内蒙古苏左旗南部早古生代蛇绿混杂岩特征及其构造意义

内蒙古苏左旗南部温都尔庙群由早元古代宝音图群(1910Ma),中、新元古代温都尔庙群(1511Ma,825Ma)和早古生代乌勒图-乌兰呼都格-查干乌拉蛇绿混杂岩(409Ma)组成。出露于乌勒图-乌兰呼都格-查干乌拉地区的乌勒图蛇绿混杂岩是由层位不全的,肢解了的蛇绿岩经构造混杂而成。其基质主要为绢云绿泥石英片岩、变质凝灰质砂岩,局部为绿泥片岩。岩块成分复杂,大小不一,形态各异,杂乱分布,主要岩石类型为白云岩、硅质岩、超基性岩、基性火山岩、灰岩。蛇绿混杂岩岩石化学分析表明,超基性岩MgO/FeO^*比值在8-13,MgO/(MgO+ FeO^*)比值在0.85-0.87 之间,与世界大多数变质橄榄岩相同。基性火山岩具大洋拉斑玄武岩特征,常量元素和稀土元素显示陆间洋盆性质。蛇绿混杂岩被晚泥盆世色日巴彦敖包组地层不整合覆盖,前者所含超基性岩块Sm-Nd 同位素等时线年龄为409Ma,表明其形成于中、晚志留世,于晚泥盆世前发生构造侵位。

FEATURES AND TECTONIC IMPLICATIONS OF THE OPHIOLITIC MELANGE IN THE SOUTHERN SUZUOQI, INNER MONGOLIA

Ondor Sum Group is situated in the southern area of Suzuoqi,Inner Mongola, which is divided into Baoyintu Group in Early Proterozoic ear(1 910Ma), Ondor Sum Group in Middle Late Proterozoic ear(1 511Ma—825Ma), and ophiolitic melange in Early Paleozoic era(409Ma).This region is a magmatic passive continent margin in Middle-Late Proterozoic ear. An ocean basin between continents is formed in Early Paleozoic era. The ophiolitic melange occurs in Wuletu-Wulanhuduge-Chaganwula area. The authors consider that the ophiolitic melange is a dismembered and broken ophiolitic suite. The matrix consists of sericite-and serpentine-schist, including many block fragments such as dolomite, silicolite, ultramafic rock, basic volcanic rock and limestone. The MgO/FeO^* and MgO/(MgO+FeO^*)ratios of the ultramafic rocks are close to those of most metaperidotites around the world. The basic volcanic rocks are similar to the oceanic tholeiite in composition. Geochemistry of major and rare-earth elements of the basic volcanic rocks indicates the features of an ocean basin between continents. The Sm-Nd whole-rock isochron for the ophiolitic melange yields an age of 409Ma. This is consistent with the fact that the ophiolitic melange is unconformably covered by the Late Devonian Seribayanaobao Formation. It is evident that the ophiolitic melange was formed in Middle-Late Silurian and tectonically emplaced between Early and Middle Devonian.