1. The Neoproterozoic East African Orogen: petrological, tectonic and geochronological investigations

During the last decade research has increased knowledge of the East African orogens by applying modern petrological, geochronological, structural geological, and geochemical techniques. The Department of Earth Sciences at the University of Graz has conducted intensive studies in these areas which were funded by the Austrian Science Foundation by FWF project P12375 “Geodynamic setting of the Panafrican Orogenesis in Eastern Africa” and FWF project P15599 “Pre-Pan-African versus Pan-African evolution in the Mozambique Belt of Tanzania”. The working group in Graz consists of Harald Fritz, Georg Hoinkes, Christoph Hauzenberger, Stefan Oberwalder, Veronika Tenczer and the project leader Prof. Wallbrecher, who retired in 2006. The ongoing studies have resulted in several publications (e.g., Hauzenberger et al., 2004,2005,2007; Fritz et al., 2005, Tenczer et al., 2006, Tenczer et al., 2007) and presentations at international conferences. A new FWF project in northern Tanzania is planned.

The new geochronological, geochemical, structural geological and petrological data have led to a better understanding of the different orogenic events which took place in East Africa. The Paleoproterozoic Usagaran Belt is found to the east of the Archaean Tanzania craton. It consists of greenschist facies metasedimentary rocks, amphibolites, eclogite lenses, some granulites as well as abundant granitoids and volcanic sequences. The Neoproterozoic Mozambique Belt (MB) reworked parts of the Archaean and Paleoproterozoic rocks. We introduce for this part of the MB in Tanzania the term "Western Granulites". They are composed of granulite facies metapsammites and metapelites, migmatic gneisses, and mafic Grt-Px-Am bearing granulites. The "Eastern Granulites", also part of the MB, are clearly distinct from the "Western Granulites" by the occurrence of a basal unit consisting of migmatic Grt-Px-Am-Pl gneisses (meta-enderbites), mafic granulites and lenses of meta-anorthosites and a cover sequence consisting mainly of marbles, calcsilicates, and metapelites. The calc-alkaline geochemical characteristic of the meta-magmatic rocks of the “Eastern Granulites” indicate an Neoproterozoic island arc setting. The main metamorphic overprint in the “Western and Eastern Granulite” terrains occurred at about 640 Ma which is attributet to the collision of Azania with East Africa (Collins & Pisarevski, 2005). In southern Kenya, the Galana Shear Zone separates the Eastern Granulites (exposed in the Taita Hills) from another granulite facies terrane (“Galana East”) which underwent metamorphism around 550 Ma. PT conditions of granulite facies rocks from the “Western and Eastern Granulites” show surprisingly very uniform PT conditions of 770 to 850°C and 1.0 to 1.3 GPa. Considering the vast areas covered by the “Western and Eastern Granulites” we are concerned about the possibility whether these estimated uniforme PT values are possibly artefacts of geothermobarometry (closure temperature of diffusion in garnet?). In order to evaluate this presumption selected samples from different areas from the “Western and Eastern Granulites” are investigated in detail by applying different geothermobarometric methods as well as interpreting observed mineral assemblages and reaction textures with calculated pseudosections.

                                        Digital elevation model of East Africa with main tectonic units and their age

1.1. Pan-African high pressure granulites from SE-Kenya: petrological and geothermobarometric evidence for a polycyclic evolution in the Mozambique belt. (Hauzenberger et al., 2004)

Two different Pan-African tectono-metamorphic events are recognised in the Taita Hill-­Tsavo East National Park/Galana river area, SE-Kenya (Mozambique belt) based on petrographic and geothermobarometric evidence. Structurally, this area can be subdivided into 4 units: (1) the easternmost part of the basement along the Galana river is characterized by subhorizontal slightly to the west and east dipping foliation planes. Migmatic paragneisses with intercalated marbles, calcsilicates and metapelites and bands of amphibolites are the dominant rock type. (2) The western part of the Galana river within the Tsavo East National Park is a ca. 25 km wide shear zone with subvertical foliation planes. The eastern part shows similar rocks as observed in unit 1, while towards west, metasedimentary units become rare and the main rock types are tonalitic gneisses with intercalated amphibolites. (3) A 10 kilometer wide zone (Sagala Hills zone) between the strike slip zone (unit 2) and the Taita Hills (unit 4) is developed. This zone is characterized by elongated and folded felsic migmatic amphibole and garnet bearing orthogneiss bodies with intercalated bands of mafic rocks. (4) The Taita Hills are a slightly to the N dipping nappe stack. The main rock type in the Taita Hills are amphibole - biotite - plagioclase - quartz ± garnet ± clinopyroxene ± scapolite bearing migmatic gneisses with mafic bands. In the southern part, metapelites, marbles and some amphibolites are common.

