朝鲜半岛西北部古元古代高温变质-深熔作用:宏观和微观岩石学以及锆石U-Pb年代学制约

朝鲜半岛左接中国大陆右连日本岛链,其地质位置重要不言而喻,对其区域地质演化历史和构造属性的准确厘定,直接关系到对整个东北亚地质的全面理解和认识。本文对朝鲜半岛狼林地块西部的南浦群和甄山群的相关岩石进行了研究。野外露头、手标本和岩相学观察表明,南浦群和甄山群岩石保存了深熔作用的宏观和微观证据,矿物组合以及矿物间的反应结构表明南浦群和甄山群混合岩经历了角闪岩相到麻粒岩相的变质作用,并且在晚期熔体结晶过程中发生了逆反应或退变质过程。7件样品的锆石U-Pb定年结果显示,朝鲜半岛西北部地区在古元古代经历了多阶段(期)的变质和深熔作用过程。南浦群岩石在1917Ma可能经历了第一阶段(期)变质作用,在1877~1855Ma经历了第二阶段(期)变质深熔和石榴石持续生长,熔体冷却结晶时代为1842Ma。甄山群样品给出的变质深熔和石榴石生长的时代为1841~1830Ma,1785Ma可能代表深熔作用中抽取的熔体冷却的时代。但是,为何南浦群和甄山群样品记录的变质和深熔作用时代显示较大的差异,尚需更进一步的研究。综述前人研究成果可知,狼林地块与华北克拉通东部辽吉活动带,在变质和深熔作用类型方面存在不同之处,然而它们所记录的古元古代高温变质-深熔时代的一致性,表明二者可能至少在古元古代之前就形成了统一的大陆。

Paleoproterozoic high temperature metamorphism and anatexis in the northwestern Korean Peninsula:Constraints from petrology and zircon U-Pb geochronology.

The Korean Peninsula is juxtaposed by the North China Craton (NCC) to the left and the Japanese island chain to the right. Its position is of both geographical and geological importance and thus the study of its geological evolution and tectonic features will greatly stimulate the thorough understanding and interpretation of northeastern Asia geology. In this paper, we present the study results of rocks sampled from the Nampo and Jungsan groups located in the western Rangrim massif. Field as well as microscopic observations show that the studied samples from both the Nampo and Jungsan groups preserve macroscopic and microscopic evidence of anatexis, like the leucosome patches, melanosome, the thin-film structure between mineral boundaries. Mineral assemblages and reaction textures of the studied samples suggest that these rocks experienced anatexis, under both fluid present (?) and fluid absent conditions and the metamorphic grade is amphibolite to granulite facies. During later cooling and melt crystallization stage, back-reaction and/or retrograde metamorphism occurred. Zircon U-Pb age results of the seven studied samples indicate episodic metamorphism and anatexis. Samples of the Nampo Group recorded an episode of metamorphism and fluid absent melting at 1917Ma and another at 1877~1855Ma. Melt cooling and crystallization probably occurred at 1842Ma. Samples of the Jungsan Group give the time of anatexis (fluid present?) and garnet growth at 1841~1830Ma and melt cooling and crystallization at 1785Ma. This study shows that the time of the Paleoproterozoic metamorphism and anatexis in the Rangrim massif is broadly similar to those in the NCC (although there might be some difference in the patterns of metamorphism and anatexis) which, in turn, might suggest that the NCC and the Rangrim massif might have evolved into a uniform continent before Paleoproterozoic.