The early interaction between the Caribbean Plateau and the NW South American Plate

The determination of accurate and precise ages for the timing of collision between oceanic plateaus and continental crust requires an understanding of how the indenting and buttressing plates respond to the collision. We present geochronological, thermochronological, geochemical and isotopic analyses of magmatic rocks from the Ecuadorian Andes, which relate to the collision of the Late Cretaceous Caribbean Plateau and Great Arc sequence with NW South America. The cessation of subduction magmatism during 65–64 Ma beneath the eastern edge of Caribbean Plateau was synchronous with accelerated surface uplift and exhumation within the buttressing continental margin during 75–65 Ma. We interpret this as the collision of the leading edge of the Caribbean Plateau and arc sequence with the South American Plate at 75–65 Ma. A U/Pb (zircon) SHRIMP age of 87.10 ± 1.66 (2σ) Ma, yielded by an accreted fragment of the plateau, precludes previous estimates of collision at 85–80 Ma if the plateau erupted above the Galápagos hotspot. Terra Nova, 18, 264–269, 2006     

Der Prozeß der Anatexis: Seine Bedeutung für die Genese der Migmatite

Zusammenfassung Gegenüber den metasomatisch-transformistischen Vorstellungen über die Genese der in situ-Migmatite wird dargelegt, daß allein Anatexis von Gneisen. ohne großräumige Metasomatose, die Entstehung der Migmatite erklärt. Der Prozeß der Anatexis wird erläutert, und es wird mittels des Q-Ab-An-Or-H₂O-Systems gezeigt, daß die Zusammensetzung der entstehenden eutektischen Erst-schmelze bei verschiedenen Gneisen verschieden und zwar um so reicher an Or-und auch Q-Komponente ist, je größer das normative An/Ab-Komponentenverhältnis des Gneises ist. Selbst wenn kein Alkalifeldspat in einem Paragneis vorkommt, enthält die jeweilige eutekiische Schmelze einen großen Anteil an Or-Komponente. Durch Reaktionen von Glimmer mit Quarz unter Beteiligung von Plagioklas entsieht nämlich bei nicht zu hohen Drucken Alkalifeldspat noch vor Beginn der Anatexis, bei höheren Drucken entstehtmit Beginn der Anatexis Or-Komponente, die in der Schmelze gelöst ist. Die Glimmer eines Paragneises sind eine potentielle Quelle für Or-Komponente bei der Anatexis. Mit steigender Temperatur ändert sich die Zusammensetzung der eutektischen granitischen Schmelze, und zwar wird sie bei den plagioklas- und biotitreichen Paragneisen granodioritisch oder sogar trondhjemitisch. Wenn dagegen in einem Gneis der K₂O-Gehalt nicht wesentlich geringer als der Na₂O-Gehalt ist, dann behält die Schmelze eine granitische Zusammensetzung.

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Summary Contrary to the metasomatic transformists' concepts, it is pointed out that the petrogenesis of in situ-migmatites can be fully understood solely by anatexis of gneisses without large scale metasomatism. The process of anatexis is outlined. It is shown with the help of the Q-Ab-An-Or-H₂O system that the chemical composition of the eutectic melt formed at the beginning of anatexis is different, depending on the normative An/Ab-ratio of any gneiss; the eutectic melt contains relatively more Or- and also Q-component the larger that ratio. Even if no alkalifeldspar is present in a paragneiss, the eutectic melt formed by anatexis does contain a large proportion of Or-component. This is due to reactions of micas with quartz, in presence of plagioclase. At pressures not too high, alkalifeldspar is formed by these reactions somewhat below the temperature of the beginning of anatexis, while at higher H₂O-pressure mica reacts together with plagioclase and quartz at the very beginning of anatexis, furnishing Or-component for the eutectic melt. Therefore, the micas are a potential source for Or-component in the process of anatexis. Depending on the composition of a gneiss system the anatectic melts existing at temperatures above the beginning of anatexis remain granitic in chemical character, or they become granodioritic or trondhjemitic.

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Herrn Prof. Dr.H. Leitmeier zum 80. Geburtstag gewidmet.     

Groundwater flow dynamics in the complex aquifer system of Gidabo River Basin (Ethiopian Rift): a multi-proxy approach

Hydrochemical and isotope data in conjunction with hydraulic head and spring discharge observations were used to characterize the regional groundwater flow dynamics and the role of the tectonic setting in the Gidabo River Basin, Ethiopian Rift. Both groundwater levels and hydrochemical and isotopic data indicate groundwater flow from the major recharge area in the highland and escarpment into deep rift floor aquifers, suggesting a deep regional flow system can be distinguished from the shallow local aquifers. The δ¹⁸O and δ²H values of deep thermal (≥30 °C) groundwater are depleted relative to the shallow (<60 m below ground level) groundwater in the rift floor. Based on the δ¹⁸O values, the thermal groundwater is found to be recharged in the highland around 2,600 m a.s.l. and on average mixed with a proportion of 30 % shallow groundwater. While most groundwater samples display diluted solutions, δ¹³C data of dissolved inorganic carbon reveal that locally the thermal groundwater near fault zones is loaded with mantle CO₂, which enhances silicate weathering and leads to anomalously high total dissolved solids (2,000–2,320 mg/l) and fluoride concentrations (6–15 mg/l) exceeding the recommended guideline value. The faults are generally found to act as complex conduit leaky barrier systems favoring vertical mixing processes. Normal faults dipping to the west appear to facilitate movement of groundwater into deeper aquifers and towards the rift floor, whereas those dipping to the east tend to act as leaky barriers perpendicular to the fault but enable preferential flow parallel to the fault plane.     

Thermochronology of allochthonous terranes in Ecuador: Unravelling the accretionary and post-accretionary history of the Northern Andes

The western cordilleras of the Northern Andes (north of 5°S) are constructed from allochthonous terranes floored by oceanic crust. We present ⁴⁰Ar/³⁹Ar and fission-track data from the Cordillera Occidental and Amotape Complex of Ecuador that probably constrain the time of terrane collision and post-accretionary tectonism in the western Andes. The data record cooling rates of 80–2 °C/my from temperatures of 540 °C, during 85 to 60 Ma, in a highly tectonised mélange (Pujilí unit) at the continent–ocean suture and in the northern Amotape Complex. The rates were highest during 85–80 Ma and decelerated towards 60 Ma. Cooling was a consequence of exhumation of the continental margin, which probably occurred in response to the accretion of the presently juxtaposing Pallatanga Terrane. The northern Amotape Complex and the Pujilí unit may have formed part of a single, regional scale, tectonic mélange that started to develop at ~85 Ma, part of which currently comprises the basement of the Interandean Depression. Cooling and rotation in the allochthonous, continental, Amotape Complex and along parts of the continent–ocean suture during 43–29 Ma, record the second accretionary phase, during which the Macuchi Island Arc system collided with the Pallatanga Terrane. Distinct periods of regional scale cooling in the Cordillera Occidental at 13 and 9 Ma were synchronous with exhumation in the Cordillera Real and were probably driven by the collision of the Carnegie Ridge with the Ecuador Trench. Finally, late Miocene–Pliocene reactivation of the Chimbo–Toachi Shear Zone was coincident with the formation of the oldest basins in the Interandean Depression and probably formed part of a transcurrent or thrust system that was responsible for the inception and subsequent growth of the valley since 6 Ma.     

Clay mineral sedimentation in high northern latitude deep-sea basins since the Middle Miocene (ODP Leg 151, NAAG)

High-resolution clay mineralogical investigations and their comparison with other sedimentological data from ODP Sites 908 and 909 (central Fram Strait) were used to reconstruct the paleoclimate and paleoceanography in the high northern latitudes since the Middle Miocene. Ice rafting has probably occurred since 15 Ma. The comparison of sand-sized components and clay mineral distribution demonstrate that both were not delivered by the same transport process. The input of the clay fraction is related to transport through sea ice and/or oceanic currents. A provenance change at 11.2 Ma is indicated by variations within clay mineral distribution and increased accumulation rates. This is interpreted as a result of an increase in water mass exchange through the Fram Strait. Decreases of the smectite to illite and chlorite ratio at Site 909 suggest a Middle Miocene cooling phase between 14.8 and 14.6 Ma, and a further cooling phase between 10 and 9 Ma. The intensification of glaciation in the Northern Hemisphere is documented by an increase of illite and chlorite from 3.4 to 3.3 Ma, which is synchronous to the onset of major Northern Hemisphere glaciation as indicated by oxygen isotope data.     

Quasielastic incoherent neutron scattering study of the rotational dynamics of the water molecules in analcime

The rotational motion of the water molecules in analcime has been investigated by quasielastic incoherent neutron scattering. Reorientational times vary from 65 ± 20 ps at 260 K to 26 ± 3 ps at 370 K, and the temperature dependence suggests an activation energy for reorientations of 780 ± 200 K. Measurements of the temperature dependence of the intensity of the elastic incoherent scattering indicate that there is no cooperative ordering at low temperatures, and that instead there is a gradual freezing-in of the motion of individual water molecules.     

Die Struktur des K2MgF4

Zusammenfassung Die Struktur des K₂MgF₄ wurde untersucht. Es ergab sich: Raumgruppe I 4/m m m ₂,a ₀ = 3,977 Å,c ₀ = 13,16 Å, Inhalt der Elementarzelle zwei Formeleinheiten. Die Struktur besteht aus Schichten zweidimensional-unendlich miteinander verknüpfter MgF₆-Koordinationsoktaeder, was rich auch in der Art der Spaltbarkeit und Optik äußert.

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Herrn Professor Dr.Carl W. Correns zum 60. Geburtstag gewidmet.