Mesozoic volcanic rocks are widely exposed in Zhenglan Banner of southeast Inner Mongolia.The volcanic rocks mainly consist of rhyolite,trachyte,porphyroclastic lava,ignimbrite,and obsidian,in which the porphyroclastic lava is the most developed.According to the results of the zircon LA-ICP-MS dating,the U-Pb zircon age of rhyolites is (141.4±0.7) Ma,that of felsitic mortar lava is (141.6±0.6) Ma and that of ignimbrite is (139.4±0.7) Ma.Within the error range,the formation ages of the three kinds of rocks are the same,which indicates that the time of the volcanic activity (from effusion to explosion and extrusion) lasted a short time.So they should be assigned to early Cretaceous Zhangjiakou Formation.Except for very few samples,the porphyroclastic lava that extruded in the late phase and the rhyolite-trachyte belonging to the early phase effusion facies are extremely consistent in the geochemical characteristics.They are rich in silicon and potassium,and poor in Ca,Mg and A1.So they could be classified as trachyte-rhyolite combination,and some samples have the features of pantellerite.They are enriched in Rb,Th,U and other largeion lithophile elements,and strongly depleted in Ba,Sr,Ti,P.The chondrite-normalized REE patterns show enrichment of light rare earth elements (LREE),strong negative Eu anomalies,and significant fractionation of LREE and HREE for the volcanic rocks (LaN/YbN=6.87-42.74,except for LQ-38).Compared with the early phase effusion facies lava,porphyroclastic lava has higher silicon and lower alkali,which shows that the magma chamber experienced obvious crystallization differentiation after effusion of the rhyolite and trachyte in the magma event.Then the magma with higher silicon and lower alkali extruded out of the earth's surface.The volcanic rocks of Zhenglan Banner area have the characteristics of A-type granites and possess the geochemical characteristics of A1 type and A1-A2 transition granite.The Nb/Ta ratio of the extrusive porphyroclastic lava and effusive rhyolites is 10.46-24.02,which is between the values of the crust and the mantle magma;Y/Nb,Ti/Yb,Ti/Zr also reflect the characteristics of crust-mantle mixing;Rb/Sr ratio is between 0.61-64.51(＞0.5),reflecting the characteristics of crust source.These data suggest that the volcanic rocks might have been produced in an extension environment of the transportation period of anorogenic and postorogenic activities,mainly being intraplate anorogenic activity.In addition,the volcanic rocks were probably derived from partial melting of the Precambrian lower crust which was strongly transformed by Mesozoic underplating and metamorphism.%内蒙古东南部正蓝旗地区中生代火山岩广泛出露,岩性主要包括流纹岩、粗面岩、碎斑熔岩、熔结凝灰岩以及黑曜岩等,以碎斑熔岩发育为显著特征.本次研究测得流纹岩、霏细质碎斑熔岩及熔结凝灰岩的LA-ICP-MS锆石U-Pb年龄分别为(141.4±0.7) Ma、(141.6±0.6) Ma和(139.4±0.7) Ma,三者在误差范围内一致,表明研究区火山活动(从溢流、爆发到侵出)持续的时间很短,时代为早白垩世早期,属张家口组.除个别样品,正蓝旗地区晚期侵出的碎斑熔岩与早期溢流相的流纹岩、粗面岩在地球化学特征上极为一致,均为富硅富钾,贫Ca、Mg和A1,属粗面岩-流纹岩组合,部分样品具有碱性流纹岩的特征;富集Rb、Th、U等大离子亲石元素,而强烈亏损Ba、Sr、Ti、P;轻稀土元素(LREE)富集,Eu强烈亏损,轻、重稀土元素分馏较强(LaN/YbN主要介于6.87～42.74).碎斑熔岩体相较于早期熔岩更加富硅贫碱,表明流纹岩-粗面岩喷溢之后,岩浆房中剩余岩浆又经过一定程度的分异,向更为富硅贫碱方向演化之后侵出地表.正蓝旗地区火山岩具有A型花岗岩的特征,为A1型为主,A1—A2过渡的地球化学特征.碎斑熔岩体与流纹岩-粗面岩的Nb/Ta介于10.46～24.02,在地壳和地幔岩浆Nb/Ta比值之间;Y/Nb、Ti/Yb、Ti/Zr均体现出壳幔混合的特点;Rb/Sr介于0.61～64.51,为壳源特征.因此,研究区火山岩是造山后向板内非造山转换并以板内非造山为主的伸展环境下,经较为强烈地幔物质改造的下地壳部分熔融的产物.
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