Origin and tectonic setting of the Cenozoic basalts in Southwest Japan

1.玄武岩的成因晚新生代玄武岩广泛分布于日本西南弧带。在Chugoku地区，具有EM I特征的富集地幔热柱从地幔深处上升。另一方面，具有EM II特征的地幔底辟缓慢上升，并携带在九州北部下方类似N-MORB的亏损地幔之上。2.岩浆作用(1)部分熔融程度和产生的岩浆：北松浦盆地(KMB)分为碱性和亚碱性(拉斑玄武岩和钙质)岩系。碱性岩系的原生岩浆是由源地幔在10-15kbar的少量部分熔融形成的，拉斑玄武岩系列的原生岩浆是由同一源区在5-10kbar的较大程度的部分熔融形成的。(2)开放系统岩浆房：在东-松浦碱性玄武岩(HMB)中，识别出三个喷发旋回。HMB的岩浆因其成分的三个旋回变化(岩浆旋回)而引起人们的注意。已发现每个岩浆旋回都始于原始玄武岩的喷发，随后是分馏玄武岩的喷发。这表明HMB的岩浆房在成分上是分层的。(3)拉斑玄武岩岩浆的分离结晶：乌库岛岩石代表四个岩浆旋回。每一旋回都始于基性岩浆的喷发，然后是安山岩-英安岩浆的喷发。每一次岩浆旋回在变化图上都呈现出连续的趋势。随着SiO_2含量的增加，锶同位素比值逐渐增大。这些证据表明，这些玄武岩的化学变化受分离结晶控制，围岩有少量的同化作用。3.构造背景这些玄武岩的成因可能与日本海的开封有关。此外，地幔底辟或岩浆的成分因与日本西南部的MORB或次大陆地幔的相互作用而发生了改变。

1.Origin of basaltLate Cenozoic basalts are widely distributed along the southwest Japan arc. In the Chugoku district, an enriched mantle plume with an EM I signature ascended from deep in the mantle. On the othe hand, mantle diapirs characterized by an EM II signature rose slowly and were entrained an overlying N-MORB like depleted mantle below the north Kyushu district.2.Magmatic processes(1) Degree of partatial melting and produced magma : Kitamatsuura basakts (KMB) is divided into the alkali and the sub-alkaline (tholeiitic and calcalkaline) rock series. The primary magam of the alkali rock series was derived by a small degree of partial melting of a source mantle at 10-15 kbar and that of the tholeiitic rock series was produced by a large degree of partial melting of the same source at 5-10 kbar. (2) Open system magma chamber : In Higashi-Matsuura alkali basalts (HMB), three eruptive cycles are recognized. The magmas of HMB attract attention by their three cyclic variation in composition (magmatic cycles). It has been found that each magmatic cycle starts with the eruption of primitive basalts followed by fractionated ones. This suggests that the magma chambers of HMB were compositionally stratified. (3) Fractional crystallization of tholeiitic magma : The rocks of Uku island represent four magmatic cycles. Each cycle began with the eruption of basic magma, followed by eruption of andesitic to dacitic magma. Each magmatic cycle shows a continuous trend on variation diagrams. Sr isotopic ratios gradually increase with increasing SiO2 contents. These lines of evidence suggest that the chemical variation of these rocks are controlled by fractional crystallization, with minor wall rock assimilation.3.Tectonic settingOrigin of these basalts were probably related to the opening of the Sea of Japan. Moreovere, the compositions of mantle diapir or magma have been modified by interaction with MORB or sub-continetal mantle beneath Southwest Japan.