Igor Broska

Replacement of primary monazite by apatite-allanite-epidote coronas in an amphibolite facies granite gneiss from the eastern Alps

Abstract Accessory monazite crystals in granites are commonly unstable during amphibolite facies regional metamorphism and typically become mantled by newly formed apatite-allanite- epidote coronas. This distinct textural feature of altered monazite and its growth mechanism were studied in detail using backscattered electron imaging in a sample of metagranite from the Tauern Window in the eastern Alps. It appears that the outer rims of the former monazites were replaced directly by an apatite ring with tiny thorite intergrowths in connection with Ca supply through metamorphic fluid. Around the apatite zone, a proximal allanite ring and a distal epidote ring developed. This concentric corona structure, with the monazite core regularly preserved in the center, shows that the reaction kinetics were diffusion controlled and relatively slow. Quantitative electron microprobe analyses suggest that the elements released from monazite breakdown (P, REE, Y, Th, U), were diluted and redistributed in the newly formed apatite, allanite, and epidote overgrowth rings and were unable to leave the corona. This supports the common hypothesis that these trace elements are highly immobile during metamorphism. Furthermore, microprobe data suggest that the preserved monazite cores lost little, possibly none of their radiogenic lead during metamorphism. Thus, metastable monazite grains from orthogneisses appear to be very useful for constraining U-Th-Pb protolith ages. On the basis of these findings and a review of literature data, it seems that monazite stability in amphibolite facies metamorphic rocks depends strongly on lithologic composition. While breaking down in granitoids, monazite may grow during prograde metamorphism in other rocks such as metapelites.

Coexisting monazite and allanite in peraluminous granitoids of the Tribeč Mountains, Western Carpathians

Abstract Monazite, a typical light rare-earth element (LREE) mineral of S-type granitoids in the Western Carpathians, was found in the peraluminous biotite granodiorite-tonalite in the Tribeč Mountains commonly containing polymineralic inclusions. These inclusions are dominated by anhedral allanite, although allanite also occurs rarely as discrete grains not enclosed by monazite. The monazite studied here is relatively homogeneous and characterized by high Th contents with proportions of huttonite (ThSiO4) and brabantite [CaTh(PO4)2] up to 14.6 and 9.3%, respectively. The discrete allanite grains are highly aluminous with a composition consistent with the peraluminous type of host rock. However, allanite included in monazite is extremely variable in LREE, Al, Fe, and Mg contents. This variation is interpreted to result from entrapment of allanite (+ melt) in monazite before local equilibrium was attained. The change from allanite to monazite as the stable LREE-rich phase is related to an overall decrease in Ca concentration caused by the onset of plagioclase crystallization. The early precipitation of allanite was possible because of the high LREE concentrations in the melt. The crystallization temperature of allanite must have been higher than monazite saturation (>856-845 °C and 798-790 °C for two analyzed samples). The Zr saturation temperature based on zircon solubility and REE thermometry based on monazite solubility reflect an increase in temperature from the edge to the center of the pluton, which coincides with an increase in the huttonite content in monazite. The primary LREE assemblage is accompanied by small grains of late huttonite(?) replacing monazite and brabantite replacing allanite.

Two types of metamorphic monazite with contrasting La/Nd, Th, and Y signatures in an ultrahigh-pressure metapelite from the Pohorje Mountains, Slovenia; indications for pressure-dependent REE; exchange between apatite and monazite?

Abstract Two monazite generations (M1; M2) were distinguished in a kyanite-garnet gneiss from the UHP terrain of the Pohorje Mountains, Slovenia. P-T estimates reveal a peak event at 760 °C/2.6 GPa and isothermal decompression down to 700 °C/0.6 GPa. M1 type provides a Th-U-Pb mean date of 100 ± 6 Ma, ThO2 contents between 3-7 wt%, Y2O3 values <0.3 wt%, and La/Nd ratios (1.2-1.4) that are clearly higher than for the whole-rock La/Nd (1.1). The absence of Y zoning in M1 and the lack of monazite inclusions in garnet indicate that M1 formed after the main stage of garnet growth (>1.2 MPa), probably close to the P-T peak. M2 type is slightly younger than M1 (74 ± 16 Ma), and has a lower La/Nd (0.3-0.9), lower ThO2 (0.1-5 wt%), and higher Y2O3 (up to 3.2 wt%). Most M2 monazites occur as tiny needles within apatite (subtype M2-a) or along apatite margins (M2-b). Parasitic growth of M2-a and -b from apatite is supported by its low ThO2 (<1 wt%) and La/Nd (<0.5). Isolated matrix grains (M2-c) and overgrowths around M1 (M2-d) have slightly higher La/Nd (0.5-0.9) and higher ThO2 (5 wt%) and were supplied from an apatite and M1 source. Elevated yttrium suggests that M2 formed during decompression, when garnet was consumed and Y was released. These observations imply that at UHP conditions MREE-rich apatite coexisted with low-MREE M1 monazite and reacted during decompression to Ca-F-apatite plus MREE-rich M2 monazite. This provides strong arguments that REE-partitioning between apatite and monazite is pressure-dependent.

