K-Ar dating of magmatic rocks from the Urup Island, Kuril Island Chain

An isotope-geochronological study of Neogene-Quaternary igneous rocks from the Urup Island (Greater Kuril Ridge) was carried out. It was established that magmatic activity in the island developed during the last 10 my and it was not interrupted by long inactive periods. K-Ar data obtained along with results of diatomic analysis are in good agreement with the regional stratigraphic scheme of Paleogene and Neogene deposits and the intraregional correlation scheme of magmatic rocks in the Kuril Islands, which are developed for the State Geologic Map, scale 1:200 000 (Second edition). In the present-day territory of the Urup Island, the earliest Late Miocene - Early Pliocene (10.5-4.5 Ma) magmatic stage was associated with formation of the Rybakovsky andesite volcanic complex, which is represented by an effusive series (Rybakovskaya Suite) and subvolcanic rocks. Actually at the same time (6.6-4.7 Ma), but at a great depth, intrusive bodies of the Prasolovsky plagiogranite-diorite plutonic complex were intruded. The Pliocene stage of magmatism in the Urup Island is characterized by formation of rocks of the Kamuysky dacitic volcanic complex (4.0-2.1 Ma). This complex is locally represented only by subvolcanic acidic bodies, and its occurrence in the island is limited. During the Pliocene - Early Neopleistocene stage of magmatism (3.0-0.8 Ma) the Fregatsky andesibasalt volcanic complex was formed in the Urup Island. This complex includes effusive series (Fregatskaya unit) and subvolcanic bodies. Quaternary time in the Urup Island is characterized by eruptive activity in subaerial conditions with formation of effusive-pyroclastic intermediate-basic rocks of the Bogatyrsky Middle Neopleistocene - Holocene complex (<0.5 Ma). Rocks of this complex formed stratovolcano cones. Pyroclastic rocks of the Rokovsky dacitic volcanic complex were erupted simultaneously. The mentioned magmatic complexes of the Urup Island well correlate with the distinguished magmatic complexes within the bounds of contiguous insular blocks of the Greater Kuril Arc and confirm uniform geologic history of magmatic development of the region.

本研究针对乌鲁普岛（Urup Island，大千岛海岭（Greater Kuril Ridge））的新近纪-第四纪火成岩（Neogene-Quaternary igneous rocks）开展了同位素地质年代学研究（isotope-geochronological study）。研究确定，该岛的岩浆活动始于距今约1000万年以来，且未被长期的岩浆静止期打断。本次研究获得的K-Ar同位素数据（K-Ar）与硅藻分析（diatomic analysis）结果，与千岛群岛（Kuril Islands）古近纪（Paleogene）-新近纪沉积岩区域地层格架（regional stratigraphic scheme），以及服务于1:20万（scale 1:200 000）国家地质图（State Geologic Map）第二版（Second edition）编制的千岛群岛岩浆岩区域内对比方案高度吻合。在乌鲁普岛现今范围内，最早的晚中新世（Late Miocene）-早上新世（Early Pliocene，10.5~4.5 Ma）岩浆活动期对应Rybakovsky安山岩火山岩套（andesite volcanic complex）的形成，该岩套以Rybakovskaya岩套（Rybakovskaya Suite）喷出序列与次火山岩（subvolcanic rocks）为典型代表。几乎同期（6.6~4.7 Ma），在深部环境中形成了Prasolovsky斜长花岗岩-闪长岩（plagiogranite-diorite）深成岩套（plutonic complex）的侵入体。乌鲁普岛的上新世（Pliocene）岩浆活动以Kamuysky英安岩火山岩套（dacitic volcanic complex，4.0~2.1 Ma）的形成为典型特征，该岩套仅在局部以酸性次火山岩体（subvolcanic acidic bodies）产出，且在岛内分布范围有限。在上新世-早更新世（Neopleistocene）岩浆活动期（3.0~0.8 Ma），乌鲁普岛形成了Fregatsky安山玄武岩火山岩套（andesibasalt volcanic complex），该岩套包含Fregatskaya单元（Fregatskaya unit）喷出序列与次火山岩体。第四纪时期，乌鲁普岛以陆上喷发环境（subaerial conditions）下的喷发活动为特征，形成了Bogatyrsky中更新世-全新世（Middle Neopleistocene - Holocene）火山岩套（<0.5 Ma）的喷出-火山碎屑型（effusive-pyroclastic）中基性岩类（intermediate-basic rocks），该岩套的岩石构成了层状火山锥（stratovolcano cones）。同期还喷发了Rokovsky英安岩火山岩套的火山碎屑岩（pyroclastic rocks）。乌鲁普岛的上述岩浆岩套与大千岛弧（Greater Kuril Arc）相邻岛块内识别出的岩浆岩套具有良好的可对比性，证实了该区域岩浆演化的统一地质历史。