秦岭群混合岩中花岗质脉体成因及锆石结晶机理

Petrogenesis of the leucogranite veins and zircon crystallization process in the Qinling migmatic complex

混合岩中淡色花岗质脉体成因机制的精细研究可为造山过程中地壳的熔融机理提供重要信息。选择秦岭群混合岩中淡色花岗质脉体进行系统的矿物化学、全岩地球化学、锆石U-Pb年代学、锆石成因矿物学研究,阐明淡色花岗质脉体的成因机制。结果表明:淡色花岗质脉体具高硅、高碱的特征,A/CNK=1.07,Eu*/Eu=0.40,富集大离子亲石元素Rb, Ba, K,Pb和不相容元素U,Th,亏损高场强元素;起源于秦岭群片麻岩在低温条件下部分熔融。淡色花岗岩脉体中锆石来源复杂,其Pb/U年龄介于970~108 Ma之间,按照锆石表观年龄分为5组,5组锆石的Ti温度分别为685,661,745,691,757℃,而全岩液相线温度为908℃,高于锆石饱和温度和锆石Ti温度,说明在岩浆初始阶段没有锆石结晶,脉体中不同锆石成因机制较为复杂。

Origin of the leucogranite in the migmatic complex can provide detailed information about the crustal melting mechanism and granite genesis.This paper carried out a systematic exploration of mineral chemistry, bulk-rock geochemistry, zircon U-Pb dating and Lu-Hf isotope for the leucogranite in the Qinling migmatic complex, so as to deduce its formation process and its petrological implication for the crustal melting mechanism in orogenic process.The leucogranite display high SiOand NaO+KO contents, with A/CNK=1.07,it is enriched in LILEs and LREE and significant negative Eu anomalies(Eu*/Eu=0.4).These features indicate that it was derived from low-temperature melting of granitic gnesis in the Qinling Group.Zircons in the leucogranite display complex origin, which have variablePb/U ages of 970 to 108 Ma.And according to the age distribution patterns, it can be subdivided into five sub-groups, with Ti-in zircon temperatures being 685,661,745,691,757 ℃,respectively.These temperatures are significantly lower than its whole-rock liquidus temperature(908 ℃),suggesting no zircons crystallized in the early stage.