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  

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

作　　者：高家昊 曾令森[1] 高利娥[1] 侯可军[3] 郭春丽[3] 

机构地区：[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

出　　处：《地质通报》2016年第1期55-70,共16页Geological Bulletin of China

基　　金：国家自然科学基金项目(批准号:41425010;41273034);中国地质调查局项目(编号:1212011502710;12120114022701)

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εHf(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,εHf(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εHf(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。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）; ②negative Eu anomalies （Eu/Eu＊=0.49-0.61） and negative Ce anomalies;③negative zircon ε Hf（t） （-3.2--0.3） and ancient two stage model ages （1.210-1.399Ga）; and （4）unradioenic to radiogen-ic initial 87Sr/86Sr （0.70424-0.71472）, and negative Nd（t） （-5.70--5.54）. The rhyolitic dikes, generated by the second episode ofmag- matism, are characterized by ①enrichment ofLILEs （K, Cs） and depletion ofHFSEs （Nb,Ti,Zr）; @weak negative Eu anomalies （Eu/ Eu＊=0.74-0.87）; and ③highly variable negative zircon 8 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 magma derived from malting of ancient continental crust, which might have resulted from roll-back of southward subduction of the Bangong-Nujiang oceanic lithosphere.

关 键 词：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 

分 类 号：P534.53[天文地球—古生物学与地层学] P597.3[天文地球—地质学]