Timing of magmatic-hydrothermal activity in the Variscan Orogenic Belt: LA-ICP-MS U–Pb geochronology of skarn-related garnet from the Schwarzenberg District, Erzgebirge

Here, we present in situ U–Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) ages of andradite-grossular garnet from four magmatic-hydrothermal polymetallic skarn prospects in the Schwarzenberg District, Erzgebirge (Germany), located in the internal zone of the Variscan Orogenic Belt. Within the geochronological framework of igneous rocks and hydrothermal mineralization in the Erzgebirge, the obtained garnet ages define three distinct episodes of Variscan skarn formation: (I) early late-collisional mineralization (338–331 Ma) recording the onset of magmatic-hydrothermal fluid flow shortly after the peak metamorphic event, (II) late-collisional mineralization (~ 327–310 Ma) related to the emplacement of large peraluminous granites following large-scale extension caused by orogenic collapse and (III) post-collisional mineralization (~ 310–295 Ma) contemporaneous with widespread volcanism associated with Permian crustal reorganization. Our results demonstrate that the formation of skarns in the Schwarzenberg District occurred episodically in all sub-stages of the Variscan orogenic cycle over a time range of at least 40 Ma. This observation is consistent with the age range of available geochronological data related to magmatic-hydrothermal ore deposits from other internal zones of the Variscan Orogenic Belt in central and western Europe. In analogy to the time–space relationship of major porphyry-Cu belts in South America, the congruent magmatic-hydrothermal evolution in the internal zones and the distinctly later (by ~ 30 Ma) occurrence of magmatic-hydrothermal ore deposits in the external zones of the Variscan Orogenic Belt may be interpreted as a function of their tectonic position relative to the Variscan collisional front.

Pure sediment-derived granites in a subduction zone

The Earth is unique in the Solar System due to significant volumes of granite in the lithosphere. However, the origins of granites are still highly debated, especially sediment-derived granites, which are often treated as a geochemical end-member of the continental crust. In the Yunnan region of South China, we identify the occurrence of pure sediment-derived granite in a subduction system. The suite of strongly peraluminous granite reported herein is interpreted to represent pure metasedimentary melts based on their whole-rock elemental and Sr-Nd-B and zircon Hf-O isotopic compositions. These Late Permian−Early Triassic (ca. 254−248 Ma) granites are characterized by radiogenically enriched Sr, Nd, and Hf isotopic signatures. They show δ11B and δ18O signatures akin to those of continental shales. Geochemical modeling indicates no contributions from the mantle that can be detected. Considering the regional tectonic evolution, these granites are suggested to be formed in a subduction zone by decompression melting of rapidly exhumed back-arc sediments. We posit that decompression melting was triggered by widespread extension and thinning of the crust prompted by rollback of the subducting oceanic crust. These granites thus provide evidence that granite formation in subduction zones does not necessarily contribute to crustal growth. These subduction-related pure sediment-derived granites have different elemental ratios and contents (e.g., Al2O3/TiO2 and Yb) from the Himalayan leucogranites. Considering their source compositions (e.g., pelitic rocks), which are similar to those of the Himalayan leucogranites, these differences are likely due to their higher formation temperature and lower pressure despite a great similarity in isotopic compositions. Identification of pure sediment-derived, strongly peraluminous granites (SPGs) in subduction systems provides an important geodynamic mechanism for crustal anatexis, which can both geochemically and tectonically complement their collisional counterparts identified in the Himalayas.

