Marion Tichomirowa

From Cadomian subduction to Early Palaeozoic rifting: the evolution of Saxo-Thuringia at the margin of Gondwana in the light of single zircon geochronology and basin development (Central European Variscides, Germany)

Abstract Saxo-Thuringia is classified as a tectonostratigraphic terrane belonging to the Armorican Terrane Collage (Cadomia). As a former part of the Avalonian-Cadomian Orogenic Belt, it became (after Cadomian orogenic events, rift-related Cambro-Ordovician geodynamic processes and a northward drift within Late Ordovician to Early Silurian times), during Late Devonian to Early Carboniferous continent-continent collision, a part of the Central European Variscides. By making use of single zircon geochronology, geochemistry and basin analysis, geological processes were reconstructed from latest Neoproterozoic to Ordovician time: (1) 660–540 Ma: subduction, back-arc sedimentation and tectonomagmatic activity in a Cadomian continental island-arc setting marginal to Gondwana; (2) 540 Ma: obduction and deformation of the island arc and marginal basins; (3) 540–530 Ma: widespread plutonism related to the obduction-related Cadomian heating event and crustal extension; (4) 530–500 Ma: transform margin regime connected with strike-slip generated formation of Early to Mid-Cambrian pull-apart basins; (5) 500–490 Ma: Late Cambrian uplift and formation of a chemical weathering crust; (6) 490–470 Ma: Ordovician rift setting with related sedimentation regime and intense igneous activity; (7) 440–435 Ma: division from Gondwana and start of northward drift. The West African and the Amazonian Cratons of Gondwana, as well as parts of Brittany, were singled out by a study of inherited and detrital zircons as potential source areas in the hinterland of Saxo-Thuringia.

Sulfide oxidation and sulfate reduction in a shallow groundwater system (Oderbruch Aquifer, Germany)

Abstract Detailed groundwater monitoring was carried out over a period of two years in an anoxic, river recharged aquifer of the Oderbruch polder, north-eastern Germany. Isotope data from wells located in a 5 km transect along the flow direction was used to determine sources and sinks of SO 4 2− in the aquifer. The SO 4 2− originates from river water infiltration and from oxidative dissolution of FeS 2 within the alluvial loam covering the aquifer sands. A change of confined hydraulic conditions near the river to unconfined conditions in the central polder effects the hydrochemistry of the aquifer. The confined areas are dominated by sulfate reduction. Increasing δ 34 S −SO 4 values suggest continuous but slow ( t 1/2 =50 years) sulfate reduction from the beginning of inflow onwards with δ 34 S −SO 4 values ranging from +1.8 to +44.7‰ versus CDT and an enrichment factor of −33‰. A zone with a strong sulfate depletion ( δ 34 S −SO 4 of up to +85.7‰) exists in a shallow microenvironment rich in solid-phase organic carbon between river and levee. In the unconfined areas of the central polder, a SO 4 2− plume with concentrations exceeding the original river water content indicates FeS 2 oxidation by O 2 and/or NO 3 − within the alluvial loam. The lowered δ 34 S −SO 4 value reflects the input of the isotopically lighter SO 4 2− from the sulfide.

