Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil

High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser ⁴⁰Ar/³⁹Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.704 and ¹⁴³Nd/¹⁴⁴Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C.     

Nd and Sr isotopic constraints on the origin of igneous rocks resulting from the opening of the Japan Sea, southwestern Japan

Strontium and Nd isotopic compositions and trace element abundances were determined for Cretaceous to late Cenozoic igneous rocks from the Japan Sea side of Southwest Japan in order to investigate the effect of the opening of the Japan Sea on igneous activity. The ⁸⁷Sr/⁸⁶Sr ratios for both high and low silica rocks decrease with decreasing age since the middle Miocene, when the opening occurred. Similarly, ¹⁴³Nd/¹⁴⁴Nd values for these rocks increase with decreasing age, and are negatively correlated with ⁸⁷Sr/⁸⁶Sr ratios. A two-component mixing process can best account for these isotopic and chemical characteristics. One end-member is likely the subcontinental lithospheric mantle (SCLM) and its derivative mafic to intermediate materials which had ɛ_Nd values of around +3. The other endmember consists of mafic to intermediate rocks with low ɛ_Nd values (e.g., −8), probably located in the lower crust. The mantle upwelling associated with the opening of the Japan Sea did not supply typical MORB or MORB-source materials to the crust, but did provide the heat that caused the melting of lithospheric mantle and lower crust. Received: 29 August 1996 / Accepted: 6 May 1997     

Grade estimation using fuzzy- set algorithms

This paper presents a new approach for estimating unknown ore grades within a mining deposit in a fuzzy environment using fuzzy c- means clustering and a fuzzy inference system. Based on a collection of cluster centers obtained from fuzzy c- means, a fuzzy rule base and fuzzy search domains are established to compute grades at these cluster centers. These cluter center- grade pairs act as control information in the fuzzy space- grade system in order to infer unknown grades on the basis of fuzzy interpolation, fuzzy extrapolation, and a defuzzification process of fuzzy control.     

Evolution of the northeastern Kaapvaal craton and the central Limpopo belt,South Africa,based on single-zircon ages (extended abstract)

Geologie en Mijnbouw -     

Parental melts of melilitolite and origin of alkaline carbonatite: evidence from crystallised melt inclusions, Gardiner complex

Perovskite and melilite crystals from melilitolites of the ultramafic alkaline Gardiner complex (East Greenland) contain crystallised melt inclusions derived from: (1) melilitite; (2) low-alkali carbonatite; (3) natrocarbonatite. The melilitite inclusion (1) homogenisation temperature of 1060 °C is similar to liquidus temperatures of experimentally investigated natural melilitites. The compositions are peralkaline, low in MgO (ca.␣5 wt%), Ni and Cr, and they are low-pressure fractionates of more magnesian larnite-normative ultramafic lamprophyre-type melts of primary mantle origin. Low-alkali carbonatite compositions (2) homogenise at 1060–1030 °C and are compositionally similar to immiscible calcite carbonatite dykes derived from the melilitolite magma. Natrocarbonatite inclusions (3) homogenise between 1030 and 900 °C and are compositionally similar to natrocarbonatite lava from Oldoinyo Lengai. Nephelinitic to phonolitic dykes which are related to the calcite carbonatite dykes, are very Zr-rich and agpaitic (molecular Na₂O + K₂O/Al₂O₃ > 1.2) and resemble nephelinites of Oldoinyo Lengai. The petrographic, geochemical and temporal relationships indicate unmixing of carbonatite compositions (ca. 10% alkalies) from evolving melilitite melt and continued fractionation of melilitite to nephelinite. It is suggested that the natrocarbonatite compositions represent degassed supercritical high temperature fluid formed in a cooling body of strongly larnite-normative nephelinite or evolved melilitite. The Gardiner complex and similar melilitolite and carbonatite-bearing ultramafic alkaline complexes are believed to represent subvolcanic complexes formed beneath volcanoes comparable to Oldoinyo Lengai and that the suggested origin of natrocarbonatite may be applied to natrocarbonatites of Oldoinyo Lengai. Received: 18 January 1996 / Accepted: 2 September 1996     

Spectrum Analysis of the Reflected Waves Observed in the Nikko Region, Central Japan

—The Nikko region, in northern Kanto, central Japan, which is a high seismicity region, is located near the volcanic front. Many seismic observations have been carried out around the Nikko region because the seismograms of the shallow earthquakes occurring in this region show an S-wave reflected phase (SxS) and an S-to-P mode-converted and reflected phase (SxP) from a mid-crustal seismic velocity discontinuity. The depth of the mid-crustal reflector was estimated to be about 9–16 km. From the amplitude analyses of SxS and SxP phases, the material of the mid-crustal layer just beneath the reflector was expected to be a low rigidity layer, which was interpreted as partially molten rocks. No evidence has been shown for the internal properties of the material of the reflector. The Vp/Vs ratio is a good measure to distinguish between the two candidates for seismic reflectors; i.e., partially molten rocks and rocks containing water. We investigate the Vp/Vs ratio of the reflector using the spectrum analyses of the data observed by a seismic array. From spectrum analysis of the data, we indicate that the Vp/Vs ratio of 2.5 is better than that of 1.8 to explain the observed data, which shows that the material beneath the reflector consists of partially molten rocks.     

High spatial and temporal resolution observations of an impulse-driven field line resonance in radar backscatter artificially generated with the Tromsø heater

The CUTLASS Finland HF radar has been operated in conjunction with the EISCAT Tromsø RF ionospheric heater facility to examine a ULF wave characteristic of the development of a field line resonance (FLR) driven by a cavity mode caused by a magnetospheric impulse. When the heater is on, striating the ionosphere with field-aligned ionospheric electron density irregularities, a large enough radar target is generated to allow post-integration over only 1 second. When combined with 15 km range gates, this gives radar measurements of a naturally occurring ULF wave at a far better temporal and spatial resolution than has been achieved previously. The time-dependent signature of the ULF wave has been examined as it evolves from a large-scale cavity resonance, through a transient where the wave period was latitude-dependent and the oscillation had the characteristics of freely ringing field lines, and finally to a very narrow, small-scale local field line resonance. The resonance width of the FLR is only 60 km and this is compared with previous observations and theory. The FLR wave signature is strongly attenuated in the ground magnetometer data. The characterisation of the impulse driven FLR was only achieved very crudely with the ground magnetometer data and, in fact, an accurate determination of the properties of the cavity and field line resonant systems challenges the currently available limitations of ionospheric radar techniques. The combination of the latest ionospheric radars and facilities such as the Tromsø ionospheric heater can result in a powerful new tool for geophysical research.