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RESEARCH FOR MAGMA SOURCES OF CRETACEOUS TO MIDDLE MIOCENE

白垩纪至中中新世岩浆来源研究

基本信息

批准号：
04453053
负责人：
KAGAMI Hiroo
金额：
$ 2.75万
依托单位：
OKAYAMA UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1992
资助国家：
日本
起止时间：
1992 至 1994
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-04453053/
关键词：
SOUTHWEST JAPAN ARC LOWER CRUST-DERIVED XENOLITH UPPER MANTLE-DERIVED XENOLITH Sr AND Nd ISOTOPIC RATIOS LATE CRETACEOUS FELSIC IGNEOUS ROCKS JURASSIC MAFICIGNEOUS ROCKS Sm-Nd AGE Rb-Sr AGE Sm-Nd年代西南日本孤後期白亜紀火成岩はんれい岩 Sm-Nd鉱物年代

项目摘要

The voluminous intermediate to felsic magmatism took place in the Southwest Japan Arc during late Cretaceous to Paleogene. The granitoid rocks, which are the most predominant rocks formed by this magmatism, have regional variations in their radiometric ages, petrography and geochemistry. Kagami et.al. (1992 ; 1993) divided the granitoid rocks into four zones (South, Transitional, North and northern Kyushu) based on the Sr and Nd isotopic data. The granitoid rocks of the South Zone have high initial Sr isotope ratios (0.7070 to 0.7088) and low initial epsilonNd values (-3.0 to -8.0) , whereas granitoid rocks from the North Zone have low initial Sr isotope ratios (0.7048 to 0.7068) and high initial epsilonNd values (+3 to -2.2) . Granitoid rocks from the northern Kyushu have low Sr isotope ratios comparable to those from the North Zone, but all have highter positive initial epsilonNd values. Most samples from the Transitional Zone have middle initial epsilon-values of the North Zone and … More South Zone. Kagami et.al. (1992) emphasized that the geographical variations in Sr and Nd isotopic date for the granitoid rocks cannot be attributed to contamination of magmas by upper crustal materials. They suggested that the Sr and Nd isotopic differences of the granitoid rocks among each zone are caused by those of the uppermantle and/or lower crust being sources of the granitoid rocks, and proposed an idea that lithosphere of each zone has independent history one another.Initial Nd isotopic ratios of the igneous rocks occurring in the North Zone gradually increase from late Cretaceous (ca.85Ma) to middle Paleogene (ca.39Ma) . On the contrary, their initial Sr isotopic ratios gradually decrease. These isotopic change can be explained by the change of the source materials of the igneous rocks.Late Gretaceous igneous activity for the South Zone commenced with intermediate magmatism (ca.110Ma) , followed by voluminous granitic plutonism which can be divided into two periods at 100-92Ma and 85-75Ma. Distributions of diabasic and gabbroic masses are conspicuous in the southern most part of the South Zone, and most of them are captured in the late Cretaceous granitoid rocks. Diabase shows metamorphic granoblastic textures, and is tentatively called metadiabase (Kagami et.al., 1985) . This rock has aluminous tholeiitic chemical compositions. The gabbro has been considered to be cumulate crystallized from tholeiitic magmas under lower crustal conditions of 6 to 8kb. Norite and hornblende gabbro, which are most predominant rock type in gabbroic masses, and metadiabase plot on a same Sm-Nd isochron which gives an age of 192? B119 Ma with an initial ^<143>Nd/^<144>Nd ratio of 0.512169? B10.000021 (Kagami et.al., 1995a) .Anorthosite and anorthositic gabbro, which occur as blocks of irregular form in the norite and hornblende gabbro, have a Sm-Nd whole rock age of 169? B129 Ma with an initial ^<143>Nd/^<144>Nd ratio of 0.512148? B10.000027 (Kagami et.al., 1995a) . These age data imply that the mafic igneous activity took place in the late Cretaceous granitic terrane during early to middle Jurassic. After late Cretaceous, field of igneous activity gradually moved to the Japan Sea Side. Igneous activity in the South Zone interrupted for 60 Ma to 70 Ma and started again at middle Miocene (ca.15Ma) .The igneous activity during middle Miocene is probably related with the opening of the Japan Sea. The middle Miocene volcanic rocks including basalt, high-magnesian andesite, andesite and dacite are sporadically distributed in the South Zone. These vokcanic rocks occasionally contain lower crustal materials as xenoliths. Less

晚白垩世至古近纪，日本西南弧发生了大量的中长英质岩浆活动。花岗岩类岩石是该岩浆作用形成的最主要岩石，其放射性年龄、岩石学和地球化学具有区域差异。Kagami等（1992 ; 1993）根据Sr和Nd同位素数据将花岗岩类岩石划分为四个带（南、过渡带、北和北方九州）。南带花岗岩类岩石具有高的初始Sr同位素比值（0.7070 ~ 0.7088）和低的初始ε Nd值（-3.0 ~-8.0），而北带花岗岩类岩石具有低的初始Sr同位素比值（0.7048 ~ 0.7068）和高的初始ε Nd值（+3 ~-2.2）。北方九州花岗岩类的Sr同位素比值与北带花岗岩类相当，但它们的初始ε Nd值均较高。过渡带样品的初始ε值大多为北带的中间值， ...更多信息南区。Kagami等（1992）强调，花岗岩类岩石Sr和Nd同位素年龄的地理差异不能归因于上地壳物质对岩浆的混染。认为各带花岗岩类岩石Sr、Nd同位素的差异是由于上地壳和下地壳是花岗岩类岩石的源区所致，北带火成岩的初始Nd同位素比值从晚白垩世开始逐渐增大，古近纪中期（约39 Ma）。相反，它们的初始Sr同位素比值逐渐减小。南带晚白垩世岩浆活动开始于中岩浆活动（约110 Ma），随后是大规模的花岗质侵入活动，可分为100- 92 Ma和85- 75 Ma两期。在南带的最南部，辉长岩和辉长岩分布明显，它们大多捕获在晚白垩世花岗岩类岩石中。闪长岩显示变质花岗变晶结构，暂时称为变闪长岩（Kagami等人，1985年）。这种岩石具有铝质拉斑玄武岩的化学成分。辉长岩被认为是在6 - 8 kb的下地壳条件下由拉斑玄武岩岩浆结晶而成的堆晶岩。苏长岩和角闪辉长岩，这是最主要的岩石类型的辉长岩群众，和变质图上的同一Sm-Nd等时线，给出了年龄为192？B119 Ma的初始^<143>Nd/^<144>Nd比为0.512169？B10.000021（Kagami等人，斜长岩和斜长辉长岩呈不规则块状产出于苏长岩和角闪辉长岩中，其Sm-Nd全岩年龄为169？B129 Ma的初始^<143>Nd/^<144>Nd比为0.512148？B10.000027（Kagami等人，1995年a）。这些年龄数据表明，晚白垩世花岗质岩浆活动发生在早、中侏罗世。晚白垩世以后，岩浆活动场逐渐向日本海一侧移动。南带的岩浆活动中断了60 ~ 70 Ma，中新世中期（约15 Ma）又重新开始，中新世中期的岩浆活动可能与日本海的打开有关。南带中中新世火山岩零星分布，包括玄武岩、高镁安山岩、安山岩和英安岩。这些火山岩偶尔含有下地壳物质作为捕虏体。少