扬子克拉通崆岭杂岩孔兹岩系同位素年代学研究及其地质意义

摘要

A series of khondalite characterized by graphite and Al-rich minerals is exposed in the Kongling Complex, overlying the Archean metamorphic crystal basement. In this study, ID-TIMS mineral-whole rock Sm-Nd isochron dating is carried out for the samples of garnet-sillimanite-quartzite in the khondalite-series. The result shows that the garnet-sillimanite-quartzite was formed at 2078±31 Ma. The Nd isotopic compositions of garnet-sillimanite-quartzite are relatively homogeneous with theeNd(t) value ranges from−4.9 to−2.3, corresponding to the two-stage model age of 2.91–2.70 Ga, indicative of a closed system. Combined with previous zircon U-Pb ages and geochemical results, we can infer that the khondalite-series in the Kongling Complex was formed in 2.13–2.08 Ga, and the provenance of the sediments was mainly the underlying ancient crustal material characterized by juvenile crustal growth in the Mesoarchean. The Paleoproterozoic high pressure granulite-facies metamorphism recognized in the interior of the Yangtze Craton, might be related to the Paleoproterozoic collisional orogen during 2.08–1.94 Ga, which is consistent with the worldwide contemporary orogeny, implying that it may have been an important component in the Paleoprotorozoic Columbia supercontinent.%崆岭杂岩出露有一套覆盖于古老变质结晶基底之上的以含石墨和富铝矿物为特征的孔兹岩系。本文对该套孔兹岩系中代表性岩石类型榴线英岩开展了同位素稀释法(ID-TIMS)矿物–全岩Sm-Nd等时线测年。研究结果表明,榴线英岩形成于2078±31 Ma。榴线英岩Nd同位素组成相对均一(εNd(t)值介于−4.9与−2.3之间),对应Nd同位素两阶段模式年龄介于2.91~2.70 Ga之间,表明大多数样品形成于封闭体系中。结合前人在同一套岩石组合中报道的锆石U-Pb年代学和岩石地球化学数据,推测崆岭杂岩孔兹岩系可能沉积于2.13~2.08 Ga,其原岩沉积物主要来自下伏的古老地壳物质,且其源区存在中太古代的初生地壳增长。扬子陆核经历的古元古代(2.08~1.94 Ga)高压麻粒岩相变质作用可能与全球广泛存在的同时期(2.1~1.8 Ga)碰撞造山事件有关,暗示其很可能是古元古代Columbia超大陆的重要组成部分。