CONNOTATION AND AGE ASSIGNMENTS OF THE WUNDURMIAO GROUP IN NEI MONGGOL AND ITS SIGNIFICANCE IN THE STRUCTURAL DEVELOPMENT OF THE PLATE CONVERGENT ZONE
Wang Dongfang Shenyang Institute of Geology and Mineral Resources
1. The Wundurmiao Group belongs to the ophiolite complex composed of meta- morphosed tholeiite, ultrabasic rocks, gabbro, diorite, plagioclase granite, spilitic lavas and dikes and siliceous rocks. They show a pseudosequence of strata and are mostly in faulted contact with each other, spanning a long time interval. Its con- ception is different from that of conventional stratigraphic groups, so it should be known as a complex. 2. The siliceous rocks in the upper part of the group contain Ordovician radiolarians. The greenschist in the mid-lower of the group has a Rb-Sr age of 509±40 m. y. and an initial ~(87)Sr/~(86)Sr ratio of 0.70434±0.0001, so it belongs to Mid -Late Cambrian deep-marine tholeiite. The K-Ar amphibole age of residual basalt fragments at the base ranges from 540-630 m. y. The emplacement time of ultra- basic rocks is pre-Silurian. The author doubts about Peng Lihong's report on the presence of hyolithis fossils in siliceous rocks that have been found in the Meishu- cun Formation at the base of the Cambrian in southern China. 3. The author considers that the absence of early Cambrian (550-600 m. y.) strata was due to Crustal compression before subduction. The Benioff zone was formed 550±30 m. y. ago. In the stable subduction stage when Cambrian-Ordovi- cian strata were deposited on the continental crust, oceanic-crustal fragments of the Wundurmiao Group as well as the Beineimiao island arc were formed. During the Silurian-Devonian, the Sino-Korean plate was obducted northwards, resulting in uplift of continental crust, and so Silurian-Davonian strata were absent and regional metamorphism occurred. Owing to the sledging mechanism of obduction, sialic crust and sima crust were mixed to form a late Caledonian to early Variscan granite zone extending along the convergent zone of plates. Finally the residual sea between plates formed a superposition basin during the Permo-Carboniferous; thus the conversion of oceanic crust to continental crust was completed.