北秦岭丹凤群向西延入祁连造山带的地质意义

依据前人及笔者近期对北秦岭的早古生代丹凤群(蛇绿岩套)追索研究表明,该带向西可延展至甘肃陇西一带,且向更西可与甘肃的雾宿山和青海的拉脊山出露的早古生代岩系相对比,从而构成位于祁连中间隆起带南侧,与北祁连褶皱带相对应的优地槽(蛇绿岩)带。本文研讨了其火山岩系的岩石系列组合与形成环境,并认为它的向西延展,显示北秦岭与祁连造山带间不但是相互连接的,而且两区具有相同的构造格架,属华北板块南部边缘的同一造山带。

THE WESTWARD EXTENTION OF THE EARLY PALEOZOIC DANFENG GROUP INTO THE QILIAN OROGEN AND ITS GEOLOGICAL SIGNIFICANCE

Rescently, the Danfeng Group and its ophiolite belt in the eastern portion of the North Qinling area has been further studied, though many researchers have corroborated it as a south ophiolite belt of the Qinling Range. It has been confirmed in this investigation that the portion from Baijiadian in Fengxian, Shaanxi to Gaojiahe in Longxi, Gansu is a continuously west-extending volcanic and ultramafic rock belt. In the portion from Guanzizhen in Tianshui County to Gaojiahe in Longxi County, apart from the volcanic rocks, there concentratedly emerge ultramafic rocks especially the Yuanyangzhen ultramafic rock body in Wushan County, Gansu, which is the largest in the eastern portion of the Qilian Mountains. According to the fact that the volcanic rocks of the area are characteristic of an island-arc association and that the ultramafic rock bodies occur mainly along fracture or secondary fracture systems, it is considered that the rock belt is made of melting ultramafic rocks derived from an island-arc root or the Earth's mantle by compressive stress and extrusion along fracture system, during a collision of the South and North China plates. Geologic data from Gansu and Qinghai Provinces show that the western portion of the Danfeng ophiolite belt extends westward through a large loess cover and reappears at the Wushushan, Gansu and the Lajishan, Qinghai and also consists of volcanic and ultramafic rocks of the Early Palaeozoic age. The westward extension of the ophiolite belt implies. It has that the North Qilian orogen has the same structural framework as the North Qinling Caledonian foldbelt and that they belong to the same orogen on the southern margin of the North China plate.