Titanite geochronology and geochemistry of the Cenozoic alkali-rich intrusions in the Ailaoshan-Red River Shear Zone, Yunnan Province

Abstract

榍石作为副矿物在哀牢山-红河剪切带新生代富碱侵入岩中广泛存在。原位获取榍石矿物内部微量元素、U-Pb年龄和Sm-Nd同位素的空间变化对获取矿物和岩石的成因演化信息具有重要意义。本文使用四级杆/多接收电感耦合等离子体质谱（Quadrupole/Multi-Collector Inductively Coupled Plasma Mass Spectrometry，Q/MC-ICP-MS）与激光剥蚀系统（Laser Ablation，LA）联用，对哀牢山-红河剪切带5个富碱侵入岩体（桃花岩体、宁蒗-永胜岩体、哈播岩体、铜厂岩体和十里村岩体）中榍石开展了微区原位微量元素、U-Pb年代学和Sm-Nd同位素研究。微量元素分析结果表明，三江富碱侵入岩中榍石为岩浆成因，亏损Rb、Ba、Pb、Sr等大离子亲石元素，富集Th、U、Nb、Ta、Zr、Hf等高场强元素。榍石的稀土配分图均表现为明显右倾，不具有或具有弱的Eu负异常。与云南北衙、马厂箐矽卡岩矿床中的热液榍石相比，本文榍石在稀土元素组成上，具有较高的稀土元素总量、较高的Th/U、LREE/HREE和Ce/Ce^*比值，具有较低的Eu/Eu^*、Nb/Ta、Zr/Hf比值。微区原位LA-Q-ICP-MS U-Pb定年结果表明，研究区富碱岩体中榍石结晶年龄在32.5~37.9Ma之间，代表了岩体形成时代，与三江地区哀牢山-红河剪切带及其附近新生代富碱岩浆活动高峰期（~35Ma）一致，属于青藏高原晚碰撞期岩浆作用的产物。榍石微区原位LA-MC-ICP-MS Sm-Nd同位素分析结果显示，榍石颗粒的Nd同位素组成均一，表明榍石结晶过程中寄主岩浆的Nd同位素组成没有发生明显变化。各个富碱岩体之间的榍石Nd同位素组成变化范围在-6.8~-2.1之间，与全岩的同位素特征一致，表明榍石的原位微区Sm-Nd同位素可以作为富碱侵入岩研究中有效的示踪手段之一。;Titanite is a common accessory mineral in the alkali-rich intrusions in the Ailaoshan-Red River Shear Zone. In situ trace elements, U-Pb ages and Sm-Nd isotopes in titanites are of great significance for obtaining genesis and evolution information of minerals and rocks. In this study, trace element compositions, U-Pb dating and Sm-Nd isotopic compositions of titanites from five alkali-rich intrusions along the Ailaoshan-Red River Shear Zone in the Sanjiang Region have been successfully analyzed by Laser Ablation-Quadrupole/Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-Q/MC-ICP-MS). Trace element chemistry of these titanites indicate that they are igneous titanites, depleted in large ionic lithophile elements such as Rb, Ba, Pb and Sr, and enriched in high field strength elements such as Th, U, Nb, Ta, Zr and Hf. Titanites in this study have relatively high total rare earth element (REE) concentrations, chondrite-normalized patterns show that they are LREE enriched with no or weak Eu negative anomaly. Titanites in this study show higher values for ∑REE, Th/U, LREE/HREE and Ce/Ce^*, and lower values for Eu/Eu^*, Nb/Ta and Zr/Hf than those of hydrothermal titanites from Beiya and Machangqing skarns. Results of in situ LA-Q-ICP-MS U-Pb dating show that the crystallization age of titanites from alkali-rich intrusions are ranging from 32.5Ma to 37.9Ma, consistent with the peak age of alkali-rich magmas along the Ailaoshan-Red River Shear Zone in the Sanjiang Region. In situ LA-MC-ICP-MS Sm-Nd isotopic analyses show that the Nd isotopic compositions of titanite grains are homogeneous, indicating that the Nd isotopic composition of host magma did not change during the crystallization of titanite. The ε_Nd(t) values of titanite in the alkaline intrusions in this study are ranging from -6.8 to -2.1, which are consistent with the Nd isotopic composition of the whole rock, indicating that in situ Sm-Nd isotopes of titanite can be used as a useful tracer in the study of alkali-rich intrusive rocks.