RESEARCH FOR MAGMA SOURCES OF CRETACEOUS TO MIDDLE MIOCENE

晚白垩世至古近纪，日本西南弧发生了大量中长英质岩浆活动。花岗岩类岩石是该岩浆活动形成的最主要的岩石，其辐射年龄、岩石学和地球化学特征具有区域性差异。Kagami出版社。（1992; 1993）根据Sr和Nd同位素资料将花岗岩划分为南九州、过渡九州、北九州和北九州4个带。南区花岗岩具有高初始Sr同位素比值（0.7070 ~ 0.7088）和低初始epsilonNd值（-3.0 ~ -8.0）的特征，而北区花岗岩具有低初始Sr同位素比值（0.7048 ~ 0.7068）和高初始epsilonNd值（+3 ~ -2.2）的特征。九州北部花岗岩类岩石的Sr同位素比值较低，但初始ε - nd值均较高。过渡带大部分样品的初始ε值为北带的中间值，南带的偏大。Kagami出版社。（1992）强调花岗岩类岩石Sr和Nd同位素日期的地理变化不能归因于岩浆受到上地壳物质的污染。认为各带花岗岩的Sr、Nd同位素差异是由上地壳和（或）下地壳作为花岗岩源区的差异造成的，并提出了各带岩石圈彼此具有独立历史的观点。在晚白垩世（ca.85Ma）至中古近纪（ca.39Ma）期间，北带火成岩的初始Nd同位素比值逐渐增大。相反，它们的初始Sr同位素比值逐渐降低。这些同位素变化可以用火成岩源物质的变化来解释。南区晚白垩世火成岩活动以中岩浆活动（约110 ma）开始，其后为大量花岗质深部活动，可分为100-92Ma和85-75Ma两个时期。南带南部大部分地区辉绿岩和辉长岩块体分布明显，大部分被捕获在晚白垩世花岗岩中。辉绿岩具有变质的花岗母岩结构，暂称其为偏辉绿岩（Kagami等）。， 1985)。这种岩石具有铝质拉斑岩的化学成分。辉长岩被认为是在6 ~ 8kb的下地壳条件下由拉斑岩浆结晶而成的。辉长岩中最主要的岩石类型为黑长岩和角闪石辉长岩，变质辉长岩位于同一Sm-Nd等时线上，年龄为192？B119 Ma，初始Nd ^<143> /^<144>Nd比值为0.512169？B10.000021 （Kagami等）， 1995)。斜长辉长岩和斜长辉长岩以不规则块状出现在黑长辉长岩和角闪辉长岩中，其Sm-Nd全岩年龄为169？B129 Ma，初始Nd ^<143> /^<144>Nd比值为0.512148？B10.000027 （Kagami等）， 1995)。这些年龄资料表明，早侏罗世至中侏罗世晚白垩世花岗岩地体中发生了基性火成岩活动。晚白垩世以后，火成岩活动区逐渐向日本海一侧移动。南区火成岩活动中断了60 ~ 70 Ma，在中新世中期（约15Ma）重新开始。中新世中期的火成岩活动可能与日本海的开放有关。南带零星分布有中中新世火山岩，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，如捕虏体。少

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