甘肅奧陶紀(jì)平?jīng)鼋M鉀質(zhì)斑脫巖地球化學(xué)和U-Pb年代學(xué)研究
發(fā)布時(shí)間：2019-01-21 16:10
【摘要】：國(guó)際“地時(shí)”(Earthtime)組織的成立,使高精度同位素地質(zhì)年代學(xué)進(jìn)入了高速發(fā)展階段,特別是CA-ID-TIMS U-Pb實(shí)驗(yàn)方法的建立,為高精度地質(zhì)年代表的完善提供了契機(jī)。火山灰層作為一個(gè)特殊的事件和時(shí)間標(biāo)志層,在古環(huán)境、古地理和古構(gòu)造分析等方面起到了極其重要的作用。本文對(duì)甘肅銀洞官莊剖面平?jīng)鼋M內(nèi)的數(shù)層鉀質(zhì)斑脫巖開(kāi)展了地球化學(xué)和高精度U-Pb放射性同位素年代學(xué)研究。結(jié)果表明,所研究樣品的SiO_2含量較高,Al2O3/Ti O_2、Zr/Hf、Ti/Th比值都顯示出酸性火山巖的特征。稀土元素配分模式呈現(xiàn)輕稀土富集,Eu負(fù)異常,無(wú)Ce異常,符合同時(shí)代鉀質(zhì)斑脫巖稀土配分模式特征。微量元素蛛網(wǎng)圖顯示Sr和Ti負(fù)異常,Pb正異常。Zr/TiO_2-Nb/Y圖表明樣品為流紋英安巖或粗面安山巖,顯示出中酸性原始巖漿親緣關(guān)系。Th/Yb-Nb/Yb圖、Th-Hf-Ta圖判別結(jié)果一致,結(jié)合微量元素特征,認(rèn)為原始巖漿形成于火山弧環(huán)境。平?jīng)鼋M覆蓋了晚奧陶世Baltonniodus variabilis和Baltonniodus gerdea牙形刺帶。通過(guò)剖面識(shí)別出的短偏心率旋回約束,兩層鉀質(zhì)斑脫巖持續(xù)時(shí)間約為2.862Myr(Fang et al.,2016)。本文在剖面底部和頂部的兩層鉀質(zhì)斑脫巖中獲得了鋯石SIMS U-Pb年齡,分別為457.6±3.8 Ma和454.8±3.8 Ma,這一結(jié)果表明平?jīng)鼋M沉積于奧陶紀(jì)桑比期,與生物地層格架對(duì)應(yīng)良好,并且兩層火山灰之間存在2.8Myr的時(shí)間間隔。因此,在誤差范圍內(nèi),GTS2012、U-Pb鋯石絕對(duì)年齡與天文年代標(biāo)尺具有良好的一致性,實(shí)現(xiàn)了不同定年方法的相互校正,為晚奧陶世桑比期的絕對(duì)年齡提供了新的證據(jù)。值得注意的是,盡管這兩組諧和年齡分別與歐洲Baltoscandia地區(qū)的Kinnekulle斑脫巖和北美的Deicke斑脫巖同時(shí),但化學(xué)成分多元分析表明它們并不同源。根據(jù)區(qū)域構(gòu)造背景、地層發(fā)育以及巖漿事件的約束等特征,表明研究區(qū)的鉀質(zhì)斑脫巖主要來(lái)自北祁連-北秦嶺的火山弧噴發(fā)。
[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 鹵3.8 Ma and 454.8 鹵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.
【學(xué)位授予單位】：中國(guó)地質(zhì)大學(xué)(北京)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：P597.3;P584
，
本文編號(hào)：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 鹵3.8 Ma and 454.8 鹵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.
【學(xué)位授予單位】：中國(guó)地質(zhì)大學(xué)(北京)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：P597.3;P584
，
本文編號(hào)：2412807
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