Gregor Markl

Composition of fluids in the lower crust inferred from metamorphic salt in lower crustal rocks

Knowledge of the rheological properties of the lower crust and the metamorphic processes that operate there is important for our understanding of orogenic processes and granite genesis. The rheological properties critically depend on whether fluids are present in the lower crust, and, if present, on their composition. Fluid-inclusion and phase-equilibria, studies of lower crustal granulites have shown that fluids with low water activities (due to the presence of dissolved components such as CH4, N2, CO, CO2 and chlorides) are present at least episodically in the lower crust. Here we report the occurrence of a solid salt solution (NaCl–KCl) found together with chlorine-rich amphibole and biotite in lower crustal granulites. A desiccation mechanism explains how salt and chlorine-rich minerals formed from an originally water-rich fluid through a short-lived series of hydration reactions in the granulites, during which chlorine was progressively enriched in the fluid. Consequently, it would appear that fluid was present in the lower crust in only small amounts and was not stable over geologically long periods of time, leading to the conclusion that the lower crust is devoid of a free fluid phase during most of its history.

Metamorphic evolution of Pan-African granulite facies metapelites from Southern Madagascar

Abstract Granulite-facies, metapelitic gneisses from five localities in Southern Madagascar exhibit very similar mineral assemblages and textures and reveal a very similar P-T evolution. They show an early assemblage of Grt+Sil+Qtz+Bio+Kfsp+Rt+Ilm±Pl±Spl which records P-T conditions of about 880±60°C at 8±1 kbar. In some samples, this assemblage is overprinted by the later assemblage Crd+Grt+Sil+Bio+Kfsp+Qtz+Mt±Crn which equilibrated at about 690±40°C and 4±1 kbar. The main Crd-producing reaction Spl + Qtz = Crd was aided by the further reactions Sil + Bio + Qtz = Crd + Kfsp + H 2 O and Grt + Sil + Qtz = Crd . Crn formed from relictic Spl, probably during cordierite formation, by the oxidation reaction 3 Hc +0.5 O 2 =3 Crn + Mt . H2O activities calculated from phase equilibria for both metamorphic events range from 0.6 to 1 which is in agreement with the observation of partial melting during the HT stage. We interpret age data from the literature to suggest that the HT/HP event occurred at about 560–580 Ma, while the MT/MP event could be related to intrusive activity between 520 and 540 Ma, possibly during the same orogenic cycle. The similarities in textures and equilibration conditions among the Malagasy samples strongly imply — in contrast to the opinion of many previous workers — a common tectonometamorphic history of the various parts of Southern Madagascar between 600 and 500 Ma. The P-T-conditions recorded by the samples and the age data correspond closely to P-T-paths deduced for other Gondwana fragments like Southern India, Sri Lanka and parts of East Antarctica. This correspondence may help to constrain the location of the suture for the collision between East and West Gondwana in the late Proterozoic.

Stability of high-Al titanite from low-pressure calcsilicates in light of fluid and host-rock composition

Abstract Titanite of variable Al and F content was found in granulite- to amphibolite-facies calcsilicates in Central Dronning Maud Land, Antarctica. The highest observed Al content corresponds to an XAl [= Al/(Al + Ti)] of 0.53. Previously, such high values of XAl were reported from high-pressure rocks, but the titanite of this study is from a lowpressure terrain. The compositional variations in titanite can be described for all samples by a set of three linearly independent exchange vectors added to the CaTiSiO5 endmember titanite. In most rocks, these vectors are Al1F1Ti-1O-1, Ti-0.25M0.25O-1OH1, and OH1F-1; in one sample, the Ti-0.25⃞0.25O-1OH1 vector is replaced by a Si-0.25⃞ 0.25O-1OH1 vector. The actual amount of exchange along these vectors and, therefore, the amount of Al in titanite, depends on P and T, on the composition of the coexisting fluid phase in terms of its H2O/HF fugacity ratio, and on host rock composition in terms of Al2O3/TiO2 activity ratio. It is inferred that, in suitable chemical environments, high-Al titanite is stable over a wide P-T range. Therefore, the Al content of titanite should not be used in geothermobarometry, even qualitatively. Additionally, because of the coupled substitutions Al1F1Ti-1O-1 and Al1OH1Ti-1O-1, the concentration of F in titanite is strongly dependent on the host rock chemistry. This rules out the easy use of titanite as a monitor of fluid composition.

