Two-Stage Late Jurassic to Early Cretaceous Hydrothermal Activity in the Sakar Unit of Southeastern Bulgaria

Southeastern Bulgaria is composed of a variety of rocks from pre-Variscan (ca. 0.3 Ga) to pre-Alpine sensu lato (ca. 0.15 Ga) time. The Sakar Unit in this region comprises a series of granitoids and gneisses formed ormetamorphosed during these events. It is cut by a series of post-Variscan hydrothermal veins, yet lacks pervasiveAlpine deformation. It thus represents a key unit for detecting potential tectonism associatedwiththe enigmaticCimmerianOrogenic episode, but limitedgeochronologyhasbeenundertaken on this unit. Here we report age constraints on hydrothermal activity in the Sakar Pluton. The investigated veins containmainly albite–actinolite–chlorite–apatite–titanite–quartz–tourmaline–epidote and accessory minerals. The most common accessory minerals are rutile and molybdenite. Apatite and titanite from the same vein were dated by U–Pb LA–ICP-MS geochronology. These dates are interpreted as crystallization ages and are 149 7Ma on apatite and 114 1Ma on titanite, respectively. These crystallization ages are the first to document two stages of hydrothermal activity during the late Jurassic to early Cretaceous, using U–Pb geochronology, and its association with the Cimmerian orogenesis. The Cimmerian tectono-thermal episode is well-documented further to the east in the Eastern Strandja Massif granitoids. However, these are the first documented ages from the western parts of the Strandja Massif, in the Sakar Unit. These ages also temporally overlap with previously published Ar–Ar and K–Ar cooling ages, and firmly establish that the Cimmerian orogeny in the studied area included both tectonic and hydrothermal activity. Such hydrothermal activity likely accounted for the intense albitization found in the Sakar Unit

Unique trace element geochemistry of pyrometamorphic apatite-supergroup minerals : a case study of fluorellestadite from burnt coal (Poland) and shale (France) post-mining waste heaps, with emphasis on boron, germanium, aluminium and titanium

Apatite-type structure is known for its flexibility towards accommodating numerous ions of different crystallographic affinities. Two samples of fluorellestadite from two pyrometamorphic rocks (slags) from burned waste heaps (BWH) from France (LdS) and Poland (RDT) were studied in terms of their trace element composition using Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Boron shows an evident, persistent enrichment in both the samples, with average/maximum levels of 497/1040 and 49/106 ppm, respectively. So is true for magnesium (884/16766 and 404/6251 ppm, i.e., respectively) and sodium (512/697 and 249/370 ppm, respectively). Germanium is clearly enriched in the first sample (29/40 ppm) and, to a lesser degree, in the second one (34 ppm on average). The LdS sample is also clearly enriched in Al (888/1238 ppm), K (385/697 ppm), Ti (515/943 ppm), V (172/347 ppm), and Cu (16/1369 ppm). The RDT sample is also rich in As (105/120 ppm) and Sr (1072/6592 ppm). An interesting feature of both samples concerns their REE pattern: Nd is the dominant element of the group, with the respective Nd/ΣLREE and Nd/(Ce+La) values of 0.43 and 0.90; and 0.37 and 0.66. In order from highest to lower average concentrations, aluminium, magnesium, titanium, boron, potassium, and germanium may be essential substituents in the BWH apatites

High-temperature fluids in granites during the Neoarchaean-Palaeoproterozoic transition: Insight from Closepet titanite chemistry and U-Pb dating (Dharwar craton, India)

International audienceThe aim of this paper is to determine the composition and age of high-temperature (HT) hydrothermal fluids in Closepet granites (Dharwar craton, India). The magmatic origin of the Neoarchaean Closepet batholith has been recognized and dated many times, whereas a HT fluid system, strong evidence for which has been partly investigated, has never been dated. Titanite was chosen for a geochemical study on its pristine and altered domains to illustrate the fluid composition; the domains were dated to recognize the timing of fluid activity. Titanite presents distinct trace element enrichments in non-altered domains and strong depletions in trace elements in marginal, irregular, patchy zones. The high total rare earth element (REE) concentrations and the marked negative Eu anomalies suggest crystallization from a residual melt. The negative Eu anomalies diminish progressively in the REE patterns of altered parts and become positive. Trace element ratios (Nb/Ta, Y/Ho, and U/Th) indicate fluid-crystal interactions. Strong U/Th fractionation might be linked to the presence of CO2 and Cl-. Zr-in-titanite thermometry yields a crystallization/recrystallization temperature of ~700 °C for both magmatic and altered domains. The titanite factor Ti+4/(Al+3 + Fe+3) reveals an affinity with significant mantle input to the fluid system. Pristine and altered titanite domains have been successfully dated by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Reversely discordant apparent ages suggest interaction with fluids, which modified the Pb composition of the magmatic titanite. The oldest semi-concordant analyses of titanite cores indicate an age of approximately 2500 Ma, which we interpret as the minimum age of magmatic titanite crystallization. Narrow dark rims of titanite with considerably lower Th/U ratios form a more coherent group of analyses defining an upper concordia intercept age of 2345 ± 0.016 Ma, interpreted as the best estimate for the time of complete titanite resetting associated with the metasomatic event. The HT fluid system appears to have remained active into the Palaeoproterozoic. Confirmation by dating that the tested fluids belonged to the Archaean hydrothermal system gives us information about this system at the time, but the information that it worked at least until the Palaeoproterozoic provides additional valuable information about the hydrothermal systems during the Neoarchaean-Palaeoproterozoic transition

Inter‐laboratory Characterisation of Apatite Reference Materials for Chlorine Isotope Analysis

Here we report on a set of six apatite reference materials (chlorapatites MGMH#133648, TUBAF#38 and fluorapatites MGMH#128441A, TUBAF#37, 40, 50) which we have characterised for their chlorine isotope ratios; these RMs span a range of Cl mass fractions within the apatite Ca10(PO4)6(F,Cl,OH)2 solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for 37Cl/35Cl homogeneity using SIMS followed by δ37Cl characterisation by gas source mass spectrometry using both dual‐inlet and continuous‐flow modes. We also report major and key trace element compositions as determined by EPMA. The repeatability of our SIMS results was better than ± 0.10% (1s) for the five samples with > 0.5% m/m Cl and ± 0.19% (1s) for the low Cl abundance material (0.27% m/m). We also observed a small, but significant crystal orientation effect of 0.38% between the mean 37Cl/35Cl ratios measured on three oriented apatite fragments. Furthermore, the results of GS‐IRMS analyses show small but systematic offset of δ37ClSMOC values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 103δ37ClSMOC values between +0.09 and +0.42 and in all cases with 1s ≤ ± 0.25.Key Points: Six apatite reference materials having various Cl mass fractions were characterised for chlorine isotope ratios by SIMS and three GS‐IRMS laboratories. A small, but significant, crystal orientation effect was recorded by SIMS analyses. Correlation of instrumental mass fractionation factor with Cl mass fraction is visible along the apatite solid solution series.Narodowe Centrum NaukiDeutscher Akademischer AustauschdienstHelmholtz Recruiting InitiativeInstitute of Geological Sciences, Polish Academy of Science