甘肃奥陶纪平凉组钾质斑脱岩地球化学和U-Pb年代学研究
发布时间：2019-01-21 16:10
【摘要】：国际“地时”(Earthtime)组织的成立,使高精度同位素地质年代学进入了高速发展阶段,特别是CA-ID-TIMS U-Pb实验方法的建立,为高精度地质年代表的完善提供了契机。火山灰层作为一个特殊的事件和时间标志层,在古环境、古地理和古构造分析等方面起到了极其重要的作用。本文对甘肃银洞官庄剖面平凉组内的数层钾质斑脱岩开展了地球化学和高精度U-Pb放射性同位素年代学研究。结果表明,所研究样品的SiO_2含量较高,Al2O3/Ti O_2、Zr/Hf、Ti/Th比值都显示出酸性火山岩的特征。稀土元素配分模式呈现轻稀土富集,Eu负异常,无Ce异常,符合同时代钾质斑脱岩稀土配分模式特征。微量元素蛛网图显示Sr和Ti负异常,Pb正异常。Zr/TiO_2-Nb/Y图表明样品为流纹英安岩或粗面安山岩,显示出中酸性原始岩浆亲缘关系。Th/Yb-Nb/Yb图、Th-Hf-Ta图判别结果一致,结合微量元素特征,认为原始岩浆形成于火山弧环境。平凉组覆盖了晚奥陶世Baltonniodus variabilis和Baltonniodus gerdea牙形刺带。通过剖面识别出的短偏心率旋回约束,两层钾质斑脱岩持续时间约为2.862Myr(Fang et al.,2016)。本文在剖面底部和顶部的两层钾质斑脱岩中获得了锆石SIMS U-Pb年龄,分别为457.6±3.8 Ma和454.8±3.8 Ma,这一结果表明平凉组沉积于奥陶纪桑比期,与生物地层格架对应良好,并且两层火山灰之间存在2.8Myr的时间间隔。因此,在误差范围内,GTS2012、U-Pb锆石绝对年龄与天文年代标尺具有良好的一致性,实现了不同定年方法的相互校正,为晚奥陶世桑比期的绝对年龄提供了新的证据。值得注意的是,尽管这两组谐和年龄分别与欧洲Baltoscandia地区的Kinnekulle斑脱岩和北美的Deicke斑脱岩同时,但化学成分多元分析表明它们并不同源。根据区域构造背景、地层发育以及岩浆事件的约束等特征,表明研究区的钾质斑脱岩主要来自北祁连-北秦岭的火山弧喷发。
[Abstract]:With the establishment of international geochronology (Earthtime), the high precision isotopic geochronology has entered a high speed development stage, especially the establishment of the CA-ID-TIMS U-Pb experimental method, which provides an opportunity for the improvement of the representative of the high precision geological year. As a special event and time marker, volcanic ash plays an important role in paleoenvironment, paleogeography and paleotectonics analysis. The geochemistry and U-Pb radioisotope dating of several layers of potash porphyry in Pingliang formation in Guanzhuang section of Yindong Gansu Province have been studied in this paper. The results show that the SiO_2 content of the samples is high, and the ratio of Al2O3/Ti O _ 2Zr / HF / Ti / Th shows the characteristics of acid volcanic rocks. The REE distribution pattern is characterized by LREE enrichment, negative Eu anomaly and no Ce anomaly. Trace element cobwebs show negative Sr and Ti anomalies, and Pb positive anomalies. Zr/TiO_2-Nb/Y diagrams show that the samples are fluvial andesite or trachyte andesite, showing the phylogenetic relationship of intermediate-acid primitive magma. Th/Yb-Nb/Yb diagram, The results of Th-Hf-Ta diagram are consistent and the trace element characteristics suggest that the primitive magma was formed in the volcanic arc environment. The pingliang formation covers the late Ordovician conodonts of Baltonniodus variabilis and Baltonniodus gerdea. According to the short eccentricity cycle constraints identified by the profile, the duration of two layers of potassium porphyry is about 2.862Myr (Fang et al.,2016). Zircon SIMS U-Pb ages of 457.6 卤3.8 Ma and 454.8 卤3.8 Ma, were obtained at the bottom and top of the section, respectively. The results indicate that the Pingliang formation was deposited in the Ordovician Sambi period. It corresponds well to the biostratigraphic framework, and there is a time interval of 2.8Myr between the two layers of volcanic ash. Therefore, within the range of errors, the absolute age of GTS2012,U-Pb zircon is in good agreement with the astronomical age scale, which realizes the mutual correction of different dating methods and provides new evidence for the absolute age of the late Ordovician period. It is worth noting that although these two groups are at the same time as Kinnekulle porphyry in Baltoscandia region in Europe and Deicke porphyry in North America, the multivariate analysis of chemical composition shows that they are not homologous. According to the characteristics of the regional tectonic setting, stratigraphic development and the constraints of magmatic events, it is shown that the potash porphyry in the study area is mainly derived from the volcanic arc eruption in the North Qilian and North Qinling Mountains.
