THE AGE OF THE LIAOHE GROUP

The Precambrian in eastern Liaoning comprises three major developmental stages: Archean, early Proterozoic and late Proterozoic. The Liaohe Group is the product of the middle stage. Its development progressed through four phases: sedimentation, metamorphism, uplift and folding, and it has a lot of salient characteristics in respects of sedimentation, metamorphism, deformation and magmatie activity. Viewing from the following aspects, the age of the Liaohe Group is early Proterozoic. A. The age of the overlying strata The overlying strata of the Liaohe Group consist of the Yongning, Xihe, Wuhangshan, Gezhenbu, Jinxian and Dalinzi Groups （from below upwards）. The facies sequence and rock formations of the first four groups are in the main similar to those in the section of Jixian, and the stratigraphic sequence and lithologic characteristics of the Dalinzi Group are similar to those of the Nantuo and Doushantuo Formations of the Sinian Wangjiawan section in Yunnan province. Therefore, they should be equivalent to the Changcheng, Nankou, Jixian, Qingbaikou and Sinian Systems, respectively. Their age is late Proterozoie. B. The age of the underlying strata The underlying strata of the Liaohe Group are representeb dy the Arche- an granite-greenstone terranes.The available geological data indicate that the granites were intruded during the synorogenic stage at the end of the Archean. They have an age range of 2800-22500 m.y. C. Evidence regarding the age of the Liaohe Group 1. Geological evidence （1） Its sedimentary characteristics （involving the sedimentary sequence, sedimentary rocks, sedimentary facies changes and abundance of trace elements） are different from those of the overlying and underlying strata. （2） For the Liaohe Group, there occurred regional metamorphism of burial-heat flow type during the subsidence phase and migmatization and granitization during the orogenic phase;while for the upper Proterozoic of the SinoKorean paraplatform, only dynamothermal metamorphism occurred in places. （3） For the Liaohe Group, two synclinoria and one anticlinoria of itself form a tight linear fold structure system（the paleozoic is not involved）; while the upper proterozoic in the Sino-Korean paraplatform invariably occurred with the Paleozoic and were not folded till Indosinian time, when it formed a relative simple fold structural system, 2. Fossil assemblages Stromatolites include Colonnella, Kussiella, Omachtenia, Eucapsiphoria,Gruneria and Pseudogymnosolen; the microflora comprises Asperatopsophosphaera umishanensis and Sphaeridium.Gruneria and Pseudogymnosolen that is analogous to Asperia are of certain significance for determining the age of the Liaohe Group. 3. Isotopic age A U-Pb age of 1955 m.y.has been determined on zircons from hornblende streak migmatites formed during the folding stage.