The Origin Of Mesozoic Adakite-like Volcanic Rocks From The Northern Taihang Mountains, North China Craton

The geochemical analysis of the samples of volcanic rocks from Tiaojishan Formation,Mesozoic of north Taihang mountains in north China craton shows that these samples havehigh K₂O contents （1.86～4.72%, K₂O/Na₂O ratio ranging from0.5to1.05）, and areplotted in the high-K calc-alkaline. The main rock types are basaltic–andesite, andesite anddecite. The volcanic rocks in this area display high content of SiO₂（55.14～69.64%）,relatively high Al₂O₃（14.5～16.1%）, low MgO （1.31～4.58%）, high Sr（864～1152ppm）,low heavy rare earth elements（Yb=1.18～2.27ppm）, Y（Y=12.4～21ppm）,highSr/Y（39.0～68.6） and La/Yb（17.3～42.3）. Volcanic rocks of Tiaojishan Formation arerich in light rare earth elements （LREEs） and large ion lithophile elements （LILE）. Theheavy rare earth elements （HREEs） and Y are strongly depleted. In Primitivemantle-normalized diagram, they display a clear LREE/HREE fractionation in thechondrite-normalized REE diagram. The above geochemical characteristics show thatvolcanic rocks of Tiaojishan Formation exactly meet geochemical criteria of adakite, whichwere defined by Defant and Drummond （1990）.The major elements of the rocks such as MgO, Fe₂O₃T, Al₂O₃, TiO₂, CaO and P2O5decreases with the increase content of SiO₂, which reflects a fractional crystallizationpattern. In the chondrite-normalized REE diagram we can find out the element Sr ispositive abnormal and the element Eu is weak positive abnormal, which indicates that thereis no plagioclase left. However, in nature system amphibole fractionation is seldom isolatedand is typically accompanied by plagioclase. The amphibole fractionation will increaseSr/Y and La/Yb ratios, whereas decrease Dy/Yb ratio. The garnet fractionation will alsoincrease the Sr/Y and La/Yb ratios, but it will increase Dy/Yb ratio. In the reseach theDy/Yb ratio decreases with the increase content of SiO₂, which indicates that amphibolefractionation was a main controlling factor on the Sr/Y ratio of the volcanic rocks. Inaddition, the content of Sr and Sr/Y ratio decrease slowly with the increase content of SiO₂,the content of Al₂O₃appears a nearly flatting trend, the content of La has no correlationwith the increase content of SiO₂,La/Sm and La/Yb ratios are strongly positive correlatedwith the increase content of SiO₂. All of these indicate that high Sr/Y and La/Yb ratios ofvolcanic rocks from Tiaojishan Formation are not dominated by plagioclase fractionationbut by amphibole fractionation.The volcanic rocks in our research reveal high Mg#（Mg-number） ranging from41.07 to57.95, moderately high initial⁸⁷Sr/⁸⁶Sr ratio ranging from0.7060to0.7062andrelatively low initial²⁰⁶Pb/²⁰⁴Pb ranging from16.58to16.81,²⁰⁷Pb/²⁰⁴Pb ranging from15.25to15.29. All of the geochemical features suggest that the materials from mantle andcrust have participated in the formation of volcanic rocks. Since early Palaeozoic, theupwelling materials of asthenosphere have been caused by subducting of the Pacific platetowards the west, then, the mantle was enriched and partially melted, in the end, maficmelts were formed. These mafic melts were mixed with derivative magmas from meltedlower-crust at bottom of lower crust, and then the mixed melts were fractionated. After theeffect of assimilation and contamination of lower crust in the rising process, these maficmelts can also be fractionated. Finally, the mixed melts ejected out of surface, and thevolcanic rocks from Tiaojishan Formation, Mesozoic of north Taihang Orogen in northChina craton were formed.