The Conjunction And Transition Of The Eastern And Western Segments Of The Proto-tethys Northern Boundary: The Tectonic Relationship Of The Qinling-qilian-helan

The Qinling-Qilian-Helan Orogenic Belt is located at the junction of the easternand western segments of the Proto-Tethys Ocean northern boundary, be characterizedby diverse geological structures and complex tectonic evolution. The Caledonian isthe key stage of ocean-continent transition in the Qinling-Qilian Orogenic Belt. TheHelanshan is located at the junction of the Qinling and the Qilian orogenic belts, andthe tectonic attributes of the Helanshan are very important for studying the transitionprocess of the Qinling and Qilian orogenic belts, even the transition process of theeastern and western segments of the Proto-Tethys Ocean northern boundary. Previousstudies mainly focused on the tectonic characteristics and evolution of the Qinling,Qilian, and Helan orogenic belts, respectively, not considering the tectonicrelationship of these three belts in the Proto-Tethys tectonic system. Combined withthe lithofacies palaeogeography, affinities of surrounding blocks, field structuralanalysis, magmatism, and the subduction process and its associated deformation, thispaper discussed the uniform tectonic attributes and evolution of theQinling-Qilian-Helan Orogenic Belt, to reveal the global tectonic background of theCaledonian. We obtained the following results:(1) The Helanshan is a continental transform boundary, consist of the northernand southern segments. The northern segment and the Zhuozi Mountain areas havesome relevance with the Khondalite Belt of the western North China Block (NCB) instratigraphic contact, magmatism, metamorphism, and sediment source, revealing thenorthern segment belonging to the NCB during the Paleoproterozoic. The southernsegment had good affinities with the South China Block (SCB)before the Ordovician, evidence by the Neoproterozoic basement of the Alashan Block(ALB). The Helanshan had biological features, stratigraphy, tectonic events of both the NCB and theSCB in the late Caledonian, following with the closure of the Proto-Tethys Ocean.However, little geological records of ophiolite, high-ultrahigh pressure metamorphic,and arc-magmatic rocks have been found around the boundary of the Helanshan andwestern margin of Ordos. In this study, the Helanshan is considered to be thetransform boundary of the NCB and the ALB: the ALB creeped along the Helanshan,and collided and amalgamated into an entity with the NCB at the Late Ordovician.(2) Micro-blocks, south of the northern boundary of the Proto-Tethys Ocean,belong to Gondwana. The North Qinling Block(NQB), the Qilian Block (QLB), theQaidam Block (QDB), and the ALB are Gondwana fragments during the breakup ofthe Rodinia, based on lithofacies paleogeography, paleontology, and detrital zirconage spectrum. The ocean-continent framework changed dramatically at the Silurian,showing the convergence of the QLB and the QDB, the convergence of the ALB andthe NCB, and the closure of part of Proto-Tethys Ocean. All micro-blocks, except theSCB, amalgamated into an entity with the NCB at the Devonian.(3) The subduction polarity of the middle segment of the northern boundary ofthe Proto-Tethys Ocean is from north to south. Along the Luonan-Luanchuan andGuyuan-Longshoushan Fault belts, to the west of the northern boundary of theProto-Tethys Ocean, its surrounding geology is characterized by strong tectonicdeformation, southward-dipping stratigraphy and faults, and strong folding S1foliation, based on tectonic analysis and previous results of Ar-Ar chronology. TheS-C fabeic, lenticular, and "σ","δ"indicate the southward subduction of the northernboundary.(4) The northern segment of the Proto-Tethys Oceanic featured bywest-younging scissor-type opening and east-younging scissor-type closure, areflection of Early Paleozoic tectonic events. The opening of the Proto-Tethys Oceanmigrated westward during the Neoproterozoic, based on the magmatism,metamorphism, and isotope chronology. A series of high-ultrahigh pressuremetamorphic belts, ophiolite belts, and corresponding arc-magmatic rocks formed in the southward subduction process of the Proto-Tethys Ocean. The closure of theProto-Tethys Ocean migrated eastward at the Early Silurian. The surroundingmicro-blocks collided with each other in the closure process, and a series ofcollision-type granites and metamorphic rocks formed. Then, the“transform boundary”effect of the Helanshan terminated. Many Gondwana mirco-blocks assembled in thesouthern margin of the NCB, constituting the Neopaleozoic active continental margin.