【作者】 余星；

【导师】 杨树锋； 陈汉林； 厉子龙； 陈仲强；

【作者基本信息】 浙江大学 ， 地质学， 2009， 博士

【摘要】 分布面积达20万km²的塔里木早二叠世火成岩一直以来是地质学家们研究的重点。本文系统研究塔里木早二叠世火成岩的年代学、岩石学、地球化学和同位素特征及岩石成因,重点探讨火成岩的演化序列及深部地质作用过程。论文取得了以下几点认识:1.在项目组提出塔里木早二叠世大火成岩省的基础上,进一步论证了塔里木大火成岩省的存在。大火成岩省岩石种类丰富,包括超基性岩类、基性岩类和中酸性岩类。超基性岩类为超镁铁质岩、爆破角砾岩和辉石岩,基性岩类包括玄武岩和辉绿岩,中酸性岩类为正长岩和正长斑岩。其中基性岩类范围最广,分布于阿瓦提坳陷、满加尔坳陷西部、塔北隆起西部、巴楚隆起和塔中隆起和塔西南坳陷等地区,估计分布面积在20万km²以上。2.高精度锆石SHRIMP U-Pb定年结果显示,柯坪玄武岩最早喷发年代在290Ma左右（289.5±2.0Ma）,最后一次喷发的时代大概在289Ma（288.9±3.4Ma）;正长斑岩的SHRIMP U-Pb年龄为278.4±2.2Ma,与正长岩体的年龄相近（277～278Ma）。结合地层古生物等资料,认为塔里木早二叠世火成岩的时间序列为:早期玄武岩喷出和超基性岩的侵入,其后辉绿岩侵入,最后是正长岩体和正长斑岩岩脉的侵入。这一演化序列得到地球化学证据的支持。3.塔里木早二叠世大火成岩省各类火成岩的SiO₂含量为31～67%,且具有双峰式火成岩特征。瓦基里塔格爆破角砾岩和辉绿岩、塔北玄武岩、小海子超镁铁质岩和辉绿岩及正长岩和正长斑岩均为碱性系列;柯坪辉绿岩与柯坪玄武岩为碱性-亚碱性过渡系列。火成岩的地球化学组成以富集大离子亲石元素和高场强元素,亏损重稀土元素为特征,与OIB微量元素特征相似。Ta/Hf比值和Nd同位素初始值指示两类火成岩:一类为柯坪玄武岩,具有负的εNd值,主要与富集的大陆岩石圈地幔有关;另一类为塔北玄武岩及各区的侵入岩,εNd值大于0,代表亏损型地幔来源。4.塔里木早二叠世大火成岩省的岩浆演化以结晶分异为主,包括橄榄石、单斜辉石及斜长石的分离结晶,基本没有受到同化混染作用的影响。塔里木早二叠世大火成岩省主要来源于两种源区:柯坪玄武岩来源于尖晶石-石榴石二辉橄榄岩约5%的部分熔融;超镁铁质岩和辉石岩均来自石榴石二辉橄榄岩,熔融程度分别为～16%和～5%左右;其他岩类均为这两种岩浆演化或相互作用的产物。5.塔里木早二叠世大火成岩省源于地幔柱的作用。地幔柱上升引起区域地壳的抬升;巨量的大陆溢流玄武岩在短时间内喷发,持续时间为1～2Ma;发育大量的二叠纪基性岩墙群;酸性岩浆的侵入,代表幔柱岩浆活动的结束;高镁岩浆岩与地幔热柱有关;微量元素比值（如Ta/Hf＞0.3）和同位素成分特征等都支持地幔柱成因假说。6.针对塔里木早二叠世大火成岩省的特征,提出塔里木大火成岩省成因模式,即早期高热的地幔柱引起了岩石圈地幔的低程度部分熔融,后期地幔柱绝热减压引起地幔柱自身熔融。它不同于巴哈纳型（单一的岩石圈熔融）和德干型（单一的地幔柱熔融）。7.塔里木早二叠世大火成岩省的演化过程为:地幔柱上升并与塔里木岩石圈地幔相互作用,引起了岩石圈地幔的低程度部分熔融,形成玄武质岩浆;当熔融的岩浆积聚到一定的压力,最终突破岩石圈,喷溢出地表,形成溢流玄武岩;玄武岩大面积喷发之后,地幔柱获得了更多的上升空间,短时间内迅速上升、减压,造成地幔柱本身开始熔融,形成高镁岩浆,可形成超镁铁质岩和爆破角砾岩筒;分离结晶出的辉石上升后形成辉石岩岩体;原始岩浆进一步演化,可以辉绿岩岩墙或塔北玄武岩的形式侵入或喷出地表;最后,玄武质岩浆经结晶分异作用演化为富碱富硅的中酸性岩浆,上侵形成正长岩体和正长斑岩岩脉。更多还原

