扬子克拉通崆岭地区现代河流碎屑锆石U-Pb和Lu-Hf同位素研究

发布时间：2018-06-02 04:30

  本文选题：扬子克拉通 + 崆岭地体　； 参考：《中国地质大学》2017年硕士论文

【摘要】：新太古代是早期地壳的形成与演化过程中的一个重要阶段,期间地球内部动力学机制可能发生重大改变,一直是前寒武纪研究的热点。目前,~2.7-2.6的岩浆和变质事件在全球各克拉通已有广泛报道和记录。然而,由于扬子克拉通内植被覆盖严重以及较少的太古宙露头,~2.7-2.6 Ga岩浆事件在扬子克拉通的分布与性质仍未得到很好地限定。近年来,锆石U-Pb和Lu-Hf同位素分析技术的成熟和应用,为扬子克拉通前寒武纪研究提供了大量资料。此外,崆岭地体作为扬子克拉通的太古宙陆核,其内保留有一系列古太古代-元古代岩石,最老可达3.45 Ga。本文对崆岭地区北部现代河流(龚家河,雾渡河和鳊鱼池溪)河沙中的碎屑锆石进行了原位微区U-Pb定年和Lu-Hf同位素分析。在三件样品中共获得323颗谐和锆石年龄和190个锆石Lu-Hf同位素数据,各样品的年龄组成类似。总体而言,可划分为以下年龄峰:3.3-3.1 Ga,3.0-2.8 Ga,2.7-2.6 Ga,2.6-2.2 Ga,2.0-1.9 Ga,1.9-1.7 Ga,1.7-1.5 Ga,和1.0-0.8 Ga,所占比例分别为5%,18%,30%,15%,27%,1%,2%和1%。在锆石的阴极发光(CL)图像显示大多锆石具有振荡环带,且Th/U比值多大于0.3,表明岩浆成因。然而部分2.0-1.9 Ga锆石无岩浆振荡环带,Th/U比值小于0.1,以及与2.7-2.6 Ga锆石近似一致的初始176Hf/177Hf(t)组成,表明其可能是由2.7-2.6 Ga锆石经后期变质重结晶作用形成。另外锆石的U-Pb和Lu-Hf同位素特征与当地岩浆岩的锆石记录相似,由此该结果可以有效反映崆岭地体的形成和演化阶段。根据以上结果,崆岭地区主体形成于中-新太古代(3.0-2.6 Ga)。Lu-Hf同位素结果显示3.0-2.8 Ga锆石εHf(t)值在-11.3和0之间,表明相应母岩浆中含有大量古老陆壳物质。而2.7-2.6 Ga锆石的εHf(t)值高达+7.4,且其中约有43%锆石εHf(t)值位于同期球粒陨石演化线上,表明岩浆活动期间有较多新生陆壳物质加入。根据2.7-2.6 Ga碎屑锆石在本文样品中所占比例,并与崆岭内变沉积岩和周边古老沉积地层中的锆石年龄结果进行对比,笔者提出2.7-2.6 Ga岩浆活动在当地太古宙岩石的形成过程中占主导地位。另外结合前人对扬子克拉通内零星2.7-2.6 Ga太古宙岩石露头,火山岩中锆石捕掳晶以及沉积地层中碎屑锆石的研究,2.7-2.6 Ga构造-热事件可能广泛影响了整个扬子克拉通,与同时期世界范围内其他克拉通保持一致,该过程可能对应早期扬子克拉通的稳定。自此以后,扬子克拉通又经历了一系列新太古代-古元古代的构造-热事件,其可能与Sclavia/Nunavutia超级克拉通(2.6-2.2Ga)和Columbia超大陆(2.1-1.5 Ga)的聚合与裂解过程有关。
[Abstract]:NeoArchean is an important stage in the formation and evolution of the early crust, during which the dynamic mechanism of the earth's interior may change greatly, and it has always been a hot spot in the Precambrian study. The magmatic and metamorphic events of 2.7-2.6 have been widely reported and recorded in various cratons around the world. However, the distribution and properties of the Archean outcrop 2.7-2.6 Ga magma events in the Yangtze craton have not been well limited due to the serious vegetation coverage in the Yangtze craton. In recent years, the maturity and application of zircon U-Pb and Lu-Hf isotopic analysis techniques have provided a lot of data for the study of the Yangtze Craton Precambrian. In addition, as the Archean continental nucleus of the Yangtze craton, the Kongling terrane contains a series of ancient Archean and Proterozoic rocks, the oldest of which is 3.45Ga. In this paper, in situ U-Pb dating and Lu-Hf isotopic analysis of zircons from sediments of modern rivers (Gonjiahe, Wudu River and bream fish pond stream) in the northern part of the Kongling area have been carried out. A total of 323 harmonic zircon ages and 190 zircon Lu-Hf isotopic data were obtained from the three samples. The age composition of each sample was similar. Overall, the following age peaks can be divided into the following age peaks: 3.3-3.1 Ga1 + 3.0-2.8 GaN 2.7-2.6 Ga + 2.6-2.2 GaN 2.0-1.9 Ga 1.9-1.7 Ga 1.7-1.5 Ga, and 1.0-0.8 Ga, respectively, in 518183030 / 15 / 2727 / 27 / 27 / 27 and 1 / 2% and 1 / 2 / 2 / 2, respectively. The cathodoluminescence (CLL) images of zircon show that most zircons have oscillating ring zones and the Th/U ratio is more than 0.3 which indicates the magma genesis. However, part of the 2.0-1.9 Ga zircon without magmatic oscillation ring has a ratio of Th- / U less than 0.1, and an initial composition of 176Hf / 177Hftwhich is approximately consistent with that of 2.7-2.6 Ga zircon, indicating that it may have been formed by the late metamorphic recrystallization of 2.7-2.6 Ga zircon. In addition, the U-Pb and Lu-Hf isotopic characteristics of zircons are similar to the zircon records of the local magmatic rocks, which can effectively reflect the formation and evolution stages of the Kongling terrane. According to the above results, the main body of the Kongling area was formed in the Meso-Neo-Neo-Archean (3.0-2.6 Ga).Lu-Hf) isotope. The results show that the zircon 蔚 Hft) values of 3.0-2.8 Ga are between -11.3 and 0, indicating that the corresponding mother magma contains a large amount of ancient continental crust material. The 蔚 Hftt value of the 2.7-2.6 Ga zircon is as high as 7.4, and about 43% of the zircon 蔚 Hft) values are located on the evolution line of the chondrites, indicating that more new continental crust materials were added during the magmatic activity. According to the proportion of 2.7-2.6 Ga detrital zircon in the samples in this paper, the results are compared with the zircon ages of the altered sedimentary rocks in the Kongling and the ancient sedimentary strata around them. The authors suggest that the magmatic activity of 2.7-2.6 Ga dominates the formation of Archean rocks. In addition, in combination with previous studies on sporadic 2.7-2.6 Ga Archean outcrops in the Yangtze craton, zircon capture in volcanic rocks and detrital zircon in sedimentary strata, the tectonic and thermal events of 2.7-2.6 Ga may have affected the whole Yangtze craton extensively. This process may correspond to the stability of the early Yangtze craton. Since then, the Yangtze craton has undergone a series of tectonic-thermal events of the NeoArchean-Gu Yuan era, which may be related to the polymerization and pyrolysis of Sclavia/Nunavutia super craton 2.6-2.2 Ga and Columbia supercontinent 2.1-1.5 Ga).
【学位授予单位】：中国地质大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P597.3

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