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It has been often debated whether all granitic gneisses associated with coesite-bearing eclogites in southern Dabieshan, China, have also been subjected to ultrahigh-pressure (UHP) metamorphism. We show here that a metagranitoid adjacent to the Bixiling eclogite-ultramafic complex has preserved primary granitic textures and an igneous mineral assemblage of biotite + plagioclase + microcline + quartz + allanite +/- amphibole. The absence of UPH recrystallization for the metagranitoid is particularly manifested by the conservation of euhedral-zoned plagioclase phenocrysts, the lack of corona garnets around igneous biotite, and the presence of an igneous mineral assemblage in zircons. The only metamorphic overprint was the epidote-amphibolite facies metamorphism characterized by the assemblage of biotite + phengiticmica + epidote + albite + K-feldspar + quartz +/- amphibole Metamorphic conditions are estimated at ca. 550degrees-680degreesC and 6-13 kbar for the metagranitoid and its amphibolitic enclave. Geochemically, the metagranitoid is similar to its country gneiss and shows an affinity to volcanic arc granitoid. Zircon U-Pb dating suggests that the Bixiling metagranitoid was emplaced during the Neoproterozoic (729+/-4 Ma), when most other granitic rocks and the protoliths of eclogite were also formed in Dabieshan. Taking into account the discovery of non-UHP granitic gneisses in other places, we argue that part of Neoproterozoic granitic rocks in the Dabieshan and Sulu terranes have escaped UHP metamorphism during the Triassic deep subduction of the continental crust as a consequence of a lack of penetrative deformation and fluid-rock interaction
Schlemaite, with the simplified formula (Cu,rectangle)(6)(Pb,Bi)Se-4, is a new mineral species from the Niederschlema-Alberoda vein-type uranium deposit at Hartenstein, Erzgebirge, Germany. It occurs as anhedral to subhedral grains with no obvious forms or twinning, in aggregates of up to several hundred mum across, with berzelianite, eucairite and clausthalite in a dolomite-ankerite matrix. Schlemaite is black with a black streak and opaque with a metallic luster. It is brittle with an uneven fracture and no observable cleavage. It has a mean VHN (25 g load) of 106 kg/mm(2), which roughly equates to a Mobs hardness of 3. In plane-polarized reflected light, schlemaite is grey, non- pleochroic with a very weak bireflectance. It has very weak anisotropy, with rotation tints in shades of very pale metallic orange and blue, and shows no internal reflections. Electron-microprobe analyses yielded a mean composition Cu 38.86, Ag 2.57, An 0.07, Hg 0.09, Pb 13.75, Bi 9.12, Se 35.11, total 99.57 wt.%. The empirical formula (based on 4 Se apfu) is (Cu5.50Ag0.21)(Sigma5.71)(Pb0.60Bi0.39)(Sigma0.99)Se-4. The calculated density is 7.54 g/cm(3) (based on the empirical formula and unit-cell parameters refined from single-crystal data). Schlemaite is monoclinic, P2(1)/m, a 9.5341(8), b 4.1004(3), c 10.2546(8) Angstrom, beta 100.066(2)degrees, V 394.72(9) Angstrom(3), a:b:c 2.3252:1:2.5009, Z = 2. The crystal structure of schlemaite was solved by direct methods and refined to an R index of 4.8% using 1303 unique reflections collected on a four-circle diffractometer equipped with a CCD detector. The structure consists of intercalated ordered and disordered layers. The ordered layer consists of ladders of Ph2+ + Bi3+ coordinated by Se, the former showing strong lone-pair-stereoactive effects, and a network of Cu+ coordinated by Se anions. The disordered layer consists of an array of sites partly occupied by Cu+ and Ag+ in a variety of coordinations, and is characterized by strong short-range order. The strongest seven lines of the X-ray powder-diffraction pattern [d in Angstrom(I)(hkl)] are: 3.189(100)(012), 3.132(100)(112), 2.601(70)(113), 2.505(50)(311), 2.151(60)(014), 2.058(80)(020) and 1.909(50)(314). Although schlemaite is chemically similar to furutobeite, (Cu,Ag)(6)PbS4, it is not isostructural with it. The mineral is named after the Schlema-Alberoda uranium ore field near Schneeberg in the ancient mining region of Saxony, Germany
Reassessment of local earthquake data from the ANCORP seismological network allowed the calculation of 3D attenuation (Q(p)) tomographic images of crust and upper mantle beneath the southern Bolivian Altiplano around 21degrees S. The images reveal a low-Q(p) middle and lower crust and a moderate-Q(p) upper mantle beneath the southern Altiplano. Beneath the recent magmatic arc, Q(p) is not significantly decreased at this latitude. The distribution of crustal Q(p) coincides with the variation of electrical resistivity, thus limiting the possible mechanisms causing the anomalies. Our findings support the hypothesis that partial melts in middle and lower crust beneath the Altiplano are present on a large scale. We see no evidence for a shallow asthenosphere beneath the southern Altiplano
High-Pressure Granulites : Formation, Recovery of Peak Conditions, and Implications for Tectonics
(2003)
Multiple landslide clusters record quaternary climate changes in the northwestern Argentine andes
(2003)
The chronology of multiple landslide deposits and related lake sediments in the semi-arid eastern Argentine Cordillera suggests that major mass movements cluster in two time periods during the Quaternary, i.e. between 40 and 25 and after 5 14C kyr BP. These clusters may correspond to the Minchin (maximum at around 28-27 14C kyr BP) and Titicaca wet periods (after 3.9 14C kyr BP). The more humid conditions apparently caused enhanced landsliding in this environment. In contrast, no landslide-related damming and associated lake sediments occurred during the Coipasa (11.5- 10 14C yr BP) and Tauca wet periods (14.5-11 14C yr BP). The two clusters at 40-25 and after 5 14C kyr BP may correspond to periods where the El Niño-Southern Oscillation (ENSO) and Tropical Atlantic Sea Surface Temperature Variability (TAV) were active. This, however, was not the case during the Coipasa and Tauca wet periods. Lake-balance modelling of a landslide-dammed lake suggests a 10-15% increase in precipitation and a 3-4 ° C decrease in temperature at ~30 14C kyr BP as compared to the present. In addition, time-series analysis reveals a strong ENSO and TAV during that time. The landslide clusters in northwestern Argentina are therefore best explained by periods of more humid and more variable climates.
