Christoph Hauzenberger

SHRIMP U–Pb zircon and Sm–Nd garnet ages from the granulite-facies basement of SE Kenya: evidence for Neoproterozoic polycyclic assembly of the Mozambique Belt

The Taita Hills–Galana River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20–30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a metamorphic overprint at 630–645 Ma, similar to areas in Tanzania. An emplacement age for the magmatic precursor rocks of 850–960 Ma was derived from zircon cores. Sm–Nd garnet–whole-rock analyses give an age of 585 Ma, interpreted as the cooling age after 630–645 Ma metamorphism. Nd crustal residence ages are between 1000 and 1500 Ma. The Galana Shear Zone east of the Taita Hills contains strongly deformed tonalitic migmatites with interlayered pegmatites that date a younger tectonometamorphic event at 560–580 Ma. East of the shear zone only a young metamorphic age of 550 Ma was found. The Nd model ages are c. 1500 Ma to c. 2900 Ma. In a continental configuration prior to Gondwana break-up our study area was located close to Madagascar, where several large shear zones are observed. One of these shear zones (Ranotsara Shear Zone) may be a continuation of the Galana Shear Zone.

SHRIMP zircon ages for post-Usagaran granitoid and rhyolitic rocks from the Palaeoproterozoic terrain of southwestern Tanzania

We report SHRIMP zircon U-Pb ages for post-Usagaran granitic- to granodioritic intrusives and a rhyolitic agglomerate from the Palaeoproterozoic terrain of southwestern Tanzania. This terrain consists of strongly deformed and metamorphosed rocks ascribed to the ca. 2 Ga Usagaran mobile belt, voluminous post-Usagaran granitoids, and minor supracrustal successions. The southeastern part of this terrain is characterized by the occurrence of little deformed and virtually unmetamorphosed rhyolithic, dacitic and andesitic volcanic rocks which overlie the older Usagaran basement. These rocks extruded between 1820 and 1921 Ma, as documented by SHRIMP zircon ages, which are in good agreement with previous ages reported from this region, from the Ubendian belt farther west, and from Palaeoproterozoic crustal fragments in the Neoproterozoic Pan-African Mozambique belt to the east. A 2519O9 Ma Archaean zircon xenocryst was found in one volcanic rock sample, also in good agreement with Archaean ages of the Tanzania craton to the northwest and Archaean rocks in the central part of the Mozambique belt. Our age data suggest that the post-Usagaran assemblage, including crustal domains farther east in the MB, was generated during a relatively short period of ~100 Ma in the late Palaeoproterozoic, probably during a crustal accretion event connected with widespread magmatic activity and generally ascribed in Africa to the Eburnian orogeny.

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 & 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.

Solubility of the assemblage albite+K-feldspar+andalusite+quartz in supercritical aqueous chloride solutions at 650 °C and 2 kbar

Abstract The solubility of the high grade pelite assemblage albite+K-feldspar+andalusite+quartz at 650 °C and 2 kbar was determined in aqueous solutions over a total chloride range of 0.01–3 mCltot using rapid-quench hydrothermal technique. The concentration of Na, K, Si, and Al was determined in the fluid phase after quench. The K/Na ratio was determined by approaching the equilibrium from below and above. It is 0.34 at low chloride concentrations and decreases slightly to 0.31 with increasing total chloride. Silica and aluminum concentrations were determined only from undersaturation. The silica solubility is found to be independent of chloride concentration and is ∼0.13 molal. Aluminum is nearly independent of chloride concentration decreasing only slightly from ∼0.0015 to ∼0.0007 molal. Comparison of the experimental data with thermodynamic model calculations demonstrates that the silica concentrations are well predicted, while significant differences exist between individual databases for Al speciation and its total concentration. Al concentrations are underestimated by up to 10 to 15 orders of magnitude using the SUPCRT92 database. Predicted K/Na ratios are underestimated by up to 30%. The best predictions achieved for this simplified high-grade pelite assemblage are those using the SUPCRT92 database with revised thermodynamic data for feldspars and K- and Na-species (J. Phys. Chem. Ref. Data 24 (1995) 1401) and additional Al-species (Am. J. Sci. 295 (1995) 1255; Geochim. Cosmochim. Acta 61 (1997) 2175). The use of ideal mixing for neutral complexes in combination with the extended Debye–Huckel activity model for the charged species yields the most compatible speciation model.