U-Pb ages of ferberite, chalcedony, agate, ‘U-mica’ and pitchblende: constraints on the mineralization history of the Schwarzwald ore district

This study deals with the use of new and unusual geochronometers applied to a variety of mineralizations in the Schwarzwald region of southwest Germany. In detail, we present new U-Pb age data of ferberite (from Clara Mine), chalcedony (from Silberbrunnle Mine), agate (Geisberg), carneol (“Markgrafler Land”), and confirm earlier geochronologic work on multistage U-mineralizations (Wittichen and Menzenschwand). Using these various geochronometers, we show both their usefulness, precision and robustness by comparing our age data to earlier work, and we can augment existing data with our new dataset to draw a picture of the timing of late- to post-Variscan hydrothermal mineralization in the region. The combined dataset reveals that vein-type mineralizations in the Schwarzwald have formed episodically over the last 300 Ma with distinct peaks of mineralization around 300 Ma (late-Variscan), several periods between 200 and 100 Ma (possibly related to the opening of the North Atlantic Ocean in the Jurassic and early Cretaceous), and around 40–20 Ma (related to the opening of the Rhinegraben). This pulsed formation of hydrothermal deposits is not unique for the Schwarzwald, as it has also been documented for the more richly mineralized Erzgebirge and other parts of post-Variscan Europe. The wide age range found within some districts or vein systems does not represent an analytical artefact, but reflects multiple reactivations of the same structures associated with changes in the stress distribution at continental margins. Comparison with the Erzgebirge shows, however, that the intensity of mineralization in a given time period shows regional differences, which reflect differences in regional tectonics.

The compositional variability of eudialyte-group minerals

Abstract Eudialyte-group minerals (EGM) represent the most important index minerals of persodic agpaitic systems. Results are presented here of a combined EPMA, Mössbauer spectroscopy and LA-ICP-MS study and EGM which crystallized in various fractionation stages from different parental melts and mineral assemblages in silica over- and undersaturated systems are compared. Compositional variability is closely related to texture, allowing for reconstruction of locally acting magmatic to hydrothermal processes. Early-magmatic EGM are invariably dominated by Fe whereas hydrothermal EGM can be virtually Fe-free and form pure Mn end-members. Hence the Mn/Fe ratio is the most suitable fractionation indicator, although crystal chemistry effects and co-crystallizing phases play a secondary role in the incorporation of Fe and Mn into EGM. Mössbauer spectroscopy of EGM from three selected occurrences indicates the Fe3+/∑Fe ratio to be governed by the hydration state of EGM rather than by the oxygen fugacity of the coexisting melt. Negative Eu anomalies are restricted to EGM that crystallized from alkali basaltic parental melts while EGM from nephelinitic parental melts invariably lack negative Eu anomalies. Even after extensive differentiation intervals, EGM reflect properties of their respective parental melts and the fractionation of plagioclase and other minerals such as Fe-Ti oxides, amphibole and sulphides.

Fluid mixing forms basement-hosted Pb-Zn deposits: Insight from metal and halogen geochemistry of individual fluid inclusions

Fluid mixing across unconformities between crystalline basement and overlying sedimentary basins is commonly invoked as an efficient chemical mechanism for ore deposition, but the origin of basement brines and the process of ore formation have rarely been linked by direct evidence. Using laser ablation–inductively coupled plasma–mass spectrometry microanalysis of individual fluid inclusions with an improved detection approach for anion components, we determined simultaneously the ore metal concentrations and the Cl/Br ratio in texturally well constrained inclusion assemblages from a basement-hosted quartz-fluorite-barite-Pb-Zn vein system. An inverse correlation between the Pb + Zn concentrations and the Cl/Br mass ratios in the fluid inclusions provides clear evidence for mixing of a basement-derived metal-rich brine and a metal-poor formation water that acquired its salinity from halite dissolution in Triassic evaporites of the sedimentary cover. This mixing of two distinct brines with comparable salinity is recorded during the growth of individual quartz crystals containing small galena inclusions, demonstrating the transient and episodic nature of fluid mixing during mineral deposition.