Eclogite-hosting metapelites from the Pohorje Mountains (Eastern Alps): P-T evolution, zircon geochronology and tectonic implications

Phase-equilibrium modelling, geothermobarometry, ion-microprobe dating and mineral chemistry of zircon have been used to constrain the P-T-t evolution of metapelitic kyanite-bearing gneisses from the ultrahigh-pressure (UHP) metamorphic terrane of the Pohorje Mountains in the Eastern Alps. These eclogite-hosting rocks are part of the continental basement of the Austroalpine nappes. Based on calculated phase diagrams in the system Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (NCKFMMnASH) and conventional geothermobarometry, the garnet-phengite-kyanite-quartz assemblages of gneisses record metamorphic conditions of 2.2-2.7 GPa at 700-800 � C. These are considered as minima because of the potential for a diffusion-related modification and re- equilibration of the garnet and phengite during early stages of decompression. It is therefore most likely that the gneisses experienced the same peak UHP metamorphism at � 3 GPa as associated kyanite eclogites. Decompression and cooling to � 0.5 GPa and 550 � C led to the consumption of garnet and phengite, and the development of matrix consisting of biotite, plagioclase, K-feldspar � sillimanite and staurolite. Textures and phase diagrams suggest a low extent of partial melting during decompression. Cathodoluminescence images as well as zircon chemistry reveal cores encompassed by two types of metamorphic zircon rims. Ion probe U-Pb dating of three zircon cores yielded Permian (286 � 10, 258 � 7 Ma) and Triassic (238 � 7 Ma) concordia ages. The zircon rims are Cretaceous with a mean concordia age of 92.0 � 0.5 Ma and some cores gave a similar age. The Cretaceous zircons all exhibit very low Th/U ratio (,0.02) typical of metamorphic origin. In these zircons, nearly flat HREE patterns, (Lu/Gd)N ¼ 1-4, and only small negative Eu anomalies indicate formation in the presence of garnet and absence of plagioclase, which is corroborated by occurrence of Mg- and Ca-rich garnet inclusions. Therefore, these zircons are interpreted to record the Cretaceous HP/UHP metamorphism. The 92.0 � 0.5 Ma age obtained in this study agrees with that (93-91 Ma) determined earlier in the Pohorje eclogites from U/Pb zircon, Sm-Nd and Lu-Hf garnet-whole-rock dating. This implies that the eclogites and their country rocks were subducted and exhumed together as a coherent piece of continental crust. There is no evidence for a melange-like assemblage of rocks, which followed different P-T-t paths, or several subduction and exhumation cycles as proposed for some other UHP metamorphic terranes.

Tourmaline nodules: products of devolatilization within the final evolutionary stage of granitic melt?

Abstract The origin of tourmaline nodules, and of their peculiar textures found in peripheral parts of the Moslavačka Gora (Croatia) Cretaceous peraluminous granite are connected with the separation of a late-stage boron-rich volatile fluid phase that exsolved from the crystallizing magma. Based on field, mineralogical and textural observations, tourmaline nodules were formed during the final stage of granite evolution when undersaturated granite magma intruded to shallow crustal horizons, become saturated and exsolved a fluid phase from residual melt as buoyant bubbles, or pockets. Calculated P–T conditions at emplacement level are c. 720 °C, 70–270 MPa, and water content in the melt up to 4.2 wt%. Two distinct occurrence types of tourmalines have been distinguished: disseminated and nodular tourmalines. Disseminated tourmaline, crystallized during magmatic stage, is typical schorl while nodular tourmaline composition is shifted toward dravite. The increase of dravite in nodular tourmaline is attributed to mixing of the fluid phase from the residual melt with fluid from the wall rocks. The pressure decrease and related cooling at shallow crustal levels can be considered as a major factor controlling fluid behaviour, formation of a volatile phase, and the crystallization path in the Moslavačka Gora granite body.

