Helene Brätz’s research while affiliated with Universitätsklinikum Erlangen and other places

Four isolated occurrences of Tertiary volcanic rocks in the northern Spessart at Beilstein, Hoher Berg, Madsteinand Kasselgrund are relics of volcanic vents or dikes. They display alkaline basalts (s.l.) with mainly trachybasalticcomposition, which, from normative mineral contents, may be designated as nepheline-bearing alkali-olivine basalts andbasanites. In part, centimetre-sized xenoliths of spinel lherzolite occur. According to Ar-Ar dating, the alkaline basalts(s.l.) from Kasselgrund have erupted at 18.1 ± 0.3 or 19.3 ± 0.4 Ma, those of Hoher Berg between c. 18 and c. 21 Ma. Theseages correspond to the Vogelsberg eruption stage I. A slightly younger Ar-Ar age of 16.8 ± 0.3 Ma was recorded for theBeilstein basalt, which is in chronological accordance to the turn of Vogelsberg eruption stages II and III. Samples of allfour occurrences reveal major and trace element compositions, which are different from those of the Vogelsberg basalts.Compositions of basalts of the stage III from Vogelsberg coincide most with the Spessart basalts. This signals a special positionof the northern Spessart volcanic rocks either as a discrete spatial part of the Vogelsberg volcanic suite or as smaller,independent eruption centres.

Imperial Porphyry, a famous dimension stone of spectacular purple color, was quarried in the Mons Porphyrites area north of Jabal Dokhan in the Eastern Desert of Egypt, from the beginning of the first until the middle of the fifth century AD. During this period, the valuable material was processed as decorative stone and was used for objects of art, reserved exclusively for the Imperial court of the Roman Empire. Later on, only antique spoils of smaller or bigger size have been re-used for these purposes. The Imperial Porphyry is a porphyritic rock of trachyandesitic to dacitic composition that occurs in the uppermost levels of shallow subvolcanic sill-like intrusions, forming a member of the Dokhan Volcanic Suite. Its purple color is mainly due to dispersed flakes of hematite, resulting from hydrothermal alteration of a dark green Common Porphyry of similar composition, underlying the Imperial Porphyry. Both, the Common Porphyry and the purple Imperial Porphyry’, are extensively exposed in the Roman quarries. Contacts between Common and Imperial Porphyry are irregular and gradational. In both rock types, intrusive breccias are frequent, indicating a complex intrusion history. U–Th–Pb zircon geochronology on two samples of Imperial Porphyry and one sample of the Common Porphyry yielded an age range of 609–600 Ma, thus confirming earlier results of radiometric dating. Geochemical evidence indicates that both the Imperial and the Common Porphyry are of medium- to high-K calc-alkaline affinity. The magmas have formed by partial melting of a subduction-modified upper mantle. The subsequent intrusion took place within a highly extended terrane (HET).

The Münchberg Massif in northeastern Bavaria, Germany is an allochthonous metamorphic nappe complex within the Saxothuringian Zone of the Variscan orogen. From top to bottom it consists of four major units: Hangend-Serie, Liegend-Serie, Randamphibolit-Serie and Prasinit-Phyllit-Serie, which show an inverted metamorphic gradient of eclogite- to amphibolite-facies (top) to greenschist-facies (bottom) and are separated from each other by thrust faults. New geochemical and U-Pb zircon data indicate that the four units host metasedimentary and meta-igneous rocks which were formed at different time and in distinct geotectonic settings during the evolution of the Saxothuringian terrane between 550 and 370 Ma. Mafic and felsic protoliths of the Hangend-Serie result from a bimodal magmatism in an evolved oceanic to continental magmatic arc setting at about 550 Ma. These rocks represent relics of the Cadomian magmatic arc, which formed a cordillera at the northern margin of Gondwana during the Neoproterozoic. The Liegend-Serie hosts slivers of granitic orthogneisses, emplaced during magmatic events at c. 505 and 480 Ma, and Early Palaeozoic paragneisses, with our samples deposited at ≤ 483 Ma. Ortho- and paragneisses were affected by an amphibolite-facies metamorphic overprint at c. 380 Ma. Granite emplacement and sediment deposition can be related to the separation of the Avalonia microterrane from the northern Gondwana margin. Amphibolite protoliths of the Randamphibolit-Serie emplaced at c. 400 Ma. They show MORB to E-MORB signatures, pointing to their formation along an oceanic spreading centre within the Rheic ocean. Mafic igneous rocks in the Prasinit-Phyllit-Serie emplaced at nearly the same time (407–401 Ma), but their calc-alkaline to tholeiitic character rather suggests formation in an intra-oceanic island arc/back arc system. This convergent margin lasted for about 30 Ma until the Late Devonian, as is suggested by a maximum deposition age of 371 Ma of associated phyllites, and by metamorphic Ar-Ar ages of 374–368 Ma. The timing of the different magmatic and sedimentary events in the Münchberg Massif and their plate tectonic settings are similar to those estimated for other Variscan nappe complexes throughout Europe, comprising the French Massif Central and NW Spain. This similarity indicates that the Münchberg Massif forms part of a European-wide suture zone, along which rock units of different origin were assembled in a complex way during the Variscan Orogeny.

