Geochemical Characteristics And Genetic Mechanism Of Wang’anzhen Complex In Taihang Mountains

Wanganzhen complex, located inZijingguan fault zone, is the representative rock massof northern Taihang Mountains. The magma source and the formative mechanism ofWanganzhen complex is the focus of the academic circle. On the basis of petrology,geochronology, rock geochemistry and isotope geochemistry studies, this paper makesinvestigation on rock type, magmatic property and magmatic origin, proposing the tectonicsetting and diagenesis model.The main rock types of Wanganzhen complex are sub-volcanic rock, basic rock andmedium-acidic rock. Rhyolite in Dawan is the main rock type of sub-volcanic rock inWanganzhen and has formed between142Ma to139Ma, which is similar to the formationtime of Volcanic Rocks of Tiaojishan Formation. Rhyolite in Dawan have the characteristicsof high Si, alkaline abundances, and Rb/Nb ratios, low Mg#, Cr, Ni, Eu and Sr content,indicating the typical continental crust origin. Combined with Sr-Nd-Pb isotopiccharacteristics, we thinks that rhyolite originated from the mid-upper crust and had thecontribution of mantle-derived components.The main rock types of basic rock in Longmengou are gabbro diorite, amphibole gabbro,and olive-gabbro-diabase, which formed between142Ma to139Ma. The basic rock areenriched in lithophile elements(the content of both Sr and Ba exceed1000×10-6) anddepleted in high field strength elements, and had negative εNd(138Ma)(-13.8～-17.3), showingmantle characteristics. Combined with the discovery of peridotite xenolith of high Mg#(92.22～94.89), low Si, having anorthite, which indicates the derivation of depleted mantle,we thinks that basic rock originated from the depleted mantle, with the contribution of thelower crustal materials and no transformation of Si rich fluid in mantle source.The medium-acidic rock, formed between129Ma to126Ma, is the main body ofWanganzhen complex. The medium-acidic rock have the characteristics of high Si and Ba, noobviously Eu anomaly, low Y, LREE enrichment, and HREE depletion. Combined withSr-Nd-Pb isotopic characteristics, we thinks that the medium-acidic rock originated from thelower crust and was the mixing production of the basic magma and granitoid magma formed by part-melting of the lower crust.On the above research, we first advanced that there were three layer magma chambers innorthern Taihang Mountains. That is the rhyolite originated from the mid-upper crust, thebasic rock originated from the depleted mantle, and the medium-acidic rock originated fromthe lower crust. The existence of three layer magma chambers may indicate this area was astrongly large thermal abnormal area during late Jurassic epoch to early cretaceous epoch.The large thermal anomaly may related with the mantle plume or asthenosphere upwellingresulted from lithosphere delamination, and we established diagenesis model on this basis.We inferred formation process of Wanganzhen complex like this: At the tectonic settingof the mantle plume or asthenosphere upwelling, with uninterrupted upward move anddecompression, lithosphere mantle experienced partial melting (up to40%) and formed theparent magma of the basic body. At the same time, lower crust experienced partial melting forthe underplating of mantle magma and formed the parent magma of the medium-acidic rock.Meanwhile, the mid-upper crust,with the help of asthenosphere or mantle plume, mantlemagma, medium-acidic magma, also experienced partial melting and formed the parentmagma of the rhyolite rock. Wherafter, the magma originating from magma chambers ofdifferent layers may mix to different degree and formed the Wanganzhen complex withdouble attribute of crust and mantle.