藏南冈底斯岩基东段晚白垩世早期朗县杂岩的岩石成因和深部地球动力学过程  

Petrogenesis and geodynamic significances of the early Late Cretaceous intrusion in the Langxian Complex,eastern Gangdese batholith of southern Tibet

作　　者：李广旭[1] 曾令森[1] 赵令浩[2] 高利娥[1] 高家昊 LI GuangXu;ZENG LingSen;ZHAO LingHao;GAO LiE;GAO JiaHao(Key Laboratory of Deep-Earth Dynamics,Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;National Research Center for Geoanalysis,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]自然资源部深部动力学重点实验室,中国地质科学院地质研究所,北京100037 [2]中国地质科学院国家地质实验测试中心,北京100037

出　　处：《岩石学报》2021年第11期3348-3376,共29页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(92055202);第二次青藏高原科学考察项目(2019QZKK0702);中国地质调查局地质调查项目(DD20190057)联合资助.

摘　　要：藏南冈底斯岩基晚白垩世早期岩浆岩保存了有关新特提斯洋的俯冲演化过程和大陆地壳生长的重要信息。本文对朗县杂岩中出露的晚白垩世早期中酸性岩(岩脉)开展了全岩元素地球化学、锆石U-Pb地质年代学、同位素(Sr、Nd和Hf)组成的研究。闪长岩和花岗闪长岩年龄为92.4~86.9Ma,花岗岩(脉)年龄为91.9~88.6Ma,均为晚白垩世早期岩浆作用的产物。闪长岩和花岗闪长岩具有高钾钙碱性偏铝质特征,具有较高的锆石Hf(εHf(t)=+8.3~+13.2,平均值+10.9)和全岩Nd(εNd(t)=+3.2~+2.7)同位素组成,为受俯冲板片流体和大洋沉积物熔体共同交代的地幔楔部分熔融形成,闪长岩岩浆源区形成深度较浅且受板片流体的交代程度较高,花岗闪长岩岩浆源区更深,受沉积物熔体交代程度较高。花岗岩(脉)在主量元素、稀土元素和微量元素组成上显示明显差异,可划分为两类,第一类花岗岩属于低钾钙碱性系列,Na_(2)O/K_(2)O>3.0,稀土总量较高,具有明显的Eu负异常,Sr/Y低(<7.2)。在主量元素组成上,该类花岗岩与闪长岩和花岗闪长岩形成较好的线性演化关系,表明它们可能是上述中酸性岩浆演化的产物。第二类花岗岩属于高钾钙碱性系列,Na_(2)O/K_(2)O较低(均<1.0),铝饱和指数较高(A/CNK=1.01~1.02),发育角闪石,稀土总量较低,具有微弱负或无Eu异常(Eu/Eu*=0.88~1.12),Sr/Y比值(33.8~55.4)较高,锆石Hf(εHf(t)=+4.1~+10.8)和全岩Nd(εNd(t)=+0.8)同位素组成都较低,为新生下地壳部分熔融形成。结合冈底斯岩基晚白垩世早期(100~87Ma)岩浆岩已有的研究结果,认为新特提斯洋板片自早白垩世以高角度俯冲,在晚白垩世早期俯冲板片发生回撤,导致软流圈物质上涌增强,诱发熔流体交代过的地幔楔较高程度的部分熔融形成镁铁质岩浆。这些镁铁质岩浆上升侵位到下地壳,发生不同程度的分离结晶作用并诱发新生下地壳部分熔融,形成晚白垩世岩�The early Late Cretaceous magmatic rocks in the Gangdese batholith provide an important record on the nature of the Neo-Tethys Ocean subduction and the crustal growth in southern Tibet.Zircon U-Pb dating yields that diorite and granodiorite formed at 92.4~86.9Ma and granite emplaced in 91.9~88.6Ma,respectively.The diorite and granodiorite are high-K calc-alkaline aluminous rocks with relatively high zircon Hf(εHf(t)=+8.3~+13.2,average+10.9)and whole rock Nd(εNd(t)=+3.2~+2.7)isotope compositions.They were originated from partial melting of mantle wedge metasomatized by fluids and melts derived from subducted materials.The diorites were derived from a shallower source metasomatised by subducted-slab-derived fluids,whereas the granodiorites from much deeper source metasomatised by sediment-derived melts.Granites can be divided into two groups according to their geochemical compositions.Group-1 is of low-potassium calc-alkaline series with high Na_(2)O/K_(2)O>3.0,high REE contents,strong negative Eu anomalies(Eu/Eu*=0.35~0.40),but low Sr/Y ratios(<7.2).Linear relationship in major elements of the granites with those in the diorite as well as in the granodiorite suggests that these granites might represent the derivative product from the intermediate-acid magma through fractional crystallization.Group-2 is of high potassium calc-alkaline series characterized by (1)low Na_(2)O/K_(2)O(<1.0)and relatively elevated aluminum saturation index(A/CNK=1.01~1.02);(2)low REE contents and weakly negative to no Eu anomalies(Eu/Eu*=0.88~1.12);(3)high Sr/Y ratios(33.8~55.4)and the presence of hornblende;and 4)relatively low zircon Hf(εHf(t)=+4.1~+10.8)and whole rock Nd(εNd(t)=+0.8)isotope compositions.Group-2 granites represents the melts derived from the newly formed lower crust.Combined with those literature data,our new data suggest that a major change in the geometry of subducted Neo-Tethys Ocean slab.High angle subduction in the Early Cretaceous was overtaken by slab retreat in the early Late Cretaceous,which resulted in in

关 键 词：冈底斯岩基 朗县杂岩 晚白垩世岩浆作用 板片回撤 新特提斯洋俯冲 

分 类 号：P588.122[天文地球—岩石学] P597.3[天文地球—地质学]