The North American-Caribbean Plate boundary in Mexico-Guatemala-Honduras

Abstract New structural, geochronological, and petrological data highlight which crustal sections of the North American–Caribbean Plate boundary in Guatemala and Honduras accommodated the large-scale sinistral offset. We develop the chronological and kinematic framework for these interactions and test for Palaeozoic to Recent geological correlations among the Maya Block, the Chortís Block, and the terranes of southern Mexico and the northern Caribbean. Our principal findings relate to how the North American–Caribbean Plate boundary partitioned deformation; whereas the southern Maya Block and the southern Chortís Block record the Late Cretaceous–Early Cenozoic collision and eastward sinistral translation of the Greater Antilles arc, the northern Chortís Block preserves evidence for northward stepping of the plate boundary with the translation of this block to its present position since the Late Eocene. Collision and translation are recorded in the ophiolite and subduction–accretion complex (North El Tambor complex), the continental margin (Rabinal and Chuacús complexes), and the Laramide foreland fold–thrust belt of the Maya Block as well as the overriding Greater Antilles arc complex. The Las Ovejas complex of the northern Chortís Block contains a significant part of the history of the eastward migration of the Chortís Block; it constitutes the southern part of the arc that facilitated the breakaway of the Chortís Block from the Xolapa complex of southern Mexico. While the Late Cretaceous collision is spectacularly sinistral transpressional, the Eocene–Recent translation of the Chortís Block is by sinistral wrenching with transtensional and transpressional episodes. Our reconstruction of the Late Mesozoic–Cenozoic evolution of the North American–Caribbean Plate boundary identified Proterozoic to Mesozoic connections among the southern Maya Block, the Chortís Block, and the terranes of southern Mexico: (i) in the Early–Middle Palaeozoic, the Acatlán complex of the southern Mexican Mixteca terrane, the Rabinal complex of the southern Maya Block, the Chuacús complex, and the Chortís Block were part of the Taconic–Acadian orogen along the northern margin of South America; (ii) after final amalgamation of Pangaea, an arc developed along its western margin, causing magmatism and regional amphibolite–facies metamorphism in southern Mexico, the Maya Block (including Rabinal complex), the Chuacús complex and the Chortís Block. The separation of North and South America also rifted the Chortís Block from southern Mexico. Rifting ultimately resulted in the formation of the Late Jurassic–Early Cretaceous oceanic crust of the South El Tambor complex; rifting and spreading terminated before the Hauterivian (c. 135 Ma). Remnants of the southwestern Mexican Guerrero complex, which also rifted from southern Mexico, remain in the Chortís Block (Sanarate complex); these complexes share Jurassic metamorphism. The South El Tambor subduction–accretion complex was emplaced onto the Chortís Block probably in the late Early Cretaceous and the Chortís Block collided with southern Mexico. Related arc magmatism and high-T/low-P metamorphism (Taxco–Viejo–Xolapa arc) of the Mixteca terrane spans all of southern Mexico. The Chortís Block shows continuous Early Cretaceous–Recent arc magmatism.

Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides)—constraints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt ☆

This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data for the major gneiss units of the Eastern Erzgebirge; (1) coarse- to medium-grained “Inner Grey Gneiss”, (2) fine-grained “Outer Grey Gneiss”, and (3) “Red Gneiss”. The Inner and Outer Grey Gneiss units (MP–MT overprinted) have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons (540 Ma) and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are zoned and document Neoproterozoic (ca. 575 Ma) source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Neoproterozoic (600–700 Ma), Paleoproterozoic (2100–2200 Ma), and Archaean (2700–2800 Ma). These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated by geological relationships, similar geochemical composition, similar Nd model ages, and inherited zircon ages. Red Gneiss occurs as separate bodies within fine- and medium-grained grey gneisses of the gneiss–eclogite zone (HP–HT overprinted). In comparison to Grey Gneisses, the Red Gneiss clearly differs in geochemical composition by lower contents of refractory elements. Rocks contain long-prismatic zircons (480–500 Ma) with oscillatory zonation indicating an igneous precursor for Red Gneiss protoliths. Geochemical data display obvious characteristics of S-type granites derived through partial melting from deeper crustal source rocks. The obtained time marks of magmatic activity (ca. 575 Ma, ca. 540 Ma, ca. 500–480 Ma) of the Eastern Erzgebirge are compared with adjacent units of the Saxothuringian zone. In all these units, similar time marks and geochemical pattern of igneous rocks prove a similar tectono-metamorphic evolution during Neoproterozoic–Ordovician time.

