松辽盆地西缘突泉地区晚侏罗世过铝质流纹岩和英云闪长玢岩的发现:从蒙古-鄂霍茨克洋闭合到陆陆碰撞的地质记录

Discovery of the Late Jurassic peraluminous rhyolites and tonalite porphyrites in the Tuquan area along the western margin of the Songliao Basin:Geological records from closure of the Mongol-Okhotsk Ocean to continental collision between the Siberian plate and the Erguna-Songliao block

蒙古-鄂霍茨克洋南向俯冲、大洋闭合和陆-陆碰撞是东北亚地区晚中生代的重要区域构造事件,它与本区的岩浆活动、变质作用、成盆和造山作用都密切相关。准确界定洋壳俯冲、大洋闭合和陆-陆碰撞这三个相互关联地质过程的时空范围是客观认识区域构造演化的前提。然而,这项研究通常难以进行,因为很难找到与之相关的合适的地质记录。作者在松辽盆地西缘突泉地区发现从大洋闭合到陆-陆碰撞相关的过铝质流纹岩和英云闪长玢岩,锆石LA-ICP-MS U-Pb定年结果显示其结晶年龄分别为156±1Ma和155±1Ma,系晚侏罗世岩浆事件产物。流纹岩和英云闪长玢岩为钙碱性过铝质岩石,具有较高的铝饱和指数A/CNK(1.32~2.13),较低的MgO+FeO T含量(0.96%~3.37%)和FeO T/MgO比值(2.84~5.02),岩矿鉴定表明它们含绢云母等原生和次生高铝矿物,在CIPW标准矿物计算中出现刚玉分子(3.77%~9.65%),结合流纹岩和英云闪长玢岩在花岗岩的S-I-M-A型分类方案相关地球化学图解投影结果,综合表明它们具有过铝质S型花岗岩特征。流纹岩和英云闪长玢岩具有较低的Rb/Sr(0.35~0.55)、Rb/Ba(0.08~0.26)和Al_(2)O_(3)/TiO_(2)(38.41~61.36)比值,表明其原始岩浆源于杂砂岩部分熔融。锆石饱和温度计算表明这两类岩石的岩浆形成温度在837~876℃之间,低于A型花岗岩岩浆形成温度(900℃)。两类岩石富集Rb、Ba、K等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素,具有较低的Y(5.29×10^(-6)~19.75×10^(-6))、Nb(7.44×10^(-6)~8.50×10^(-6))、Sr(60.6×10^(-6)~154.9×10^(-6))和Yb(0.53×10^(-6)~2.40×10^(-6))含量,具有弧岩浆属性。在Y-Nb图解中流纹岩和英云闪长玢岩投点在火山弧和同碰撞型花岗岩区域内;在Rb/10-Hf-Ta×3图解中投点在火山弧和碰撞型花岗岩区域内;在R1-R2构造判别图解中样品投点在同碰撞和造山期区域内;在Sr-Yb图解中样品投点在地壳加厚区域内。结合全球典型俯冲岛弧和同碰撞酸性岩地球化学特征和同时期本区域岩浆岩成因,表明流纹岩和英云闪长玢岩形成于洋壳俯冲和陆-陆碰撞转换背景,暗示其形成与蒙古-鄂霍茨克茨洋闭合和陆-陆碰撞作用有关。流纹岩(La/Yb)N值为6.62~8.77,指示源区起源深度为40~46km,英云闪长玢岩(La/Yb)N值为7.93~13.39,指示源区起源深度为44~55km,表明在156±1Ma~155±1Ma地壳处于持续加厚过程。结合区域地质资料,构建了蒙古-鄂霍茨克洋从俯冲闭合到陆-陆碰撞的构造-岩浆演化模型。本文认为蒙古-鄂霍茨克构造体系的影响范围到达了松辽盆地西缘突泉地区,156±1Ma~155±1Ma研究区处于大洋闭合到陆-陆碰撞转换阶段。

