Low-Grade Metamorphic Rocks in the Dabie-Sulu Orogenic Belt: A Passive-Margin Accretionary Wedge Deformed during Continent Subduction

Abstract

Greenschist-facies metasedimentary and meta-igneous rocks occur continuously along the northern margin, and sporadically in the interior, of the Dabie-Sulu orogenic belt in east-central China. An integrated study of geochronological, petrological, and paleontological observations demonstrates that precursors of flysch-facies metasedimentary rocks were deposited along the northern, passive continental margin of the Yangtze plate prior to the Triassic, and that protoliths of the meta-igneous rocks are a product of Middle Neoproterozoic bimodal magmatism along the northern margin of this plate. Except for the striking contrast in metamorphic grade, these low-grade rocks generally can be correlated in protolith origin and age with ultrahigh-pressure metamorphic rocks within the orogenic belt. Relationships in time and space between these rocks of contrasting grades can be reasonably interpreted through an accretionary wedge model that links their evolution with continent subduction. The low-grade metamorphic rocks of the subducting accretionary wedge consist of two parts: (1) large masses of metasedimentary rocks (including slates, schists, phyllites, metasandstones, and marble) along the northern margin of the Dabie-Sulu orogenic belt and deformed igneous rocks of Middle Neoproterozoic age; (2) sporadic outcrops in the interior of the belt of metavolcanics, metaclastics, phyllite, and marbles. During Triassic subduction of the Yangtze plate, the sedimentary cover and its underlying basement were partly scraped off by the overthrusted North China plate. The scraped-off materials accumulated in front of the overriding plate, forming an accretionary wedge that underwent deformation and metamorphism under greenschist-facies conditions. The present study also provides a constraint on the location of the Triassic suture zone between the North China and Yangtze plates. It is located below, or north of, the accretionary wedge (i.e., the Beihuaiyang zone in the Dabie region and the Wulian-Penglai zone in the Sulu region) rather than along the northern margin of the ultrahigh-pressure metamorphic zones.