massif.Materials and methods. The microprobe analysis of mineral composition was performed using Tescan Vega3 sbu and REMMA202M scanning microscopes equipped with microanalyzers. The content of major, trace and rareearth elements (REE) in rock samples was determined by the methods of AAS and ICP-MS.Results. The garnet-amphibole miaskites under study are characterized by a rare mineral paragenesis, i.e. garnet-amphibole-pyroxene-nepheline-plagioclase. The mafic minerals exhibit a high ferruginosity (f = 70–99), while the accessory minerals have high Al, F and low REE contents. The garnetamphibole miaskites contains high concentrations of Al, Fe3+, Ca, Na, Be, Rb, Mo, Tl and low concentrations of LILE, HFSE, REE and transit elements.Conclusions. According to the garnet composition and its ferruginosity (f = 95– 99), high contents of Al and F in accessory minerals, the prevalence of Fe3+, as well as negative Eu/Eu* and positive Ce/ Ce* anomalies, the garnet-amphibole miaskites under study are assumed to be the product of acid-alkaline metasomatism occurring under the oxidizing conditions of petrogenesis. The low ratios of Cr/V and Ni/Co indicate the immobility of transit elements during metasomatism, and their clarke of concentration corresponds to the content in metaterrigenous and metacarbonate rocks, which suggests crustal substratum for garnet–amphibole miaskites. Garnet-amphibole miaskites are the markers of the interaction of crustal material with deep fluids, which occurred during the stage of shear tectonics development (270–240 Ma) due to the broad permeability of the rocks composing the Ilmenogorsky miaskite massif.

Eclogites of the Great Caucasus (nature of protolite and it geodynamical typification)

