The evolution of the Palim granite in the Bastar tin province, Central India

Summary The Palim granite, hosted by the metasedimentary country rocks in the Bastar tin province, is a heterogeneous pluton that comprises hornblende granite, biotite granite and two-mica granite. Spherical inhomogeneous surmicaceous enclaves occur within the granites with coarse grained cores of muscovite mantled by finer muscovite-quartz-biotite (± sillimanite) rims. Geochemical features imply that the granites are highly evolved and geochemically distinct. Petrographic and geochemical considerations point towards a transition from metaluminous I-type hornblende-bearing granite in the south to peraluminous volatile-enriched S-type like lithologies (biotite and two-mica granites) towards north. Modeling of highly incompatible elements such as Nb and Cs, implies 31 to 33% assimilated fractional crystallization of a melt with an initial composition close to that of the hornblende granite to form the two-mica granite. Hornblende geobarometry, plagioclase-hornblende thermometry (in hornblende granite) and phengite barometry (in two-mica granite), yield P-T estimates of 5–7 kb/725°–760 °C, and 6 kb/700 °C, respectively. The study further implies that a genetic link exists between granite magmatism and the formation of tin pegmatites in the region. The preponderance of peripheral pegmatites to the north-east of the Palim granite is regarded a result of outward crystal-melt fractionation and tectonic tilting of the pluton. Received October 21, 1999; revised version accepted December 12, 2000     

Recent large-magnitude floods and their impact on valley-floor environments of northeastern Yellowstone

The Lamar River watershed of northeastern Yellowstone contains some of the most diverse and important habitat in the national park. Broad glacial valley floors feature grassland winter range for ungulates, riparian vegetation that provides food and cover for a variety of species, and alluvial channels that are requisite habitat for native fish. Rapid Neogene uplift and Quaternary climatic change have created a dynamic modern environment in which catastrophic processes exert a major influence on riverine–riparian ecosystems. Uplift and glacial erosion have generated high local relief and extensive cliffs of friable volcaniclastic bedrock. As a result, steep tributary basins produce voluminous runoff and sediment during intense precipitation and rapid snowmelt. Recent major floods on trunk streams deposited extensive overbank gravels that replaced loamy soils on flood plains and allowed conifers to colonize valley-floor meadows. Tree-ring dating identifies major floods in 1918, ca. 1873, and possibly ca. 1790. In 1996 and 1997, discharge during snowmelt runoff on Soda Butte Creek approached the 100-year flood estimated by regional techniques, with substantial local bank erosion and channel widening. Indirect estimates show that peak discharges in 1918 were approximately three times greater than in 1996, with similar duration and much greater flood plain impact. Nonetheless, 1918 peak discharge reconstructions fall well within the range of maximum recorded discharges in relation to basin area in the upper Yellowstone region. The 1873 and 1918 floods produced lasting impacts on the channel form and flood plain of Soda Butte Creek. Channels may still be locally enlarged from flood erosion, and net downcutting has occurred in some reaches, leaving the pre-1790 flood plain abandoned as a terrace. Gravelly overbank deposits raise flood-plain surfaces above levels of frequent inundation and are well drained, therefore flood-plain soils are drier. Noncohesive gravels also reduce bank stability and may have persistent effects on channel form. Overall, floods are part of a suite of catastrophic geomorphic processes that exert a very strong influence on landscape patterns and valley-floor ecosystems in northeastern Yellowstone.     

K-Ar dating of white micas from the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa: timing of post-depositional alteration

Summary White micas from metamorphosed and hydrothermally-altered basaltic lavas, conglomerates, quartzites and shales in and around the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa, were dated by the K-Ar method to constrain post-depositional thermal and mineralization processes. The minerals were separated into various grain sizes between < 0.4 and 10m, and characterized in terms of composition, paragenetic sequence and texture, by XRD, SEM and TEM techniques. The K-Ar isochron of all white micas in the basaltic lavas suggests an age of 1994 ± 60 Ma, and that of the smaller mica particles (< 2m) in the quartzites defines a younger age of 1917 ± 66 Ma. This range is considered to reflect the timing of long-lived hydrothermal alteration in the Ventersdorp Contact Reef. The older age is slightly younger than the intrusion of the Bushveld Complex (2060–2054 Ma) and the Vredefort catastrophism (2020 Ma), which are well-documented events that were superimposed onto the Witwatersrand Basin. The younger age may be associated with the Eburnian orogenesis along the western edge of the Kaapvaal Craton resulting in continental-scale fluid migration and hydrothermal activity that extended throughout the Griqualand Basin and even into the Witwatersrand Basin. The K-Ar ages obtained here for the white mica fractions of the Ventersdorp Contact Reef in the Witwatersrand Basin confirm that the period between 2.0 and 1.9 Ga was significant, as far as alteration, and possibly also gold mobilization, was concerned.

