RESEARCH FOR MAGMA SOURCES OF CRETACEOUS TO MIDDLE MIOCENE

晚白垩世至古近纪，日本西南弧区发生了规模庞大的中-长英质岩浆作用。花岗岩类岩石是这种岩浆作用形成的最主要的岩石，其放射性年龄、岩石学和地球化学具有区域性差异。Kagami等人。(1992；1993)根据锶、钕同位素资料将花岗岩类划分为南、过渡、北、北四个带。南带花岗岩具有较高的初始锶同位素比值(0.7070~0.7088)和较低的初始epsilon Nd值(-3.0~-8.0)，而北带花岗岩具有较低的初始锶同位素比值(0.7048~0.7068)和较高的初始epsilon Nd值(+3~-2.2)。来自九州北部的花岗岩类岩石具有与北带相当的低的锶同位素比值，但都具有较高的正初始epsilon nd值。过渡带的大多数样品具有北带和…的中等初始epsion值更多的是南区。Kagami等人。(1992)强调，花岗岩类岩石的锶、钕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。他们认为，各带花岗岩类岩石的锶、钕同位素差异是由于上地壳和/或下地壳是花岗岩类岩石的源区所致，并提出各带岩石圈具有相互独立的历史的观点，北带火成岩的初始Nd同位素比值从晚白垩世(约85 Ma)至古近纪中期(约39 Ma)逐渐增大。相反，它们的初始锶同位素比值逐渐降低。南带晚白垩世火成岩活动始于中等岩浆作用(约110 Ma)，随后又经历了100~92 Ma和85~75 Ma两个阶段的花岗质深成作用。南带南部大部分地区以辉绿岩和辉长岩类块体的分布最为明显，主要集中在晚白垩世花岗岩类岩石中。辉绿岩为变质花岗闪长岩结构，暂称为变质辉绿岩(Kagami等，1985)。这块岩石具有铝质拉斑玄武岩的化学成分。辉长岩被认为是在6-8Kb的下地壳条件下由拉斑玄武岩岩浆堆积结晶而成。辉长岩中以辉长岩和角闪辉长岩为主的辉长岩和变质辉绿岩在同一Sm-ND等时线上的年龄为192？B119 Ma，初始Nd143Nd/144Nd值为0.512169？B10.000021(Kagami等，1995A)斜长辉长岩和斜长辉长岩呈不规则块状产出于辉长岩和角闪辉长岩中，其Sm-ND全岩年龄为169？B129 Ma，初始Nd143Nd/144Nd值为0.512148？B10.000027(Kagami等人，1995A)。这些年龄数据表明，镁铁质火成岩活动发生在早、中侏罗世的晚白垩世花岗质地体中。晚白垩世以后，火成活动区逐渐向日本海一侧转移。南带火成岩活动中断了60~70 Ma，中新世中期(约15 Ma)又开始活动，可能与日本海的打开有关。南带零星分布中中新世火山岩，包括玄武岩、高镁安山岩、安山岩和英安岩。这些洞穴状岩石偶尔含有下地壳物质，如捕虏体。较少

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