В структурно формационной зоне Передового хребта Большого Кавказа, в разрезах пород «балканской» и «лабарданской» свит встречаются тела эклогитов омфацит-гранатового состава с примесью амфибола, эпидота, цоизита и кианита. Целью работыявлялось изучение петрохимических и геохимических особенностейэклогитов в разрезах пород Большого Кавказа.Методы исследования.Расчеты, проведенные на основании гранат-клинопироксенового термометра, определяют интервал температур стабильности наблюдаемой ассоциации гранат + омфацит в пределах 580–650 °С, а проведенные оценки давлений, по растворимости жадеитов в клинопироксенах, дают максимальные давления наблюдаемого парагенезиса порядка 13,5 кбар и минимальное – 8,5 кбар. Результаты работы.Проведено геохимическое изучение эклогитов и гранатовых амфиболитов и приведены результаты их RFA, ICP-MS анализов, а также дано краткое петрографическое описание изучаемых пород. Рассмотрены петро-геохимические характеристики эклогитов, определена их дометаморфическая природа, а также расшифрована наиболее вероятная геодинамическая типизация исходного протолита. Показано, что эклогиты по составу соответствуют магматическим породам базальтового типа, с отношением изотопов 87Sr/86Sr равным 0,7035.Эклогиты образовались по умеренно-титанистым, умеренно-глиноземистым, умеренно-магнезиальным, низко-калиевым, вулканитам с натровым типом щелочности. Предполагается, что исходный расплав основного состава был образован при 8–15% плавлении шпинелевых перидотитов, а Ni/Со отношение ∑/n равное 2,9 соответствует показателю мантийных выплавок, варьирующему в пределах 2,5–5,0. Низкие значения Mg#=0,55 указывают на возможные явления дифференциации исходного расплава. Положительные европиевая Eu/Eu*=0,95–2,75 и стронциевая аномалии допускают изначальную аккумуляцию плагиоклаза в вулканитах. Анализ петрогенетических диаграмм показывает, что фигуративные точки эклогитов распологаются в полях базальтов Е–MORBтипа, а также толеитов островных дуг,базальтов задуговых котловин (окраинные моря) и срединно-океанических хребтов. Геохимическая специализация исходных расплавов– сидерофильная. Несовместимые элементы и REEнормированные по N-MORBи хондриту, образуют спектры линий, близких к N-MORB, а Laн/Ybн =0,6-1,7 при ∑/nREE=34 г/т. В тоже время полученные спектры отличаются от эталона N-MORBчеткими отрицательными аномалиями высокозарядных элементов (Nb, Ta, Zr, Hf), что указывает на их надсубдукционную природу. По совокупности полученных результатов,с учетом палеотектонических реконструкций Кавказского региона,предполагается, что формирование исходных вулканитов происходило в раннем палеозое, в условиях задугового бассейна расположенного на северной границе палеотетиса In structural-formation zone of Front ridge of the Great Caucasus in rock sections of “balkanskoy” and “labardanskoy” suites bodies of eclogites of omphacite-garnet composition with a admixture of amphibole, epidote, zoisite and clinopyroxene are occur. Aim of the work was to study the petrochemical and geochemical features of eclogites in sections of the Greater Caucasus rocks.Methods.Calculation carried out on the basis of garnet-clinopyroxene thermometer, are assess interval temperature of stability of observed association garnet + omphacite within 580-650°С, and carried out assessments of pressure according solubility of jadeites in the clinopyroxene, give maximum pressure of observed paragenesis is 13,5 kbar and minimum - 8,5 Kbar. Results.Geochemical investigation of eclogites and garnet amphibolites was carried out and results of it RFA, ICP-MS analysis and also briefly petrographyc discription of investigated rocks have been done. Petro-geochemical characteristics of eclogites have been treated and it premetamorphic nature was determined and the most likely geodynamical typification of initial protolite was deciphed. It was shown, that eclogites correspond on composition to magmatic rocks of basaltic type with a relation of isotopes of 87Sr/86Sr equal to 0.7035. Eclogites were formed from moderate-titanoferous, moderate-aluminiferous and moderate-magnesioferous, low-potassic volcanites with a sodium type of alkalinity. It is suggested, that initial melt of basic composition, was formed at a 8-15% melting of spinelian peridotite. Ni/Соratio ∑/n equal 2,9 correspond to index of mantle smelted, varying within 2,5 –5,0. Low values Mg# =0,55 denotes on possible phenomenon of differentiation of initial melt. Positive europium Eu/Eu*=0,95-2,75 and strontium anomalies allows of primary accumulation of plagioclase in volcanites. Analysis of petrogenic diagrams are show, that dots of eclogites are located in field of basalt Е–MORB type, and also - tholeite of arc island, basalt of backarc basin (margin sea) and medial ocean ridge. Geochemical specialization of the initial melts is siderophile. Incompatible elements and REE are normalized to N-MORB and chondrite, are formed spectraof lines, close to MORB type basalt; Laн/Ybн =0,6-1,7 when ∑/n REE=34 ppm. At the same time obtained spectrums are distinguish from standart of N-MORB by clear negative anomalies of high-charged elements (Nb, Ta, Zr, Hf), that indicate of their oversubduction nature.According to tofality of obtained results and accounting of paleotectonic reconstruction of the Caucasus region, have supposedly, that forming of initial volcanites was occurred in Early Paleozoic in condition of back-arc basin which located on the northern margin of Paleo-Tethys

Raman Microspectroscopy of Garnets from S-Fibulae from the Archaeological Site Lajh (Slovenia)

Garnets (19 pieces) of Late Antique S-fibulae from the archaeological site at Lajh-Kranj (Slovenia) were analysed with Raman microspectroscopy to obtain their mineral characteristic, including inclusion assemblage. Most garnets were determined as almandines Type I of pyralspite solid solution series; however, three garnets showed a higher Mg, Mn and Ca contents and were determined as almandines Type II. Most significant Raman bands were determined in the range of 169–173 cm−1 (T(X2+)), 346–352 cm−1 (R(SiO4)), 557–559 cm−1 (ν2), 633–637 cm−1 (ν4), 917–919 cm−1 (ν1), and 1042–1045 cm−1 (ν3). Shifting of certain Raman bands toward higher frequencies was the result of an increase of the Mg content in the garnet composition, which also indicates the presence of pyrope end member in solid garnet solutions. Inclusions of apatite, quartz, mica, magnetite, ilmenite, as well as inclusions with pleochroic or radiation halo and tension fissures (zircon), were found in most of the garnets. Rutile and sillimanite were found only in garnets with the highest pyrope content. Spherical inclusions were also observed in two garnets, which may indicate the presence of melt or gas residues. The determined inclusion assemblage indicates the formation of garnets during medium- to high-grade metamorphism of amphibolite or granulite facies. According to earlier investigations of the garnets from Late Antique jewellery, the investigated garnets are believed to originate from India.

