利用原位微区元素和Pb同位素分析技术研究环斑长石成因：以都庞岭印支期花岗岩体为例

It has been well documented that rapakivi granites are an important indicator for mineralization of tungsten-tin, and bear key information for crust-mantle interaction, magmatic process, and deep-crust fluid. Such information is crucial for understanding the links between deep-crust fluid, petrogenesis of granitoids and the mineralization of tungsten-tin in the Indosinian-age rapakivi granites in the Dupangling area. However, due to the lack of the elemental and isotopic analyses of rapakivi feldspar, the genetic link between magmatic process, deep-crust fluid and mineralization in this region is poor constrained. Here, this study will apply in-situ techniques to analyze element and isotopes (Sr-Hf-O-Pb) isotopes of rapakivi feldspar, apatite and zircon to decipher physical (pressure, temperature, phase diagram) and chemical conditions (fluid and oxygen fugacity) for generation of these minerals. This result will be integrated with detailed petrographic examination and convectional whole-rock analyses to constrain the origin of rapakivi feldspar in the Dupangling area. Through integrating the outcomes with continued updated datasets, we will unravel source components, crust-mantle interaction, magmatic processes (fractional crystallization, crustal contamination and magma mixing) and fluid-rock interaction. Such information is essential for constraining the fluid recycling process in deep-earth. Together with available data, the outcomes of this study will provide new perspectives on petrogenesis of rapakivi feldspar and its hosted granites. Combined with spatial and temporal distribution of tungsten-tin mineralization, this study will help to establish a genetic link between magmatism, fluid cycling and mineralization in Dupangling area, and provide an independent constraint on the Indosinian mineralization event in South China.

环斑花岗岩记录了壳幔作用、岩浆过程和地壳流体活动等关键信息，同时是钨锡多金属矿床的重要找矿岩石学标志。前期工作表明都庞岭印支期花岗岩体与钨锡多金属成矿的时间一致且发育环斑长石，但缺乏对环斑长石原位微区元素和同位素组成的系统研究，制约了我们揭示岩浆演化、地壳流体与成矿作用的成因联系。因此，本项目拟以环斑长石为重点研究对象，开展环斑长石与共生的石英、磷灰石、锆石主微量元素分析，明确各世代矿物形成的温压条件，重建矿物反应相图；重点开展环斑长石Pb-磷灰石Sr-锆石Hf-O同位素分析，结合长石、磷灰石和锆石微量元素和相图分析，试图建立岩浆源区组成、壳幔演化、岩浆过程（岩浆氧化性、结晶分异、地壳混染和岩浆混合）、地壳流体与环斑长石的成因联系。结合已有的成果，本研究将揭示环斑长石及寄主花岗岩的成因，为探寻岩浆过程、流体活动与成矿作用之间的成因联系提供新的观察和资料，为理解华南印支期成矿提供新的约束。

研究环斑花岗岩的成因对于揭示壳幔相互作用、岩浆过程与深部动力学过程具有重要意义。本项目以广西都庞岭印支期环斑花岗岩为主要研究对象，运用MC-LA-ICPMS锆石U-Pb年代学、锆石原位Hf同位素以及单矿物成分分析方法，结合全岩主微量和Nd同位素组成，系统厘定了都庞岭地区印支期环斑花岗岩体及其岩脉的形成时间，划分了该套环斑花岗岩的岩石地球化学类型，揭示了母岩浆源区特征和岩浆过程，获得了都庞岭环斑花岗岩环斑长石及共生矿物的化学组成和结构特征，并结合全球环斑长石成因分析，提出岩浆混合作用是都庞岭地区印支期环斑花岗岩环斑长石的重要生长方式。定年结果显示都庞岭地区印支期环斑花岗岩及其岩脉的侵位时间为215~210 Ma，表明桂东北地区晚三叠世发育了空间规模较大的长英质岩浆活动。都庞岭地区印支期环斑花岗岩SiO2含量（76.1~78.2 wt%）较高，为高演化准铝质至弱过铝质花岗岩。环斑花岗岩呈现出类似海鸥型的稀土元素配分模式特征，轻稀土元素弱富集，重稀土元素相对平坦，Eu具有明显负异常（δEu = 0.05~0.38）。相对高的Ce含量和1000Ga/Al值，表明环斑花岗岩具有A型花岗岩的特征。与实验熔体相比，环斑花岗岩样品主要落在变杂砂岩熔体范围内，表明其起源于砂质岩源区。全岩样品εNd(215 Ma)值相对较低（-8.45~-7.91），锆石颗粒εHf(215 Ma)值变化范围较大（-9.3~-1.0），对应的二阶段平均地壳Hf模式年龄(TDM2)为1.84~1.32 Ga，表明印支期环斑花岗岩是都庞岭地区元古代壳源岩石部分熔融的产物。矿物学分析结果显示都庞岭环斑花岗岩中环斑长石由内部钾长石巨晶（Or91-97）和钠长石外壳（Ab88-98）组成。钾长石巨晶发育条纹结构，并包裹有钙长石、环带斜长石、石英和磁铁矿等矿物包裹体。岩石样品中钾长石、斜长石和石英为多个世代的结晶矿物。环斑长石An值从核部到边部呈现先升高后降低的变化规律。环斑长石、环带长石和反环带长石相互共生的特征反映在长石结晶过程中岩浆房内发生了偏基性岩浆的补给。因此，岩浆混合作用是都庞岭地区印支期环斑花岗岩环斑长石的形成机制。