Experimental study on the solubility of the model -pelite mineral assemblage albite + K-feldspar + andalusite + quartz in supercritical chloride-rich aqueous solutions at 0.2 GPa and 600°C

A total of 34 solubility experiments using the “model”-pelite mineral assemblage microcline + low albite + andalusite + quartz were performed at 600°C and 0.2 GPa over a total chloride range of 0.03–2.9 molal. The concentrations of sodium, potassium, aluminum, and silica were measured and the results are compared with four different thermodynamic datasets. The K/Na ratio was approached from below and above for the thermodynamically buffered mineral assemblage microcline + low albite + andalusite + quartz. Tight brackets were obtained for experiments performed in up to 1 molal chloride concentration. From 0.03 to ∼1 molal chloride concentration, a constant K/Na ratio of 0.33 was obtained. At 3 molal chloride concentration the K/Na ratio decreases to 0.28. Experiments accidentally running out of quartz show a different trend. The K/Na ratio systematically decreases from 0.39 to 0.27 from low to high chloride concentrations. Silica and aluminum concentrations were only approached from below. The silica concentration of 0.1 molal is constant over the investigated total chloride concentration. Aluminum is more concentrated at low Cltot, decreasing from 0.001 to 0.0005. Comparison of experimental results with computed aqueous speciation generated with four different thermodynamic databases shows good qualitative agreement. For the most part, predicted total concentrations using the extended Debye-Huckel activity model for charged solutes, and setting the Setchenow coefficient to zero for neutral species, are generally within an order of magnitude of those measured, which is likely within the uncertainties of the calculations.

THERMAL HISTORY: A new software to interpret diffusive zoning profiles in garnet

Mineral grains can record the cooling history of metamorphic terrains by preserving characteristic chemical zoning profiles caused by diffusion. A range of analytical and numerical models have been used to describe the relationship between the cooling rate and the shape of chemical zoning profiles. Most of these models are characterized by a deficit of usability to external users. This problem is overcome by the code THERMAL HISTORY presented here. The code is platform independent and runs without compilation or a hard disk install. Model results are stored in a basic database and displayed graphically. The code is controlled by an intuitive graphical user interface and uses a very fast diffusion algorithm. THERMAL HISTORY can be used to model zoning profiles as a function of a series of cooling histories, and is written so that it is particularly applicable for the Fe-Mg exchange between garnet and biotite. The code takes into account mass balance so that the volumetric ratio of garnet and biotite can be considered explicitly and it provides a facility to calculate sections through the grains. As some of these facilities have never been published before, the impact of cooling histories, mass balance and section position is explained in some detail. As an application example, THERMAL HISTORY is used to demonstrate that highly non-linear cooling histories, small biotite-garnet ratios and the section effect may result in zoning profiles that are misleading if interpreted in terms of the cooling history.

Mesoproterozoic and Paleoproterozoic subcontinental lithospheric mantle domains beneath southern Patagonia: Isotopic evidence for its connection to Africa and Antarctica

New isotopic studies on mantle xenoliths from Santa Cruz Province, southern Patagonia, Argentina, reveal that at least three discrete subcontinental lithospheric mantle (SCLM) domains—the Deseado Massif, Tres Lagos, and Pali Aike—form the southernmost part of South America. Re-Os systematics yield early Paleoproterozoic (up to 2.5 Ga) SCLM formation ages (rhenium depletion ages, T RD ) for Pali Aike spinel peridotites, while samples from the Deseado Massif and Tres Lagos indicate a younger SCLM origin with Neoproterozoic to Mesoproterozoic (0.9–1.3 Ga) and Mesoproterozoic to late Paleoproterozoic (1.3–1.9 Ga) T RD ages, respectively. Hf-Sr-Nd isotopic compositions indicate metasomatic overprinting of the majority of the samples, which, however, has not affected the Os isotopic system. Based on similar formation ages, the geological evolution of the Deseado Massif is most likely connected to the evolution of the Namaqua-Natal belt of South Africa. T RD ages from SCLM domains underneath Tres Lagos and Pali Aike indicate a common origin with crustal sections from Shackleton Range, Antarctica, positioning the southern tip of South America closer to west Antarctica in the reconstructed Rodinia supercontinent than previously assumed.

