兴安地块下泥盆统古地磁结果对其古地理位置的制约  

Paleomagnetic results from Lower Devonian sandstones of the Niqiuhe Formation in the Duobaoshan area and its constraints on paleoposition of the Xing'an block

作　　者：张东海[1] 黄宝春[1] 赵千 张也[1] Donghai Zhang;Baochun Huang;Qian Zhao;Ye Zhang(Key Laboratory of Orogenic Belt and Crust Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China)

机构地区：[1]北京大学地球与空间科学学院造山带与地壳演化教育部重点实验室,北京100871

出　　处：《科学通报》2018年第15期1502-1514,共13页Chinese Science Bulletin

基　　金：国家重点基础研究发展计划(2013CB429802);国家自然科学基金重大项目(41190071)资助

摘　　要：兴安地块对中亚造山带的晚古生代演化过程具有重要意义,然而其古地理位置目前仍无可靠古地磁数据限制.本文选取黑龙江省多宝山地区的一个下泥盆统泥鳅河组紫红色粉砂岩剖面开展了系统古地磁学研究,11个采点均显示高低温双分量特征.低温分量在地理坐标系下与现代地磁场接近,应为近代获得的黏滞剩磁或热黏滞剩磁;高温分量显示单一负极性特征,在35.7%褶皱展开处获得最大集中,经与同时代乌里雅斯太陆缘和中蒙古额尔古纳地块的古地磁数据进行对比,认为其可能遭受了附近309~299 Ma(石炭-二叠纪负极性超时期间)的花岗岩浆侵入热事件改造而获得重磁化特征剩磁分量:D/I=28.6°/-33.2°,k=18.6,α95=10.9°,对应的古地磁极为λp/φp=17.3°S/97.1°E(dp/dm=7.0°/12.4°).结合同时代周边地块的古地磁数据分析认为:晚石炭世中蒙古-额尔古纳地块、乌里雅斯太陆缘和兴安地块纬度差异小于3°,可能已形成统一陆块,位于28°~30°N附近;此时西伯利亚位于45°~50°N附近而华北和松辽-锡林浩特地块则位于赤道低纬度地区;古亚洲洋此时仍未关闭,主洋盆为贺根山洋而非索伦洋.Two common but contradictory views toward the timing of the final closure of the eastern Paleo-Asian Ocean（PAO） are either the Pre-Late Devonian or the Late Permian to Early Triassic. Previous paleomagnetic studies support a consolidation of the unified North China and Mongolia by the Late Permian. However, Pre-Late Permian paleogeography of the eastern Central Asian Orogenic Belt（CAOB） is not well understood due to the scarcity of reliable paleomagnetic records. The Xing＇an block plays a key role in the evolution of the CAOB, however, no paleomagnetic results are available so far to provide constraints on its Late Paleozoic paleo-position. We performed a systematic paleomagnetic study on a purple siltstone section of the Lower Devonian Niqiuhe Formation near the Duobaoshan County, Heilongjiang Province.Representative fresh end samples were subjected to rock magnetic analysis to identify their dominant magnetic carriers and particle size distribution. Rock magnetic experiments include measurements of magnetic susceptibility variation dependence of temperature（κ-T） curves, hysteresis loops, acquisition of isothermal remanent magnetization（IRM） and back-field demagnetization of saturated IRM（SIRM） curves. The results indicate that the magnetic remanence carriers are dominant by hematite with minor contributions of magnetite. 112 cylindrical specimens were thermally demagnetized using an ASC TD-48 S furnace and measured in a 2 G-755-4 K SQUID system in the Magnetotectonics Lab of Peking University. A total of 11 sites revealed identical bi-components character. The low temperature component resembles the present geomagnetic field in geographic coordinates, while the high temperature characteristic remanence（Ch RM）component was reversed polarity both before and after tilt correction. Optimal concentration is achieved at 35.7%unfolding percentage, which suggests the magnetization was acquired during the late stage of a folding process. A previous structural study indicated that stress field

关 键 词：古亚洲洋 兴安地块 同褶皱重磁化 下泥盆统泥鳅河组 古地磁学 

分 类 号：P318.44[天文地球—固体地球物理学] P534.44[天文地球—地球物理学]