Seismic wave velocity of Archaeozoic felsic rocks from North China and its existing location in the crust

This paper contains two contents. The first is seismic velocity of felsic crystalline rocks of North China at room temperature and high pressures and at both high temperatures and pressures. The second is heating acoustic emission of felsic rocks at atmosphere pressure and the temperature of the quartz (-( transition. The results of these experiments show that velocities of the felsic crystalline rocks are obviously lower than that of the basic rocks and no visible relationship with metamorphic phase. The velocity curves of rocks containing quartz display peaks of the (-( phase reaction, which are different from other rocks in configuration. When the heating temperature is up to the phase transition temperature of quartz at the atmosphere pressure, felsic hypometamorphic rocks and magma granite produce acoustic emission. While the other kind of the granite generated by metasomatism does not produce acoustic emission. These results have the following implications. It explains the crustal constitution of the North China craton in combination with other geoscientific data, and clarifies the existing space of ( quartz and ( quartz. It also indicates that the (-( quartz transition and dehydration melting of amphibole and biotite in the lower crustal rocks of the Cenozoic tectonic subsidence area are likely associated with faulting (tensional fault) and seismic activities.

Geochemistry of the Eocene Felsic Porphyric Rocks and High-Mg Potassic Rocks along JARSZ:Implication for the Tectonic Evolution in Eastern Tibet

Eocene felsic porphyric rocks and the high-Mg potassic volcanic rocks（HMPR） occur along the Jinshajiang-Ailao Shan-Red River shear zone（JARSZ） in eastern Tibet.Compared with the HMPR,which are generally believed to be sourced from an enriched mantle,the felsic porphyric rocks show similar K_2O contents,enrichment in LREE and LILE,particularly radiogenic isotope（e.g.Sr and Nd） features much similar to the former,implying generation of the felsic porphyric rocks most likely related to the HMPR,although they both have clearly different major and trace element compositions. The close relationship in spatial-temporal distribution and similar Sr-Nd characteristics between the felsic porphyric rocks and HMPR in eastern Tibet indicate that both of them were possibly formed by a similar tectonic process（event）.Combining the basic dikes in southern and eastern Tibet,we suggest that the break-off of north-dipping Neo-Tethyan slab in southern Tibet during 50-40 Ma,triggered formation of high-Mg potassic magma.This led to developing felsic porphyric magma production by partial melting of underplating HMPR in the lower crust,or fractionation crystallization of the high-Mg potassic magmas.The break-off of slab in the Eocene may also have contributed to the abundant ore-forming material related to earlier subduction events,resulting in formation of the porphyric deposits along JARSZ in eastern Tibet.

The possible source of uranium mineralization in felsic volcanic rocks,Eastern Desert,Egypt of the Arabian-Nubian Shield:Constraints from whole-rock geochemistry and spectrometric prospection

The present work deals with the detailed geology,mineralogy,geochemistry,and spectrometric prospection of the felsic volcanic rocks at the Eastern Desert,Egypt of the Arabian-Nubian Shield.Felsic volcanic rocks are an essential source for rare earth elements(REEs)and uranium occurrences in this area.They are compositionally uniform with tholeiitic to calc-alkaline affinities,peraluminous and belong to the series of rhyolite with high-K melt.They exhibit more enrichment in high field strength elements(HFSE,e.g.Zr,Ta,Nd,Th,and U)and large-ion lithophile elements(LILE,e.g.Pb and Rb)compared to the country rocks of the studied area,with REE ranging from 188.20 to 442.70 ppm and strong depletion in Ti,Sr,P with deep negative Eu oddities.The felsic volcanic rocks were mostly generated from the partial melting of quartz-amphibolite facies accreted during the Neoproterozoic.Positive oddities of Zr-U-Th for the felsic volcanic rocks determine the involvement of crustal materials.Felsic volcanic rocks are found in A-type suites of magma and represent highly fractionated rocks derived from rhyolitic magma,with insignificant interaction with continental crust in the low-pressure environment and during fractional crystallization.Felsic volcanic rocks have higher values of radioactivity in which eU range from 0.5 to 121 ppm and eTh from 1.0 to 415.10 ppm.The high values of eU and eTh can be ascribed to the mineralization of uranium and the presence of accessory minerals of radiogenic nature such as uranophane,uranothorite,zircon,and monazite.Uranophane is considered as the mineral with most enriched uranium contents in the studied felsic volcanic rocks in which(UO2=87.30 wt%).Also,they are enriched with REE-bearing accessory minerals comprising allanite,titanite,and apatite.The geological investigations of the felsic volcanic rocks in the studied areas are inappropriate to clear the feasible economic potentialities of rare earth elements and U occurrences;itemized and invaluable explorational work is as yet needed.Whilst,the environmental impact of mineralization,owing to U and Th and their radiogenic daughter products,is observed and must be elaborated minutely.

The Bafoussam volcanic series:origin and evolution of the volcanism along the Cameroon volcanic line

The Bafoussam area in western Cameroon is part of the central Cameroon Volcanic Ligne(CVL).This study presents the mineralogy,major and trace element compositions,Sr-Pb-Hf isotopes,and new K-Ar geochronological data about mafic and felsic volcanic rocks.These rocks belong to two different series:A transitional series made of basalts,basaltic andesite,and trachytes and an alkaline mafic series with basalts,hawaiites,and basanites.New age data show that the transitional series belongs to the oldest part of the CVL and was emplaced between 47 and 35 Ma.The alkaline volcanism is younger,with ages ranging from10 to 4.5 Ma.Magmatic evolution in both series is accomplished through a fractional crystallization process,with the removal of olivine and clinopyroxene,while plagioclase does not seem to be a major crystallizing phase.All the samples are enriched in incompatible trace elements,but the rocks from the alkaline series have more fractionated REE patterns and high Nb content compared to the transitional mafic lavas.Alkaline lavas have lower initial^(87)Sr/^(86)Sr and higher^(176)Hf/^(177)Hf and Pb isotopic ratios than the transitional lavas.Low La/Nb and high^(87)Sr/^(86)Sriratio are among chemical characteristics that show that some samples from the transitional series have interacted with a crustal component during their evolution in the crust.They cannot be used for discussing the mantle source of the volcanic rocks from this series.Trace elements show that primary magmas for both series formed in a garnet-bearing mantle source,with higher partial melting degrees(3-5%)for the transitional magmas than for the alkaline magmas(<2.5%).Combining trace elements and isotopic ratios,we show that the Bafoussam lavas formed from two different mantle sources.Transitional magmas formed from a pyroxenite-bearing enriched mantle with low Pb isotopic composition.This mantle source is present in all the oldest lavas from the CVL.Alkaline magmas formed from an HIMU-like mantle source,different from the Mt Cameroon HIMU mantle source.The depleted asthenospheric mantle is not involved in the Bafoussam magmatism and the two mantle sources are probably located in the lithospheric mantle,in agreement with recent geophysical models presenting the CVL as a consequence of the partial melting of the lithospheric mantle in response to edge convection along the margin of the Congo craton.