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K-Ar Datierung von Hellglimmern aus dem hentersdorp Contact Reef des Witwatersrand Beckens, Südafrika: die zeitliche Eingrenzung der nach der Sedimentablagerung erfolgten Alteration

Zusammenfassung Hellglimmer von metamorph und hydrothermal überprägtem Basalt, Konglomerat, Quarzit und Pelit aus dem Ventersdorp Contact Reef im Witwatersrand Becken, Südafrika, wurden mittels der K-Ar Methode datiert, um das Alter der postsedimentären thermischen Überprägung einzugrenzen. Mineralkörner wurden in einzelne Fraktionen zwischen < 0.4 und 10m separiert und auf deren Zusammensetzung, Paragenese und Textur mittels Röntgendiffraktometrie und Elektronenmikroskopie untersucht. Das Isochronenalter für alle Hellglimmerproben aus dem Metabasalt liegt bei 1994 ± 60 Ma, jenes für die Glimmer mit einer Korngröße < 2m im Quarzit bei 1917 ± 66 Ma. Diese Altersspanne wird als Ausdruck einer lang anhaltenden hydrothermalen Veränderung des Ventersdorp Contact Reef interpretiert. Das ältere Alter ist etwas jünger als die Intrusion des Bushveld Komplex (2060–2054 Ma) und die Vredefort Katastrophe (2020 Ma) - zwei gut dokumentierte Ereignisse, die das Witwatersrand Becken erfaßten. Das jüngere Alter könnte mit der eburnischen Orogenese entlang des Westrandes des Kaapvaal Kratons in Zusammenhang stehen, während der es zu Fluid Migration und hydrothermaler Aktivität quer über den Kontinent vom Griqualand Becken bis in das Witwatersrand Becken kam. Die neuen K-Ar Alter bestätigen somit, daß die Zeitspanne von 2.0–1.9 Ga wesentlich für die Alteration und möglicherweise auch für die Mobilisierung von Gold im Witwatersrand Becken war.

     

Evaporation of Accretion Disks around Black Holes: The Disk-Corona Transition and the Connection to the Advection-dominated Accretion Flow

We apply the disk-corona evaporation model (Meyer & Meyer-Hofmeister) originally derived for dwarf novae to black hole systems. This model describes the transition of a thin cool outer disk to a hot coronal flow. The mass accretion rate determines the location of this transition. For a number of well-studied black hole binaries, we take the mass flow rates derived from a fit of the advection-dominated accretion flow (ADAF) model to the observed spectra (for a review, see Narayan, Mahadevan, & Quataert) and determine where the transition of accretion via a cool disk to a coronal flow/ADAF would be located for these rates. We compare this with the observed location of the inner disk edge, as estimated from the maximum velocity of the Halpha emission line. We find that the transition caused by evaporation agrees with this determination in stellar disks. We also show that the ADAF and the "thin outer disk + corona" are compatible in terms of the physics in the transition region.     

Inelastic dynamic analysis of tall buildings

The importance of inelastic action of frame structures subjected to strong ground motions has been recognized by engineers for many years. However, the dynamic analysis of buildings undergoing inelastic deformations requires the solution of many theoretical problems, as well as the development of computer software which makes such analyses economically feasible in a design office–in spite of the extraordinary amount of computation involved. In this paper, some of the principal theoretical problems are briefly described. These are the load-deformation relationship, yield capacity reduction, ductility, P– δ effect, viscous damping, panel zone distortions, numerical integration techniques, energy analysis and the effect of non-structural elements. Special consideration is given to questions associated with the practical implementation of this theory. These questions arose during the development of a computer program, called NLDYN, capable of analysing the non-linear dynamic behaviour of tall buildings in an engineering office environment. The capabilities of this computer program are illustrated with the results of the analysis of a 60 storey office building currently under construction in downtown Los Angeles.     

