The Liujiatai Formation in the Huangyuan Group is a middle-high grade metamorphic series of low amphibolites facies mainly composed of felsic gneiss with biotite granulite.The protolith of the felsic gneiss is acid intrusion.They have low CaO (1.80~1.85 wt%), but high SiO2 (68.78~71.58 wt%) and alkali contents (Na2O+K2O=(7.6~7.7 wt%).These samples are relatively enriched in LREE and LILE (e.g.Ba and Sr), but depleted in HFSE (e.g.Nb, Ta and Ti), with the presence of notably negative Eu anomalies.They are mainly characterized by high HFSE abundances, with Zr+Nb+Ce+Y=(402~420)×10-6 (ppm) [>350×10-6(ppm)] and 10 000Ga/Al=3.2~3.3 (>2.6).All the geochemical data indicated that these gneisses show A-type affinities.LA-ICP-MS zircon U-Pb dating give a upper intercept age of 1 124±29 Ma, implying that the Huangyuan Group was formed in the Mesoproterozoic.The new age data is evidently younger than previously thought.%湟源群中的刘家台组为一套以长英质片麻岩为主,夹黑云变粒岩的低角闪岩相的中-深变质岩系.长英质片麻岩的原岩为酸性侵入岩体,其具有低CaO(1.80~1.85 wt%)、高SiO2(68.78~71.58 wt%)和碱含量(Na2O+K2O=7.6~7.7 wt%).样品相对富集轻稀土,亏损Nb、Ta、Ti等高场强元素,富集Ba、Sr等大离子亲石元素,并且具有明显的Eu负异常(δEu=0.23~0.38).它们最主要的特征是具有较高含量的高场强元素,使得Zr+Nb+Ce+Y=(402~420)×10-6 (ppm) [>350×10-6 (ppm)],而 10 000Ga/Al=3.2~3.3 (>2.6).这些岩石地球化学特征表明这套片麻岩的原岩为A型花岗岩.单颗粒锆石 U-Pb同位素测年获得的不一致曲线的上交点年龄1 124±29 Ma,属中元古代,比前人认为的湟源群形成时代古元古代要年轻.
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