Intensive low-temperature tectono-hydrothermal overprint of peraluminous rare-metal granite: a case study from the Dlhá dolina valley (Gemericum, Slovakia)

Abstract A unique case of low-temperature metamorphic (hydrothermal) overprint of peraluminous, highly evolved rare-metal S-type granite is described. The hidden Dlhá dolina granite pluton of Permian age (Western Carpathians, eastern Slovakia) is composed of barren biotite granite, mineralized Li-mica granite and albitite. Based on whole-rock chemical data and evaluation of compositional variations of rock-forming and accessory minerals (Rb-P-enriched K-feldspar and albite; biotite, zinnwaldite and di-octahedral micas; Hf-(Sc)-rich zircon, fluorapatite, topaz, schorlitic tourmaline), the following evolutionary scenario is proposed: (1) Intrusion of evolved peraluminous melt enriched in Li, B, P, F, Sn, Nb, Ta, and W took place followed by intrusion of a large body of biotite granites into Paleozoic metapelites and metarhyolite tuffs; (2) The highly evolved melt differentiated in situ forming tourmaline-bearing Li-biotite granite at the bottom, topaz-zinnwaldite granite in the middle, and quartz albitite to albitite at the top of the cupola. The main part of the Sn, Nb, and Ta crystallized from the melt as disseminated cassiterite and Nb-Ta oxide minerals within the albitite, while disseminated wolframite appears mainly within the topaz-zinnwaldite granite. The fluid separated from the last portion of crystallized magma caused small scale greisenization of the albitite; (3) Alpine (Cretaceous) thrusting strongly tectonized and mylonitized the upper part of the pluton. Hydrothermal low-temperature fluids enriched in Ca, Mg, and CO2 unfiltered mechanically damaged granite. This fluid-driven overprint caused formation of carbonate veinlets, alteration and release of phosphorus from crystal lattice of feldspars and Li from micas, precipitating secondary Sr-enriched apatite and Mg-rich micas. Consequently, all bulk-rock and mineral markers were reset and now represent the P-T conditions of the Alpine overprint.

Oriented inclusions in apatite in a post-UHP fluid-mediated regime (Tromsø Nappe, Norway)

We report pyrrhotite, anhydrite and dolomite crystal rods in fluorapatite occurring in silicate-bearing carbonate rocks associated with UH P eclogites in the Tromso Nappe of the Scandinavian Caledonides in Norway. The apatite-rich rock (up to 10 vol. %) is composed of Mg-rich calcite-dolomite exsolutions, almandine-grossular garnet, low-jadeite clinopyroxene, magnesiohornblende, phlogopite, and accessory minerals represented mainly by zircon, Fe-Ti oxides and allanite. Fluorapatite occurring as euhedral crystals in the carbonate matrix and as inclusions in garnet and clinopyroxene shows up to 45 mol. % of the hydroxylapatite component, traces of CO 3 2− , probably CN − and small amounts of the britholite and ellestadite components. Pyrrhotite occurs as crystallographically oriented rods parallel to the c axis of the host hydroxyl-bearing fluorapatite either as a dense trellis or in the form of scarce inclusions. Precipitation of pyrrhotite in the fluorapatite was probably facilitated by a volatile sulphur phase ( e.g ., H 2 S), which was enclosed within the apatite nano-channels and interacted with Fe in apatite. Anhydrite and dolomite rods have also been identified in the apatite, pointing to the presence of HCO 3 − in the fluids. The anhydrite is also trapped by exsolved dolomite from calcite in the carbonate matrix. Crystallisation of anhydrite, and probably also the associated pyrrhotite, at about 550–650°C was deduced from calcite–dolomite thermometry. At these amphibolite-facies, post-UH P conditions rapid pyrrhotite precipitation in the host apatite is presumed. Relaxation of the fluorapatite structure in the a -axis direction during decompression facilitated the formation of the oriented inclusions in apatite.

Genesis and stability of accessory phosphates in silicic magmatic rocks: a Western Carpathian case study

Genesis and stability of accessory phosphates in silicic magmatic rocks: a Western Carpathian case study The formation of accessory phosphates in granites reflects many chemical and physical factors, including magma composition, oxidation state, concentrations of volatiles and degree of differentiation. The geotectonic setting of granites can be judged from the distribution and character of their phosphates. Robust apatite crystallization is typical of the early magmatic crystallization of I-type granitoids, and of late magmatic stages when increased Ca activity may occur due to the release of anorthite from plagioclase. Although S-type granites also accumulate apatite in the early stages, increasing phosphorus in late differentiates is common due to their high ASI. The apatite from the S-types is enriched in Mn compared to that in I-type granites. A-type granites characteristically contain minor amounts of apatite due to low P concentrations in their magmas. Monazite is typical of S-type granites but it can also become stable in late I-type differentiates. Huttonite contents in monazite correlate roughly positively with temperature. The cheralite molecule seems to be highest in monazite from the most evolved granites enriched in B and F. Magmatic xenotime is common mainly in the S-type granites, but crystallization of secondary xenotime is not uncommon. The formation of the berlinite molecule in feldspars in peraluminous melts may suppress phosphate precipitation and lead to distributional inhomogeneities. Phosphate mobility commonly leads to the formation of phosphate veinlets in and outside granite bodies. The stability of phosphates in the superimposed, metamorphic processes is restricted. Both monazite-(Ce) and xenotime-(Y) are unstable during fluid-activated overprinting. REE accessories, especially monazite and allanite, show complex replacement patterns culminating in late allanite and epidote formation.

