Transmission electron microscopic study of antiphase domains in CaAl2Ge2O8-Feldspar

Transmission electron microscopic study of synthetic CaAl₂Ge₂O₈-feldspar revealed two types of antiphase domains: type “b”-antiphase domains with the displacement vector 1/2 [110] and type “c”-antiphase domains with the displacement vector 1/2 [111]. The “b”-domains were on the order of 0.1 μm in size, while the “c”-domains displayed wall to wall distances mostly between 0.1 and 1 μm. The formation of the two types of antiphase domains is due to reductions in translational symmetry associated with the phase transitions \(C\bar 1 \to I\bar 1 \to P\bar 1\) . The antiphase domain textures of CaAl₂Ge₂O₈-feldspar are very similar to the textures found in natural and synthetic anorthite, CaAl₂Si₂O₈.     

Correlation of the Quaternary terrace sequence in the Lower Rhine valley and northern Alpine foothills of central Europe

Thirteen glacial terraces are known from the western part of the northern Alpine foothills between the Lech and Iller Rivers. In the Lower Rhine region of West Germany, a similar number of terraces are capped by interglacial floodloams and soils. Whereas the environment during individual interglaciations did not differ substantially, the glaciations were progressively more severe. The Main Terrace system of the Rhine may be an exception. The duration of the Quaternary, starting at the base of Praetiglian, is estimated at approximately 2 million yr by paleomagnetic dating. The major cold-warm climatic cycles of the earliest Pleistocene lasted approximately 100,000 yr, the same as those of the Brunhes Chron. The intervening Main Terrace system has not yet been climatically subdivided. Correlation with the Netherlands is possible because of an abundance of paleobotanic and paleomagnetic evidence. In the Alpine foothills, stratigraphically useful indicators of warm climates are missing, but analogies in terrace development permit comparison with the Lower Rhine and Danube. The terrace sequence in the Alpine foothills is incomplete, as are those along most of the other rivers in Europe. Some of the older terraces may have been eroded.     

Impacts of Present and Future Climate Change and Climate Variability on Agriculture in the Temperate Regions: North America

The potential impact of climate variability and climate change on agricultural production in the United States and Canada varies generally by latitude. Largest reductions are projected in southern crop areas due to increased temperatures and reduced water availability. A longer growing season and projected increases in CO₂ may enhance crop yields in northern growing areas. Major factors in these scenarios analyzes are increased drought tendencies and more extreme weather events, both of which are detrimental to agriculture. Increasing competition for water between agriculture and non-agricultural users also focuses attention on water management issues. Agriculture also has impact on the greenhouse gas balance. Forests and soils are natural sinks for CO₂. Removal of forests and changes in land use, associated with the conversion from rural to urban domains, alters these natural sinks. Agricultural livestock and rice cultivation are leading contributors to methane emission into the atmosphere. The application of fertilizers is also a significant contributor to nitrous oxide emission into the atmosphere. Thus, efficient management strategies in agriculture can play an important role in managing the sources and sinks of greenhouse gases. Forest and land management can be effective tools in mitigating the greenhouse effect.     

Archaean high-K granitoids produced by remelting of earlier Tonalite–Trondhjemite–Granodiorite (TTG) in the Sangmelima region of the Ntem complex of the Congo craton,southern Cameroon

We present a geochemical and isotopic study that, consistent with observed field relations, suggest Sangmelima late Archaean high-K granite was derived by partial melting of older Archaean TTG. The TTG formations are sodic-trondhjemitic, showing calcic and calc-alkalic trends and are metaluminous to peraluminous. High-K granites in contrast show a potassic calc-alkaline affinity that spans the calcic, calc-alkalic, alkali-calcic and alkalic compositions. The two rock groups (TTG and high-K granites) on the other hand are both ferroan and magnesian. They have a similar degree of fractionation for LREE but a different one for HREE. Nd model ages and Sr/Y ratios define Mesoarchaean and slab-mantle derived magma compositions respectively, with Nb and Ti anomalies indicating a subduction setting for the TTG. Major and trace element in addition to Sr and Nd isotopic compositions support field observations that indicate the derivation of the high-K granitic group from the partial melting of the older TTG equivalent at depth. Geochemical characteristics of the high-K granitic group are therefore inherited features from the TTG protolith and cannot be used for determining their tectonic setting. The heat budget required for TTG partial melting is ascribed to the upwelling of the mantle marked by a doleritic event of identical age as the generated high-K granite melts. The cause of this upwelling is related to linear delamination along mega-shear zones in an intracontinental setting.     

