A newly found Paleozoic Japan-type subduction–accretion system

Abstract

Accretionary orogenic belts are complicated collages composed of magmatic arcs, accretionary complexes, ophiolitic melanges, fore-arc/back-arc basins and microcontinents formed at convergent plate margins, which are key sites of continental growth through earth history [1]. Arcs are important orogenic components of accretionary orogenesis, but the types of active margin with old continental basement and intraoceanic arcs without continental basement and their roles in accretionary orogenesis are different. The Japan-type arc is a special kind of arc rifted from active margin, a special intraoceanic arc with continental basement that has been neglected in the anatomy of accretionary orogeny [2]. It is noted that the Qilian orogenic belt has been considered to be developed from a Japan-type or west Pacific-type-arc–back-arc system [3]. The Central Asian Orogenic Belt, one of the largest accretionary orogenic systems in the world, is characterized by the appearance of various kinds of magmatic arcs and associated accretionary complexes that are formed diachronously from ca. 1000 Ma to ca. 250 Ma [4, 5], although some authors consider that the final collision occurred as late as 230–220 Ma [6]. Such long-evolved orogenic belts have been found elsewhere in China indicated by accurate geochronologic research, such as the Tongbai orogen [7] and the Trans-North China orogen [8, 9]. The Beishan orogenic collage, which connects the Tianshan orogenic belt to the west and the Inner Mongolia– Solonker suture to the east [4], is a key area for understanding the accretionary tectonics of the southern Central Asian Orogenic Belt. Extensive metamorphic complexes exist in the central segment of the Beishan, which are pivotal for understanding the subduction–accretion processes of the Beishan orogen. However, the timing and tectonic attribute for metamorphic terranes from the Beishan orogenic collage is highly ambiguous. Up to date, there has been no detailed study on the metamorphic complexes and associated volcanic rocks, which has hampered man’s understanding of the architecture of the Beishan orogen. The Tectonics Research Group led by Wenjiao Xiao from the Tethys Research Center, Institute of Geology and Geophysics, Chinese Academy of Sciences, have carried out detailed studies from field mapping and geochronology for four cross sections on the metamorphic complexes and volcanic rocks from the central segment of Beishan [10] (Fig. S1). Field mapping shows that all the metamorphic complexes have undergone polyphase ductile deformation (Fig. S2), with mainly NNE-dipping foliations, while the volcanic rocks are weakly deformed and the primary pyroclastic structures are preserved [10]. High-resolution geochronological data reveal that the ages of the protolith of the metamorphic complexes range from *465 to *400 Ma, but were not formed in the Precambrian as previously regarded, which probably represent the deeper parts of arcs or accretionary complexes. The volcanic rocks are formed at 446–441 Ma, with Precambrian detrital zircon ages [10]. The age information from these rocks may indicate continental materials in the generation of the magmatic arcs. However, it excludes the possibility that large-scale crystallized basement exists in the Beishan orogenic collage. Combined the temporal and spatial distribution of ophiolites and fore-arc complex in this area, they proposed that during the Paleozoic there was a JapanElectronic supplementary material The online version of this article (doi:10.1007/s11434-015-0956-z) contains supplementary material, which is available to authorized users.