Stratigraphically well-defined volcanic rocks in Palaeozoic volcano-sedimentary units of the Frankenwald area (Saxothuringian Zone, Variscan Orogen) were sampled for geochemical characterisation and U–Pb zircon dating. The oldest rock suite comprises quartz keratophyre, brecciated keratophyre, quartz keratophyre tuff and basalt, formed in Upper Cambrian to Tremadocian time (c. 497– 478 Ma). Basaltic volcanism continued until the Silurian. Quartz keratophyre shows post-collisional calc-alkaline signature, the Ordovician–Silurian basalt has alkaline signature typical of continental rift environments. The combined datasets provide evidence of Cambro-Ordovician bimodal volcanism and successive rifting until the Silurian. This evolution very likely resulted from break-up of the northern Gondwana margin, as recorded in many terranes throughout Europe. The position at the northern Gondwana margin is supported by detrital zircon grains in some tuffs,with typical Gondwana-derived age spectra mostly recording ages of 550–750 Ma and minor age populations of 950–1100 and 1700–2700 Ma. The absence of N-MORB basalt in the Frankenwald area points to a retarded breakoff of the Saxothuringian terrane along a continental rift system from Uppermost Cambrian to Middle Silurian time. Geochemical data for a second suite of Upper Devonian basalt provide evidence of emplacement in a hot spot-related ocean-island setting south of the Rheic Ocean. Our results also require partial revision of the lithostratigraphy of the Frankenwald area. The basal volcanic unit of the Randschiefer Formation yielded a Tremadocian age and, therefore, should be attributed to the Vogtendorf Formation. Keratophyre of the Vogtendorf Formation, previously assigned to the Tremadoc, is most likely of Upper Devonian age.

Drill cores from the Lower Saxony Basin display iron and base metal sulfides in the Zechstein Staßfurt Carbonate (Ca2). The general geologic setting and geochemical conditions at which mineralizing events took place are currently being investigated within the framework of the BMBF-funded MinNoBeck project — 'Resource potential of covered hydrothermal mineralization in the North German Basin'. The major aim is to assess the potential for economically significant ore deposits that contain strategic elements such as Ga, Ge and In. This MSc thesis focusses on a detailed petrographic and geochemical study of the drill core samples and will contribute to a comprehensive genetic model of the ore-forming processes. Of particular interest are the sources of metals and sulfur, and the fluid-rock interactions. Therefore, sulfides, such as ZnS, PbS, FeS2, and associated gangue minerals are being investigated. Here, we present preliminary petrographic and geochemical results. Ore microscopy provides first details about the mineralogy and textures that are present in the drill core samples. Electron-Probe Micro-Analysis provides information about the chemistry of ore minerals as well as compositional patterns. In addition, the samples are being analyzed by laser ablation-ICP-MS in order to detect trace element abundances. Petrographic observations show almost no (hydrothermal) alteration and grain boundaries between sulfides and the Ca 2 dolostone are generally sharp. Geochemical data indicate that at least three distinct varieties of sphalerites are present — (1) low-Fe sphalerite ranging from 10-50 ppm; (2) moderate-Fe sphalerite ranging from 0.2-0.8 wt.-%; (3) high-Fe sphalerite ranging from 1 – 2.8 wt.-% of total iron. The Co/Ni ratios in pyrites are overall comparatively low and indicate a sedimentary to hydrothermal origin of the pyrite (Fig. 1). Preliminary analyses provide data for geochemical fingerprinting which assists in creating a genetic model. Figure 1: Co/Ni-plot based on LA-ICP-MS data for pyrites from mineralized zones in sediments of the North-German Basin. Colored fields after Misra (2000) and Co/Ni-ratio lines after Price (1972) — Co/Ni ratios above 1 indicate a hydrothermal origin (red samples point) Misra, K. C. (2000): Understanding Mineral Deposits. Kluwer Academic Publishers, 845 p. Price, B. J. (1972): Minor elements in pyrites from the Smithers map area, BC and exploration applications of minor element studies. Doctoral dissertation, University of British Columbia.