Although the geological structures are different in units 1 and 2, the calculated PT conditions are similar with peak PT of 760-820°C and 7.5-9.5 kbar. Temperatures in unit 3 (Sagalla Hills zone) and unit 4 (Taita Hills) are slightly higher ca. 760-840°C, but pressure is significantly higher, ranging from 10 to 12 kbar. Sillimanite growth around kyanite, garnet zonation pattern, mineral reaction textures, and PT calculations constrain a "clock-wise" PT-path with near isobaric cooling following the peak of metamorphism. The different PT conditions, tectonic setting, and a different age of metamorphism are evidence that units 1 and 2 (Galana river) belong to a different tectono-metamorphic event than unit 3 (Sagala Hills zone) and 4 (Taita Hills). The major shear zone (unit 2) marks a tectonic suture dividing the two different tectono-metamorphic domains. It is also likely that it played an important role during exhumation of the granulite facies rocks from units 3 and 4.

View of the Taita Hills, SE-Kenya. The nappe tranport towards south can be seen from the outline of  the
Taita Hills and in outcrops. To the right there are typical thinsections of graniltes (top) and metapelites
(bottom).

1.2. Garnet Zoning in High Pressure Granulite-Facies Metapelites, Mozambique belt, SE-Kenya: Constraints on the Cooling History

Three metapelitic samples from the granulite facies Taita Hills, part of the Neoproterozoic Mozambique belt in SE-Kenya, contain nearly pure almandine-pyrope garnets. These garnets show a diffusional zoning of X_Fe = Fe/(Fe+Mg) at the rim over a distance of ~200-500 µm if in contact with biotite. Garnet-biotite Fe-Mg exchange thermometry yields closure temperatures between 530 - 735 °C. Diffusion zoning profiles in garnets are used to estimate cooling rates using a numerical model. For the calculations a metamorphic peak temperature and pressure of 820 °C and 1.15 GPa are obtained from mafic granulites. Matching of numerically modelled and observed zoning profiles indicates cooling rates between 1-3°C/my. Comparison with cooling rates estimated with the analytical approach of Ehlers and Powell (1994) and with geochronologically derived cooling rates shows that the volumetric ratio of biotite to garnet was about 0.5 during closure. This is consistent with the volumetric ratio observed in thin section, but inconsistent with microprobe analyses that indicate that only biotite in the immediate vicinity of garnet equilibrated with garnet. Conversely, significant garnet zoning only occurs where in contact with biotite. We suggest that these inconsistencies can be explained with changes in the grain boundary processes during cooling: in the thermal evolution above the closure temperature around 735°C a fast grain boundary model applied so that all biotite in the thin section equilibrated with garnet. At lower temperatures local zoning developed, but did not influence the composition of the garnet grain centers. The change in grain boundary process from fast to slow diffusing grain boundaries may correlate with the solidus temperature of the rock.

            BSE image and two-dimensional chemical composition map of
            garnet in contact with biotite and quartz (left).

            Variation of chemical composition with cooling rate and observed
            chemical composition of garnet (right).

2. Petrological investigations of the crystalline basement rocks between Graz and Vienna

The crystalline basement rocks between Graz and Vienna are not well known because outcrops are rare and the geological units are not well defined. Two bachelor students (Barbara Puhr, Manfred Roeggla) investigated the Radegund crystalline, which is part of the Rappold complex. A Master thesis is currently done by Manfred Roeggla about the Anger crystalline, which consists of three different complexes, the Woelz complex, Rappold complex and the Schoberkogel complex. This work is done in cooperation with the "Geologischen Bundesanstalt", Dr. Ralf Schuster.

