北祁连中段高压/低温混杂带——岩石学、地球化学和年代学证据

High pressure/low-temperature mélange in the central North Qilian Mountains:Evidence from Petrology, geochemistry and geochronology

详细的野外研究结果表明,北祁连中段清水沟-香子沟高压低温变质带中出露的变质岩主要有榴辉岩、蓝片岩、多硅白云母石英片岩、变硅质岩、大理岩和蛇纹岩。地球化学测试结果表明,榴辉岩原岩为大洋中脊玄武岩和洋岛拉斑玄武岩;蓝片岩有两种类型,第1种蓝片岩原岩为基性-中基性火山岩,第2种蓝片岩原岩主要形成环境为大洋岛弧和/或大陆岛弧的沉积岩;多硅白云母石英片岩的原岩主要形成于大陆边缘环境;变硅质岩的原岩为形成于远离大陆边缘的沉积环境中的热水成因硅质岩;大理岩原岩为灰岩。榴辉岩锆石SHRIMP定年结果显示其原岩年龄为500±4 Ma,第1种蓝片岩原岩年龄为529±5 Ma。锆石Lu-Hf同位素表明,榴辉岩原岩明显受到了古老地壳的影响,基性蓝片岩的原岩源区为亏损地幔,后期没有或者极微弱地受到地壳物质的影响。结合已有研究资料,认为北祁连高压/低温变质带中变质岩的原岩类型具有明显的多样性特征,且原岩时代具有多期性特征。上述研究结果表明,古祁连洋在早古生代向北俯冲过程中,携带了不同性质和时代的岩石进入俯冲带深部,形成高压/低温变质条件下的混杂带,代表了早古生代与洋壳俯冲有关的俯冲隧道。

Detailed geological mapping reveals that metamorphic rocks outcropped in Qingshuigou-Xiangzigou area of the North Qilian Mountains mainly consist of eclogite, blueschist, phengite-quartz schist, metachert, marble and serpentinite. Geochemical data show that the protolith of eclogite was composed of MORB or OIB. Two types of blueschists were identified： the protolith of type Ⅰ was basic volcanic rock, that of type Ⅱ was sedimentary rock derived from ocean island arc and/or continental arc; phengite-quartz schist was derived from terrigenous clastic rocks formed at continental margin. Protolith of metachert was hydrothermal chert deposited far away from the continental margin. Marble came from limestone. The SHRIMP Ⅱ zircon U-Pb dating results show the protolith of eclogite and that of type Ⅰ blueschist are 500 ± 4 Ma and 529 ± 5 Ma, respectively.Zircon Lu-Hf isotopic values indicate that the protolith of eclogites has been heavily influenced by ancient crust,and the protolith of type Ⅰ blueschist was derived from depleted mantle. Combined with the previous data, the authors hold that the protolith of metamorphic rocks in the high-pressure and low-temperature metamorphic belt shows heterogeneous features, i. e., with different protolith natures and various protolith ages. These data suggest that the rocks with different protolith natures and various ages were carried into the great depth of the subduction zone during the northward subduction of paleo-Qilian Ocean in early Paleozoic, and formed mélange under the highpressure/low-temperature condition, thus representing an early Paleozoic subduction channel related to oceanic subduction.