北秦岭中、晚元古代地质演化特征及其有关问题讨论

THE MIDDLE AND LATE PROTEROZOIC GEOLOGICAL EVOLUTION OF NORTH QINLING WITH DISCUSSION ON SOME RELATED PROBLEMS

北秦岭地区较广泛发育不同岩类组合、不同古构造环境的中、晚元古代地质体，并构成断续残留的古构造岩相岩带，它们是在晋宁期（１０００Ｍａ±）碰撞造山过程中形成于华北古板块南部的古造山带的组成部分。研究表明，华北古板块南部曾经历中元古代早期板块裂解、向洋发展；中元古代晚期洋盆俯冲、消减；中无古代末－晚元古代初（晋宁期）碰撞造山的构造作用过程。鉴于秦岭造山带显生亩板块运动的分隔性，在未恢复显生宙板块运动的前提下，北秦岭和南秦岭在晋宁运动期间形成的结构、构造特征不能以现今状态作简单的时、空对比和分析。

Lies across the central section of China's Mainland, the Qinling Orogenic Belt is a Phanerozoic continental orogenic belt, with the Shangdan belt as its suture zone, and to its north is generally called North Qinling. The study of geological evolutionary features of the well developed Middle and Late Proterozoic geological bodies in this district is of great significance to the investigation of early crustal formation and evolution of China continent. The Middle and Late Proterozoic geological bodies in North Qinling are chiefly of preSinian metamorphic rock series which had experienced polyphase reconstruction of complicated deformation and metamorphism, and can be divided into several diverse rock associations as follows from north to south based on the present rock associations as wed as their distribution in time and space.1. Early. Middle Proterozoic (1800 - 1400Ma ) sedimentary--volcanic rock belt of rift,with the Xiong'er Group as an example;2. Sedimentary belt of stable continental margin in the late stage of Middle Proterozoic (1400-- 1000Ma), represented by a set of terrigenous elastics and carbonate of Gaoshanhe Group and Luonan Group in Shaanxi Province, Guandaokou Group and Ruyang Groups in Henan Province, which overlain upon the Xiong'er Group;3. Middle Proterozoic (1800-1000Ma) sedimentary-volcanic belt of rift-oceanic basin environment, exemplified by the Kuanping group;4. Late stage of Middle Proterozoic to initial stage of Late Proterozoic accreted volcanic complex belt of various attributes, constituted by Songshugou ophiolite sheet and parts of the sedimentary volcanic rocks of Danfeng Group.The existence of tectonic factes in good time and space sequences indicated that, after the construction of North China palaeocontinental block through lateral accretion and vertical overgrowth of North China cratonic nucleus at the end of Early Proterozoic, its southern part splitted and developed into an ocean in the early Middle Proterozoic (1800Ma), with its flourish developing stage of oceanic basin to be 1400f Ma, and it is only to the late Middle Proterozoic did the oceanic basin started to underthrust and subduct.A comprehensive study of the deformation, metamorphism, magmatism and isotopic age data of the Middle and Late Proterozoic geological bodies suggests that this district had undergone an important orogenic event in 1000Ma± The fact that Songshugou ophiolite sheet was exotic block tectonically emplaced in 983 ± 140Ma (Sm--Nd Isochron age),accompanied with high (superhigh?) pressure metamorphism constituting a PTt evolutionary locus of clockwise relief of pressure of ITD type by means of its structural analysis, in addition to the occurrence of collision granite of 1000Ma ± in this area, suggests that North Qinling had experienced an orogenic process of oceanic closure and continental collision in 1 000Ma ±.As a whole, in the Middle and Late Proterozoic, the North Qinling was a palaeoaccretion belt similar to modern plate tectonics, which includes splitting from the southern part of North China palaeo--continent, gradually evolved to an ocean, and finally subducted.