Petrologische Untersuchungen im Angerkristallin, Steiermark  (Manfred Röggla, Christoph Hauzenberger, Ralf Schuster)
Das Angerkristallin befindet sich etwa 30 km nordöstlich von Graz und erstreckt sich über eine Gesamtfläche von 90 km². Neubauer (1982) kartierte ein Umschwenken der Schöckelkalke ins Angerkristallin, wodurch er eine präalpine Zusammengehörigkeit interpretierte und stellte somit das Angerkristallin zum Oberostalpin. Geländepetrographische Untersuchungen ergaben, dass das Angerkristallin aus wahrscheinlich 3 Einheiten besteht, die nicht dem Oberostalpin zuzuordnen sind: (1) Der südliche Teil zeigt Ähnlichkeiten mit Gesteinen der Gleinalm und wird dem Rappold Komplex zugeordnet. Dieser Komplex besteht aus dunkelgrauen Glimmerschiefern und Paragneisen. (2) Im Norden sind Gesteine zu finden die dem Wölz Komplex entsprechen. Charakterisiert wird dieser Teil des Angerkristallins durch Granatglimmerschiefer mit Amphiboliten, welche teilweise Granat-führend sind, sowie aus Garbenschiefern (3) Der zentrale Bereich wird nach der Lokalität als Schoberkogel Komplex bezeichnet und besteht aus monotonen Paragneisen, Glimmerschiefern und Einschaltungen von Amphiboliten. Granate aus dem Rappold Komplex zeigen deutliches zwei- und manchmal auch dreiphasiges Granatwachstum mit hohem Ca-Gehalt im Kern, kontiniuierliche Abnahme zum Rand, wo ein sprunghafter Anstieg festzustellen ist. Die Almandinkomponente ist genau gegenläufig. Optisch kann man die Granatgenerationen deutlich aufgrund ihres Graphitgehaltes unterscheiden. Der Kern ist Graphit-frei, Rand1 ist Graphit-reich, Rand2 ist am Graphit-reichsten. Einschlüsse in den Granaten sind neben Quarz hauptsächlich Akzessorien wie Turmalin, Epidot, Zirkon, Monazit sowie Ilmenit. Staurolith kommt stabil mit dem äußersten Granatrand vor.  Die Matrix besteht vor allem aus Muskovit, Biotit, Chlorit, Plagioklas und Quarz. Bildungesbedingungen konnten mit 550-600°C und 8-10 kbar abgeleitet werden. Der Wölz Komplex kann aufgrund der Granatzonierung in ein- und zweiphasige Granate eingeteilt werden. Die Granate der zweiphasigen Wölzer sind typischerweise größer als 5mm wobei sowohl der Granatkern und der Granatrand idiomorphe ausgebildet sein können. Der Kern besitzt einen deutlich niedrigeren Grossulargehalt von 5 Mol%, der diskontinuierlich randlich auf 15 Mol.% ansteigt. Einschlüsse in den Granaten sind neben Quarz hauptsächlich Akzessorien wie Turmalin, Epidot, Zirkon, Monazit sowie Ilmenit. Die Matrix besteht zum größten Teil aus Glimmern, wobei Muskovit und Biotit dominieren und Paragonit nur selten vorkommt. Deutlich zu erkennen ist ein Plagioklas-Blasten Wachstum über die Hellglimmer. Chlorit tritt meist als retrograde Phase auf, wobei dieser Granat vollständig pseudomorph ersetzten kann. Akzessorien sind Zirkon, Epidot, Turmalin, Ilmenit, Rutil und Apatit. Druck und Temperaturberechnungen ergaben Werte von 540 – 570°C und 10-14 kbar. In den einphasigen Wölzer Glimmerschiefern kommt idiomorpher einschlußarmer Granat bis zu 5 mm vor. Die Plagioklas-Blasten treten untergeorndnet auf. PT Bestimmungen sind denen der 2 phasigen Wölzer Gesteine änhlich und haben 520-550°C und ca. 11-12 kbar Der Schoberkogel Komplex kann analog zu den Wölzer Komplex in Einheiten mit ein- und mehrphasigem Granatwachstum eingeteilt werden. Die einphasigen Granate ähneln in ihrer Zusammensetzung jenen der Wölzer Einheit. Die mehrphasigen Granate zeigen Ähnlichkeit zu den Granaten des Rappold Komplexes. PT Bestimmungen ergaben 620-650°C und 11-12 kbar.

View of the southern part of the Anger crystalline complex. Note the difficult outcrop situation.