Geochemistry of agates: a trace element and stable isotope study

Abstract Agate samples of acidic, intermediate and basic volcanics from 18 localities around the world and of different age (Precambrian to Tertiary) were studied by trace element and stable isotope analysis to provide information about the process of agate formation and the origin of mineral-forming fluids. Trace element data are similar for agates from acidic and basic volcanics. The general chondrite-normalized REE distribution pattern is characterized by a slope from La to Lu with enriched LREE and a positive Eu anomaly in some samples. The similarity in the shape of the REE patterns between agates and the parent volcanic rocks suggests that the elements are mobilized by circulating fluids during syn- and postvolcanic alteration of the volcanic wall rocks. Observed positive Eu anomalies in agates probably originate from feldspar alteration. Deuterium and oxygen isotope analyses of agates and associated quartz incrustations (δD: −44‰ to −130‰; δ18O: +16.4‰ to 33.4‰) reveal variations in isotopic composition between samples of different localities but also within single agate samples (up to 10‰ for δ18O). In general, oxygen isotope compositions become heavier as volcanic host rocks grow more acidic. Furthermore, agates have higher δ18O values than associated quartz incrustations. Variations within single agate samples can be explained either by kinetic effects during isotope fractionation (e.g., the formation of agate from a noncrystalline precursor) or by mixing processes of meteoric and magmatic fluids. Remarkably high δ18O values of the parent volcanic rocks (up to+19.5‰) suggest that the circulation of 18O enriched hydrothermal fluids originated from heated meteoric water and/or residual magmatic fluids. This conclusion is supported by δ13C and δ18O data of paragenetic calcite. The temperature of agate formation was calculated for different fluid compositions, and they indicate a temperature range of ca. 50°C to 250°C.

Effects of natural radiation damage on back-scattered electron images of single crystals of minerals

Abstract Generally, it has been assumed that signal intensity variations in back-scattered electron (BSE) images of minerals are mainly controlled by chemical heterogeneity. This is especially true for images of single crystals, where effects of different crystal orientations with respect to the incident beam on the observed BSE are excluded. In contrast, we show that local variations of the structural state within single-crystals (i.e., degree of lattice order or lattice imperfectness) may also have dramatic effects on the back-scattering of electrons. As an example, we present BSE images of single-crystals of natural zircon, ZrSiO4, whose intensity patterns are predominantly controlled by structural heterogeneity, whereas effects of chemical variations are mostly negligible. In the case of natural zircon, structural heterogeneity affecting the BSE patterns is predominantly due to heterogeneous accumulation of radiation damage. We attempt to explain our observations with lowered penetration and channeling and, thus, enhanced back-scattering of electrons in more radiation-damaged internal zones and microareas. Back-scattered electron contrast of natural zircon is, therefore, considered as a special case of electron channeling contrast. This phenomenon seems to have been generally underappreciated in the discussion of BSE images of radiation-damaged minerals thus far.

Building the Pamir‐Tibetan Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 2. Timing and rates

Geothermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before ~35 Ma and peaked at ~23–20 Ma, reflecting top-to- ~N thrust-sheet and fold-nappe emplacement that tripled the thickness of the upper ~7–10 km of the Asian crust. Multimethod thermochronology traces cooling through ~700–100°C between ~22 and 12 Ma due to exhumation along dome-bounding normal-sense shear zones. Synkinematic minerals date normal sense shear-zone deformation at ~22–17 Ma. Age-versus-elevation relationships and paleoisotherm spacing imply exhumation at ≥3 km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene and enhanced cooling (7–31°C/Myr) starting between ~23 and 12 Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome, implies (i) regionally distributed, Paleogene crustal thickening; (ii) Pamir-wide gravitational collapse of thickened crust starting at ~23–21 Ma during ongoing India-Asia convergence; and (iii) termination of doming and resumption of shortening following northward propagating underthrusting of the Indian cratonic lithosphere at ≥12 Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep-seated processes, e.g., slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.

