摘要:
辽吉古元古代造山带位于华北克拉通东部,造山带的沉积构造背景和地层格架长期存在争议。最近的研究表明,辽吉古元古代造山带的沉积环境为一活动大陆边缘弧后盆地,经历了早期弧后盆地扩张和晚期盆地收缩-俯冲-造山的演化过程。北辽河群的浪子山组—里尔峪组(含老岭群达台山组)与南辽河群的里尔峪组和老岭群的蚂蚁河组应是同时异相的产物,前者沉积在弧后盆地大陆边缘一侧,以陆缘碎屑沉积为主夹少量酸性火山岩;后者发育在弧后盆地岩浆弧一侧,火山作用强烈。集安群(荒岔沟组和大东岔组)和南辽河群的中上部(南高家峪组—大石桥组)所构成的孔兹岩系为弧后盆地收缩-俯冲阶段的沉积建造(相当于弧前构造背景),遭受了~1.9 Ga的变质作用,并被~1.9 Ga的石榴石花岗岩侵入。河栏—草河口地区的“大石桥组一段”为一套大理岩夹变质基性火山岩的沉积建造,变质基性火山岩和变质基性岩脉均具有岛弧拉斑玄武岩特征,形成于弧后盆地的构造背景。依据其中基性火山岩~1.87 Ga的SHRIMP锆石U-Pb年龄,推测其为弧后盆地俯冲收缩阶段的产物。狭义的盖县组及其相当地层的碎屑锆石反映出大量古元古代的年龄信息,太古宙年龄信息相对较少,故推测物源区主要来自南侧的岩浆弧,所谓的“狼林地块”不是一个太古宙陆块,而主要表现为一条古元古代岩浆弧。
Abstract:
Paleoproterozoic Liao–Ji orogen is located on the eastern margin of the North China Craton, and its sedimentary tectonic setting and stratigraphic framework are topics of much controversy. Recent studies indicate that the sedimentary environment of Paleoproterozoic Liao–Ji orogen was an active continental margin back-arc basin, which went through an evolutional process of early basin expansion and late contraction-orogeny. The lower North Liaohe Group (Lanzishan Formation and Lieryu Formation, including the Dataishan Formation of Laoling Group) and the lower South Liaohe Group (Lieryu Formation, including the Mayihe Formation of Jian Group) were formed in different tectonic environments simultaneously, the former was a suite of clastic sediments with a small amount of acidic volcanic rocks formed on the inland side of back-arc basin, whereas the latter was developed on the side of the magmatic arc with intense volcanism. The khondalite composed of the Ji'an Group (Huangchagou Formation and Dadongcha Formation) and the upper South Liaohe Group (Gaojiayu Formation and Dashiqiao Formation) was formed in an sedimentary environment similar to one of the fore-arc basin during contraction and subduction of the back-arc basin, which was subjected to ~1.9 Ga metamorphism and the intrusion of ~1.9 Ga garnet-bearing granite. The lower Dashiqiao Formation of North Liaohe Group in the Helan–Caohekou area, a metamorphic sedimentary formation composed of marble and mafic volcanic rocks, was formed also in a tectonic setting of back-arc basin, as shown by the fact that the metamorphic mafic volcanic rocks and metamorphic mafic dikes have geochemical characteristics of island-arc tholeiite. According to SHRIMP U-Pb zircon age (~1.87 Ga) of the tholeiite which is regarded as the magmatic crystallization age, the authors hold that the tholeiite was formed at the stage of back-arc basin contraction-subduction. Because the detrital zircons from the narrow sense Gaixian Formation and equivalent strata recorded plenty of Paleoproterozoic age information but only recorded less Archean information, the authors consider that the clastics were mainly from magmatic arc on the south side of Liao–Ji Orogen, and hence the Rangrim Block is basically a Paleoproterozoic magmatic arc rather than an Archean block.
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