| 藏北班公湖-怒江缝合带西段沙木罗组火山岩年代学、Hf同位素及地球化学特征 |
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| 引用本文： | 吴建亮,刘文,尹显科,雷传扬,王波.藏北班公湖-怒江缝合带西段沙木罗组火山岩年代学、Hf同位素及地球化学特征[J].地球科学,2021,46(2):444-459. |
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| 作者姓名： | 吴建亮 刘文 尹显科 雷传扬 王波 |
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| 作者单位： | 四川省地质调查院稀有稀土战略资源评价与利用四川省重点实验室, 四川成都 610081 |
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| 摘 要： | 班公湖-怒江缝合带在中生代的构造岩浆演化一直以来都是青藏高原基础地质研究中的热点之一.在该缝合带南部的冈底斯北部地区发育大量的中生代火山岩及相关岩浆岩，其岩石成因与成岩地球动力学背景长期以来存在较大的争议.以班公湖-怒江缝合带西段阿翁错地区沙木罗组新发现的火山岩夹层为研究对象，对沙木罗组的形成时代、火山岩的成因、构造环境以及动力学背景进行了探讨.沙木罗组火山岩由下部扎果安山岩和中部昂母过安山质晶屑凝灰岩组成.锆石U-Pb定年结果表明下部扎果安山岩锆石结晶年龄为141.3±1.7 Ma，中部昂母过安山质晶屑凝灰岩的锆石结晶年龄为134.0±0.4 Ma，与区域上沙木罗组产出的化石及碎屑锆石约束时代较为一致.研究区沙木罗组下部安山岩以低硅（SiO2=52.58%~55.35%）、低铝（Al2O3=14.43%~15.44%，A/CNK=0.58~0.75）、富镁（MgO=6.12%~8.51%）、富钠（Na2O=1.55%~5.03%，Na2O/K2O=1.81~3.61）为特征，属于高镁安山岩范畴；中部安山质晶屑凝灰岩以相对高硅（SiO2=55.83%~61.88%）、低镁（MgO=2.73%~4.51%）、高铝（Al2O3=17.75%~19.62%，A/CNK=0.90~1.08）、富钠（Na2O=1.55%~5.03%，Na2O/K2O=1.81~3.61）为特征，由下往上沙木罗组火山岩的SiO2、Al2O3、Na2O含量逐渐升高，MgO、K2O、Fe2O3含量逐渐降低，属于钠质低钾钙碱性系列岩石.二者均明显富集轻稀土（LREE）和大离子亲石元素（LILE），亏损高场强元素（HFSE），显示出典型的岛弧岩浆岩特征.下部扎果安山岩的锆石εHf（t）为+15.5~+18.7，中部昂母过安山质晶屑凝灰岩的锆石εHf（t）为+13.8~+16.2，指示安山岩、安山质晶屑凝灰岩的形成有新生地壳物质的参与.扎果高镁安山岩具有低的Y（11.9×10-6~13.5×10-6）、Yb（1.32×10-6~1.43×10-6）含量和较高的Sr（481×10-6~794×10-6）含量，以及较高的Sr/Y（35.6~65.6）和LaN/YbN（8.2~10.4），其形成可能与俯冲板片熔体交代地幔橄榄岩有关.综上所述，研究区沙木罗组火山岩形成于俯冲岛弧环境下，为早白垩世早期班公湖-怒江洋壳南向俯冲消减的岩浆响应.
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| 关 键 词： | 沙木罗组 高镁安山岩 班公湖-怒江缝合带 藏北 地球化学 地质年代学 |
| 收稿时间： | 2020-02-09 |
Geochronology,Zircon Hf Isotope and Geochemistry of Volcanic Rocks from Shamuluo Formation in Western Banggongco-Nujiang Suture Zone,North Tibet |
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| Wu Jianliang,Liu Wen,Yin Xianke,Lei Chuanyang,Wang Bo.Geochronology,Zircon Hf Isotope and Geochemistry of Volcanic Rocks from Shamuluo Formation in Western Banggongco-Nujiang Suture Zone,North Tibet[J].Earth Science-Journal of China University of Geosciences,2021,46(2):444-459. |
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| Authors: | Wu Jianliang Liu Wen Yin Xianke Lei Chuanyang Wang Bo |
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| Affiliation: | (Evaluation and Utilization of Strategic Rare Metals and Rare Earth Resources Key Laboratory of Sichuan Province,Sichuan Geological Survey,Chengdu 610081,China) |
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| Abstract: | The tectonic-magmatic evolution of the Banggongco-Nujiang suture zone is one of the hottest scientific problems related to fundamental geology of the Tibetan Plateau.The Mesozoic volcanic-intrusive rocks are widely distributed in the North Gangdese belt,which is also located in the southern of the Banghongco-Nujiang suture zone.However,the petrogenesis and geodynamic setting of those rocks remain under debate.It reports the newly found vocalic rock dissections from Shamuluo Formation in Awengcuo area,which are located at the west segment of Bangongco-Nujiang suture zone to explore these issues.Those rocks consist of lower dissection named Zhaguo andesite and central dissection named Angmuguo andesistic crystal tuff.Zircon U-Pb dating indicates that the Zhaguo andesite was emplaced at 141.3±1.7 Ma,and the Angmuguo andesistic crystal tuff was formed in the age of 134.0±0.4 Ma,which provides new chronological data for the Shamuluo Formation.The Zhaguo andesite exhibits SiO2contents of 52.58%-55.35%,Al2O3contents of 14.43%-15.44%with A/CNK ranging from 0.58 to 0.75,MgO contents of 6.12%-8.51%,Na2O contents of 1.55%-5.03%,with Na2O/K2O ratios of 1.81-3.61 and Mg numbers of 57.6-63.2(average in 59.8),which can be assigned to high-Mg andesite.The Angmuguo andesistic crystal tuff has contents of SiO2(55.83%-61.88%),MgO(2.73%-4.51%),Al2O3(17.75%-19.62%)with A/CNK of 0.90-1.08,Na2O(1.55%-5.03%),and Na2O/K2O(1.81-3.61).All these samples are sodic low-K calc-alkaline series,and enriched in large ion lithopile elements(LILE)and depleted in high field strength elements(HFSE),showing a similar character of island arc-type magmas.The zirconεHf(t)of Zhaguo andesite values are+15.5 to+18.7,and the Angmuguo andesistic crystal tuff values from+13.8 to+16.2,indicating that these rocks came from the partial melting of newly-formed lower crust.The Zhaguo high-Mg andesite has low contents of Y(11.9×10-6-13.5×10-6),Yb(1.32×10-6-1.43×10-6),and high Sr(481×10-6-794×10-6)with the ratios of Sr/Y(35.6-65.6)and LaN/YbN(8.2-10.4).The melt of subducted slab and metasomic mantle peridoite played a great role on its forming process.Based on the above data,it is proposed that the volcanic rock magma from Shamuoluo Formation formed in inland-arc setting,which can be the early magma response of Bangongco-Nujiang Tethyan Ocean’s southward subduction. |
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| Keywords: | Shamuluo Formation high-Mg andesite Bangongco-Nujiang suture zone North Tibet geochemistry geochronology |
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