Geological setting and chemistry of kimberlite clan rocks in the Dharwar Craton, India

Diamond exploration in India over the past decade has led to the discovery of over 80 kimberlite-inferred and lamproite-related intrusions in three of the four major Archean cratons that dominate the subcontinent. These intrusions are Proterozoic (1.1 Ga), and are structurally controlled: locally (at the intersections of faults); regionally (in a 200 km wide, 1000 km long diamond corridor); and globally (in the reconstructed supercontinent of Rodinia). The geochemistry of 57 samples from 13 intrusions in the southern Dharwar Craton of Andhra Pradesh has been determined by XRF spectrometry. The bodies are iron-rich with mg#=50–70 and are neither archetypal kimberlites nor ideal lamproites; this may be the underlying reason that conventional exploration techniques have thus far failed to locate the primary sources of India's historically famous diamonds. The two major fields of kimberlite-clan rocks (KCR) in the Dharwar Craton, Wajrakur and Narayanpet, are separated by a NW–SE trending, transcontinental (Mumbai-Chennai) gravity lineament. About 80% of intrusions in Wajrakur are diamondiferous, but diamonds have not yet been reported in Narayanpet. The gravity anomaly may mark the boundary of an architectural modification in the keel of the sub-continental lithosphere, a suggestion that is supported by differences in kimberlite mineralogy, chemistry, mantle xenoliths, structural setting and crustal host rocks.     

Iron site geometry in orthopyroxene: Multiple scattering calculations and XANES study

The iron site geometry in orthopyroxenes (OPX) and synthetic FeO, has been investigated by X-ray Absorption Near Edge Structure (XANES) spectroscopy. Multiple scattering calculations have been used to determine how the iron polyhedral geometry affects the absorption spectra. The results reported here demonstrate that this approach is effective in determining the site geometry around the absorbing atom, allowing the assignment of features in the XANES spectrum to specific structural aspects of the coordination environment. In the case of the orthopyroxenes this method allows discrimination between the multiple scattering contributions to the absorption spectrum due to the two octahedral sites M1 and M2 of the OPX structure. The results obtained with the MS calculation were used to evaluate the effects of polyhedral distortion, symmetry changes and iron site occupancies on the experimental spectra. The results indicate a more quantitative way to interpret XANES experimental spectra and give insights into the interpretation of site geometries of more complex or unknown structures and amorphous materials.     

Climatic change in southern Africa: Past and present conditions and possible future scenarios

A brief review of climatic changes over the last 800 000 years in southern Africa is presented. The greatest emphasis is placed on those changes occurring during the Holocene and the period of meteorological record. Twentieth-century rainfall variations and their spatial manifestations are examined, and scenarios of possible future conditions are presented.     

Palynofacies investigation of Callovian (Middle Jurassic) sediments from DSDP Site 534, Blake-Bahama Basin,Western Central Atlantic

During the extension of Deep Sea Drilling Project (DSDP) Leg 76 a new and previously unpenetrated lithological unit composed mainly of claystones was cored above basalt basement at Site 534 in the Blake-Bahama Basin. The Callovian part of the new unit contains interbedded ‘black shales’ which were hitherto unexpected in this part of the section. This Paper presents a brief palynological examination of lithofacies-kerogen relationships in these sediments and shows that their organic content is almost entirely a function of the re-deposition of terrestial and marine organic matter versus the ambient redox conditions of the depositional environment. Allochthonous organic matter inputs are highest in the interbedded turbidites and decline progressively toward the pelagic black shales in which marine organic matter is comparatively well preserved. The significance of various kerogen and palynomorph indices are discussed. The study emphasizes the absolute necessity for sedimentologically-aware sampling in all palynological and geochemical work on lithologically heterogeneous sequences.     

Changing Gradients of Climate Change in Southern Africa during the Past Millennium: Implications for Population Movements

Climates of equatorial East Africa and subtropical Southern Africa have varied inversely over long periods of time. The high-resolution ¹⁸O stalagmite record from Cold Air Cave in the Makapansgat valley in South Africa and a similar resolution lake-level record for Lake Naivasha in Kenya have been in anti-phase for much of the last thousand years. A similar relationship is evident in the twentieth century meteorological record. The changes in rainfall in the two regions on multi-decadal to centennial scales have influenced both settlement patterns and livelihoods of Iron Age agriculturalists. The resulting latitudinal gradient of change may have been a significant factor in promoting southward migration of Sotho-Tswana speaking people from equatorial East Africa during the first few centuries of the last millennium and earlier. This would have occurred at times when environments in the north were deteriorating and those to the south were ameliorating.     

The relationship between major element chemistry and tectonic environment of basic and intermediate volcanic rocks

This work reports the preliminary results of a study of the relationship between tectonic environment and major element chemistry of volcanic rocks. Using a file of 8400 analyses of rocks of varying ages and geographic distribution it has been found that a simple ternary plot of MgOFeO(total)Al₂O₃ may be used to distinguish five tectonic environments: ocean ridge and floor, ocean island, continental, orogenic, and spreading center island. Subalkaline “basaltic-andesites” (51–56% SiO₂ calculated anhydrous) are used in this study. Alkaline rocks do not generally show the simple patterns of tholeiitic or subalkaline rocks. The ocean island suite is bimodal with one mode tholeiitic and the other alkaline. A bimodal age/chemistry relationship in the continental suite is tentatively related to different conditions during and after active breaking up of continents (rifting and drifting stages).Most of the Archean rocks in our file generally do not fall in the orogenic field. It is unlikely that modern island arc models can be generally applied to Archean greenstone belts. Locally, however, there may be Archean rocks analogous in some respect to modern calc-alkali rocks.     

Petrology of the Renard igneous bodies: host rocks for diamond in the northern Otish Mountains region, Quebec

The Renard igneous bodies were discovered in late 2001 as part of a regional diamond exploration program launched by Ashton Mining of Canada and SOQUEM. Nine bodies have been discovered within a 2-km-diameter area, and are comprised of root zone to lower diatreme facies rocks including kimberlitic breccia, olivine macrocrystic hypabyssal material, and brecciated country rock with minor amounts of kimberlitic material. Many mineralogical and petrographic features are common to both kimberlite and melnoite, and strict assignment of the rocks as kimberlite is not possible with these criteria alone. Whole rock trace element compositions suggest a closer affinity to Group I kimberlite, with derivation from a garnet-bearing mantle. Exceptions to conventional classification of the rocks along petrographic or mineralogical lines may be due in part to assimilation of felsic country rock into the Renard magmas at the time of emplacement. The Renard magmas were emplaced into northeastern Laurentia at 630 Ma, when the supercontinent was undergoing a change from convergent margin magmatism to rifting, the latter being associated ultimately with the opening of the Iapetus ocean.     

Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks

Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd.