依兰陨石坑:我国东北部一个新发现的撞击构造

Yilan crater, a newly identified impact structure in northeast China

依兰陨石坑,一个直径1.85 km的碗形撞击构造,发现于我国黑龙江省中部的低山丘陵地区.该陨石坑坐落在白垩纪花岗岩基岩上,大部分坑缘保存状态良好,坑体南部占总长度约三分之一的坑缘缺失.坑底堆积的花岗岩质撞击角砾岩中石英多组面状变形页理(PDFs)的发现,提供了确凿的冲击变质证据,证实了该坑的星球撞击起源.坑区地形地貌、地质构造、撞击角砾岩和坑底湖相沉积物产状等均与一个星球撞击场所的特征相吻合.距今大约1万年前,南部坑缘在特定外动力地质作用下发生方向性的侵蚀和搬运,造成坑缘大规模缺失.

This study reports the discovery of Yilan crater,a newly identified impact structure in northeast China.The crater has a rimto-rim diameter of 1.85 km,and is located in Yilan County of Heilongjiang Province.The latitude and longitude coordinates of the crater are 46°23′03″N and 129°18′40″E,respectively.The crater lies in a low mountainous and hilly area of the southwest border of the Xiaoxinganling mountain range.Most of the crater is covered with dense forests.Geotectonically,the area belongs to the eastern Central Asian Orogenic Belt where strong tectonic and magmatic activities have occurred since the late Proterozoic.The crater was entirely formed within the bedrock of Cretaceous granite and appears as a bowl-shaped structure.Approximately one-third of the southern part of the crater rim has been significantly eroded and mostly removed;otherwise,most of the crater rim is well preserved.In a panoramic view,the crater appears as an unclosed ring mountain surrounding a circular depression,and it has steep walls and a relatively flat floor.The crater rim raises approximately 70–120 m above the surrounding terrain.The maximum elevation of the crater rim over the present crater floor is approximately 150 m.Most of the crater rim is composed of granite impact breccia varying from granules to boulders in size.The crater’s interior has been filled by a large amount of granite impact breccia at the bottom and lacustrine deposits at the top.The target rock of granite contains abundant quartz.This study investigated the planar deformation features(PDFs)in the quartz,which is one of the commonest shock metamorphic features in numerous terrestrial impact structures.For the investigation,granite impact breccia was collected from the crater floor.Most of the samples were made into polished thin sections for identifying the PDFs in quartz.Some fine-grained fragments of quartz were also collected from the samples for observation and analysis.The crystallographic orientations of the quartz PDFs were observed on a 4-axis universal stage under an optical microscope.No PDFs in quartz were observed in the quartz of large granite breccia fragments,but many PDFs appeared in the quartz of the fine-grained samples.One to four sets of PDFs in quartz were observed.The forms of the PDFs with Miller indices,such as(0001),{1011}and{1013},were indexed.The observed multiple sets of PDFs in quartz provide unambiguous evidence of shock-metamorphism and confirm an impact origin of the structure.The topographic and structural features of the crater,as well as the occurrences of crater-filled materials(impact breccia and lacustrine deposits),characterize a bolide impact site.Based on14C dating of the lacustrine deposits from the crater floor,the Crater Lake disappeared approximately 10 ka ago;this disappearance can be related to the loss of crater rim in the southern part of the crater.The large-scale absence of the crater rim cannot be explained by tectonic activity,normal weathering,or erosional process,and must correspond to directional erosion caused by a specific exogenic geological process after the formation of the impact crater.The geologic agent,mechanism,and history of the strong erosion of the crater rim require further investigation.The Yilan crater is the second confirmed impact structure in a large country(China)with an area of 9.6 million square kilometers.