秦岭三叠系分带及印支期发展史

秦岭及共邻区的三叠系自北而南可分为四带.北秦岭三叠系具有富含植物化石的陆相上三叠统,其下的优地槽型细碧角斑岩系时代未定.中秦岭下三叠统为复理石夹多层砾状灰岩,后者系斜坡沉积,物源可能来自北方,安尼期为复理石.南秦岭北带在二叠纪晚期已裂陷接受复理石及以砾状灰岩为代表的斜坡沉积.早三叠世至安尼期为深水相黑色板岩、薄层灰岩、复理石并夹火山岩.南秦岭南带及巴顿喀喇从早三叠世至安尼期为扬子地台的一部分,岩相及化百群与之一致,具有发育良好的安尼期陆棚边缘生物滩.从拉丁期开始裂陷．出现鱼鳞蛤页岩、砾状灰岩及巨厚复理石,后者延续至晚三叠世,有放射虫为证.整个中,南秦岭呈现一个由二叠纪晚期开始,延续于印支期的裂陷槽发育史.它的北部—中秦岭和南秦岭北带于二叠纪末及三叠纪初先后裂陷,并于拉丁期褶皱回返.它的南部—南秦岭南带及巴颜喀喇于拉丁期裂陷,并于三叠纪末回返.这个裂陷槽是否构成印支期秦岭的主体,抑或它仅是“北秦岭小洋盆”在扬子大陆边缘的弧后扩张盆地,取决于北秦岭是否存在早、中三叠世优地槽沉积.后者尚未证实.

TRIASSIC BELTS AND INDOSINIAN DEVELOPMENT OF THE QINLING MOUNTAINS

The Triassic of the Qinling Mts. and its adjacent areas can be subdi-vided into four belts. The North Qinling Belt (Fig. l-b) yields terrestrial Upper Triassic with abundant plants. The eugeosynclinal spilite-keratophyre sequence, seemingly underlying the Upper Triassic comformably, has recently been suggested to be Lower-Middle Triassic but not yet confirmed. The Lower Triassic of the Central Qinling Belt (Fig. 1-c) consists of flysch interbedded with breccio'a beds which are regarded as olistostromes on aulacogen slope; source of elastics may be from the north. The Anisian are also flysch deposits. In the Northern South Oinling Belt riftogenesis with flysch and olisto-stromic brecciola originated since Late permian. Lower Triassic and Anisian are bathyal black slates, flysch and interbedded volcanics. Deposition ceased since Ladinian in both belts. Noting the Ladinian regression in North China, margin. South Qilian and Burhan Budai (East Kunlun) areas, these two belts must have been folded and uplifted by Ladinian. The Southern South Oinling Belt and the neighboring Bayan Har Mts. (Fig. 1 e) were parts of the Yangtze Platform during Early Triassic and Ani-sian, with similar lithofacies and faunas. The well-developed Anisian shelf-margin bioclastic bank is especially interesting. Rift fanlting began from Ladinian and formed successively Daonella shales, olistostromic brecciolas and very thick flysch sequences which extended to Late Triassie as indica-ted by ladiolarians. The Central and South Qinling regions underwent riftogenesis from Late Permian to Triassic. Its northern part-the Central and Northern South Qinling Belts began to rift in Early Triassic and Late Permian respectively and folded up in Ladinian, while its southern part-the Southern South Qin-ling and Bayan Har began to rift in Ladinian and folded up by the end of Triassic. This aulaeogeu may be either the main part of the Triassic Qin-ling Geosyncline or only a backare tension gash basin on the Yangtze con-tinental slope, with the almost totally subducted North Qinling micro-oceanic basin as the main part. Existence of the latter depends on whether the North Qinling spilite-keiatophyre sequence is of Early-Middle Triassic age.