Vestiges of the Mesoproterozoic Events in the Neoproterozoic Mozambique Belt: the East African Perspective in the Rodinia Puzzle

Abstract Most of the geological and palaeogeographical models consider the Neoproterozoic supercontinent Gondwana (∼650-550 Ma) as the direct offspring of the disintegrated Mesoproterozoic supercontinent Rodinia (∼1300-750 Ma). One of the main classical sutures along which the dispersing Rodinia fragments were fused into a new supercontinent (Godwana) is identified as the Mozambique belt of East Africa. The calc-alkaline magmatism (∼1200-950 Ma) in northern Mozambique, southern Malawi and southern Tanzania is regarded as the sole evidence for fragmentation of Rodinia, which is traced within this Neoproterozoic orogenic belt. There are no unequivocal Mesoproterozoic (Kibaran) sediments in this orogen. Concrete evidence for Kibaran metamorphism and deformation is missing. Thus, these solitary documented Kibaran magmatic vestiges in the belt do not ascribe to a true complete orogenesis, which involved the disintegration and dispersal of Rodinia. Consequently, the available sparse Mesoproterozoic (Kibaran) geological and isotopic data from the Mozambique belt of East Africa contentiously suggest its involvement in the aggregation of the supercontinent Rodinia at about 1300-1100 Ma ago.

Trattnerite, (Fe, Mg)2(Mg, Fe)3[Si12O30], a new mineral of the milarite group : mineral data and crystal structure

The new mineral trattnerite, (Fe, Mg) 2 (Mg, Fe) 3 [Si 12 O 30 ], a member of the milarite group, occurs in small cavities of a Sirich xenolith at the hauyne-nephelinite quarry Stradner Kogel, eastern Styria, Austria. Trattnerite is hexagonal and forms hypidiomorphic, short prismatic to tabular crystals up to 1 mm. Associated minerals are sanidine, tridymite, quartz, hematite, ortho-and clinopyroxene and clinoamphibole. The colour is pleocroitic deep blue (ω) to yellowish-green (ϵ), the streak is white and the crystals have a vitreous luster. Optically, the mineral is uniaxial (−), ω = 1.589(1), ϵ = 1.586(1) at 589 nm. The cleavage parallel to (001) is good, poor parallel to (100), and the tenacity is brittle. Trattnerite is hexagonal, P6/mcc , with unit cell parameters a = 10.050(1) A, c = 14.338(2) A, V = 1254.1(1) A 3 , Z = 2 and D calc. = 2.68 g/cm 3 . The strongest lines in the X-ray powder diffraction pattern ( d obs in A, ( hkl ), I ) are: 8.70, (100), 97; 7.17, (002), 100; 5.535, (102), 96; 5.026, (110), 61; 3.207, (211), 85. Two single-crystal structure refinements ( R ( F ) = 0.028 and 0.025) of trattnerite yielded following site occupancies: the T1 tetrahedron of the double-ring unit is fully occupied by Si, the T2 tetrahedron and the A octahedron have a mixed (Mg, Fe) occupation with Fe predominantly in A and Mg in T2. The T2-O and A-O bond lengths indicate most of the iron to be in the trivalent state which is in accordance with the empirical formula: c (K 0.07 Na 0.01 ) 0.08 A, T2 (Mg 2.46 Fe 3+ 1.99 Fe 2+ 0.30 Mn 0.08 Zn 0.05 A1 0.04 Ti 0.01 ) 4.93 T1 [Si 12 O 30 ], obtained by a combination of electron microprobe data and LA-ICP-MS. The C-site located between consecutive double-ring units is occupied only by small amounts of K, the remaining 88–95% being vacancies, which lead to increased C-O distances of 3.123 and 3.118 A. Trattnerite could be described as an end member in the series merrihueite-trattnerite with the chemical substitution Fe 3+ + □ = Fe 2+ + (K+Na) + .

Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan, North Qaidam belt, NW China

abstract An integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust.