Open-system behaviour of detrital zircon during weathering: an example from the Palaeoproterozoic Pretoria Group, South Africa

Detrital zircon in six surface samples of sandstone and contact metamorphic quartzite of the Magaliesberg and Rayton formations of the Pretoria Group (depositional age c. 2.20–2.06 Ga) show a major age fraction at 2.35–2.20 Ga, and minor early Palaeoproterozoic – Neoarchaean fractions. Trace-element concentrations vary widely, with Ti, Y and light rare earth elements (LREEs) spanning over three orders of magnitude. REE distribution patterns range from typical zircon patterns (LREE depletion, heavy REE enrichment, well-developed positive Ce and negative Eu anomalies) to patterns that are flat to concave downwards, with indistinct Ce and Eu anomalies. The change in REE pattern correlates with increases in alteration-sensitive parameters such as Ti concentration and (Dy/Sm) + (Dy/Nd), U–Pb discordance and content of common lead, and with a gradual washing-out of oscillatory zoning in cathodoluminescence images. U and Th concentrations also increase, but Th/U behaves erratically. Discordant zircon scatters along lead-loss lines to zero-age lower intercepts, suggesting that the isotopic and chemical variations are the results of disturbance long after deposition. The rocks sampled have been in a surface-near position (at least) since Late Cretaceous time, and exposed to deep weathering under intermittently hot and humid conditions. In this environment, even elements commonly considered as relatively insoluble could be mobilized locally, and taken up by radiation-damaged zircon. Such secondary alteration effects on U–Pb and trace elements can be expected in zircon in any ancient sedimentary rock that has been exposed to tropical–subtropical weathering, which needs to be considered when interpreting detrital zircon data.

Insight on the possible use of black shales for geological storage of radioactive wastes

This study was undertaken to investigate the natural materials before and after appropriate physicochemical treatments. The samples were collected at different depths from the outcrop Lamsied located in the Tarfaya-Boujdour basin. This work concerns a mineralogical and geochemical characterization of local black shale. For instance, mineralogical and granulometric analysis showed that the local black shale is composed essentially of calcite, and the texture does not depend neither on the depth nor on the lithology. The distribution of stable elements such as rare earth elements (RRE) and other trace and major elements was determined. Different techniques of analysis were used for the characterization of the samples. Enrichment or depletion of major elements was observed. NASC-normalized REE patterns revealed a heavy REE (HREE) enrichment, a light REE (LREE) depletion, a positive Eu anomaly and a negative Ce anomaly. The result indicates reduction conditions. Results of correlation analysis suggest the association of La, Ce, Dy, Ho, Er, Yb and Lu with terrigenous minerals and of Eu, Sm and Tm with carbonates and TOC (total organic carbon).

Lithospheric mantle beneath the Vogelsberg volcanic field (Central Germany)

<p>Vogelsberg is a Cenozoic volcanic field situated at the northern tip of the Upper Rhine Graben. It stretches over two major Variscan basement units: the Rheno-Hercynian Zone in the NW and the Saxo-Thuringian Zone in the SE. We studied peridotite xenoliths from Breitenborn, Nidda and Dreihausen (SE, central and NW part of Vogelsberg, respectively) in order to reveal the evolution of the subcontinental lithospheric mantle (SCLM) rejuvenated during a Cenozoic rifting episode.</p><p>The Vogelsberg xenoliths are spinel harzburgites and clinopyroxene-poor spinel lherzolites. Most samples show grain size reduction leading to serial or porphyroclastic texture, or slight to well-defined foliation. All studied sites have similar major elements chemistry: olivine Fo 89.3-91.7%; orthopyroxene (opx) Mg# 0.89-0.92 and 0.06-0.25 atoms of Al pfu (per formula unit); clinopyroxene (cpx) Mg# 0.89-0.93 and 0.10-0.33 atoms of Al pfu. Spinel Cr# is highly variable: 0.18-0.45 for Breitenborn, 0.14-0.57 for Nidda and 0.11-0.61 for Dreihausen.</p><p>Vogelsberg peridotites exhibit a diversity of REE patterns:</p><p>(1) opx with a sinusoidal pattern, no cpx (Nidda, Dreihausen);</p><p>(2) cpx with flat patterns; coexisting opx with strong LREE-depletion, (La/Lu)<sub>N</sub> ~0.02 (Nidda, Dreihausen)</p><p>(3) cpx with flat, spoon-shaped patterns with La-Ce-enrichment (La/Pr)<sub>N</sub> ~4.3; opx similar to (2) but partly spoon-like, (Nd/Lu)<sub>N</sub> ~0.02 (Nidda, Breitenborn)</p><p>(4) cpx with different degree of LREE-enrichment, (La/Lu)<sub>N</sub>­ of 4-21.4; coexisting opx with mild LREE-depletion, (La/Lu)<sub>N</sub> of 0.1-0.3 (Breitenborn, Nidda, Dreihausen)</p><p>(5) cpx with flat HREE pattern and strongly LREE-depleted, (La/Eu)<sub>N</sub> ~0.03; coexisting opx similar to (2) but with (Ce/Lu)<sub>N</sub> ~0.001 (Breitenborn)</p><p>Temperatures calculated using REE content (T<sub>REE</sub>) [1] for the Breitenborn peridotites exhibit two ranges: 930-990°C and 1050-1130°C, for the Nidda ones: 880-930°C, 1000-1050°C and 1110-1150°C and for Dreihausen ones: 1140-1190°C. Temperatures calculated on the basis of pyroxene major element contents (T<sub>BKN</sub>) [2] are 40-90°C lower than T<sub>REE</sub> in Breitenborn and Nidda and lower by 10-55°C in Dreihausen.</p><p>The most common pyroxene REE patterns (type 4) are products of two-phase metasomatism: by Vogelsberg alkali basalt followed by a highly LREE-rich melt that further increased LREE contents in cpx, up to observed abundances. Strongly LREE-depleted opx (types 2, 3, 5) and cpx (type 5) patterns could be residues after partial melting of a fertile protolith, or products of metasomatism by melts derived from depleted MORB mantle. Cpx patterns of type 2 and 3 might have been once similar to type 5 but were later affected by the second phase of metasomatism: highly LREE-rich melt that increased chromatographically their LREE contents to variable degrees. The diversity of REE patterns and calculated temperatures shows that the SCLM beneath Vogelsberg is highly heterogeneous, probably due to spatial variability of deformation and percolation of hot melts connected with Cenozoic rifting.</p><p> </p><p>This study was funded by Polish National Science Centre to MZ (UMO-2018/29/N/ST10/00259) and JP (UMO-2014/15/B/ST10/00095). EPMA analyses were done thanks to the Polish-Austrian projects WTZ PL/16 and WTZ PL 08/2018. MZ acknowledges the DAAD fellowship at Goethe University Frankfurt.</p><p>References</p><p>[1] Liang Y. et al. (2013). GeochimCosmochimActa 102, 246–260.</p><p>[2] Brey G. & Köhler T. (1990). JPetrol 31, 1353–1378.</p>