【Abstract】 Huge quantities of Early Permian igneous rocks were found in Tarim Basin, Xinjiang Province,NW China.The overall coverage of the igneous rocks was up to 200,000km².These igneous rocks have always been the key interest to geologists.In this thesis,systematic researches have been carried out as to Early Permian igneous rocks,focusing on the magma evolution and deep geological processes of the large igneous province.Up to now,several points have been addressed as follow:1.Based upon the nomination of Early Permian Tarim LIP by our research group, more works have been down to support the conclusion.Early Permian Tarim LIP consists of ultrabasic-basic-felsic rock types.The ultrabasic rocks are ultramafic rocks, explosive breccia and pyroxenite.Basalt and diabase are the main components of basic series,while felsic series outcropping as syenite and syenite porphyry.Basalts were widely erupted across the west,northwest,southwest part of the basin,with estimated area of more than 200,000km².2.According to Zircon SHRIMP U-Pb dating,The lowest unit of Keping basalts erupted at about 290Ma（289.5±2.0Ma）,while the top unit reveals an age of 288.9±3.4Ma,which means the basalt eruption probably ended at 289Ma.SHRIMP U-Pb age of syenite porphyry is 278.4±2.2Ma,which is of the same age as syenite （277～278Ma）.So the temporal series for Tarim LIP are:basalt eruption came first, ultramafic intrution,then diabase followed,syenite porphyry and syenite intruded at last.3.Early Permian Tarim LIP consists of a range of igneous rocks,with SiO₂ content ranging from 31%to 67%.All the igneous rocks are rich in large ion lithosphile elements（LILE） as well as high field-strength elements（HFSE）,depleted in HREE.The geochemical features are to some extent similar to OIB.Wajilitag explosive breccia and diabase,Xiaohaizi ultramafic rocks and diabase,syenite, syenite porphyry are belonging to alkali series,while Keping basalt and diabase are at the boundary between alkali and sub-alkali series.Based on the apparent difference ofεNd and Ta/Hf value,two types of magma source were brought in to explain their petrogenesis:one is Keping basalts（εNd＜0） derived from lithospheric mantle;another is intrusive rocks and Tabei basalts（εNd＞0） related to depleted mantle.4.Magma evolution of Tarim LIP mainly happened in the way of crystal fractionation,including the fractionation of olivine,clinopyroxene and plagioclase. There are seldom captures found in Tarim igneous rocks,which means no significant crust contamination happened to Early Permian Tarim igneous rocks.Early Permian Tarim igneous rocks are mainly from two magma sources,ultramafic rocks and pyroxenite were respectively from～16%and～5%melting of garnet lherzolite; Keping basalts were probably from 5%melting of spinel-garnet lherzolite.All of the other igneous rocks are from these two magma sources.5.Early Permian Tarim large igneous province was caused by the action of mantle plume.A lot of evidences show that there was a plume under Tarim block during early Permian.Uprising of the plume resulted in uplift of the crust.Such huge quantities of flood basalts were erupted during such a short period（1～2Ma）.A lot of dyke swarms occurred in Tarim basin.At the final stage of plume-caused magmatism, felsic magma was evolved in and intruded into the crust,ultra high MgO content magmatic rocks are related to hot mantle plume.Trace elements ratios（e.g.Ta/Hf＞0.3） and isotopic data also support the existence of mantle plume.6.According to the characteristics of Early Permian igneous rocks in Tarim basin,we put forward a petrogenic model for the Tarim large igneous province.At the early stage,hot plume caused the low-level partial melting of lithosphere mantle;later, sudden uplift of plume lead to melting of itself due to its adiabatic pressure release. This is different from Bahana model and Deccan model,either by partial melting of lithosphere or mantle plume.7.The general evolution process of Tarim LIP should be like this:First,the uprising mantle plume met with the lithosphere mantle,causing the low degree partial melting of lithosphere mantle,forming the magma for Keping basalts.After eruption of basaltic magma,mantle plume got more room to move up,leading to pressure release and followed by plume melting.The magma can feed ultrarnafic intrusions and explosive breccia.Also,the evolved magma can be the parental magma for Tabei basalts and diobases,and even syenite porphyry and syenite.Pyroxenite can be the accumulates from the fractional crystallization of the primitive magma.更多还原