Temporal changes of isotopes in igneous rocks occurring in the ciucum Japan Sea regions

日本海地区火成岩中同位素的时间变化

• 批准号: 05452086
• 负责人: SHUTO Kenji
• 金额: $ 3.46万
• 依托单位: Niigata 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: 1993
• 资助国家: 日本
• 起止时间: 1993 至 1994
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-05452086/
• 关键词: Sr isotope; Nd isotope; Temporal change; Northeast Japan arc; High field strength elements; HFS element depletion; OIB source; MORB source; Zr; Y比; 東北日本; 奥尻海嶺; 上部マントル; 白亜紀; 第三紀

The results obtained from this research project are summarized as follows.(1) On the basis of Sr and Nd isotopic data for Tertiary to Quaternary volcanic rocks from the NE Japan arc, the mantle magma sources beneath the NE Japan arc can be divided into two regions ; one is the isotopically undepleted sub-island arc upper mantle and another is the depleted asthenosphere (called the depleted source) which was upwelled to the undepleted sub-island arc upper mantle at around 15 Ma.Temporal change of HFS element (high field strength element) character two different mantle parts beneath the NE Japan arc, the undepleted and depleted sources, is investigated using four HFS elements Nb, Ti, Y and Zr in 103 basalitc samples (MgO*5%, SiO2*53%) from the trench side (including some samples from the transitional zone) and 127 ones from the back-arc side. Systematic differences are observed in HFS element composition of the older basalts relative to the younger ones. Basaltic rocks derived from the u … More ndepleted source exhibit a gradual decrease in Zr/Y ratio and a gradual increase in Ti/Zr and Zr/Nb ratios with decreasing age from -35 Ma to the present, whereas those from the depleted source show similar variations in these HFS element ratios with decreasing age from -15 Ma to the present.Plotting of basalts with minimum Zr/Y ratio among respective basaltic rocks in 2Nb-Zr/4-Y diagram proposed by Meschede (1986) suggests that the undepleted source in the mantle wedge of the NE Japan arc has gradually changed its HFS element character, from a highly incompatible HFS element-enriched OIB source type through N-MORB source type to a highly incompatible HFS element-depleted IAB source type, with the decrease in age. Gradual depletions of HFS elements of greater incompatibility in the undepleted source may be due to the successive extraction of basaltic magmas from the source since -35 Ma ago. Examination of basalts derived from the depleted source in Meschede's diagram also indicates the changing from the source depleted slightly in Zr and enriched slightly in Nb compared with N-MORB source to that depleted slightly both in Zr and Nb compared with N-MORB source during -15 Ma to the present. Variations of Ti/Zr and Zr/Nb ratios of basaltic rocks from the NE Japan arc are not compatible with the presence of residual Ti, Nb-enriched minerals such as sphene, rutile and amphibole in the source mantle.(2) The initial ^<87>Sr/^<86>Sr and ^<143>Nd/^<144>Nd ratios (Sri and Ndi ratios) were determined for the late Miocene to Pliocene volcanic rocks from twenty-six locations in the NE Japan arc and a spatial-temporal variation of these ratios was also examined using the newly obtained and published isotope data. Sri and Ndi ratios of volcanic rocks except few samples during 12-0 Ma show linear trends across the NE Japan arc from the undepletedisotopic signature in the trench side volcanics to the depleted one in the back-arc side volcanics. This can be ascribed to a across-arc isotopic heterogeneity in their source mantle material due to mixing of isotopically undepleted island arc mantle with depleted asthenospheric mantle which was up-welled to the former beneath the back-arc side during the opening of the Japan Sea. Less

