秦岭造山带是印支碰撞造山带吗?

国内外一些学者认为秦岭是一个印支碰撞造山带.但迄今为止，秦岭尚未发现三叠纪或古生代延续到三叠纪的洋盆存在的任何痕迹.秦岭泥盆系-三叠系为滨、浅海相沉积，没有远洋沉积，更没有镁铁质和超镁铁质岩石及与之密切相关的放射虫硅质岩组成的蛇绿岩套.泥盆系与下伏地质体之间有一个清楚的区域性角度不整合.商丹断裂并不是印支期，而是加里东期的板块缝合带；其两侧，中朝板块南缘和扬子板块北缘均有十分清楚的加里东造山作用的记录.沉积于扬子板块北缘的中上泥盆统刘岭群的放射性铅同位素组成与北秦岭相近，碎屑锆石年龄谱系亦证明其物质主要来自中朝板块南缘的北秦岭造山带.所谓勉略印支缝合带中的勉略和三里岗蛇绿混杂岩中的镁铁质岩，同位素测年均为元古代之产物，后者又被南华系-震旦系沉积覆盖.所谓勉略缝合带，实为一区域性大断裂带.早古生代，其北侧属扬子板块北部被动边缘；南侧为扬子板块核心部分的扬子准地台（小克拉通）.所以，秦岭的印支造山作用，并不是洋盆消失后的陆陆碰撞造山作用，而是海盆消失后的中朝与扬子2个小陆块间逆冲-叠覆造山作用.作为秦岭东延的大别山超高压变质带被认为是秦岭印支碰撞造山的重要证据之一，但大别山超高压变质岩是在造山作用过程中动态超高压条件下形成的，仅用简单的静岩压力来计算其形成深度，显然是不符合实际情况的.野外地质观察、构造地质学、变质岩石学、同位素地质学、地球化学、地球物理学以及物理实验等方面的实际资料和研究结果均说明超高压变质作用并不是在上地幔而是在地壳内进行的.南秦岭-大别山的地壳构造层次，上地壳自上而下依次为:未变质的沉积岩层、绿帘-蓝片岩层、高压变质岩层、超高压变质岩层；下地壳为未卷入超高压变质作用的麻粒岩相-高角闪岩相变质杂岩.含柯石英的超高压单位只是位于上地壳下部的厚约10~12 km的席状构造岩片.初步认为上地壳这一从低压到高压再到超高压的构造系统，是印支造山期间，南秦岭-大别山的上地壳以下地壳顶部为主剪切滑动面，多层次剪切作用造成的.上地壳下部的超高压变质岩，则可能是强烈剪切引起的频繁地震的震源区瞬时超高压作用的结果. 

Is the Qinling Orogen an Indosinian Collisional Orogenic Belt?

Since the late 1980s, the Qinling orogen has been considered an Indosinian collisional orogenic belt by some Chinese and foreign geologists. However, no trace of a Triassic oceanic basin or a Paleozoic oceanic basin that continued to Triassic has been found as yet in the Qinling area. It has been suggested that the Devonian-Triassic of the Qinling area is characterized by coastal to shallow marine sediments, and there are no pelagic sediments or ophiolites. There is a clear regional angular unconformity between the Devonian and its underlying rocks. The Shangdan fault is not an Indosinian but a Caledonian suture zone, both sides of which (i.e. the southern margin of the Sino-Korean plate and the northern margin of the Yangtze plate) have a clear record of the Caledonian orogeny. The Pb isotopic compositions of the Liuling Group of the Middle-Upper Devonian, deposited on the northern margin of the Yangtze plate, are similar to those of the North Qinling belt. The detrital zircon age spectrum also suggests that the sediments came from the North Qinling orogenic belt. Mafic rocks from the Mianlue and Sanligang ophiolitic melanges within the Mianlue Indosinian suture zone show a Proterozoic age, and the Sanligang ophiolitic melange is overlain by the sediments of the Nanhuan-Sinian. The so-called Mianlue suture zone is actually a huge regional fracture zone, to the north of which was the passive margin of the Yangtze plate in Early Paleozoic and to the south lay the Yangtze paraplatform (small craton), the core of the Yangtze plate. Therefore, the Indosinian orogen of the Qinling should be attributed to the thrusting and overlapping of the continental crust rather than the continent-continent collision after the disappearance of the oceanic basin. The ultrahigh pressure (UHP) metamorphic belt of the Dabieshan, the eastern extension of the Qinling orogen, is considered an important piece of evidence suggesting that the Qinling was an Indosinian collisional orogen. However, the Dabieshan UHP metamorphic rocks were formed under dynamic UHP conditions during the orogenic process. Therefore, it is inappropriate to convert the estimated thermodynamic pressure to depth by assuming that the pressure was static simply due to burial. Accumulating data from field geological observations, structural geology, metamorphic petrology, isotope geology, geochemistry, geophysics, and physical experiments all indicate that the UHP metamorphism did not occur in the upper mantle but in the earth crust. Structurally, the upper crust of the South Qinling-Dabie region can be subdivided, from top to bottom, into four layers:unmetamorphosed sedimentary rocks, epidote-glaucophane schists, high pressure metamorphic rocks, and UHP metamorphic rocks. The lower crust is represented by the upper amphibolite-granulite facies rocks which were not affected by the UHP metamorphism. The coesite-bearing metamorphic rocks are merely sheeted slices of about 10-12 km thick in the lower part of the upper crust. Based on these observations, our preliminary conclusion is that the low-high-ultrahigh pressure tectonic system of the upper crust in the South Qinling-Dabie region was the result of multilayered decollement above the lower crust, acting as a main shear slide plane during the Indosinian orogeny. The UHP metamorphic rocks of the lower part of the upper crust were formed in response to repeated transient UHP events in the focal area, where frequent earthquakes were likely caused by severe shearing.