The age of Proterozoic granulite facies metamorphism and deformation in the Strangways Metamorphic Complex (SMC) of central Australia is determined on zircon grown in syn-metamorphic and syn-deformational orthopyroxene-bearing, enderbitic, veins. SHRIMP zircon studies suggest that M1-M2 and the correlated periods of intense deformation (D1-D2) are part of a single tectonothermal event between 1,717-2 and 1,732-7 Ma. It is considered unlikely that the two metamorphic phases (M1, M2) suggested by earlier work represent separate events occurring within 10-25 Ma of each other. Previous higher estimates for the age of M1 granulite metamorphism in the SMC (Early Strangways event at ca. 1,770 Ma) based on U-Pb zircon dating of granitic, intrusive rocks, are not believed to relate to the metamorphism, but to represent pre-metamorphic intrusion ages. Conventional multi-grain U-Pb monazite analyses on high-grade metasediments from three widely spaced localities in the western SMC yield 207Pb/235U ages between 1,728-11 and 1,712-2 Ma. The age range of the monazites corresponds to the SHRIMP zircon ages in the granulitic veins and is interpreted to record monazite growth (prograde in the metasedimentary rocks). The data imply a maximum time-span of 30 Ma for high-grade metamorphism and deformation in the SMC. There is, thus, no evidence for an extremely long period of continuous high- temperature conditions from 1,770 to ca. 1,720 Ma as previously proposed. The results firmly establish that the SMC has a very different high-grade metamorphic history than the neighbouring Harts Range, where upper amphibolite facies metamorphism in the Palaeozoic caused widespread growth or recrystallization of monazite.
We modeled the two most extreme highstands of Lake Naivasha during the last 175 k.y. to estimate potential precipitation/ evaporation changes in this basin. In a first step, the bathymetry of the paleolakes at f135 and 9 k.y. BP was reconstructed from sediment cores and surface outcrops. Second, we modeled the paleohydrologic budget during the highstands using a simplified coupled energy mass-balance model. Our results show that the hydrologic and hence the climate conditions at f135 and 9 k.y. BP were similar, but significantly different from today. The main difference is a f15% higher value in precipitation compared to the present. An adaptation and migration of vegetation in the cause of climate changes would result in a f30% increase in precipitation. The most likely cause for such a wetter climate at f135 and 9 k.y. BP is a more intense intertropical convergence and increased precipitation in East Africa.
Variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long-term fluctuations in African lake levels. The understanding of such relationships is essential for designing climate-prediction models for the tropics. Our assessment of the nature and timing of East African climate change is based on lake-level fluctuations of Lake Naivasha in the Central Kenya Rift (0°55'S 36°20'E), inferred from sediment characteristics, diatoms, authigenic mineral assemblages and 17 single-crystal 40Ar/39Ar age determinations. Assuming that these fluctuations reflect climate changes, the Lake Naivasha record demonstrates that periods of increased humidity in East Africa mainly followed maximum equatorial solar radiation in March or September. Interestingly, the most dramatic change in the Naivasha Basin occurred as early as 146 kyr BP and the highest lake level was recorded at about 139 to 133 kyr BP. This is consistent with other well-dated low-latitude climate records, but does not correspond to peaks in Northern Hemisphere summer insolation as the trigger for the ice- age cycles. The Naivasha record therefore provides evidence for low-latitude forcing of the ice-age climate cycles.
Present erosion and sediment flux in the semi-arid intramontane Santa Maria Basin, NW Argentina are compared with conditions during a period of wetter and more variable climate at about 30,000 14C yrs ago. The results suggest that the influence of climate change on the overall erosional sediment budget is significant, mainly because of a change in the erosion regime coupled with an increase in mass movements. The most effective mechanism to increase landslide activity in this environment is a highly variable climate on inter-annual timescales. In contrast, Quaternary changes in erosional budgets due to variations in moisture regimes is small in the Santa Maria Basin. Since the magnitude of a potential increase in background erosion as well as enhanced landsliding is smaller than typical levels of uncertainty of erosional budgets for such large basins, it is not likely that climate-driven erosional unloading can influence tectonic style and rates in this semi-arid environment on time scales of several 103 to 104 years.