Gabbroic xenoliths in tuff-breccia pipes from the Hyblean Plateau: insights into the nature and composition of the lower crust underneath South-eastern Sicily, Italy

Summary Crust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided into three groups: gabbros (plagioclase + clinopyroxene + Fe–Ti oxides ± apatite ± amphibole ± Fe-rich green spinel), diorites (An-poor plagioclase, clinopyroxene ± Fe–Ti oxides ± orthopyroxene) and mafic granulites (plagioclase + clinopyroxene + green spinel ± orthopyroxene ± Fe–Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe–Ti-rich melts related to tholeiitic-type fractional crystallisation. Incompatible element ratios (Zr/Nb = 5–26; Y/Nb = 1.4–11) indicate that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important, metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well-known geodynamic settings show that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment.     

Long-term variations in the asymmetry of the magnetic field of the sun and heliosphere

The asymmetry of the magnetic field of the Sun and its manifestation in the interplanetary magnetic field (IMF) are studied. The dominant magnetic polarity of the radial component of the IMF alternates from cycle to cycle, but with an overall systematic dominance of polarity directed toward the Sun. The global asymmetry is also manifest in the component of the IMF perpendicular to the plane of the solar equator. The dominance of positive values of B _z together with an appreciable linear trend in the cumulative sum of this quantity is interpreted as a manifestation of a relic solar magnetic field. The strength of this relic magnetic field near the Earth is estimated to be 0.048 ± 0.015 nT, based on the growth of the linear component of the cumulative sum of B _z . Time intervals, in which negative values of the B _z component of the IMF dominate and enhanced geomagnetic activity is observed, are identified. Our analysis of solar and heliospheric magnetic fields in an integrated representation has enabled us to compare various types of measurements and estimate their stability.     

Ecological characteristics of Vrishabhavathy River in Bangalore (India)

River Vrishabhavathy, a tributary of Cauvery River was studied for 12 physico-chemical parameters at four sites over a distance of 50 km for a period of 2 years (1999–2001) at monthly intervals. Water was faintly alkaline, with pH showing negative correlation with temperature. The dissolved oxygen content increased downstream with negative correlation to biological oxygen demand (BOD), chemical oxygen demand (COD) and turbidity. Bicarbonate alkalinity was very low compared with carbonate alkalinity. Carbonate alkalinity, total hardness, total dissolved solids, total suspended solids, electrical conductivity, BOD and COD decreased downstream, with an upward trend in the middle reaches due to the introduction of raw sewage. The seasonal and yearly trends are also discussed. The river is a sewer collector undergoing self-purification.     

Hydrothermal sulfide deposits of the Lucky Strike vent field, Mid-Atlantic Ridge

Several hydrothermal sulfide structures were sampled using Mir manned submersibles in the relatively shallow Lucky Strike vent field, Mid-Atlantic Ridge; the bathymetric position of these structures varies by approximately 100 m. The investigation of the chemical and mineral compositions of hydrothermal ore occurrences led to the conclusion that the initial high-temperature ore-bearing solution ascending toward the surface became unstable and experienced phase separation beneath the ocean floor. The phase separation was responsible for the bathymetric control of hydrothermal ore formation in the ocean.     

The correlation of Mesozoic gold ore districts at the Sino-Korean and Aldan-Stanovoi shields

The geological structure and gold ore potential of the activized Aldan-Stanovoi and Sino-Korean shields of East Asia are compared. These two regions show similar tendencies in their geological evolution during the Archean, Proterozoic, and Phanerozoic epochs but differ in types of tectonic structure and associations of ore deposits. According to recent studies by Russian and Chinese geologists, the Mesozoic complexes of these shields possess higher gold ore potential than was suggested before. As a result of these studies, the amount of conditions favoring the formation of large gold districts and deposits in the activized shields has strongly increased. Some of these deposits are polychronic and polygenetic (the Bam deposit), others are associated with J-K alkaline magmatism (the Central Aldan district), a third group of deposits are related to granites of the same age (the East Shandong district), and a fourth group includes stratiform deposits in the lower part of the udokan series (Ugui district). The various Mesozoic hydrothermal ore deposits of the northern framework of the Sino-Korean Shield are especially interesting. The study of problems of gold metallogeny was initiated in Russian geological science by Yu.A. Bilibin (1935–1940) in the central part of the Aldan Shield. Some new data concerning the gold ore potential of the Sino-Korean Shield extend our knowledge of gold ore districts in East Asia and make clear the necessity of more careful and systematic study of the gold ore potential of the Aldan-Stanovoi Shield.