| The northern segment of Taihang Mountains belt is the concentration area of the large-scale,Mesozoic magmatism in Eastern China.In this region,the intermediate-acid intrusions are exposed as plutons in the bead-like pattern along the major faults whereas the mafic rocks develop mostly as dykes along the two sides of Wulonggou fault.In recent years,although these dykes have attracted a wide spread attention,the systematic research is relatively weak.In this paper,synthesizing the characteristics of field investigations,the analysis of geochemistry and the systematic determination of geochronology,we can draw some conclusions as follows:The mafic dykes exposed in the northern segment of Taihang Mountains belt strike mainly NE trending and secondly NW trending,which are characterized by the steep dip angle,the small width,scarce condensing edge and baking edge and more xenolith of country rocks.The dykes which are the product of the rapid emplacement of magma under the extensional environment are mainly distributed in the east of Wulonggou fault or the hanging wall,less in the west of Wulonggou fault or the footwall,which indicates that the dykes are strictly controlled by the Wulonggou fault which has a stronger impact on the hanging wall than the footwall.The lithology of the mafic dykes are composed of lamprophyre and sillite which is magmatic rocks characterized by rich in volatile matter and titanium,high alkalinity and has a petrographic characteristics of quasi-aluminous-alkaline-shoshonite.There are hydrous minerals such as hornblende,biotite in the mafic dykes through the observation under the polarizing microscope.According to the geochemical analysis of the different trending mafic dykes,they are characterized by the positive correlation between the compatible elements in the dykes and MgO,the positive correlation among La/Yb,La/Sm and La,and La/Sm>5.The chondrite-normalized REE patterns are characterized by enrichment in LREEs,depletion in HREEs,steep LREE patterns and a non-obvious Eu anomaly.Furthermore,the dykes have a feature of enrichment in LREE and large ironic lithophile element,depletion in highfield-strength element.It is indicated that the Mesozoic mafic dykes in the Taihang Mountains belt are the product of partial melting of the mantle in extension environment which experience a certain extent of crystal fractionation and is subject to the hybridization of crust during the rising emplacement of the mantle,as well as the mafic dykes are derived from the same magmatic source-the I-type enrich mantle accompanied by the effect of mantle metasomatism.Synthesizing the acid dykes with a intersected correlation with the mafic dykes,the ages of the country rocks and mafic dykes,it is concluded that the emplacement age of the NE trending mafic dykes are about 128 Ma and the NW trending mafic dykes about 116~122 Ma.It is obvious that the emplacement of the NW trending mafic dykes is later than the NE trending mafic dykes reflecting that the magma intrusion diagenesis has occurred under the multistage extension environment in this area during the Early Cretaceous.The Mesozoic mafic dykes in the northern segment of Taihang Mountains belt are the product of intracontinent extensional setting.The formation of the mafic dykes possibly has a correlation with gravity collapse in the post-orogeny,which are exactly described by the gravity instability of the thickened crust in the orogenic period due to extension in the post-orogeny and the rapid emplacement of magma along the sliding surface of tension-shear structures affected by the Wulonggou fault during the process of mantle upwelling resulted from the delamination of the lithosphere. |
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