Tschermaks mineralogische und petrographische Mitteilungen

Upper amphibolite facies gneisses in the southern Indian Shield show local transformation into veins, clots and patches of orthopyroxene-bearing dry granulites (“incipient charnockites”). Depending upon the protolith composition, these desiccated zones are classified into ortho- and para-charnockites and have developed within rocks of distinct mineralogy and chemistry at different time intervals through the structurally-controlled influx of carbon dioxide-rich fluids. Our geochemical investigations at five critical quarry sections indicate that the incipient charnockites have undepleted chemistry and very low K/Rb values. In the paracharnockite localities, where granulite formation is characterized by consumption of garnet, biotite and quartz to produce orthopyroxene, loss of Rb and Ba and enrichment of Ti are observed. In contrast, the orthocharnockite localities show marked LILE enrichment with gain of K, Rb and Ba and loss of CaO, suggesting extensive replacement of plagioclase in the gneisses by K-feldspar in the charnockite through K-Na-Ca exchange reactions with influxing carbonic fluids. The marked depletion in Fe, Mg, Ti and P in these rocks correlates with progressive dissolution of hornblende, biotite, magnetite and accessory apatite. Our study indicates that gneiss to granulite transformation, even if on a local scale, is not an isochemical phenomenon, but attended by distinct element mobilities, although they are contrastingly different from the geochemical trends in some regional high grade terrains.

Retrograde eclogites and serpentinites from the Hochgrössen massif, Styria, are parts of the Speik complex in the Austroalpine basement nappes of the Eastern Alps. They are in tectonic contact with pre-Alpine gneisses, amphibolites, and Permo-Triassic quartz phyllites (Rannach Series). The eclogites are derived from ocean-floor basalts with affinities to mid-ocean ridge and back-arc basin basalts. Fresh eclogites are rare and contain omphacite with a maximum of 39 mol% jadeite content, garnet (Py15–19) and amphibole. Retrograde eclogites consist of amphibole and symplectites of Na-poor clinopyroxene (5–8 mol% Jd) + albite ± amphibole. Amphiboles are classified as edenite, pargasite, tschermakite, magnesiohornblende and actinolite. In relatively fresh eclogite, edenite is a common amphibole and texturally coexists with omphacite and garnet. An average temperature of 700 °C was obtained for eclogite facies metamorphism using garnet-pyroxene thermometry. A minimum pressure of 1.5 GPa is indicated by the maximum jadeite content in omphacite. Thermobarometric calculations using the TWEEQ program for amphibole in textural equilibrium with omphacite and garnet give pressures of 1.8–2.2 GPa at 700 °C. The equilibrium assemblage of Na-poor clinopyroxene, albite, amphibole and zoisite in the symplectites gives a pressure of about 0.6–0.8 GPa at 590–640 °C. 40Ar/39Ar radiometric dating of edenitic amphibole in textural equilibrium with omphacite gave a plateau age of 397.3 ± 7.8 Ma, and probably indicates retrograde cooling through the closure temperature for amphibole (∼500 °C). The age of the high-pressure metamorphism thus must be pre-Variscan and points to one of the earliest metamorphic events in the Austroalpine nappes known to date.

Der im Altkristallin des mittleren Ötztales gelegene “Winnebachgranit” ist ein Migmatit mit granitisch-granodioritischem Chemismus. Der kontinuierliche Übergang vom Migmatit in die umgebenden Paragneise und der ähnliche Chemismus von Biotit-Plagioklas-Gneis-Schollen und Mobilisat lassen sich am besten durch eine Anatexis in situ erklären. Die aus den bekannten wasserhältigen granitischen Systemen abgeleiteten Schmelztemperaturen der Winnebachgesteine wurden durch Experimente bestätigt. Der früheste Schmelzbeginn erfolgt in Gesteinen mit den geringsten CaO-Gehalten bei ca. 660°C (4 kb). Unveränderte Gneisschollen im Migmatit deuten an, daß die Maximaltemperatur der Anatexis 685°C (4kb) nicht überschritten hat. 1–2 km entfernte Biotit-Plagioklas-Gneise zeigen bei einem, für die frühe Anatexis geeigneten Chemismus, keinerlei Aufschmelzung. Diese Beobachtung wird, unter Annahme von Wassersättigung im gesamten Komplex, durch eine um mindestens 20°C geringere Temperatur als im Zentrum des Migmatits erklärt. Die Bildung des Migmatits gehört als letztes Großereignis in die voralpidische Metamorphose.

Polymetallicmajor veins of the West Carbery district (County Cork) are compared with the nearby stratiform-disseminated copper mineralization in metasedimentary rocks, containingminor veins (metamorphic quartz veins and veinlets). These stratiform deposits are hosted by non-marine Devonian sediments (Old Red Sandstone), metamorphosed in the Hercynian orogeny. In sulphides from the stratiform deposits and minor veins, isotopic compositions of sulphur (δ34S) range from − 21.00 to + 5.14%0, consistent with the vein sulphide being remobilized stratiform-disseminated sulphide, and the latter being of diagenetic bacteriogenic origin. Sulphate (barite), found in veins separate from the sulphides, has δ34S + 12.3 to + 15.7%0. consistent with groundwater origin. In minor-vein quartz, fluid inclusions have homogenization temperatures consistent with trapping under the estimated peak-metamorphic conditions (300–400°C, 1–3 kbar). In the major veins, sulphide δ34S (−15.8 to −4.2 0) suggest remobilization of diagenetic sulphide. Oxygen and hydrogen isotopes suggest deposition from metamorphic fluids (calculated δ18OH2O approximately + 8 to + l3%0, measured range of δD −52.2 to −27.3%0). Immiscible C02-bearing fluids were trapped in the temperature range 280–350°C with fluid pressure < ca. 600 bar. The inferred pressure-temperature history is attributed to late-metamorphic uplift, with fluid pressures falling below lithostatic. The sulphide-bearing veins are interpreted as a small-scale example of redistribution of mineral deposits by metamorphic fluids.

Es wird über die erste elektronenoptische Abbildung von Mineralien (Bleiglanz, Kupferkies) berichtet. Die Abbildung erfolgte unter Benutzung einer magnetischen Linse mit lichtelektrisch im Hochvakuum ausgelösten Elektronen, die auf rund 25 kV beschleunigt wurden.