Monazite Microstructures and Their Interpretation in Petrochronology

The phosphate mineral monazite (LREE,Y,Th,Ca,Si)PO4 occurs as an accessory phase in peraluminous granites and Ca-poor meta-psammopelites. Due to negligible common Pb and very low Pb diffusion rates at high temperatures, monazite has received increasing attention in geochronology. As the monazite grain sizes are mostly below 100 μm in upper greenschist to amphibolite facies meta-psammopelites, and rarely exceed 250 μm in granulite facies gneisses and in migmatites, microstructural observation and mineral chemical analysis need the investigation by scanning electron microscope and electron probe microanalyzer, with related routines of automated mineralogy. Not only the microstructural positions, sizes and contours of the grains, but also their internal structures in backscattered electron imaging gray tones, mainly controlled by the Th contents, can be assessed by this approach. Monazite crystallizes mostly euhedral to anhedral with more or less rounded crystal corners. There are transitions from elliptical over amoeboid to strongly emarginated grain shapes. The internal structures of the grains range from single to complex concentric over systematic oszillatory zonations to turbulent and cloudy, all with low to high contrast in backscattered electron imaging gray tones. Fluid-mediated partial alteration and coupled dissolution-reprecipitation can lead to Th-poor and Th-rich rim zones with sharp concave boundaries extending to the interior. Of particular interest is the corona structure with monazite surrounded by apatite and allanite, which is interpreted to result from a replacement during retrogression. The satellite structure with an atoll-like arrangement of small monazites may indicate re-heating after retrogression. Cluster structures with numerous small monazite grains, various aggregation structures and coating suggest nucleation and growth along heating or/and enhanced fluid activity. Microstructures of monazite fluid-mediated alteration, decomposition and replacement are strongly sutured grain boundaries and sponge-like porosity and intergrowth with apatite. Garnet-bearing assemblages allow an independent reconstruction of the pressure-temperature evolution in monazite-bearing meta-psammopelites. This provides additional potential for evaluation of the monazite microstructures, mineral chemistry and Th-U-Pb ages in terms of clockwise and counterclockwise pressure-temperature-time-deformation paths of anatectic melting, metamorphism and polymetamorphism. That way, monazite microstructures serve as unique indicators of tectonic and geodynamic scenarios.

Solubility of Monazite-Cheralite and Xenotime in Granitic Melts, and Experimental Evidence of Liquid-Liquid Immiscibility in Concentrating REE

We provide new experimental data of monazite, xenotime and U-Th-bearing cheralite solubility in slightly peralkaline to peraluminous granitic melts using dissolution and reverse (i.e., recrystallisation after dissolution) experiments in water-saturated and flux-bearing (P+F+Li) granitic melts, at 800 °C and 200 MPa. Although a positive correlation between REE solubility and melt peralkalinity is confirmed, monazite solubilities reported here are much lower than the values previously published. We suggest that the presence of elevated phosphorus concentrations in our melts depresses monazite solubility, principally because phosphorus complexes with Al and alkali which normally depolymerise the melt through the formation of non-bridging oxygens. The new solubility data provide an explanation for the very low REE concentrations generally encountered in phosphorus-bearing peraluminous granites and pegmatites. This accounts for the compatibility of REE in peraluminous systems, as the early crystallisation of REE-bearing minerals (mainly monazite and zircon) leads to progressive REE depletion during liquid differentiation. In addition, dissolution and reverse experiments of U-Th-bearing cheralite-monazite display liquid-liquid immiscibility processes in our slightly peralkaline glass. The immiscible liquid forms droplets up to 10 µm in diameter and hosts in average 35 wt.% P2O5, 25-30 wt.% F, 22 wt.% Al2O3, 4 wt.% CaO, 5 wt.% Na2O, 2 wt.% La2O3, and 12 wt.% ThO2+UO2. We believe that the droplets formed during the runs and may have coalesced to larger droplets during quenching. We suggest that liquid-liquid immiscibility is a possible mechanism of REE concentration in highly-fluxed melts and should be considered in natural systems where REE are extremely concentrated (up to thousands of µg/g) in magmatic rocks.

Partial melting as an efficient mechanism to produce rare metal granite?

