The Mamfe Basin is located in the SW of Cameroon and is an extension of the much bigger Benue Trough in the SE of Nigeria. Along the Asenem River and its tributaries in the western part of the Mamfe Basin (close to the border with Nigeria) gem placers yielding big zircon grains were found in recent river sediments close to Nsanaragati. In order to determine the source area and to establish a possible correlation between the zircons found in the Nsanaragati placers and rocks surrounding the Mamfe Basin 56 detrital zircon grains were analysed regarding their U–Pb ages and selected trace element contents by LA‐ICP–MS techniques. Possible source areas are rocks from the Benue Trough in the West and from the Cameroon Volcanic Line (CVL) in the SE of the study area. Based on microscopic analyses it was possible to distinguish two groups of zircons: reddish and non‐reddish ones, where the latter group comprises color variations from brown to orange, yellow to even colorless. In general, the detrital zircons show high hafnium values (4576 to 12565 ppm) and very variable thorium (7.8 to 1565 ppm) and uranium values (13.4 to 687 ppm). The Th/U ratios vary from 0.4 to 2.3, allowing correlations for some zircon grains to kimberlitic, granitic or syenitic affinities. It was also possible to distinguish zircon grains crystallised in mafic mantle‐derived melts from those crystallised in felsic melts, e.g. from continental rift‐related magmatic systems. In general, the U–Pb zircon ages obtained range from 11.7 to 1949 Ma. All zircons of the reddish group yielded almost similar ages resulting in a Concordia age of 12.4 Ma (Serravallian), an age unknown from the Mamfe Basin so far. The group of non‐reddish zircons showed various ages ranging from Serravallian to Orosirian. It was possible to correlate the youngest ages with rocks known from intrusions along the CVL, dated with K/Ar or Ar/Ar methods. The most probable sources were Mount Bamenda and Mount Bambouto in the east of the Mamfe Basin. Cretaceous ages are interpreted as re‐recycled clastic sediments whose original source had been rocks in the south and the north of the Benue Trough and who had been eroded and deposited within the catchment area of the Asenem river system in the Mamfe Basin in post‐Cretaceous times. The oldest ages are assumed to represent the pan‐African and pre‐pan‐African basement of the Mamfe basin. 相似文献
Mineralogy and Petrology - The Dengjitun Cu deposit, located in the central Xing’an Block, represents a key target for medium- to large-sized porphyry Cu deposit exploration. The... 相似文献
The na lidar-observed temperature diurnal tidal perturbations, based on full-diurnal-cycle observations from 2002 to 2008, are compared with tidal wave measurements by the TIMED/SABER instrument to elucidate the nature of diurnal tidal-period perturbations observed locally. The diurnal amplitude and phase profiles deduced by the two instruments are in very good agreement most of the year. However, the lidar-observed diurnal amplitudes during winter months and early spring are considerably larger than SABER observations, leading to the existence of a significant amplitude maximum of 12 K near 90 km in February and a different seasonal structure of temperature diurnal amplitude from the two instruments. The lidar-observed diurnal phase shows propagating wave characteristics during equinoctial months, but exhibit “evanescent wave” behavior in winter months, whereas SABER diurnal tidal phase exhibits propagating diurnal tidal character all year long with small seasonal variation. This anomalous tidal characteristic from the lidar observations repeats almost every winter. The exact mechanism behind this tidal feature is not fully understood, therefore further investigation and more experimental observations are necessary. 相似文献
This paper describes the operational principles, design and field testing of a new, compact, Faraday filter-based spectrometer to measure the D2 (589.158 nm) to D1 (589.756 nm) intensity ratio of the sodium nightglow. This work was motivated by the observations of Slanger et al. (2005) who reported an annual variation in D2/D1 with values ranging from 1.2 to 1.8. Their proposed explanation, a modified Chapman mechanism, requires that the intensity ratio is related to the concentration ratio of atomic oxygen [O] to molecular oxygen [O2]. Our method of measuring D2/D1 utilizes narrowband Na vapor Faraday filters, which can yield observations on the fractional contributions of the two chemical pathways of the modified Chapman mechanism. Since delineation of the two chemical pathways requires a spectral resolution of 0.0002 nm, this is not possible with any other existing instrument. 相似文献
A new strategy for inversion of high-frequency wave radiation condition on the fault plan is exhibited. One-dimensional source model of large earthquake was divided into subfaults, each subfault contains a series of subsources to express high-frequency wave radiation. Envelope of large earthquakes can be expressed as a root-mean-squared with combination of envelope attenuation relationship from all subsources. The envelope attenuation relationship is considered as the empirical Green’s function. Distribution of subsources is estimated by envelope inversion. According to this method, the high-frequency (>?1 Hz) wave radiation areas of 2008 Wenchuan earthquake are generally inverted by the differential evolution using acceleration data from 27 near-field stations, while acceleration waveforms of the Lushan earthquake from 43 near-field stations were utilized to create attenuation envelope. High-frequency waves radiated in: (1) surface rupture areas, including Yingxiu and Beichuan areas; (2) close to the boundaries of asperities near Yingxiu, Yuejiashan, Beichuan and Nanba areas; (3) within 30 km length near the fault northeastern tip; (4) around Qingchuan area.