Phlogopite in calc-alkaline lamprophyres of Northern England

Summary Phlogopite mica is the major unaltered phase in minette and kersantite dikes in the Cross Fell and Taythes inliers in northwestern England. The compositional zonation trend in the micas from Mg-rich, Cr-bearing cores to Fe, Ti, Ba-rich, low Cr margins, or rims, is similar to that reported for micas from minettes and kersantites worldwide. Complex mantling and zoning trends suggest that the phenocrystal mica assemblage is a hybrid one formed by the mixing of magmas of broadly similar composition which have undergone different amounts of fractional crystallization. The magmas are considered to have beeen produced in a continuously replenished fractionating magma chamber. The lamprophyres have a common source and were probably derived by the extensive fractionation of a basic mantle-derived magma.

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Phlogopit in kalk-alkalischen lamprophyren von Nord-England

Zusammenfassung Phlogopit ist die wichtigste, unveränderte Phase in Minetten und kersantitischen Gängen in den Cross Fell and Taythes Gebieten von Nordwest-England. Die Zonierung der Zusammensetzung in den Glimmern von Mg-reichen, Cr-führenden Kernen zu Fe, Ti, Ba-reichen Rändern mit niedrigen Cr-Gehalten ist ähnlich der, die von Glimmern in Minetten und Kersantiten weltweit ermittelt wurde. Komplexe Ummantelung und Zonierung weisen darauf hin, daß die Glimmer-Vergesellschaftung, die weithin idiomorph ist, durch die Mischung von Magmen weithin ähnlicher Zusammensetzung entstanden ist, welche in verschiedenem Ausmaß von Kristallisationsfraktionierung betroffen waren. Die Magmen dürften in einer kontinuierlich aufgefüllten, fraktionierten Magmenkammer entstanden sein. Die Lamprophyre haben einen gemeinsamen Ursprung und gehen wahrscheinlich auf die extensive Fraktionierung eines basischen Magmas von Mantel-Herkunft zurück.

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With 5 Figures     

Torsional instability in structures

Torsional instability under harmonic excitation was detected in structures with cubic softening member stiffness. A variational procedure was introduced to perform the analysis for the general problem formulation. Single-storey structures with zero eccentricity were studied first; previous findings were checked and corrected, and the behaviour was examined in greater detail. Two-storey structures with large eccentricity were also examined, and further regions of torsional instability were detected. Time history analysis was then employed to quantify the behaviour of various typical structures. An unbounded growth in rotation or column displacement was not observed. Rather, the instability appeared to be related to the jump in response which exists in single degree-of-freedom softening oscillators subjected to harmonic excitation. The practical significance of torsional instability is therefore rather weak, although prudent practice would recommend its avoidance.     

Interstitial acid glass and chlorophaeite in Iceland basalts

Rhyolitic glass occurs as an interstitial phase in Tertiary basaltic dikes from northwestern Iceland forming up to 8% of the mode. Chlorophaeite occurs as globules within the glass as well as in interstitial vugs and vesicles. The existence and textural relations of these iron-rich globules in a silica-rich glass is suggestive of liquid immiscibility such as observed in synthetic systems. Trace element data on these naturally occurring phases is, however, inconsistent with experimentally determined partition coefficients for, for example, Ti, P, and Zr in immiscible liquids indicating that the chlorophaeite does not represent an immiscible phase and is more likely an alteration product. The similarity of the interstitial acid glasses to Iceland rhyolites is suggestive evidence of an origin for at least some Icelandic rhyolites by shallow-level fractional crystallization of basaltic magmas.     

Age of the last major scabland flood of the Columbia Plateau in eastern Washington

Pumice layers of set S from Mount St. Helens can be correlated with certain ash beds associated with young flood deposits of the channeled scabland. The correlation points to an age of about 13,000 ¹⁴C yr B.P. for the last major flood to have crossed the scabland. Until recently, the last major episode of flooding was thought to be closer to 20,000 yr B.P., an age inferred chiefly from the relation of the flood to glacial events of the northern Rocky Mountains. Several investigations within the last few years have suggested that the last major flood occurred well after 20,000 yr B.P. Tentative correlations of ash beds of the scabland with set S pumice layers, the relations of flood and glacial events along the northwestern margin of the Columbia Plateau, and a radiocarbon date from the Snake River drainage southeast of the plateau all indicate an age much younger than 20,000 yr. The postulated age of about 13,000 yr B.P. is further supported by a radiocarbon date in the Columbia River valley downstream from the scabland tract. Basal peat from a bog on the Portland delta of Bretz, which is a downvalley deposit of the last major scabland flood, has been dated as 13,080 ± 300 yr B.P. (W-3404).