The southeastward subduction of the Mongol-Okhotsk Ocean,oceanic closure,and continental collision were important regional tectonic events in the Late Mesozoic of Northeast Asia.They are closely related to magmatic activity,metamorphism,basin formation and orogeny in the area.Accurately defining the spatiotemporal range of the interrelated geological processes of the above three events is a prerequisite for understanding the tectonics of the region in the Late Mesozoic.However,it is difficult to conduct this kind of investigation as it is difficult to find suitable geological records related to these events.We discovered peraluminous rhyolites and tonalite porphyrites in the Tuquan area along the western margin of the Songliao Basin.These samples are probably related to the oceanic closure and continental collision.The zircon LA-ICP-MS U-Pb dating results show that their crystallization ages are 156±1Ma and 155±1Ma,respectively,indicating they are the products of Late Jurassic magmatic events.They are calc-alkaline peraluminous rocks with high aluminum saturation index A/CNK(1.32~2.13)and low content of MgO+FeO T(0.96%~3.37%)and low ratio of FeO T/MgO(2.84~5.02).Thin-section work shows that they contain high aluminum minerals such as sericite.Corundum molecules(3.77%~9.65%)appear in CIPW standard mineral calculation,and these rhyolites and tonalite porphyrites are mapped in the geochemical diagrams related to the S-I-M-A classification scheme of granites.These results show the characteristics of S-type granites.The low ratios of Rb/Sr(0.35~0.55)and Rb/Ba(0.08~0.26)of rhyolites and tonalite porphyrites indicate that the primary magma of rhyolites and tonalite porphyrites were originated from poor argillaceous arenaceous rocks.Zircon saturation temperature calculation shows that the magmatic crystallization temperatures of rhyolites and tonalite porphyrites range from 837℃to 876℃,and the samples have high ratio of Al_(2)O_(3)/TiO_(2)(38.41~61.36),these temperatures are lower than the formation temperature of A-type granite(900℃).These rhyolites and tonalite porphyrites are richen in large ion lithophile elements such as Rb,Ba,K,and lack high field strength elements such as Nb,Ta,P,Ti.They also show low contents of Y(5.29×10^(-6)~19.75×10^(-6)),Nb(7.44×10^(-6)~8.50×10^(-6)),Sr(60.6×10^(-6)~154.9×10^(-6))and Yb(0.53×10^(-6)~2.40×10^(-6)),showing arc magmatic properties.In the R1-R2 major element tectonic discrimination diagram,the samples are mainly projected within the range of collisional and orogenic periods.The Nb-Y diagram shows that the samples are dotted in volcanic arcs and syn-collision granites.The Rb/10-Hf-Ta×3 diagram shows that the samples are located in the regions of volcanic arcs and collision type granite.The Sr-Yb diagrams indicate that rhyolites and tonalite porphyrites were formed in the stage of crustal thickening.The tectonic discrimination for the rhyolites and tonalite porphyrites in the Tuquan area is suggested that the Late Jurassic in the area was a period of the volcanic arc and continental crust collision environments.It implies that their formation is related to oceanic crust subduction and continental collision during the closure process of the Mongol-Okhotsk Ocean.The ratio of rhyolites(La/Yb)N range from 6.62 to 8.77,indicating a depth of 40~46km in the source area.The ratio of tonalite porphyrites(La/Yb)N range from 7.93 to 13.39,indicating a depth of 44~55km in the source area.These results indicate that a continuous thickening process of the crust at 156±1Ma to 155±1Ma.These characteristics of rhyolites and tonalite porphyrites provide key igneous rock evidence for evolution from oceanic subduction to continental collision.Combined with the regional geological data,the tectonic-magmatic evolution model of the Mongol-Okhotsk Ocean from the subduction and closure to the continental collision is constructed.In this paper,the influence range of the Mongol-Okhotsk Ocean tectonic system reached the Tuquan area in the western margin of the Songliao Basin,the Mongol-Okhotsk Ocean was closed at 156±1Ma,and the study area at 155±1Ma was at the stage of continental collision and crust thickening after ocean closure.