北秦岭-南祁连早古生代裂谷造山带火山岩与小洋盆蛇绿岩套特征及纬向对比

CHARACTERISTICS AND LATITUDINAL COMPARATIVE RESEARCH ON THE EARLY PALAEOZOIC VOLCANIC ROCKS OF RIFTED OROGENIC BELT AND SMALL OCEAN BASIN OPHIOLITE SUIT FROM NORTHERN QINLING MOUNTAINS AND SOUTHERN QILIAN MOUNTAINS

北秦岭—南祁连位于我国著名巨型纬向造山带的中段，是以寒武—奥陶纪时期为主形成的裂谷造山带。它经历了板内大陆裂谷→陆间裂谷→火山弧→造山带的开合过程。寒武纪拉张形成碱性、拉斑玄武岩系列及小洋盆蛇绿岩套，奥陶纪闭合形成钙碱性与钾玄岩系列中酸性火山岩及火山弧岩套，并见高镁安山岩。南祁连为单裂谷，北秦岭为双裂谷。北秦岭裂谷从东到西，拉张速度和距离变小，闭合速度变大，火山爆发强度及火山岩的酸度、钾量、壳源组分也顺序增大。其拉张距离与红海型相似，而闭合速度近似安第斯型。古地幔以多种类型叠加为特征，曾有过先强烈亏损，后又富集的过程。从大别山→北秦岭→南祁连→柴北缘纬向对比可知：本区的基性与酸性火山岩浆，分别为地幔与地壳部分熔融形成；中性岩主要为基性岩浆ＡＦＣ作用产物。从东到西，裂谷依次张开，火山喷发时代逐渐变新，拉张速度和距离变大，闭合速度也变大，从而火山岩的碱度变小，蛇绿岩套从无到有。古地幔由ＤＭＭ与ＥＭⅠ混源为主，变为ＤＭＭ与ＥＭⅡ混源为主。

Northern Qinling Mountains and southern Qilian Mountains are located in the middle part of the famous latitudinal Qinling mega-orogenic belt. It belongs to rifted orogenic belt of the Cambrian-Ordovician period. It underwent a series of tectonic opening-closing including intraplate continental rift→intercontinental rift→volcanic arc→orogenic belt. During the Cambrian, the alkaline and tholeiitic basalt, and also small ocean basin ophiolite suit were formed under an extension environment. In the Ordovician, the tectonic setting was closing. The intermediate-acidic volcanic rocks of calc-alkaline and shoshonite series , as well as the volcanic arc suit rocks were formed. The boninites were also formed at this time . Southern Qilian belongs to the single rift system; however, northern Qinling belongs to the double rift system. In northern Qinling rift system, from east to west and from south to north, the distance of extension decreased , the velocity of closing increased, and the intensity of volcanic eruptions, acidity, K content, crustal materials of volcanic rocks were gradually getting larger. The distance of extension was similar to the Red sea's; however, the velocity of closing was similar to the Andes type. The characteristics of old mantle were superimposed. In the Cambrian period the mantle was depleted in crustal materials; however,in the Ordovician period the mantle was enriched in crustal materials. Through the latitudinal comparative resaerch between Dabei Mountains→northern Qinling→southern Qilian→northern margin of the Chaidamu Basin, we found that the basic and acidic magmas of northern Qinling and southern Qilian were mainly formed through partial melting of mantle and crustal respectively; whereas the intermediate rocks were formed through assimilation-fractional crystallization(AFC)of the basic magama. From east to west, the rift opened gradually; The ages of volcanic eruptions were getting younger; the duration of volcanic eruptions was getting shorter; the rift system changed from single to double character; the distance of extension was getting larger; the velocity of extension and closing was becoming larger; the alkalinity of volcanic rocks was getting smaller.There was no ophiolite suit in the east,whereas it appeared in the west;The sources of old mantle were changed from mixed DMM and EMⅠ to mixed EMM and EMⅡ.