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

本研究项目的研究结果总结如下：(1)根据日本东北弧第三系至第四纪火山岩Sr、Nd同位素资料，将日本东北弧下地幔岩浆源区划分为两个区域；一个是同位素未枯竭亚岛弧上地幔，另一个是在15ma左右上涌至未枯竭亚岛弧上地幔的枯竭软流圈（称为枯竭源）。利用海沟侧（含部分过渡带）103个玄武岩样品（MgO*5%, SiO2*53%）和弧后侧127个样品的HFS元素Nb、Ti、Y、Zr等4种HFS元素，研究了东北缘日本弧下未枯竭源和枯竭源两个不同地幔部分HFS元素的时间变化特征。老玄武岩与新玄武岩的HFS元素组成存在系统差异。从-35 Ma到现在，贫源的玄武岩的Zr/Y比值逐渐降低，Ti/Zr和Zr/Nb比值逐渐升高，而从-15 Ma到现在，贫源的玄武岩的HFS元素比值也呈现出类似的变化。Meschede（1986）提出的2Nb-Zr/4-Y图对各玄武岩中Zr/Y比值最小的玄武岩进行了标绘，表明日本东北弧地幔楔形未枯竭源的HFS元素特征随着年龄的减小逐渐发生了变化，从高不相容的富HFS元素OIB源型到N-MORB源型再到高不相容的贫HFS元素IAB源型。在未枯竭源中，不相容性较大的HFS元素逐渐枯竭可能是由于-35 Ma以前玄武岩岩浆从源中连续提取所致。对Meschede图中贫源玄武岩的考察也表明，从-15 Ma至今，与N-MORB源相比，贫源Zr略贫，贫源Nb略富，而与N-MORB源相比，贫源Zr和Nb略贫。日本东北弧玄武岩Ti/Zr和Zr/Nb比值的变化与源幔中残余钛、富铌矿物如榍石、金红石、角闪洞等的存在不相容。(2)测定了日本东北弧26个地点晚中新世至上新世火山岩的初始^<87>Sr/^<86>Sr和^<143>Nd/^<144>Nd比值（Sri和Ndi比值），并利用新获得和已发表的同位素资料分析了这些比值的时空变化规律。12 ~ 0 Ma期间火山岩的Sri和Ndi比值除少数样品外，在日本东北弧上呈现出从海沟侧火山岩的非耗尽异位特征到弧后侧火山岩的耗尽异位特征的线性趋势。这可能是由于同位素未亏缺的岛弧地幔与在日本海打开期间从弧后侧上涌而来的亏缺的软流圈地幔混合在一起，形成了岛弧源幔物质的跨弧同位素不均匀性。少

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