对云南西部侵入于原西盟群帕可组的花岗片麻岩开展主量元素、锆石 U-Pb 年龄及原位 Hf 同位素组成分析。岩石地球化学特征显示其为高钾钙碱性强过铝质 S 型花岗岩，两件代表性样品分别给出了461.4±2.5 Ma 和461.5±3.3 Ma 的206Pb/238U 加权平均年龄，相应的锆石εHf(t)值分别介于−6.45~−1.90和−5.60~−1.44之间，二阶段模式年龄(tDM2)分别为1.57~1.86 Ga 和1.54~1.90 Ga。这些数据表明西盟地区侵入于原西盟群帕可组内的花岗片麻岩并非前人所认为的中元古代岩石，而是中奥陶世产物，其源岩可能是元古宙变质杂砂岩。结合前人研究成果，作者推测西盟地区早古生代花岗片麻岩是环冈瓦纳大陆边缘安第斯型造山作用的产物。%A set of whole rock major oxides, zircon U-Pb geochronological and in-situ Lu-Hf isotopic data is presented for the granitic gneisses from the Ximeng area, Southwest Yunnan province. Our results show that the granitic gneisses, which were taken from the previously-mapped Pake Formation of the Ximeng Group, are high-K calc-alkaline, strongly peraluminous S-type granites. Zircon separates from two representative samples (11ML-73 and 11ML-74) yield the U-Pb weighted mean ages of 461.4±2.5 Ma and 461.5±3.3 Ma, respectively, which are interpreted as their crystallization ages. Their corresponding εHf(t) values range from −6.45 to −1.90 for 11ML-73 and −5.60 to −1.44 for 11ML-74 with mean values of −4.73±0.36 and −4.46±0.57, respectively. The two-stage Hf model ages range from 1.57 Ga to 1.86 Ga and 1.54 Ga to 1.90 Ga, respectively. These data indicate that the granitic gneisses in the Pake Formation of the Ximeng Group are of Ordovician origin rather than previously-mapped Mesoproterozoic period. They might have been derived from ancient metasedimentary rocks. These data, together with other observations, suggest that the Ximeng granitic gneisses were the elements of the Paleozoic igneous belt of the Andean-type orogen along the northern margin of Gondwana.
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