本研究取得的成果如下：(1)根据日本东北弧第三纪至第四纪火山岩的Sr、Nd同位素资料，将日本东北弧下方的地幔岩浆源区划分为两个区域；一个是同位素未贫化的亚岛弧上地幔，另一个是贫化软流圈（称为贫化源），它在 15 Ma 左右上升到未贫化的亚岛弧上地幔。日本东北弧下方两个不同地幔部分（未贫化源和贫化源）的 HFS 元素（高场强元素）特征的时间变化采用四种方法进行研究 沟槽侧103个玄武岩样品（MgO*5%，SiO2*53%）（包括过渡区部分样品）和弧后侧127个玄武岩样品中HFS元素Nb、Ti、Y和Zr。相对于较年轻的玄武岩，较古老的玄武岩的 HFS 元素组成存在系统差异。源自贫铀源的玄武岩随着年龄从 -35 Ma 到现在的减小，Zr/Y 比率逐渐降低，Ti/Zr 和 Zr/Nb 比率逐渐增加，而来自贫铀源的玄武岩则随着年龄从 -15 Ma 到现在的减小，这些 HFS 元素比率表现出类似的变化。 绘制具有最小 Zr/Y 的玄武岩 Meschede(1986)提出的2Nb-Zr/4-Y图解中各玄武岩的比例表明，日本东北弧地幔楔中未贫化源区的HFS元素特征逐渐改变，从高度不相容的富含HFS元素的OIB源类型经N-MORB源类型转变为高度不相容的贫HFS元素的IAB源类型，随着 年龄。未耗尽源中不相容性较大的 HFS 元素的逐渐耗尽可能是由于自 -35 Ma 前以来玄武岩浆从源中连续提取所致。对Meschede图中贫化源的玄武岩的检查也表明，从-15 Ma至今，与N-MORB源相比，Zr略有贫化，Nb略有富集的源变化为与N-MORB源相比，Zr和Nb均略有贫化的变化。日本东北岛弧玄武岩 Ti/Zr 和 Zr/Nb 比值的变化与源地幔中残余富 Ti、Nb 矿物如榍石、金红石和角闪石的存在不相容。(2) 初始 ^<87>Sr/^<86>Sr 和 ^<143>Nd/^<144>Nd 比值（Sri 和 确定了日本东北弧 26 个地点的中新世晚期至上新世火山岩的 Ndi 比率，并使用新获得和发布的同位素数据检查了这些比率的时空变化。除 12-0 Ma 期间的少数样本外，火山岩的 Sri 和 Ndi 比率显示出日本东北弧上从海沟侧火山岩中未贫化同位素特征到弧后侧火山岩中贫化同位素特征的线性趋势。这可以归因于其源地幔物质中的跨弧同位素异质性，这是由于同位素未耗尽的岛弧地幔与耗尽的软流圈地幔的混合，在日本海开放期间，软流圈地幔在弧后侧下方上升到前者。较少的

项目成果

Kenji Shuto: "Relationships between drastic change of Sr isotopes in magma sources beneath the NE Japan arc and the spreading of the Japan Sea back-arc basin" Mineralogy and Petrology. 49. 71-90 (1993)

Kenji Shuto：“日本东北弧下方岩浆源中锶同位素的剧烈变化与日本海弧后盆地扩张之间的关系”矿物学和岩石学。

Jun'ichi Ohki: "K-Ar ages of the Miocene Ryozen basalts from the northern margin of the Abukuma Highland, Japan" J.Min.Pet.Econ.Geol.88. 313-319 (1993)

Junichi Ohki：“日本阿武隈高原北缘中新世 Ryozen 玄武岩的 K-Ar 年龄”J.Min.Pet.Econ.Geol.88。

Naoki Watanabe: "K-Ar ages of the Miocene volcanic rocks from the Tomari area in the Shimokita Peninsula,Northeast Japan arc" 岩鉱. 88. 352-358 (1993)

Naoki Watanabe：“日本东北部下北半岛泊地区中新世火山岩的 K-Ar 年龄”《岩石矿山》，88. 352-358 (1993)。

Hiroo Kagami: "Temporal variation of Sr and Nd isotopic compositions of Jurassic to Miocene igneous rocks occurring in the Seto Inland Sea Kinki districts of the Ryoke belts, Southwest Japan arc" Memoirs Geol.Soc.Japan. 44 (submitted).

Hiroo Kagami：“日本西南弧 Ryoke 地带濑户内海近畿地区侏罗纪至中新世火成岩的 Sr 和 Nd 同位素组成的时间变化”Memoirs Geol.Soc.Japan。

Ohki Jyun-ichi: "K-Arages of the Miocene Ryoiecn baseets from the northern margin of the Abukuma Highland,Japan." 岩鉱. 88. 313-319 (1993)

Ohki Jyun-ichi：“日本阿武隈高原北缘的中新世 Ryoiecn 基群的 K-Arages。”88. Iwako (1993)。