U-Pb dating of skarn garnets from Bulgarian deposits

&lt;p&gt;Calcic garnets from grossular-andradite (grandite) series have proven their ability to record the conditions and timing of their formation processes. Typically these minerals occur in skarn systems, together with other calc-silicates (diopside, epidote) and commonly host economic Cu, Zn-Pb-Ag, Au, Sn, W or Mo mineralization. Based on the U-content in the garnet structure, we used in-situ LA-ICP-MS U-Pb geochronology to determine the age record in more than 15 skarn deposits from different tectonic zones in Bulgaria. The data is partly complemented with ID-TIMS dating. The mineralogical, geochemical and petrological characteristics of the materials were described additionally. Both contact and infiltration skarns were studied.&lt;/p&gt;&lt;p&gt;The obtained data revealed that the garnet composition in terms of major elements does not affect the precision of age determination. Both andradite and grossular members yield age data with very high accuracy. The dating results, however, depend on the geochemical signature of the garnets and especially on the U-content and U/Pb ratio. Our data shows that skarn samples from the vicinities of magmatic bodies or along contacts of causative pegmatite veins usually have increased U-incorporation from several to more than 70 ppm, as suggested by their proximal position to the source. The contact skarn garnets formed by intrusion of silicate melts (or pegmatites) onto carbonate-rich hosts mostly produce precise ages, which are in good agreement with the geochronological zircon data about the magmatism in the studied regions (e.g. Central Pirin, Teshevo, Plana, Gutsal, Rila-West Rhodope, Sv. Nikola etc. plutons). The infiltration skarns, though, generally reveal ages with low accuracy and significant errors, mainly due to U-content below 1 ppm. The reason for the low U-concentration and U/Pb ratio is either connected with a primary U-deficit and its depletion in the garnet-precipitating fluids with time and space but might be also related to garnet retrograde hydrothermal alteration.&lt;/p&gt;&lt;p&gt;The time span of the Bulgarian skarn garnets is closely connected with the causative magmatic bodies. The studied skarns reveal Paleogene (~30-42 Ma - Central Pirin and Teshevo plutons and pegmatites from Rila-West Rhodope batholith; Djurkovo, Murzian and Zvezdel Pb-Zn deposits; ~ 58 Ma - skarns from Western Rila Mts., ~ 68 Ma &amp;#8211; Babyak Mo-Ag-Au-W-Bi-Cu-Pb-Zn deposit), Cretaceous (~ 76 Ma- Gutsal pluton, 81 Ma - scheelite bearing skarns from the Plana pluton, 86 Ma &amp;#8211; Iglika skarn deposit) and Paleozoic (~ 303 Ma &amp;#8211; Martinovo Fe-skarn deposit) ages. Given the occurrence of Ca-garnet in contact rocks and hydrothermal ore deposits, our results highlight the potential of grandite as a powerful U-Pb geochronometer for dating magmatism and skarn-related mineralizations.&lt;/p&gt;&lt;p&gt;&lt;em&gt;Acknowledgements.&lt;/em&gt; The study is partly supported by the DNTS 02/15 bilateral project between Bulgaria and the Russian Federation, financed by the Bulgarian National Science Fund.&lt;/p&gt;

Phase relations at interaction of phlogopite with carbonate melt at Р = 3.8 GPa

The phase relations in the phlogopite-carbonate system were studied at P = 3.8 GPa, T = 1200-1300 C. The interaction of phlogopite with carbonate melt resulted in the formation of a polymineral association of relic and newly formed phases of the phlogopite-carbonate-clinopyroxene-spinel-garnet composition coexisting with carbonate melt. By increasing the temperature from 1200 to 1300 C in the carbonate melt increases the solubility of phlogopite and the concentrations of its components - Si, Al, Mg, K. The phase composition of the quenching phases of the carbonate melt varies from substantially carbonate with isolated microcrystals of apatite and phlogopite at 1200 C to phlogopite-carbonate with a variety of texture ratios at 1300 C, reflecting the spontaneous crystallization of the carbonate melt during quenching. In the studied P-T, close to the mantle adiabate, phlogopite remains stable in the presence of silicate-carbonate melt.