<p>Rare metal (HFSE such Sn, W, Ta, Nb and LILLE such Li, Rb) granite represent the most enriched magmatic rocks on Earth. This is especially true for some elements that belongs either to the European list of critical raw materials and/or the conflict minerals (eg. Li, Sn, W, Nb, Ta). Rare metal granites generally emplace in the vincinity of S-type granites during late orogenic stages. The fraction crystallisation mechanism is postulated to be the unique way to produce enriched silicate melt that later leads to ore deposits due to a combination of magmatic/hydrothermal processes. However, some problems persist in the explanation of the genesis of rare metal granite: crystal fractionation alone does not lead to the very high rare metal concentrations; field discrepancies exist between rare metal granites and their supposed parent peraluminous granites that in some cases are unknown. An alternative model - based on the integration of geochemical, experimental, paleogeographical and structural studies – suggests that low degree partial melting could be an efficient mechanism to produce critical metals enriched silicate melts enriched. To test whether this hypothesis makes sense, we present a study of the behaviour of W, Sn, Nb and Ta in metamorphic minerals from various metapelitic rocks. The selected samples do not present anomalous bulk concentrations of these elements with respect to an average continental crust. They formed at different pressure temperature conditions, in different orogenic belts. The rock collection comprises (i) amphibolite-facies staurolite bearing rocks, (ii) sillimanite-bearing rocks and (iii) granulite-facies orthopyroxene-bearing rocks. These samples represent the three main stages of the classical evolution of a collisional gradient leading to partial melting: they respectively belong to the muscovite + biotite domain, the muscovite-out reaction and the biotite-out reaction. We first estimate pressure-temperature conditions of formation of the rocks using pseudosection modelling. We then expose a set of LA-ICP-MS data to identify the critical metal carriers minerals in our samples. Meanwhile, we investigate the behaviour of W, Sn, Nb and Ta during the muscovite out reaction with two piston cylinder experiments (a partial melting experiment and a crystallization experiment). The protolith consists of a staurolite-bearing metapelite that did not suffer partial melting. In the light of these new data, we discuss the framework of the production of critical metal enriched silicate melts. We show that the main carrier of W is muscovite (up to 30 ppm) and that biotite handle Sn at high temperature (up to 40ppm). Using both the data from the natural sample and the experiments, we highlight that muscovite releases W during its destabilisation ant that Sn enters in biotite until the mineral breaks. We finally discuss the implication of multiple low degree partial melting / melt extraction as efficient way to produce enriched silicate melts.</p>

Thermodynamic properties, crystal structure and phase relations of pushcharovskite [Cu(AsO₃OH)(H₂O) ⋅ 0.5H₂O], geminite [Cu(AsO₃OH)(H₂O)] and liroconite [Cu₂Al(AsO₄)(OH)₄ ⋅ 4H₂O]

The phases pushcharovskite, geminite and liroconite were synthesized or acquired and characterized by powder X-ray diffraction, infrared spectroscopy, electron microprobe analysis, thermogravimetric analysis and optical emission spectrometry, as needed. Their thermodynamic properties were determined by a combination of acid-solution calorimetry and relaxation calorimetry, resulting in Gibbs free energies of formation (ΔfGo, all values in kilojoules per mole) of -1036.4±3.8 (pushcharovskite, Cu(AsO3OH)(H2O)⋅0.5H2O) and -926.7±3.2 (geminite, Cu(AsO3OH)(H2O)). For the natural liroconite (Cu2Al[(AsO4)0.83(PO4)0.17](OH)4⋅4H2O), ΔfGo=-2996.3±9.2 kJ mol−1. The estimated ΔfGo for the endmember Cu2Al(AsO4)(OH)4⋅4H2O is −2931.6 kJ mol−1. The crystal structure of liroconite was refined (R1=1.96 % for 962 reflections with I>3σ(I)) by single-crystal X-ray diffraction and the positions of H atoms, not known previously, were determined. Liroconite is a rare mineral, except for several localities, notably Wheal Gorland in England. Thermodynamic modelling showed that liroconite will be preferred over olivenite if the Al(III) concentration in the fluid reaches levels needed for saturation with X-ray amorphous Al(OH)3. We assume that such fluids are responsible for the liroconite formation during contemporaneous oxidation of primary Cu–As ores and pervasive kaolinization of the host peraluminous granites. pH had to be kept in mildly acidic (5–6), and the activities of dissolved silica were too low to form dioptase. The main stage with abundant liroconite formation was preceded by an acidic episode with scorodite and pharmacosiderite and followed by a late neutral to mildly basic episode with copper carbonates. Geminite and pushcharovskite, on the other hand, are minerals typical for very acidic solutions. At the studied site in Jáchymov (Czech Republic), extremely acidic water precipitates arsenolite; sulfate is removed by formation of gypsum. Geminite associates with other acidic minerals, such as slavkovite, yvonite and minerals of the lindackerite group. Pushcharovskite is metastable with respect to geminite and probably converts quickly to geminite under field conditions.