Abstract:

The ages and types of orogenic belts in China vary greatly, where subduction accretionary complexes and ophiolite suites of various styles are developed through complex and changeable ocean-continent transition processes. How to study the orogenic processes such as ocean basin evolution and ocean-continent evolution has always been an intractable issue. To this end, the Regional Geology of China compilation project team has proposed the geological study of oceanic plate in an attempt to systematically study the geological construction and the structure of oceanic lithospheric plates such as orogenic subduction accretionary complex zone and ophiolite belt, and subsequently reconstruct the whole geological process from plate formation in the mid-ocean ridge to plate subduction in trench to eventual transformation of oceanic plate into land. This paper introduces the main research progress on the geology of oceanic plate focusing on the following four aspects. First, a preliminary geological framework of oceanic plate has been established. According to this model, the geological study of oceanic plate mainly concerns with the material composition of subduction-accretion complex, ophiolite type and its tectonic environment, unique sedimentary assemblages in oceanic plate and their characterization, paleoorogenic OPS sequence reconstruction, forearc volcanic assemblage in subduction zone, ocean-continent transition process and mechanism, and mineral formation and evolution in ocean-continent transition zone. Second, 62 subduction-accretion complex belts are identified, which include Beishan Niuquanzi-Mazongshan, Jiayin-Yilan, Chencai, East Kunlun Buqingshan-Animaqing, Yingyangguan, Dahongshan, Ganzi-Litang, and Xinyu Shenshan-Xingan Shenzhengqiao subduction-accretion complex belts, and so on. Subduction-accretion complex is the key to understanding the spatial-temporal structure, composition, and evolution of orogenic system. Third, intra-oceanic arc volcanic assemblages have been discovered in Qilian, which provides a petrological basis for the study of ocean-continent transition process. The Qilian orogenic belt is a classic area for studying the geology of oceanic plates. The study shows that a complete set of intra-ocean arc rock assemblages are exposed in Dangjinshan area. These rocks record the whole process of intra-ocean arc development from initial subduction to mature subduction in the study area. On this basis, the tectonic evolutionary history of Early Paleozoic in southern Qilian and northern Qaidam has been reconstructed, which provides a new basis for discussing the tectonic evolution of the Proto-Tethys Ocean in the Qilian orogenic belt. Fourth, the compilation scheme of the geological structure map of the oceanic plate is formulated. The compilation content contains five main categories: subduction-accretion complex zone; magmatic arc properties and attributes; high - ultra high pressure zone; deformation structural elements in subduction and collision periods; and tectonic evolution. The mapping unit is divided into three levels: accretionary complex (1^st level), rock slice (2^nd level), and matrix and rock block levels (3^rd level). During the mapping process, it is necessary to clarify the nature and attributes of the magmatic arc as well as to clarify which ophiolite mélange (including subduction-accretion complex) or ocean is matched by a magmatic arc on the map. The structural element expression on the map focuses on distinguishing subduction and collision stages. The structural deformation traces in subduction and collision periods are identified by studying the temporal, phase and position states of structural deformation. Based on these preliminary research results, the project team in the next step will discuss composite evolution of the oceans in China and its constraint on continental evolution. In addition, the next research direction is to explore fundamental geological issues such as the Tethys Ocean, and the relationship between the Paleo-Pacific/Pacific transformation in eastern China and Mesozoic-Cenozoic mineralization. At present, the geological study of oceanic plates is still in its infancy. Major geological problems such as the genetic relationship between ocean plate geology and mineralization need in-depth study in the future.

Key words: ocean plate geology, orogenic belt, ophiolite, subduction accretionary complex, intra-oceanic arc, initial intra-oceanic subduction, forearc volcanic assemblage, tectonic map of oceanic plate