浅变质岩在示踪大别—苏鲁造山带大陆板块俯冲与折返过程中的意义

TECTONIC IMPLICATIONS OF THE LOW-GRADE METAMORPHIC ROCKS FOR THE SUBDUCTION OF A CPMTOMEMTAL PLATE IN THE DABIE-SULU OROGEN

中国大别—苏鲁造山带为大陆板块俯冲形成的碰撞造山带,该带北缘和内部产有原岩时代为新元古代—晚古生代的浅变质岩。这些浅变质岩对应于扬子板块北缘前寒武变质基底和扬子板块北缘古生代大陆架沉积物,形成过程于印支期扬子板块向北俯冲过程中的刮削作用密切相关,与大洋板块俯冲过程中刮削形成的加积楔具有类似的动力学过程。对大别—苏鲁造山带浅变质岩的深入研究,不仅有助于揭示大陆板块俯冲过程中高压—超高压岩石形成与折返过程,而且确定了扬子板块与华北板块之间的缝合线位置位于大别造山带北淮阳带的北部和苏鲁造山带的五莲—蓬莱群的北侧。

In the margin area of subduction of oceanic plate, a great deal of marine sediments and underlying rocks were scraped off from the subducting oceanic plate to accumulate as wedge-shaped mass. It is called the accretionary wedge and uniquely develops at the boundary of convergent plates. This tectonic unit frequently occurs in orogenic belts, which were formed by subduction of oceanic plate, but it is unclear whether it also occurs in orogenic belts due to subduction of continental plate. The Dabie-Sulu Orogen in East-central China is a typical continent-continent collision orogen due to subduction of the Yangtze craton beneath the Sino-Korean craton at Triassic. Ultrahigh-pressure (UHP) metamorphism has been recognized by occurrence of micro-diamond and coesite-bearing eclogites and gneisses within this orogen. There are a great deal of low-grade metasedimetary and metaigneous rocks to occur not only in the northern margin of the UHP metamorphic belt but also in its interior. They consist of two parts: (1) large masses in the northern margin of the belt which are composed of slates, schists, phyllites, metasandsones and marble as well as intrusions; (2) sparotic outcrops in the interior of the belt which are composed of metaclastics, phyllite, and marbles. All of the rocks suffered strong deformation and dynamic metamorphism mostly at greenschist facies. Palaeotological, petrologic and geochronological investigations show that the metasedimentary rocks were deposited in the northern passive continental margin of the Yangtze plate as a flysch facies prior to Triassic, and that the intrusions are of Neoproterozoic ages and thus the product of rift magmatism in the northern margin of the Yangtze craton at Late Precambrian. Both Middle Paleozoic and Triassic events were dated for the low-grade rocks, being concordant with tectonic events experienced by the ultrahigh-pressure rocks in the Dabie-Sulu orogen. Apparently, the low-grade metamorphic rocks were scraped off from the subducting Yangtze plate at Triassic and thus correspond to a part of the accretionary complexes of the continental plate subduction. Within the framework of the accretionary wedge of continental plate subduction, the low-grade metamorphic rocks in the northern margin and the interior and of the Dabie-Sulu orogenic belt can be unified as a whole in comparison with the UHP metamorphic rocks. Relationship in time and space between the low-grade and the UHP metamorphic rocks is studied in order to understand the geodynamics of the Yangtze plate subduction.