The distinction between regional metamorphic and metasomatic mineral assemblages may be occasionally challenging when the two phenomena show a very close spatial and temporal association. The Khetri complex in NW India, provides an excellent example of a geological terrane that has been significantly affected by metamorphism followed by extensive metasomatism. The albitic quartzite, orthoamphibole-cordierite quartzite and chlorite metapelite in northern Khetri complex are the outcome of Na-Cl-Fe-Mg metasomatism. The andalusite metapelite and associated scapolite-bearing metasediments, however, do not show any significant metasomatic overprint and the former rock unit has been used to deduce the metamorphic conditions, nature of provenance and weathering processes. The peak P-T conditions inferred from the conventional geothermobarometry and pseudosection modeling are consistent at 550°C and 3.5 kbar. The geochemical data suggest that the source rocks of the metapelites were compositionally mature and experienced intense chemical weathering in a passive margin setting. The metasomatic overprint postdates the regional metamorphism and, the new monazite U-Th-Pb age data constrains the age of metasomatism at 900-850 Ma and that of the regional metamorphism at ca. 975 Ma in the Khetri complex.

About the characterisation and the origin of central European ”High-magnesium glass”

Cu- and Mn-bearing tourmalines from Brazil and Mozambique were characterised chemically (EMPA and LA-ICP-MS) and by X-ray single-crystal structure refinement. All these samples are rich in Al, Li and F (fluor-elbaite) and contain significant amounts of CuO (up to ~1.8 wt%) and MnO (up to ~3.5 wt%). Structurally investigated samples show a pronounced positive correlation between the distances and the (Li + Mn$^{2+}$ + Cu + Fe$^{2+}$) content (apfu) at this site with R$^2$ = 0.90. An excellent negative correlation exists between the distances and the Al$_2$O$_3$ content (R$^2$ = 0.94). The samples at each locality generally show a strong negative correlation between the X-site vacancies and the (MnO + FeO) content. The Mn content in these tourmalines depends on the availability of Mn, on the formation temperature, as well as on stereochemical constraints. Because of a very weak correlation between MnO and CuO we believe that the Cu content in tourmaline is essentially dependent on the availability of Cu and on stereochemical constraints.

Copper-bearing tourmalines are highly prized for their vivid coloration. We analysed the major and trace elements of some gem-quality Cu-bearing tourmalines (e.g. blue, greenish blue, yellowish green, green, violet and pink) from Brazil, Mozambique and Nigeria. Most of them contained significant amounts of Cu, Mn or a combination of both elements. There was no clearcut correlation of the Cu and Mn contents with coloration. Blue colour was in most cases due to Cu²⁺. Pink and violet coloration (due to Mn³⁺) was shown by Mn-bearing tourmalines that contained no significant Fe. Green colour in the Nigerian tourmaline was most probably due to a combination of Mn, Cu and Fe. Some of the green samples from Brazil contained up to 0.6 wt.% V2O3. Among the trace elements, remarkable contents of Pb (up to 4,000 ppm) and Bi (up to 2,900 ppm) were detected rarely in samples from all three countries. Based on a comparison of unheated pink and violet samples with data for blue Paraíba-type tourmalines, CuO/MnOtot is usually ≥0.5 for unheated blue samples. Hence, we suggest that blue Cu-bearing tourmalines with CuO/MnOtot <0.5 may have been heat treated to reduce the contribution of the reddish component of Mn³⁺.