秦岭造山带武当地区古生代伸展构造

PALEOZOIC EXTENSIONAL TECTONICS OF THE WUDANG BLOCK IN THE QINLING OROGEN, CHINA

武当地块位于勉 (县 )略 (阳 )地区东侧 ,南部逆冲于扬子板块北缘之上 ,中 晚古生代期间曾经历过一期伸展构造作用。伸展构造系统以现今中元古代武当山群与新元古代耀岭河组之间的界面为主滑脱面 ,盖层系统由南往北滑脱拆离。沿主滑脱面上、下侵位了大量的基性岩席 ,其单颗粒锆石U Pb年龄为 40 1± 1 4Ma和 40 7± 1 2Ma。滑脱系统上部次级滑脱面上新生白云母的40 Ar 39Ar年龄为 2 82± 8.5Ma和 2 61± 0 .2 5Ma,表明伸展作用可能一直持续到二叠纪。考虑到邻区同时代的碱性岩浆作用 ,作者认为伸展构造是伴随着古生代大规模的上地幔基性岩浆的底侵作用发生的 ,很可能代表了勉略洋打开的早期阶段。

The ophiolite along the Mian Lüe suture zone of Qinling orogen suggested that the southern Qinling belt was extended to be an ocean during the Middle Late Paleozoic. But up to now, there was no enough evidence showing that the suture extended both to the west and the east to Mian Lüe area. The Wudang Block located at the east to Mian Lüe area. The study on the tectonics of the block in this paper could be concluded that the block went through a phase extension before the thrusting of it onto the northern margin of the Yangtze plate. The sedimentary cover, composed of the Upper Proterozoic, the Lower Paleozoic and the Devonian, was detached from south to north along a main detachment plane which was a present boundary of the Middle Proterozoic Wudangshan Group and Upper Proterozoic Yaolinghe Formation. A large number of mafic magma intruded into the Wudangshan Group and Yaolinghe Formation near the main detachment plane. Two U Pb ages of zircons from the mafic sills yielded 401±14 Ma and 407±12 Ma respectively, and the 40 Ar 39 Ar ages of the new formed muscovite from the sub order detachment planes developed in the Sinian system were 282±8.5 Ma and 261±0.25 Ma. Considering the simultaneous intrusion and eruption of alkaline magma in the adjacent area, the authors believed that the extension was accompanied by the underplating of the mafic magma from the upper mantle, and the extension in the Wudang Block would be an early stage of the Mian Lüe ocean' openning.