北祁连榴辉岩相变沉积岩的特征及其构造意义

Characteristic of Eclogitic Metasedimentary Rocks in the North Qilian Mountains and Its Geodynamic Implications

在北祁连造山带中,出露典型的高压/低温变质岩石,前人对其中的低温榴辉岩已做过较多的研究,但对其中的变沉积岩研究涉及很少。本文展示了榴辉岩相变质沉积岩的岩石学、地球化学、锆石U-Pb年代学和Hf同位素方面的一些新的研究结果。变沉积岩含有榴辉岩相的矿物组合,峰期温压条件为t=450～520℃,p=1.9～2.3GPa,与相邻榴辉岩的温压条件一致。地球化学显示这些岩石的原岩为不成熟的沉积岩,可能形成于大陆边缘或大陆岛弧环境。变沉积岩中的碎屑锆石U-Pb年龄主要集中在1800Ma左右和540～600Ma之间,结合锆石Hf同位素特征,表明其原岩的碎屑来源既有周缘陆块的前寒武纪变质基底物质,又有新元古代—早古生代新生洋壳或增生物质。同时,这些数据也表明北祁连早古生代洋壳俯冲过程中发生了活动大陆边缘的构造剥蚀作用,即形成于上盘的沉积物(弧前盆地或增生楔)被构造作用运移到俯冲带中,并俯冲到60～70km深处,遭受榴辉岩相变质作用,然后折返到地表。

In this paper, we present new petrological, geochemical and LA-MC-ICPMS data for metasedimentary rocks from the Baijingsi area, North Qilian Mountains, in which typical early-Paleozoic ophiolitic sequences and HP/LT metamorphic rocks have been previously recognized. The studied metasedimentary rocks contain eclogitic facies assemblages reflecting PT conditions of 450 - 520℃ and 1. 9 - 2. 3 GPa, consistent with those of adjacent eclogites. Geochemical data indicate that the precursor rocks of the metasedimentary rocks were immature sediments formed in continental margin or island arc environments. U-Pb dating of detrital zircons from metasedimentary rocks is concentrated around 1800 Ma and 540-600 Ma, along with zircon Hf isotopic characteristic, suggesting that they derived from both the mixture of Paleoproterozoic metamorphic basement and Late Neoproterozoic-early Paleozoic oceanic materials, which were probably accreted into active continental margin during subduction. These data imply that tectonic erosion of the active continental margin of North Qilian occurred during the Mesozoic ocean and crustal subduction, resulting in the formation of hangingwall sediments in the upper plate （fore-arc basin or accretionary prism）, which were then transported into the subduction zone to a depth of 60-70 km and exhumed after eclogite-facies metamorphism.