Genesis of LREE-enriched zircons and their highly radiogenic Hf compositions: A case study from Zhuopan alkaline complex in western Yunnan

Abstract

卓潘碱性杂岩体位于思茅地块的西缘，在岩体的霞石正长岩、正长岩、辉石岩中发现多种具不同CL发光特征的锆石。其中CL图下呈灰白色的锆石（Ⅰ类）的Th、U含量低，轻稀土元素亏损，重稀土元素富集，具典型碱性岩岩浆锆石特征，U-Pb年龄约35.7Ma，代表岩体的成岩年龄；不发光或边部具微弱环带的锆石（Ⅱ类）其Th、U、稀土元素含量高，U-Pb年龄为34.2~35.1Ma；呈杂乱海绵状结构的锆石内部不透明，无法获得谐和的U-Pb年龄；震荡环带发育的锆石为捕获锆石，具有较老的U-Pb年龄。本文依据稀土元素含量与配分模式将Ⅱ类锆石进一步分为三种不同类型：A型锆石轻稀土含量低，有明显左倾特征，其LREE<1170×10^-6，（La/Gd）_N<0.09，（Tm/Gd）_N>2.2；B型锆石轻稀土亏损但中-重稀土分馏较小，其（La/Gd）_N<0.009，（Tm/Gd）_N<2；C型锆石轻稀土元素含量明显升高且中-重稀土分馏较小，其LREE>1150×10^-6，（Tm/Gd）_N<2。这些异常的稀土元素特征并非由分析到磷灰石或榍石等矿物包裹体导致，而是与热液作用过程中流体成分与反应条件有关。由于晶格损伤导致放射性Pb丢失，热液锆石的年龄略小于岩浆锆石，没有明确的地质意义。尽管在本文中两类不同CL发光特征的锆石的年龄相差不大，但不加区分地计算平均年龄可能无法获得准确的成岩时代。卓潘碱性岩体热液锆石的Hf同位素变化范围极大（ε_Hf（t）=0.1~100），这种异常的Hf同位素特征可能是由于流体与围岩反应过程中溶解了富Lu矿物（如磷灰石）或具高¹⁷⁶Hf/¹⁷⁷Hf值的矿物（如石榴子石）所导致的。;The Zhuopan alkaline complex is located at the western Simao block. From nepheline syenite, syenite, and pyroxenite in this intrusion, two different types of zircons are recognized according to their CL images. Zircons with bright CL images (type Ⅰ) have low Th and U abundances, depleted LREE and enriched HREE, which is typical for magmatic zircons, and they give a weighted mean ²³⁶Pb/²³⁸U age of 35.7Ma, representing the intrusive age. In contrast, grains that show dark CL images with weakly oscillatory zoning in the outer rims occasionally (type Ⅱ) contain comparatively higher Th, U and sum REE concentrations and yield the U-Pb ages of 34.2~35.1Ma. Opaque zircons display spongy internal textures and failed to obtain concordia U-Pb ages. Moreover, some inherited zircons with clear oscillatory zoning in CL images can also be observed. The chondrite-normalized REE patterns of type Ⅱ zircons in this study can be further divided into three groups: type A zircons show LREE-depleted pattern with LREE<1170×10^-6,(La/Gd)_N<0.09 and (Tm/Gd)_N>2.2; type B zircons have similar increasing slope from La to Sm but no distinct fractionation between MREE and HREE, and they have (La/Gd)_N<0.009 and (Tm/Gd)_N<2; and type C zircons are characterized by the flat REE patterns and elevated LREE concentrations, with LREE>1150×10^-6 and (Tm/Gd)_N<2. These differences of REE characteristics are result from various compositions of hydrothermal fluids and alteration conditions, rather than involvement of mineral inclusions such as apatite and titanite. Hydrothermal zircons are younger than magmatic ones because of radiogenic Pb loss caused by radiation damage. Therefore, average U-Pb ages calculated from unfiltered zircon data may not give an accurate intrusive age, although the impact is limited in this study. A wide range of ε_Hf(t) (0.1~100) are observed in Zhuopan hydrothermal zircons. The extremely radiogenic Hf compositions can be generated by dissolution of Lu-rich (such as apatite) or high ¹⁷⁶Hf/¹⁷⁷Hf minerals (such as garnet).