【作者】 吴海林；

【导师】 朱文斌；

【作者基本信息】 南京大学 ， 构造地质学， 2012， 硕士

【摘要】 塔里木克拉通是中国三大古老克拉通之一,古老的前寒武纪基底在库鲁克塔格隆起有着很好的出露。本研究对库鲁克塔格隆起西缘库尔勒地区的兴地塔格群石榴云母片岩及斜长角闪岩进行了锆石U-Pb及Lu-Hf同位素和矿物电子探针分析,此外还对斜长角闪岩进行了岩石地球化学分析,旨在获得兴地塔格群的沉积时代和变质作用时代,兴地塔格群及斜长角闪岩变质温压条件,源区地壳增生时间及斜长角闪岩形成的大地构造背景。兴地塔格群中的四块石榴云母片岩中的锆石U-Pb定年表明,它们都记录到1.85Ga的高级变质作用。其中三个样品中的碎屑锆石发生溶解或经历固相重结晶作用,形成变质新生锆石,这些锆石具有一致的1.85Ga的上交点年龄,并且每个样品中锆石具有近于一致的176Hf/177Hf值。库尔勒地区1.85Ga高级变质作用是塔里木北缘对Columbia超大陆聚合的响应。其中的一块样品(T1)除了记录到1.85Ga的变质作用,还保留了原岩中碎屑锆石的年龄信息。该样品中的碎屑锆石最年轻的峰值年龄为2.0Ga,因此兴地塔格群在2.0-1.85Ga之间发生沉积.属于古元古代晚期地层。兴地塔格群中的斜长角闪岩的岩石地球化学研究表明,它们的原岩可分为三种类型：碱性OIB. E-MORB、受到地壳组分混染的基性岩。元素的地球化学模拟表明,碱性OIB和E-MORB样品是来自较原始地幔还要富集的地幔源区。而受到地壳组分混染的基性岩,部分可能是通过AFC作用形成,但是其岩浆源非本研究中的碱性OIB或E-MORB样品,而是类似于N-MORB成分。对两个斜长角闪岩样品(09T-2、09T-10)进行了锆石U-Pb定年及Lu-Hf同位素研究。其中样品09T-2的原岩结晶年龄为2.7Ga,并经历了2.35Ga和1.85Ga的变质作用,其锆石的Hf同位素表明,在2.7Ga时塔里木克拉通北缘有初生地壳的形成,且该期岩浆在侵位过程受到≥3.5Ga的地壳物质的混染。另一块斜长角闪岩样品09T-10,记录到2.21Ga的变质事件,锆石的Hf同位素指示其源区地壳的平均残留年龄为3.6-3.1Ga。对石榴云母片岩和斜长角闪岩的温压估算结果为：1)石榴云母片岩的变质温度为590-800℃,压力为10-17Kbar(尽管压力的估算可能偏高),表明兴地塔格群经历了高级的变质作用,使得原岩中碎屑锆石的U-Pb及Lu-Hf同位素体系被重置；2)斜长角闪岩的变质温度为580-790℃,除两个样品的变质压力为2.7和2.9Kbar较低之外,其余样品为5.8-8.4Kbar,表明样品经历了角闪岩相变质,与基性变质岩中角闪石+斜长石矿物组合所指示的角闪岩相变质相吻合。兴地塔格群中的石榴云母片岩与斜长角闪岩是否为同一期变质作用的产物,区域尺度上兴地塔格群属于何种变质相系以及变质作用形成于怎样的大地构造背景(变质作用是碰撞造山所致还是伸展环境下的岩浆底侵作用所致),还需要更进一步的研究。更多还原

【Abstract】 The Tarim Craton is one of the three old cratons in china. Precambrian basement are well outcropped in the Kuruktag uplift. We carried out zircon U-Pb and Lu-Hf isotope and mineral composition microprobe measurements on garnet-mica schists and amphibolites, as well as geochemical analysis on amphibolites, of the Xingditage Group in order to obtain the maximum depositional age, the metamorphic age and P-T condition of the Xingditage Group, reveal the crustal growth history of the studied area and infer the tectonic setting of the protoliths of amphibolites.Zircon U-Pb age results of four garnet-mica schists indicate that they all record the1.85Ga high grade metamorphic event. The detrital zircons in samples09T-7,8,9either were dissolved to form the new metamorphic zircons or experienced the solid state recrystallization. These zircons have the same upper intercept age of1.85Ga and the nearly identical176Hf/177Hf. This metamorphic event of the Tarim Craton was response to the amalgamation of Columbia supercontinent. Sample T1preserved the source information of the detrital zircons with four small age peaks. The youngest age peak of detrital zircon in sample T1is2.0Ga, indicating that the Xingditage Group was deposited between2.0-1.85Ga and is firstly constrained as the Late Paleoproterzoic strata.The protoliths of amphibolites can be subdivided to three types according to their geochemical character:alkaline OIB, E-MORB and basic rocks contaminated by the crust materials. Element geochemical modelling indicates that both alkaline OIB and E-MORB samples were generated from the enriched mantle source. Some of basic rocks contaminated by the crust materials could be formed by the AFC process, but the magma source are N-MORB compostion, rather than alkaline OIB or E-MORB compostions. Two amphibolite samples(09T-2,09T-10) were carried on zircon U-Pb and Lu-Hf isotope study. The results show that sample09T-2was crystallied at2.7Ga, and then experienced metamorphism at2.35Ga and1.85Ga, respectively. The Hf isotope data indicate that the Tarim Craton has juvenile crust input at2.7Ga, which was mixed by the≥3.5Ga old crust materials. The other amphibolite sample09T-10records the2.21Ga metamorphic event, the average crust residence time of the source of this sample is3.6-3.1Ga.The metamorphic P-T conditions of garnet-mica schists and amphibolites are:1) the metamorphic temperatures of garnet-mica schists are590-800℃and the metamorphic pressures are10-17Kbar (maybe higher than the truth), indicating that the Xingditage Group have experienced high-grade metamorphism and caused the reset of U-Pb and Lu-Hf isotope of detrital zircons;2)The metamorphic temperatures of amphibolites are580-790℃, and metamorphic pressures are5.8-8.4Kbar, except for two samples having lower pressures of2.7Kbar and2.9Kbar. The P-T data indicate the samples have experienced amphibolite facies metamorphism, which is in agreement with the typical amphibolite facies mineral assamblages of amphibole+plagioclase. Did the metamorphism of garnet-mica schists and amphibolites of the Xingditage Group occur at the same episode, which metamorphic face series did the Xingditage Group reach and from which tectonic setting was the metamorphism result (collisional orogen or extentional setting that the magma underplate results in metamorphism) are several major problems needed to be further investigated.更多还原