High-Field-Strength Elements in Mafic Volcanics from North China Craton: Implications for Archean-Proterozoic Boundary and Source Composition

Archean to Cenozoic mafic volcanic rocks from the North China craton are studied. They show Archean Proterozoic (Ar Pt) boundary and geochemical anomalies in Cenozoic basalts. Proterozoic mafic volcanics are enriched in most of the high field strength elements (HFSE) compared with Archean ones. Nb, Ta and Th show a distinct sequence of incompatibility in Archean and Proterozoic. The Cenozoic basalts are enriched in HFSE and Ni and their REEs are strongly differentiated with positive Eu anomalies ( δ (Eu)=1.14). The Ar Pt boundary could be related to change in oxygen fugacity and requires an increasing importance of enriched mantle source. The geochemistry of Cenozoic basalts implies a mantle source similar to OIB. Residuum from subducting partial melting of old basaltic oceanic crust and continental crust is likely to contribute to the formation of the enriched mantle.

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.

Geochronology and geochemistry of the Heihe mafic pillow lavas in the Qinling Mountains,China

The Qinling Mountains in central China are the joint orogenic zone between the Sino-Korean(or North China)and the Yangtze craton blocks.The age and genesis of the Danfeng mafic volcanics in the north of the Shangzhou-Danfeng fault zone,I.e.The main suture zone in the Qinling orogenic belt,have been controverted for a long time because their age is closely related to the converged time of two blocks.The ages and the geochemical data of the Heihe pillow lavas for the Danfeng mafic volcanics in the Heihe River area in the Qinling orogen are reported in this paper.The obtained isochron age by the Sm-Nd isotopic data of the 13 whole-rock samples for the mafic pillow lavas is 963±130(2σ)Ma,corresponding to I_(nd)=0.51173±16(2σ),ε_(Nd)(T)=+6.6,MSWD0.57.However,the Rb-Sr isotopic analytical results for the same samples as the Sm-Nd whole-rock ones are disperse.For the Sm-Nd isotopic systems were interfered during the later geological functions,the Sm-Nd isochron age for the whole-rock sample(Q9511WR)and the mineral phenocrystal sam-ples:amphiboles(Hb)and plagioclases(Plag)presents the better uncertainty,whereas isochron ages of 930 Ma and 437 Ma are given if the WR-Plag and WR-Hb are calculated respectively,and their Rb-Sr isochron age is 268±47(2σ)Ma,I_(sr)=0.70475±11(2σ),MSWD0.96.The major and trace elements for the lavas show that they were formed in the quasi-N-MORB setting.