西藏拉萨地体西北部革吉地区两期早白垩世岩浆作用——锆石U-Pb年龄、Hf同位素和地球化学特征

高家昊; 曾令森; 高利娥; 侯可军; 郭春丽

西藏拉萨地体西北部革吉地区两期早白垩世岩浆作用——锆石U-Pb年龄、Hf同位素和地球化学特征

1. 大陆构造与动力学国家重点实验室/中国地质科学院地质研究所, 北京 100037;
2. 中国地质大学地球科学与资源学院, 北京 100083;
3. 中国地质科学院矿产资源研究所, 北京 100037

收稿日期: 2015-09-15

修回日期: 2015-10-27

刊出日期: 2016-01-15

Two episodes of Early Cretaceous magmatism in Geji area of the Lhasa Block, Tibet

1. State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China;
3. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China

Received Date: 15 September 2015

Revised Date: 27 October 2015

Publish Date: 15 January 2016

摘要

摘要: 邦巴岩体位于拉萨地块西部革吉地区,由主体花岗岩、花岗闪长岩、闪长质包体及一系列近平行、南北向展布的花岗斑岩脉体组成。野外地质调查和LA-MC-ICP-MS锆石U-Pb定年表明,革吉地区白垩纪的两期岩浆活动分别发生在131~132Ma和127Ma。早期岩浆作用形成主体花岗岩、花岗闪长岩及闪长质包体,具有以下特征:①明显富集K、Cs等大离子亲石元素,亏损Nb、Ti、Zr等高场强元素;②具有明显的负Eu异常(Eu/Eu^*=0.49~0.61)及负Ce异常;③具负ε_Hf(t)值(-3.2~-0.3)及古老的地壳模式年龄(1.210~1.399Ga);④初始Sr同位素比值为0.70424~0.71472,ε_Hf(t)值为-5.70~-5.54。晚期岩浆作用形成花岗斑岩脉体,具有以下特征:①富集K、Cs等大离子亲石元素,强烈亏损Nb、Ta、Ti等高场强元素;②基本不具有负Eu异常或具有轻微的负Eu异常(Eu/Eu^*=0.74~0.87);③具有更老的Hf同位素地壳模式年龄(1.226~1.576Ga)及更负的大范围变化的ε_Hf(t)值(-6.1~-0.7)。晚期岩浆作用锆饱和温度(777~796℃)及轻稀土元素不饱和温度(794~812℃)均高于早期岩浆的锆饱和温度(661~762℃)及轻稀土元素不饱和温度(750~769℃)。上述特征表明,两期岩浆作用均为中拉萨地块古老基底部分与地幔物质混染部分熔融的产物,随着岩浆作用的持续进行,岩浆中的古老地壳组分增加,熔体温度也增加,可能与南向俯冲的班公-怒江洋壳回转驱动的地幔岩浆活动向北迁移有关。
- 革吉地区 /
- 邦巴岩体 /
- 两期岩浆作用 /
- 锆饱和温度 /
- 轻稀土元素不饱和温度 /
- LA-MC-ICP-MS锆石U-Pb年龄
Abstract: A series of sub-parallel, NS-trending dikes of rhyolitic composition intruded the Bangba granitic complex in Geji area, western central Lhasa Block. Field investigation and LA-MC-ICP-MS zircon U-Pb dating indicate that two episodes of Early Cretaceous magamatism occurred at 131~132Ma and 127Ma respectively in this area. Earlier episode magmatism is represented by the formation of the dioritic enclaves and their hosts of granodiorite and granite. The magmatic rocks of this episode are characterized by ① enrichment of LILEs(K, Cs) and depletion of HFSEs(Nb, Ti, Zr); ② negative Eu anomalies(Eu/Eu^*=0.49~0.61) and negative Ce anomalies; ③ negative zircon ε_Hf(t)(-3.2~-0.3) and ancient two stage model ages(1.210~1.399Ga); and(4) unradioenic to radiogenic initial ⁸⁷Sr/⁸⁶Sr(0.70424~0.71472), and negative Nd(t)(-5.70~-5.54). The rhyolitic dikes, generated by the second episode of magmatism, are characterized by ① enrichment of LILEs(K, Cs) and depletion of HFSEs(Nb, Ti, Zr); ② weak negative Eu anomalies(Eu/Eu^*=0.74~0.87); and ③ highly variable negative zircon ε_Hf(t)(-6.1~-0.7) and more ancient two stage model ages(1.226~1.576Ga). Both the zircon saturation thermometer(777~796℃) and monazite saturation thermometer(794~812℃) yielded slightly higher magma temperature for the rhyolitic dike than the temperatures(661~762℃ and 750~769℃) either for granodiotite or for the mafic enclave. Both episodes of magmatism are considered to have been the consequence of mixing of mantle-derived magma with the mag-ma derived from melting of ancient continental crust, which might have resulted from roll-back of southward subduction of the Ban-gong-Nujiang oceanic lithosphere.
- Geji area /
- Bangba complex /
- two episodes of magmatism /
- zircon saturation thermometer /
- monazite saturation thermome-ter /
- LA-MC-ICP-MS zircon U-Pb ages

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西藏拉萨地体西北部革吉地区两期早白垩世岩浆作用——锆石U-Pb年龄、Hf同位素和地球化学特征

1. 大陆构造与动力学国家重点实验室/中国地质科学院地质研究所, 北京 100037;2. 中国地质大学地球科学与资源学院, 北京 100083;3. 中国地质科学院矿产资源研究所, 北京 100037

Two episodes of Early Cretaceous magmatism in Geji area of the Lhasa Block, Tibet

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1. 大陆构造与动力学国家重点实验室/中国地质科学院地质研究所, 北京 100037;
2. 中国地质大学地球科学与资源学院, 北京 100083;
3. 中国地质科学院矿产资源研究所, 北京 100037