SOME VIEWS ON THE STRATIGRAPHY OF THE “QINLING AXIS”

On the basis of observations of field sections combined with studies and analyses of the data of predecessors, this paper puts forwards a series of new views on the stratigraphy of the “Qinling axis" （which is actually the southern part of the Qinling Mountains area）. Within the scope of the "axis" （s.l.）,two unconformities have been found.According to the characteristics of the rock formations and their metamorphic grade,the strata within the "axis" may be divided into three parts,i.e.;the upper and lower formations of pre-Sinian age and the upper Marble of Sinian-Paleozoic age.Nroth of the " axis",the greenstone series is separated from the Erlangping Group and assigned to the Paleozoic;while south of the "axis", the Zhoujinggou Formation is replaced at the Liuling Group,whose age is post-Paleozoic.The remaining strata are respectively correlated with the pre-Sinian strata within the"axis".Thus,on the basis of the rock association,metamorphic grade,contact relationships and distribution features,the strata in the southern part of the Eastern Qinling Mountains area may be viewed as a temporally and spatially unified and interconnected whole.Meanwhile,the scheme of classification of related strata in the area has been incorporated in a single table of stratigraphic classification and correlation. On the basis of the stratigraphic classification and proceeding from the attitudes of the strata,it is proposed that folds are the basic structural type and control the distribution of the strata in the area,while a series of longactive deep fractures only play a role of modifying the distribution to some extent and do not exert controlling effects.The major fracture at both sides of the "axis"（s.l.）do not represent the natural demarcation lines between different stratigraphic units,and the "axis" itself is merely an anticlinorium in the fold structure in the area. This paper also suggests that the pre-Sinian strata in the area do not possess the characteristics of geosynclinal sediments. They are small in thickness and simple in rock association, and in the main continental sediments with small quantities of volcanic constituents; the folds are gentle. The Sinian-Paleozoic strata are only distributed sporadically and mostly occur in downfaulted basins or the synclinal part of the folds. They possess the characteristic of platform-type sediments. So it is considered that the Eastern Qinling area should be a part of the North China platform itself. The fold structure in the area, particularly the gigantic fractures, are characterized by polyphase activity; the phenomena of banded migmatization and granitization occurring on the basis of low-grade regional metamorphism imply the possibility of "activization" at the late stage in the area, i. e. tectonometamorphism was strong, too. The authors consider that the stratigraphic sequence of the Douling Group south of the "axis" is not reversed but normal. The ophiolite-like sequence and its overlying strata of the Sinian System （the Yaoyingzhai and Matoushan Formations） should be the elements of the Yangtze platform. They underlie the older Douling Group of the North China platform and contain the "fragments" of the latter, the contact line being uneven. So it is considered that this area might represent the juncture （or suture） between the north and south landmasses of China, i. e. the North China platform and the Yangtze platform, and that the latter was subducted beneath the former.So it is concluded that there might exist a subduction zone along the Jingziquan-Zhechuan line. It should occur in late Sinian time. This further explains why the "activization" and metamorphism weaken gradually from south to north in the Eastern Qinling Mountains area. This paper also roughly discusses the problems of gold source beds and metallogenic processes in the Eastern Qinling Mountains area and proposes the law of dual, i. e. stratigraphic and structural, control of primary, secondary and multiple concentrations, where stratigraphical horizon is the basis and structure is the condition.