Genesis and evolution of the mantle melts of the devonian mafic-ultramafic rocks from the Eastern Azov region (Dnieper-Donetsk rift, Ukraine) based on the clinopyroxene geochemistry study

The Devonian magmatic association of the Eastern Azov region, which is part of the Pripyat-Dnieper-Donetsk rift zone, was studied. The association includes gabbroids, peridotites, pyroxenites, and lamprophyre dikes of the Pokrovo-Kireevsky massif (PKM) and picrites, picrobasalts and basalts of the Anton-Taramskaya suite (ATS). The clinopyroxenes of different generations from the micaceous PCM gabbro and the alkaline ATS picrite were studied. It was obtained the information on the mantle source composition and the evolution of melts, who determined the close spatial-temporal location of kimberlites, basites, ultramafic rocks, including alkaline ones, in the Eastern Azov region. Clinopyroxenes from micaceous gabbro are composed of Cpx1 (Mg# = 0.870.88) or Сpx2 (Mg# = 0.800.81) cores and Cpx3 external zones (Mg# = 0.700.76). Clinopyroxenes in alkaline picrite are composed of Сpx2 cores (Mg # = 0.800.84) and external Cpx3 zones (Mg# = 0.710.78). The multielement spectra of clinopyroxenes are generally dome-shaped in nature, with enrichment with LREE, depletion of Ba, Nb, TREE, Zr-Hf negative anomaly, a negative Sr-anomaly appears in Cpx2 and Cpx3 also. The resulting compositions of the model melt for Сpx2 from the micaceous gabbro are very close to the composition of this gabbro, and the compositions of the model melt for Cpx2 from the alkaline picrite coincide with those of this picrite. The high Mg# value and concentrations of Cr in Cpx1 cores indicate that the earliest weakly differentiated composition close to the primary could serve as the equilibrium melt. The presence in the Cpx1 geochemical spectra of a negative Zr-Hf anomaly at ZrPM HfРМ may be evidence of the origin of melts that once contained these clinopyroxenes, due to the melting of metasomatized, possibly carbonated garnet-contained peridotites. Probably, the Cpx1 cores are relics of phenocrysts crystallized from the earliest melt during the formation of the PCM and the ATS. An important feature of the Eastern Azov rocks is a very high content of Ti (up to 7.3 wt.% TiO2) in the high-Mg (Mg# = 0.480.65) and deep (CaO/Al2O3 0.8) melts, which formed picrobasalts and lamprophyres. The geochemical features of the early Cpx1 cores compared with the geochemistry of clinopyroxenes from ilmenite-containing mantle metasomatites are consistent with the assumption that carbonated ilmenite-containing peridotites, possibly also phlogopite-containing (PIC), are the source of ultrahigh-Ti primary melts for the Eastern Azov lamprophyres.