Regional and temporal (1992–2004) evolution of air-borne sulphur isotope composition in Saxony, southeastern Germany, central Europe†

The isotopic composition of air-borne sulphur was investigated in Saxony, Southeast Germany – a region with formerly very high atmospheric SO2 concentrations. In addition, data from various authors were compiled for different Saxonian locations, spanning from 1992 to 2004, i.e., a time of decreasing SO2 concentrations in the atmosphere. There were no obvious temporal changes in the mean δ34S value of bulk precipitation. However, the variability of monthly mean δ34S values decreased. The mean sulphur isotope composition of sulphate from bulk precipitation after the year 2000 converges in Saxony towards 4–5‰, with similar values for different locations. Mean values of different forms of sulphur show the following enrichment order: δ34S of SO2 < δ34S of weathering crusts ≤δ34S of sulphate from bulk precipitation ≤δ34S of dust. Judging from local differences on sulphate crusts and corresponding isotope values of sources, the δ34S value of SO2 as well as for crusts mainly reflects local point sources. The mean δ34S value of bulk precipitation represents more regionally well-mixed SO2 sources and is therefore an ideal tool for monitoring regional atmospheric change. †Revised version of a paper presented at the 27th Annual Meeting of the German Association for Stable Isotope Research (GASIR), October, 4–6, 2006, Freiberg, Germany.

SULFUR AND OXYGEN ISOTOPE GEOCHEMISTRY OF ACID MINE DRAINAGE - THE POLYMETALLIC SULFIDE DEPOSIT ''HIMMELFAHRT FUNDGRUBE'' IN FREIBERG (GERMANY)

Abstract We investigated physical, chemical and isotope (S, O) parameters of sulfate from acid mine drainage from the polymetallic sulfide ore deposit Freiberg (Germany), which was mined for more than eight hundred years. Two main groups of water were distinguished: 1. Flowing mine water with sulfate concentrations of less than 9000 mg/1 and pH values higher than 3.2 2. Pore water in weathered low grade ores and pools with sulfate concentrations higher than 9000 mg/1 and pH values below 3.2. The sulfur and oxygen isotope composition of sulfate from flowing mine waters reflects mixing of sulfate from two sulfur sources: a) atmospheric sulfur from precipitation and b) sulfate formed as a result of sulfide oxidation processes. Sulfur isotope values of mine water sulfate were used to estimate the contribution of sulfate derived through oxidation of sulfides. The sulfur isotope composition of pore water sulfate and precipitated sulfate (jarosite) from weathered low grade ore samples is identical to the sulfur isotope composition of primary sulfides. The oxygen isotope composition of pore water sulfate from low grade ore samples indicates that the oxidation process proceeds relatively slowly in O2-depleted waters, probably without significant microbial catalysis.

The influence of pyrite grain size on the final oxygen isotope difference between sulphate and water in aerobic pyrite oxidation experiments

Oxidation experiments with different pyrite grain sizes (63–100, 100–140, 140–180 μm) were carried out to investigate the oxygen and sulphur isotope composition of sulphate produced under aerobic acid conditions, which may help to understand oxidation mechanisms and to interpret data from natural sites. Long-term experiments with grain size 63–100 μm showed that constant δ 18OSO4 values were not achieved before 100 days. The final oxygen isotope difference between water and sulphate indicates that a small proportion of molecular oxygen is incorporated into sulphate even in the later course of the oxidation due to sulphite oxidation by molecular oxygen. However, most of the sulphate oxygen derives from water. Similar δ 18OSO4 values from experiments with grain sizes 63–100, 100–140, and 140–180 μ m indicate similar oxidation mechanisms for all three grain sizes. These results differed from previous results of identical experiments with grain size<63 μ m, where higher δ 18OSO4 values were obtained. We propose that the greater proportion of molecular oxygen in sulphate from oxidised fine-grained pyrite is caused by an intensified adsorption of molecular oxygen on sulphur sites of ultrafine pyrite particles. Hence, the formation of sulphate from the (initial) reaction on sulphur sites of pyrite and from sulphite oxidation should be more dominant if ultrafine material is present. The δ 34SSO4 values (2.0–2.7) obtained from experiments with the coarser grain sizes agreed with the δ 34S value of pyrite (2.4), whereas sulphur isotopes of sulphate obtained from previous experiments with fine-grained pyrite showed an initial 32S enrichment compared with pyrite. Due to the lack of δ 34SSO4 values from the beginning of the experiments with coarser grain sizes, it remains speculative that sulphur isotopes indicate at least initial differences in oxidation mechanisms between fine and coarser pyrite grain sizes.