朝鲜北部狼林地块构造归属与地壳形成时代

朝鲜半岛北部的狼林地块一直被认为是中朝克拉通的重要组成部分。传统认为,它向南与我国辽东半岛的辽南太古宙地体相接,与其北部的鞍山-辽北-吉南太古宙地体(龙岗地块)具有基本类似的物质组成。两大太古宙地体之间是著名的以辽河群、集安群和老岭群为代表的辽吉古元古代岩系。辽吉岩系目前最主要的学术争论是,它是原本一体的太古宙地体的裂解产物,还是两个性质不同的太古宙地体拼合的结果。无论采用何种模型,学术界都普遍接受狼林地块主要由太古代岩石组成这一基本假定。为准确厘定狼林地块的地壳性质与形成时代,本文选择大同江、清川江、城川江、长津江、厚州川、厚昌江和秃鲁江中的河沙样品作为研究对象。这些河流均发源于狼林山脉,是狼林地块的核心区域,因而这些河流沉积物能够较好地全面反映狼林地块的物质组成情况。上述河流不同部位8件样品的分析结果显示,狼林地块主要由18~19亿年的古元古代岩石组成,太古宙岩石比例极为有限。但锆石Hf同位素模式年龄集中在28亿年左右,与华北克拉通全岩样品的Nd同位素模式年龄基本一致。结合狼林地块大量麻粒岩相变质表壳岩系和古元古宙花岗岩的发育,本文认为狼林地块是与辽吉岩系基本类似的古元古代地体,它可能是华北克拉通在古元古代期间东南大陆边缘的巨型造山带,我们可将其简称为辽-吉-朝古元古代造山带。因此,先前认为狼林地块主要由太古宙岩石组成的观点需要重新检查和认识。

Crustal growth and evolution of the Rangnim Massif, northern Korean Peninsula.

Rangnim Massif, located in northern part of the Korean Peninsula, has long been considered as a coherent unit of the Sino-Korean or North China craton. It has been accepted that this massif is the eastward continuation of the Liaonan (South Liaoning) Archean massif in the Liaodong Peninsular, and has the similar Archean basement exposed to the Anshan-Liaobei-Jinan (Longgang) massif to the north. Between the Rangnim and Longgang massifs, a well-known Paleoproterozoic Liaoji rock unit is developed, represented by the Liaohe Group in Liaoning Province and Ji''an and Laoling groups in Jilin Province. However, it is controversial in terms of the tectonic setting of this huge Paleoproterozoic belt, as being either a rift within an Archean continent or a collisional belt between two individual Archean massifs. In this paper, detrital zircons from sands of the Taedong, Chongchon, Songchon, Changjin, Huju, Houchang and Tongno rivers were collected for U-Pb and Hf isotopic analyses, in order to constrain the crustal growth and evolution of the Rangnim massif since all these rivers are originated and run through the massif. The results from 10 samples indicated that the Rangnim massif is mostly composed by 1800~1900Ma rocks, with little component of Archean material. Field investigation concluded that the Rangnim massif is mostly composed by granulite-facies metamorphosed supracrustal rocks and related granites. Therefore, it is proposed here that the Rangnim is not an Archean massif, but a Paleoproterozoic unit like that of the Liaoji belt. Considered that the analyzed zircons show an Hf isotopic model age similar to that of the whole-rock in the North China craton, indicating a crustal reworking process within the North China craton during the Paleoproterozoic, it is argued that the above Paleoproterozoic unit could be an orogenic belt along the southeastern margin of the North China craton, and we therefore name it as the Liaoning-Jilin-Korea (Liaojichao) Paleoproterozoic orogenic belt. The traditional point of view that the Rangnim is of Archean should be re-considered, and further works are pressingly needed.