Proterozoic eclogites from the Lofoten islands, northern Norway

Abstract Massif lenses and bands of variably retrogressed eclogites occur within the granulite facies Proterozoic basement of the Lofoten islands in northern Norway. Eclogites are often related to shear and mylonite zones suggesting that deformation and fluid access was important for their formation. The eclogitic rocks are restricted to gabbroic or troctolitic rock compositions and show, in the least retrogressed samples, the typical assemblage garnet + omphacite + rutile. The widespread occurrence of eclogite shows that the whole group of the Lofoten islands was affected by high pressure metamorphism. Eclogites were found on three of the five major islands so far (Flakstadoy, Vestvagoy, Austvagoy). Thermobarometry reveals minimum conditions during eclogite formation of 680 °C/15 kbar on Flakstadoy and 540 °C/14 kbar on Austvagoy. Geological, petrological and geochemical data suggest the following succession of events: (a) Intrusion of mafic to mangeritic magmas (at about 1.8 Ga; Griffin et al., 1978). (b) Incomplete recrystallization under high-pressure granulite facies conditions. Associated with granulitization was the formation of Cl-rich hydrosilicates. Granulite formation was incomplete except for zones of intensive deformation. (c) Eclogites formed in response to a significant pressure increase in the same high strain zones. However, scattered eclogite lenses and boudins also occur in gneisses. (d) Post-eclogite retrogression occurred under middle to high pressure amphibolite facies conditions (probably the Leknes event at 1.1 Ga of Griffin et al., 1978). Eclogite formation was related to crustal thickening during the final stages of early Proterozoic magmatism and granulite facies metamorphism.

A geochemical and Sr-Nd-O isotopic study of the Proterozoic Eriksfjord Basalts, Gardar Province, South Greenland: Reconstruction of an OIB signature in crustally contaminated rift-related basalts

Abstract Basalts from the volcano-sedimentaiy Eriksfjord Formation (Gardar Province, South Greenland) were erupted at around 1.2 Ga into rift-related graben structures. The basalts have compositions transitional between tholeiite and alkaline basalt with MgO contents <7 wt.% and they display LREE-enrichment relative to a chondritic source. Most of the trace element and REE characteristics are similar to those of basalts derived from OIB-Iike mantle sources. Initial 87Sr/86Sr ratios of clinopyroxene separates range from 0.70278 to 0.70383 and initial εNd values vary from -3.2 to +2.1. The most unradiogenic samples overlap with the field defined by carbonatites of similar age and can be explained by mixing of isotopically depleted and enriched mantle components. Using AFC modelling equations, the Sr-Nd isotope data of the more radiogenic basalts can successfully be modelled by addition of <5% lower crustal granulite-facies gneisses as contaminants. δ18Ov-smow values of separated clinopyroxene range from +5.2 to +6.0%o and fall within the range of typical mantle-derived rocks. However, up to 10% mixing with an average lower crustal component are permitted by the data.

New Caledonian eclogite province in Norway and potential Laurentian (Taconic) and Baltic links

Field observations and 4 0 Ar/ 3 9 Ar isotopic dating indicate that eclogites exposed in the Lofoten Islands, north Norway, formerly presumed to be Proterozoic features, most likely formed as a result of early to middle Paleozoic, i.e., Caledonian, metamorphism. The Lofoten eclogites occur in shear zones that cut Baltic Precambrian continental basement. This unusual style of occurrence is shared only with Caledonian shear-zone eclogites of the allochthonous Bergen arcs of western Norway. Our findings help to link Scandinavian eclogites with those on the Laurentian side of this collisional zone in East Greenland. Ordovician to Silurian eclogites also are found locally throughout the southern, Appalachian continuation of the orogen in eastern North America. We compare the pressures, ages, and tectonic and structural settings of the eclogites along the ∼10,000 km length of the Appalachian-Caledonian system. Our synthesis supports the idea that Laurentian Taconic elements may be preserved in high-level nappes in Norway. The rare, deep-crustal metamorphic relicts also appear to be shared between Baltica and Laurentia, offering a new perspective in which to view the geodynamic evolution of this once-Earth-spanning orogenic system.

Fluid mixing from below in unconformity-related hydrothermal ore deposits

Unconformity-related hydrothermal ore deposits typically form by mixing of hot, deep, rock-buffered basement brines and cooler fluids derived from the surface or overlying sediments. Current models invoking simultaneous downward and upward flow of the mixing fluids are inconsistent with fluid overpressure indicated by fracturing and brecciation, fast fluid flow suggested by thermal disequilibrium, and small-scale fluid composition variations indicated by fluid inclusion analyses. We propose a new model where fluids first descend, then evolve while residing in pores and later ascend. We use the hydrothermal ore deposits of the Schwarzwald district in southwest Germany as an example. Oldest fluids reach the greatest depths, where long residence times and elevated temperatures allow them to equilibrate with their host rock, to reach high salinity, and to scavenge metals. Youngest fluids can only penetrate to shallower depths and can (partially) retain their original signatures. When fluids are released from different levels of the crustal column, these fluids mix during rapid ascent in hydrofractures to form hydrothermal ore deposits. Mixing from below during ascent provides a viable hydromechanical mechanism to explain the common phenomenon of mixed shallow and deep fluids in the formation of hydrothermal ore deposits.