| 西准噶尔西南地区古海洋地壳与板块构造 |
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| ANCIENT OCEANIC CRUST AND PLATE TECTONICS IN SOUTHWESTERN SECTION OF WESTERN JUNGGAR |
| According to the petrologic assemblage, sedimentary formation of various periods and relationship between them, this paper discusses the presence of ancient oceanic crust in the southwestern section of western Junggar.There are three ancient oceanic crusts from south to north, formed during the Middle Ordovician, Middle-Late Silurian and Middle Devonian respectively.. They share several characters in common: 1) rather thin oceanic crust; 2) roughly similar assemblage; 3) the layer 2 and layer 3 belonging to spilite-tholeiite system; 4) occurring as ophiolitic melanges and being controlled by north dipping thrusts; and 5) andesitic lava developed more or less in the upper part. It is worth noticing that the total content of alkali in these three crusts decreases gradually from south to north (from older to younger in age) and they had a distribution of LREE different from that of oceanic mid-ridge. It is suggested here that these crusts were probably formed in similar tectonic background, and might be the products of partial spreading ridge of sea basin. Since the Middle Ordovician oceanic crust is regarded as the oldest in this region, plate movements may be traced back to the Early Paleozoic.Based on the above-mentioned facts and combined with their distribution and the polar marks of the plate, an evolutional model of Junggarian ocean has been proposed. The ancient ocean located between the Tarim and Siberian plates underwent sea-spreading and migration of ancient oceanic cryst since the Ordovician. The ancient ocean retracted gradually from south to north, while the ancient continent accreted constantly seawards, on the margin of which were ancient oceanic crusts of various periods and related geosynclinal foldbelts. Two subduction events took place at the end of the Middle Ordovician and of the Middle-Late Silurian respectively, resulting in the Caledonian foldbelts. During the Middle Devonian the ancient oceanic crust subducted still southward beneath the northern margin of the Caledonian foldbelts. It did not closed until the end of the Early Carboniferous, thus ending the development of the ancient ocean.In the light of 1) geological characteristics of occurrence of ultramafics, 2) contact relation of ultramafics with the surrounding rocks, 3) the location of the ultramafics in the column of ancient oceanic crust, and 4) the evolutional model of the ancient ocean, it is suggested that the ultramafics belong actually to tectonic emplacement and were formed prior to the Paleozoic rather than in the Variscian cycle. |
| ZHU Bao-qing.1984.ANCIENT OCEANIC CRUST AND PLATE TECTONICS IN SOUTHWESTERN SECTION OF WESTERN JUNGGAR[J].Acta Geoscientica Sinica,6(3):137-150. |
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