Formation of the Vergenoeg F–Fe–REE Deposit (South Africa) by Accumulation from a Ferroan Silicic Magma

Situated in the centre of the Paleoproterozoic Bushveld Large Igneous Province (LIP) of South Africa the Vergenoeg F–Fe–REE deposit is one of the largest, but at the same time most unusual, fluorite deposits on Earth. In situ major and trace element analyses of fayalite, magnetite, ilmenite, fluorapatite, fluorite and allanite from fayalite-rich rocks are combined with oxygen isotope data for fayalite, magnetite and ilmenite to unravel the complex evolution of the deposit. Textural and compositional characterization of the fayalite-rich rocks supports a magmatic formation as cumulates and an intense late hydrothermal overprint. Fayalite accumulated together with minor Ti-rich magnetite, ilmenite, fluorapatite and allanite from a highly evolved, H2O-poor felsic melt at low oxygen fugacity. Chondrite-normalized rare earth element (REE) patterns of fayalite and the recalculated parental melts, using fayalite–rhyolite partition coefficients, exhibit positive trends with strong enrichment of the heavy REE (HREE) relative to the light REE (LREE). Apart from the LREE depletion the patterns are similar to those of highly fractionated high-silica REE rhyolites that often occur in siliceous LIPs. We attribute the LREE depletion to crystallization of accessory allanite, the main host of the LREE in the cumulates. Chondrite-normalized REE patterns of the parental melt prior to fayalite accumulation, recalculated using allanite–rhyolite partition coefficients, resemble the composition of the rhyolites of the Rooiberg Group and therefore document a petrogenetic link to the Bushveld LIP. High δ18O values of fayalite (up to ≈7·4 ‰) are consistent with its crystallization in a rhyolitic melt that has formed by extensive fractionation from basic melts of the Rustenburg Layer Suite, the mafic member of the Bushveld LIP. Primary fluorite crystallized together with rare quartz, and a second generation of fayalite, magnetite and ilmenite from rare intercumulus melt in interstices between cumulate fayalite. Textural and mineral compositional data, as well as the generally negative δ18O values of magnetite (–2·9 to 0 ‰), are in agreement with the main magnetite–fluorite ore formation in Vergenoeg being related to a hydrothermal overprint, which was responsible for further F and Fe enrichments of the rocks. Fluorine-rich fluids, released from the crystallizing granites of the felsic member of the Bushveld LIP (Lebowa Granite Suite), caused the extensive alteration of fayalite to bowlingite and its replacement by Ti-poor magnetite and quartz. The hydrothermal overprint was associated with the widespread formation of secondary fluorite and minor fluorapatite. Our new petrogenetic model for the Vergenoeg deposit, as constrained from the primary fayalite cumulates, implies that the formation of the Vergenoeg deposit was directly linked to the evolution of the Bushveld LIP.

GEOCHEMISTRY AND TECTONIC SETTING OF ECLOGITE PROTOLITHS FROM KECHROS COMPLEX IN EAST RHODOPE (N.E. GREECE)

Eclogites and partially amphibolitized eclogites from the metamorphic Kechros complex in East Rhodope are studied in order to provide the geodynamic framework for the origin of their protoliths. Geochemical evidence from whole rock major and trace element concentrations shows two distinct protolith groups. The low-Fe-Ti eclogites (Charakoma locality) have low-TiO2 content (<0.67 wt%), negative Nb anomalies, positive Sr anomalies, small negative Zr and Hf anomalies and variable enrichments in LILE (e.g. Rb and Ba). The REE patterns are characterized by strong LREE enrichment (LaN/YbN=5.45-5.81), HREE depletion (GdN/YbN=1.60-1.63) and HREE abundance within the rangeof 9-10 × chondrite. The high-Fe-Ti eclogites (Kovalo and Virsini locality) have variable Sr contents, small to moderate LILE enrichment, HREE`s similar to MORB values and absence of Nb anomalies. The REE patterns of the Kovalo and Virsini eclogites are characterized by LREE depletion and relative flat MREE HREE patterns at approximately 20-30 × chondrite concentrations. Our results suggest that the protoliths of the Low-Ti eclogites show a continental rifting tectonic environment. In contrast, the protoliths of the High-Ti eclogites indicate formation of their protoliths by partial melting in an extensional oceanic environment.

LA-ICP-MS U-Pb dating and REE patterns of apatite from the Tatra Mountains, Poland as a monitor of the regional tectonomagmatic activity

This study presents apatite LA-ICP-MS U-Pb age and trace elements concentrations data from different granite types from the Tatra Mountains, Poland. Apatite from monazite and xenotime-bearing High Tatra granite was dated at 339 ± 5 Ma. The apatite LREE patterns reflect two types of magmas that contributed to this layered magma series. Apatite from a hybrid allanite-bearing diorite from the Goryczkowa Unit was dated at 340 ± 4 Ma with apatite LREE depletion reflecting the role of allanite and titanite during apatite crystallization. Apatite crystals from a hybrid cumulative rock from the Western Tatra Mountains were dated at 344 ± 3 Ma. Apatite is one of the main REE carriers in this sample and exhibit flat REE patterns. Taking into account the relatively low closure temperature of the U-Pb system in apatite (350–550°C), the c. 340 Ma apatite ages mark the end of high temperature tectonometamorphic activity in the Tatra Mountains.