摘要
新特提斯洋长期俯冲消减作用在早白垩世可能经历二次俯冲启动或板片俯冲几何形态的重大转换。确定西藏南部冈底斯岩基早白垩世岩浆作用的岩石地球化学特征和作用方式是甄别上述过程的关键,对理解新特提斯洋的俯冲演化过程至关重要。本文就冈底斯岩基东段朗县杂岩中保存的各类早白垩世岩浆岩,开展了锆石U-Pb地质年代学和Hf同位素、全岩元素和同位素(Sr-Nd)组成分析。数据结果表明:1)基性岩侵位时代为早白垩世晚期(103.6~100.8Ma),为高钾钙碱性偏铝质岩石,锆石ε_(Hf)(t)=+0.3~+5.7,全岩ε_(Nd)(t)=-0.8和-0.3,暗示其岩浆源区具有大量俯冲沉积物或流体的混入,为沉积物熔体和流体交代的地幔楔物质部分熔融的产物,经历了一定程度的角闪石分离结晶作用;2)中性岩形成于99.8~97.6Ma,略晚于基性岩,其主量元素与基性岩具有较好的线性关系,全岩ε_(Nd)(t)=+1.1,具有较多的地幔物质参与,为基性岩浆进一步演化形成;3)酸性岩(脉体)记录了多阶段岩浆作用(124.1~95.3Ma),根据同位素组成不同进一步划分为两类,第一类具有较低的全岩ε_(Nd)(t)值(-8.3~-6.0),其岩浆源区显示富集特征,t DM2=1385~1586Ma,由古老地壳物质的再熔融形成;第二类的锆石ε_(Hf)(t)值(-2.8~+3.2)变化较大,岩脉的锆石ε_(Hf)(t)=+0.4~+8.1,t DM=428~906Ma,全岩ε_(Nd)(t)=+0.1和+0.8,表明岩浆源区具有不均一性,为古老地壳物质被富流体地幔岩浆改造形成;和4)镁铁质包体的主量元素与寄主花岗岩具有较好的线性关系,锆石的Hf同位素组成变化较大(ε_(Hf)(t)=-9.3~+4.1),变化范围可达13个ε单位,为岩浆混合成因。寄主花岗岩和角闪辉长岩分别作为酸性和基性端元,是基性岩浆与其诱发古老地壳熔融形成的花岗质岩浆经混合形成。结合冈底斯岩基早白垩世岩浆岩的研究结果,朗县杂岩在早白垩世(124~97Ma)的岩浆作用具有明显的岩浆混合现象,�
The subduction of the Neo-Tethyan oceanic lithosphere might experienced a major shift in the subdcution geometry in the Early Cretaceous time during its long-lasting subducion.Knowing the geochemical nature and the mode of the Early Cretaceous magmatism in the Gangdese batholith is critical to deccriminate the subdcution processes in this time.Data from zircon U-Pb geochronology and geochemical(whole-rock element and isotope,zircon Hf)analyses on various rock types preserved in the Langxian complex show that:1)the mafic rocks formed at 103.6~100.8Ma are of high-K calc-alkaline aluminous in composition.They are characterized by relatively low zirconε_(Hf)(t)(+0.3~+5.7)and bulkε_(Nd)(t)(-0.8 and-0.3),indicating that they were drived from partial melting of depeled mantle wedge metasomatized by fluids from subducted sediment;2)the intermediate rocks formed at 99.8~97.6Ma and have slightly higher Nd isotope compositions withε_(Nd)(t)=+1.1.They display a good linear relationship in major oxides with those in the mafic ones,suggesting that they are derivative products from the mafic suite;3)the felsic rock(pluton and dike)crystallized at 124.1~95.3Ma.They could be subdivided into two types based on their isotopic compositions.The first type has a lowerε_(Nd)(t)(-8.3~6.0)but higher t DM2(1385~1586Ma).They formed by remelting of ancient crustal materials.In contrast,ε_(Hf)(t)andε_(Nd)(t)in the second type granitic rocks vary widely withε_(Hf)(t)ranging from-2.8 to+3.2 for the plutonic granite and from+0.4 to+8.1 for the dike,respectively.Together with relatively low Nd isotope(ε_(Nd)(t)=+0.1 and+0.8)and t DM(428~906Ma),these characteristics suggest a relative young source region due to modification of ancient crustal materials by melts derived from fluid-enriched mantle;4)enclaves hosted by the granitic rocks show good linear relationship in major oxides with the host rocks.They display a rather large shift of~13 epsilon units in zircon Hf isotope compositions(ε_(Hf)(t)=-9.3~+4.1)and might represent the melts fo
作者
李广旭
曾令森
高利娥
高家昊
赵令浩
LI GuangXu;ZENG LingSen;GAO LiE;GAO JiaHao;ZHAO LingHao(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)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2021年第10期2995-3034,共40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.
关键词
冈底斯岩基
朗县杂岩
早白垩世岩浆作用
岩浆混合
新特提斯洋二次俯冲
Gangdese batholith
Langxian complex
Early Cretaceous magmatism
Magma mixing
Neo-Tethys ocean subduction re-initiation