Pegmatites in two suites of Variscan orogenic granitic rocks (Western Carpathians, Slovakia)

SummaryTwo rare-element (Be-Nb-Ta) granitic pegmatite populations have been observed in the Western Carpathian granitoids: (1) pegmatites with Ti- and Mg-poor mineral assemblages, and (2) pegmatites carrying Ti- and Mg-enriched phases (Nb-Ta oxide minerals, garnet, beryl). Mineral chemistry of the pegmatites reflects the primary composition of the parental granitic rocks. The first pegmatite type is derived from monazite-bearing orogenic granites (MOG), and the second from allanite-bearing orogenic granites (AOG). The MOG produced an abundance of pegmatites, whereas in the AOG group the pegmatites are less evolved and relatively scarce. The two kinds of pegmatites support the subdivision of the Western Carpathian granitoids into two principal genetic groups.ZusammenfassungIn den Granitoiden der West-Karpathen kommen zwei Populationen von Selten-Element (Be-Nb-Ta) granitischen Pegmatiten vor: (1) Pegmatite mit Ti- und Mg-armen Mineralvergesellschaftungen und (2) Pegmatite mit Ti- und Mg-angereicherten Phasen (Nb-Ta Oxyde, Granat, Beryll). Die Mineralchemie der Pegmatite spiegelt die primäre Zusammensetzung der granitischen Ursprungsgesteine wider. Der erste Pegmatit-Typ stammt von Monazit-führenden orogenen Graniten (MOG) ab, und der zweite von Allanit-führenden orogenen Graniten (AOG). Die MOG sind für eine Vielzahl von Pegmatiten verantwortlich, während die Pegmatite der AOG-Gruppe weniger entwickelt und relativ selten sind. Das Vorkommen dieser zwei Arten von Pegmatiten unterstützt die Unterteilung der Granitoide der West-Karpathen in zwei genetische Hauptgruppen.

Variscan thrusting in I- and S-type granitic rocks of the Tribeč Mountains, Western Carpathians (Slovakia): evidence from mineral compositions and monazite dating

Abstract The Tribeč granitic core (Tatric Superunit, Western Carpathians, Slovakia) is formed by Devonian/Lower Carboniferous, calc-alkaline I- and S-type granitic rocks and their altered equivalents, which provide a rare opportunity to study the Variscan magmatic, post-magmatic and tectonic evolution. The calculated P-T-X path of I-type granitic rocks, based on Fe-Ti oxides, hornblende, titanite and mica-bearing equilibria, illustrates changes in redox evolution. There is a transition from magmatic stage at T ca. 800–850 °C and moderate oxygen fugacity (FMQ buffer) to an oxidation event at 600 °C between HM and NNO up to the oxidation peak at 480 °C and HM buffer, to the final reduction at ca. 470 °C at ΔNN= 3.3. Thus, the post-magmatic Variscan history recorded in I-type tonalites shows at early stage pronounced oxidation and low temperature shift back to reduction. The S-type granites originated at temperature 700–750 °C at lower water activity and temperature. The P-T conditions of mineral reactions in altered granitoids at Variscan time (both I and S-types) correspond to greenschist facies involving formation of secondary biotite. The Tribeč granite pluton recently shows horizontal and vertical zoning: from the west side toward the east S-type granodiorites replace I-type tonalites and these medium/coarse-grained granitoids are vertically overlain by their altered equivalents in greenschist facies. Along the Tribeč mountain ridge, younger undeformed leucocratic granite dykes in age 342±4.4 Ma cut these metasomatically altered granitic rocks and thus post-date the alteration process. The overlaying sheet of the altered granites is in a low-angle superposition on undeformed granitoids and forms “a granite duplex” within Alpine Tatric Superunit, which resulted from a syn-collisional Variscan thrusting event and melt formation ~340 Ma. The process of alteration may have been responsible for shifting the oxidation trend to the observed partial reduction.