【学位授予单位】：中国地质大学(北京)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P597.3;P584
，
本文编号：2412807
[Abstract]:With the establishment of international geochronology (Earthtime), the high precision isotopic geochronology has entered a high speed development stage, especially the establishment of the CA-ID-TIMS U-Pb experimental method, which provides an opportunity for the improvement of the representative of the high precision geological year. As a special event and time marker, volcanic ash plays an important role in paleoenvironment, paleogeography and paleotectonics analysis. The geochemistry and U-Pb radioisotope dating of several layers of potash porphyry in Pingliang formation in Guanzhuang section of Yindong Gansu Province have been studied in this paper. The results show that the SiO_2 content of the samples is high, and the ratio of Al2O3/Ti O _ 2Zr / HF / Ti / Th shows the characteristics of acid volcanic rocks. The REE distribution pattern is characterized by LREE enrichment, negative Eu anomaly and no Ce anomaly. Trace element cobwebs show negative Sr and Ti anomalies, and Pb positive anomalies. Zr/TiO_2-Nb/Y diagrams show that the samples are fluvial andesite or trachyte andesite, showing the phylogenetic relationship of intermediate-acid primitive magma. Th/Yb-Nb/Yb diagram, The results of Th-Hf-Ta diagram are consistent and the trace element characteristics suggest that the primitive magma was formed in the volcanic arc environment. The pingliang formation covers the late Ordovician conodonts of Baltonniodus variabilis and Baltonniodus gerdea. According to the short eccentricity cycle constraints identified by the profile, the duration of two layers of potassium porphyry is about 2.862Myr (Fang et al.,2016). Zircon SIMS U-Pb ages of 457.6 卤3.8 Ma and 454.8 卤3.8 Ma, were obtained at the bottom and top of the section, respectively. The results indicate that the Pingliang formation was deposited in the Ordovician Sambi period. It corresponds well to the biostratigraphic framework, and there is a time interval of 2.8Myr between the two layers of volcanic ash. Therefore, within the range of errors, the absolute age of GTS2012,U-Pb zircon is in good agreement with the astronomical age scale, which realizes the mutual correction of different dating methods and provides new evidence for the absolute age of the late Ordovician period. It is worth noting that although these two groups are at the same time as Kinnekulle porphyry in Baltoscandia region in Europe and Deicke porphyry in North America, the multivariate analysis of chemical composition shows that they are not homologous. According to the characteristics of the regional tectonic setting, stratigraphic development and the constraints of magmatic events, it is shown that the potash porphyry in the study area is mainly derived from the volcanic arc eruption in the North Qilian and North Qinling Mountains.
【学位授予单位】：中国地质大学(北京)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P597.3;P584
，
本文编号：2412807
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