Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ18O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648.
The initial 143Nd/144Nd ratios, expressed by εNd(T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [δ18O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of 18O/16O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ18O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ18O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally. 相似文献
Environmental or land quality indicators are being developed internationally as a means whereby the ‘State of the Environment’ (SoE) can be assessed and trends monitored. In Australia, the use of indicators in SoE reporting is in its early stages of development. Indicators have been developed, in relation to agriculture, for flora and fauna, soil, chemical contamination, surface waters and groundwater. Internationally, the World Bank has listed ‘indicators of pressure’ on the land, ‘indicators of state and impact’, and ‘indicators of response’ for each of seven major issues of land degradation. Late in 1996, an ‘indicators of catchment health’ workshop in Australia identified a relatively small number of key indicators relating to farm productivity, soil health, water quality and ‘landscape integrity’, later adding social indicators, and also distinguishing between indicators at farm and catchment scales. A broad range of other indicators was also considered by various participants in the workshop, which concluded by supporting a nationally co‐ordinated effort and the establishment of a national steering committee. Although most indicators are static measures, those from which predictions can be made require a greater emphasis on the functioning of the system. This implies that indicators are only one aspect of system modelling, and that environmental or land assessment needs to progress further than the search for quality indicators. Since this involves both biophysical and human processes, geographers should be playing a central role. 相似文献
The present communication addresses the potential use of damage features, observed in the ancient ruins of the Avdat archeological
site (Negev Desert, Israel) as a tool to identify the seismic origin of the destruction there and roughly to determine the
direction of seismic wave propagation.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献