Leakage below dam sites in limestone terrains by enhanced karstification: a modeling approach

Unnaturally high hydraulic gradients below dam sites enhance dissolutional widening of fractures in limestone. A model is presented which suggests that under unfavorable conditions, leakage rates could become unbearably high during the lifetime of the dam. At the beginning when water is impounded, leakage rates are low and increase slowly. A positive feedback loop, however, causes a sudden dramatic increase of leakage within a few years. Deep grouting becomes necessary to prevent such failures. Inefficiencies in grouting may leave some open fractures in the grouting curtain. These fractures widen faster than pathways below the grouting curtain, and reduce the effect of the grouted region located below. Therefore, open fractures act in a similar way as reduction of the grouting depth.     

Zircon age and Nd–Hf isotopic composition of the Yunnan Tethyan belt,southwestern China

The Baoshan block of the Tethyan Yunnan, southwestern China, is considered as northern part of the Sibumasu microcontinent. Basement of this block that comprises presumably greenschist-facies Neoproterozoic metamorphic rocks is covered by Paleozoic to Mesozoic low-grade metamorphic sedimentary rocks. This study presents zircon ages and Nd–Hf isotopic composition of granites generated from crustal reworking to reveal geochemical feature of the underlying basement. Dating results obtained using the single zircon U–Pb isotopic dilution method show that granites exposed in the study area formed in early Paleozoic (about 470 Ma; Pingdajie granite) and in late Yanshanian (about 78–61 Ma, Late Cretaceous to Early Tertiary; Huataolin granite). The early Paleozoic granite contains Archean to Mesoproterozoic inherited zircons and the late Yanshanian granite contains late Proterozoic to early Paleozoic zircon cores. Both granites have similar geochemical and Nd–Hf isotopic charateristics, indicating similar magma sources. They have whole-rock T _DM(Nd) values of around 2,000 Ma and zircon T _DM(Hf) values clustering around 1,900–1,800 and 1,600–1,400 Ma. The Nd–Hf isotopic data imply Paleoproterozoic to Mesoproterozoic crustal material as the major components of the underlying basement, being consistent with a derivation from Archean and Paleoproterozoic terrains of India or NW Australia. Both granites formed in two different tectonic events similarly originated from intra-crustal reworking. Temporally, the late Yanshanian magmatism is probably related to the closure of the Neotethys ocean. The early Paleozoic magmatism traced in the Baoshan block indicates a comparable history of the basements during early Paleozoic between the SE Asia and the western Tethyan belt, such as the basement outcrops in the Alpine belt and probably in the European Variscides that are considered as continental blocks drifting from Gondwana prior to or simultaneously with those of the SE Asia.     

Ophiolites of the Eastern Peninsulas zone (Eastern Kamchatka): Age, composition, and geodynamic diversity

Abstract   The geological, geochemical and mineralogical data of dismembered ophiolites of various ages and genesis occurring in accretionary piles of the Eastern Peninsulas of Kamchatka enables us to discriminate three ophiolite complexes: (i) Aptian–Cenomanian complex: a fragment of ancient oceanic crust, composed of tholeiite basalts, pelagic sediments, and gabbroic rocks, presently occurring in a single tectonic slices (Afrika complex) and in olistoplaques in Pikezh complex of the Kamchatsky Mys Peninsula and probably in the mélange of the Kronotsky Peninsula; (ii) Upper Cretaceous complex, composed of highly depleted peridotite, gabbro and plagiogranite, associated with island arc tholeiite, boninite, and high-alumina tholeiitic basalt of supra-subduction origin; and (iii) Paleocene–Early Eocene complex of intra-island arc or back-arc origin, composed of gabbros, dolerites (sheeted dykes) and basalts produced from oceanic tholeiite melts, and back-arc basin-like dolerites. Formation of the various ophiolite complexes is related to the Kronotskaya intra-oceanic volcanic arc evolution. The first ophiolite complex is a fragment of ancient Aptian–Cenomanian oceanic crust on which the Kronotskaya arc originated. Ophiolites of the supra-subduction zone affinity were formed as a result of repeated partial melting of peridotites in the mantle wedge up to the subduction zone. This is accompanied by production of tholeiite basalts and boninites in the Kamchatsky Mys segment and plagioclase-bearing tholeiites in the Kronotsky segment of the Kronotskaya paleoarc. The ophiolite complex with intra-arc and mid-oceanic ridge basalt geochemical characteristics was formed in an extension regime during the last stage of Kronotskaya volcanic arc evolution.