准噶尔盆地西北缘二叠系-下侏罗统碎屑岩骨架组分及其物源与构造背景演化示踪

Skeleton components of Permian-Lower Jurassic clastic rocks in NW margin of Junggar basin:tracing to provenance and tectonic settings evolution

紧邻西准噶尔造山带的准噶尔盆地西北缘多年勘探钻揭了二叠系-下侏罗统砂砾岩地层,这为源-汇系统研究提供了较齐全的沉积记录。笔者基于砂砾岩43个岩心和37个薄片样品,采用岩心观察与镜下碎屑骨架成分统计、多类型碎屑及端元成分分析、典型物源区碎屑组合模式及特征参数分析等多种方法,对其物源类型与构造背景演化开展了全层系示踪。其砂砾岩骨架碎屑组成较为复杂,砾石成分有岩浆岩砾石(30%~80%)、沉积岩砾石(10%~40%)、变质岩砾石(0%~30%)三类;岩屑成分均以凝灰质、安山质、霏细岩类岩浆岩岩屑和砂泥岩、粉砂质粘土岩、燧石类沉积岩屑为主,仅见少量石英岩、片岩岩屑,表明本区物源主要来自岩浆岩分布区,也有遭剥蚀再沉积的物源供应。其沉积物源主要来自西准噶尔造山带(扎依尔山、哈拉阿拉特山和德仑山),沉积记录反映了物源区母岩类型、组合特征及其隆升剥蚀过程。由二叠系至下侏罗统,砾岩碎屑组分和含量变化呈现"两个组骤变、三段式增减"的特点,砂岩碎屑成分和含量也有变化,表明源-汇系统经历了多次构造抬升与沉降、深部岩石被多次暴露剥蚀与沉积的幕式演化。划分出P1j-P1f、P2w、P3w、T1b-T2k、J1b-J1s五个构造-碎屑沉积响应旋回,其中P2w旋回和P3w旋回,火山岩和沉积岩砾石含量彼此消长和跳跃式变化,反映构造活跃和快速堆积;P1j-P1f、T1bT2k、J1b-J1s三个旋回,火成岩和沉积岩砾石含量均呈由高到低减少趋势,显示早期构造活跃、晚期沉积稳定的构造-碎屑沉积响应。准噶尔盆地西北缘在二叠纪伸展-挤压→三叠纪继承性挤压→早侏罗世弱伸展的构造演化发展中,物源区构造背景经历了由弧造山带→弧造山带+冲断造山带→未切割弧造山带及碰撞造山带→切割弧造山带+冲断造山带+基底隆起区的发展演变,物源类型组合经历了由单陆源为主→陆源+内源叠加→多陆源为主→多陆源+再旋回物源混合的演化和响应。该成果为原型盆地和区域地质演化分析提供了新的依据,为油气勘探和矿产预测提供了基础资料。

Many years of exploration in the northwest margin of Junggar basin,which is adjacent to the West Junggar Orogenic zone,revealed the Permian to Lower Jurassic glutenite strata.This provides complete sedimentary records for the study of the source to sink system.Based on 43 cores and 37 thin section samples of sandstone and glutenite,its clastic components and whole-strata tracing to provenance types and tectonic settings evolution have been analyzed for the first time by means of multiple methods such as core observation and microscopic statistics of clastic skeleton composition,multi-type clastic and endelement components(Qm,Qp,P,K,Lv,Ls)study,analysis of clastic assemblage patterns and characteristic parameters in typical provenance areas,etc.The study shows that the skeleton clastic composition of sandstone and conglomerate are relatively complex,with magmatic rock gravel(30%~80%),sedimentary rock gravel(10%~40%),metamorphic rock gravel(0%~30%).The components of the debris are mainly volcanic debris(such as tuffaceous,andesite,and feldspar)and sedimentary debris(such as sandy mudstone,silty claystone,and chert),with a small amount of quartzite and schist debris.Thus indicates that the provenance of this area mainly come from magmatic rocks,and also come from the source supply of denuded and re-deposited materials.Its Permian-Lower Jurassic sediment sources mainly come from the western Junggar orogenic zone(Zaire Mountain,Hala’alat Mountain and Delun Mountain).The sedimentary records reflect the types and assemblages of parent rocks in the provenance area of the western Junggar orogenic zone,and their uplift and denudation processes.From the Permian to the lower Jurassic,the composition and content of conglomerate clastic show the characteristics of"two groups change abruptly and three-stage increase and decrease".The composition and content changes of sandstone debris show increasing content of feldspar debris,schist,gneiss debris and chert,three-group abrupt changing of felsite and andesite debris content,two-group total content of sandstone debris exceeds the total content of magmatic rock debris.Thus shows that the source to sink system has experienced episodic evolution with multiple tectonic uplift and subsidence,multiple denudation and deposition of deep rocks.Five tectono-clastic sedimentary response cycles,P1 j-P1 f、P2 w、P3 w、T1 b-T2 k、J1 b-J1 s,are classified.Among them,the changes of volcanic rocks and sedimentary rocks in P2 wcycle and P3 wcycle fluctuated and leaped with each other,reflecting active structure and rapid accumulation;the contents of volcanic gravel and sedimentary gravel in P1 j-P1 f、T1 b-T2 k、J1 b-J1 s cycle are decreasing from high to low,showing the response characteristics of tectonic-clastic sediments with active early tectonics and stable late sedimentation.In the structural evolution and development of the northwest margin of Junggar basin from extension-compression in Permian to successive compression in Triassic and weak extension in early Jurassic,the tectonic setting of the provenance underwent the development and evolution from arc orogenic belt to arc orogenic belt+thrust orogenic belt→uncut arc orogenic belt and collision orogenic belt→cut arc orogenic belt+thrust orogenic belt+basement uplift area,and the provenance type combination experienced the evolution and response from mainly single terrigenous supply→terrigenous supply superposited by endogenous supply→give priority to multi terrigenous supply→multiple terrigenous supply mixed by recyclic sources.The results provide a new basis for the analysis of prototype basin and regional geological evolution,and provide basic data for oil and gas exploration and mineral prediction.