Early Cretaceous magmatism of the Langxian complex in the eastern Gangdese batholith, southern Tibet: Neo-Tethys ocean subduction re-initiation

Abstract

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，锆石Hf和全岩Sr-Nd同位素组成变化较大，可达13个ε单位，其岩浆源区复杂且富含流体，代表了新特提斯洋在早期（240~144Ma）经历漫长的俯冲之后，在早白垩世时期（~120Ma）俯冲带发生跃迁或俯冲角度达到临界点，导致大量俯冲沉积物和流体沿俯冲带俯冲下去，与发生部分熔融的地幔楔物质混合，底侵导致上覆古老地壳物质的再熔融，形成早白垩世复杂的岩浆岩组合，很可能是新特提斯洋二次俯冲开始的标志。;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 zircon ε_Hf(t) (+0.3~+5.7) and bulk ε_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 with ε_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 lower ε_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) and ε_Nd(t) in the second type granitic rocks vary widely with ε_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 formed by various degrees of mixing an acidic end member of the hosted granite wih a mafic end member of hornblende gabbro. Combined with literature data of the Gangdese batholith, magmatic rocks from the Langxian complex preserved a key record of a major magmatic process during the Early Cretaceous (122~97Ma). Melting and mixing of various source components (ancient crustal material, fluid-enriched mantle, and depleted mantle) is one of the important features of the Early Cretaceous magmatism due to subduction of the Neo-Tethyan ocean and resulted in large shifts (up to ~13 epsilon units) in zircon Hf as well as in bulk Sr-Nd isotope compositions. To introduce various components into and achieve a large isotopic heterogeneity in the source region, it requires a major change in the subduction geometry in the long subduction of the Neo-Tethyan subdction system. Since the initation of the subduction at ~240Ma, subdcition of the Neo-Tethyan Ocean might experinecd a critical reorganization enabling the introduction of an increased amount of sediments and fluids into the subduting system and the modification of the mantle wedge in the Early Cretaceous (~120Ma). The mafic magmas caused remelting of the overlying ancient crustal material and finally formed the complex magmatic activity in the Early Cretaceous, which is likely to be a sign of the beginning of the second subduction of the Neo-Tethyan Ocean.