北秦岭老君山和秦岭梁环斑结构花岗岩长石微区原位铅同位素组成特征及其对岩浆源区和演化过程的制约

With the key of accurate and precious determination of lead isotopic ratios in feldspar by femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fsLA-MC-ICP-MS), the aims of this project are to determine the lead isotopic ratios in different feldspars in enclaves and host granitoids from Qinlingliang and Laojunshan rapakivi-textured granites, to confirm the difference of lead isotopic compositions for the two magmatic sources combined with the lead isotope of coeval lamprophyre dykes. Taking account of other data from the Paleoproterozoic basement and Paleozoic - Mesozoic granitoids of Northern and Southern Qinling belt, the research is to decipher the sources for the rapakivi-textured granites. Pb isotopic zoning in plagioclase-mantled alkali feldspar megacrysts by fsLA-MC-ICP-MS combined with field petrographic observations and the major and trace elements provide an ideal opportunity to use in situ techniques to assess the extent of open-system processes and better characterize the components involved in the genesis of complex magma systems. Sr isotopic data of the rim and core in the same megacryst using microdrilled sampling are determined by solution nebulizer MC-ICP-MS. The decoupling of lead isotope and Sr isotope provides general support magma mixing. At last, the lead isotopic compositions of synthesized matrix-matched K-feldspar glass are determined using fsLA-MC-ICP-MS. Therefore, synthesized K-feldspar with homogeneous lead isotope could serve as an external calibration standard for in situ lead isotope measurements.

本课题以飞秒激光剥蚀多接收等离子体高精度准确测定长石微区铅同位素为基本切入点，对北秦岭老君山和秦岭梁环斑结构花岗岩中寄主岩和包体中长石铅同位素组成进行测定，结合同一地区同时代的煌斑岩脉铅同位素组成特征，查明不同岩浆端元中铅同位素组成的差异性，并与南、北秦岭造山带中的元古代结晶基底和古生代-中生代长石铅同位素组成比较，分析可能的物质来源。对发育斜长石外壳的碱性长石进行完整剖面的铅同位素分析，测定边部和核部的差异性，结合岩相学、微区主微量元素确认长石在动态岩浆系统的结晶过程，反演开放体系中岩体复杂的形成过程。拟用微钻取样结合溶液法测定环斑长石边部和核部的Sr同位素组成，查明Sr同位素和Pb同位素是否呈现负相关关系，进一步确认岩浆混合作用。最后利用fsLA-MC-ICP-MS技术对合成的钾长石玻璃进行铅同位素均一性检验，确定其参考值，可作为长石铅同位素微区分析的外部标准物质。

本研究利用激光剥蚀多接收等离子质谱和电子探针技术对秦岭造山带中环斑结构花岗岩的自形-半自形碱性长石和发育斜长石外壳的碱性长石进行了主、微量元素和铅同位素组成分析。自形-半自形碱性长石主(Or = 91.81-98.17%)、微量元素和铅同位素比值(208Pb/204Pb = 37.928±0.027、207Pb/204Pb = 15.525±0.012、206Pb/204Pb = 17.738±0.014) 比较均一。发育斜长石外壳的卵球状碱性长石可见成份和结构的不平衡现象，即具有钾长石核部 (Or = 93.30-98.17%) 和更长石的幔部 (An = 9.75-24.28%)，而且主、微量元素、铅同位素组成变化与熔蚀面一致。核部卵球状钾长石和自形-半自形碱性长石具有相似的铅同位素组成，边部更长石相对富集高放射性成因铅。核部钾长石Rb、Ga、Ba、Pb和Rb/Sr高于边部斜长石，Sr略低于边部。沿熔蚀面呈现明显环带分布的微量元素和Pb同位素组成记录了壳幔岩浆混合作用：核部代表早期酸性岩浆结晶的碱性长石, 晚期温度较高的来自地幔的基性岩浆注入到含碱性长石斑晶的早期酸性岩浆中, 碱性长石未能与基性岩浆完全反应, 发生部分熔融形成以卵球状为主的碱性长石斑晶, 较基性的岩浆附着在其上快速结晶形成斜长石外壳。这些研究为环斑结构花岗岩是起源于地壳的酸性岩浆和起源于地幔的基性岩浆形成的混合岩浆结晶的产物提供了新的证据。同时利用该项目资助，完成了铁锰结核微区原位Pb同位素分析测试、低Mg含量岩石样品的Mg同位素准确测试分析方法、单次溶样先后分离地质和生物样品的Mg-Ca同位素组成分析方法、无需离子交换树脂分离，利用硫酸和碳酸盐岩反应，MCICPMS直接测定Mg同位素组成分析方法、Mg同位素标准溶液研制、无需离子交换树脂分离，MCICPMS直接测试富铜矿物的Cu同位素组成分析方法等分析方法，进一步服务于国内外地球科学研究人员的分析测试平台。