Rare-earth-element compositions of rocks from Quaternary volcanoes in Northeast Japan arc and their magmagenetic implications.

日本东北部第四纪火山岩石的稀土元素组成及其岩浆成因意义。

基本信息

批准号：
02640620
负责人：
FUJINAWA Akihiko
金额：
$ 1.28万
依托单位：
Ibaraki University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1990
资助国家：
日本
起止时间：
1990 至 1992
项目状态：
已结题

项目摘要

Contents of large-ion-lithophile (LIL), rare earth (RE), and high-field-strength (HFS) elements are determined for representative rock samples from volcanoes at Moriyoshi and Chokai zones, situated in back-arc side of Northeast Japan arc. Based on these results together with the preceeding research made by myself ( Grant-in-aid No. 63540643 ), across-arc inhomogeneity in magma source region with respect to incompatible elements is investigated.Across-arc variation in respect of lighter REE / heavier REE ratios is clearly recognized among basaltic members of the magma suites; the ratios increase systematically from trench side toward back-arc side zone. This tendency becomes distinct more and more as the relevant magmas enrich in silica. Since different degrees of partial melting of common magma source can not account for the systematic variations, it seems plausible that across-arc variation with regard to REE exists within source mantle region. Further, role of magma mixing is more important in magmatic evolution process for back-arc side volcanoes, with acidic end-member magma suffering fractionation ( or remnant ) of some HREE-bearing minerals such as amphibole, clinopyroxene, and garnet.Detailed examination of the LIL-RE-HFS systematics for concurring magma suites at Kampu, Adatara, and Funagata volcanoes revealed source heterogeneity in respect of incompatible element geochemistry. The inferred chemical characteristics for metasomatically added agent(s) are well-reconciled with the two-stage stepwise wedge dehydration model postulating two pressuresensitive dehydration reactions ( proposed by Tatsumi; 1989 ), suggesting strongly that differential infiltration of slabderived fluid into the mantle source is a crucial process in producing a variety of magmas.

对来自日本东北部弧后侧的森吉和鸟海地区火山的代表性岩石样品测定了大离子亲石元素 (LIL)、稀土 (RE) 和高场强 (HFS) 元素的含量弧。在此基础上，结合本人前期研究（资助号：63540643），对岩浆源区不相容元素的弧间不均匀性进行了研究。轻稀土/重稀土的弧间变化岩浆套玄武岩成员中的稀土元素比率得到了明确的认可；该比率从沟槽侧向弧后侧区域系统地增加。随着相关岩浆富含二氧化硅，这种趋势变得越来越明显。由于普通岩浆源的不同程度的部分熔融不能解释系统的变化，因此源地幔区域内稀土元素的跨弧变化似乎是合理的。此外，在弧后侧火山的岩浆演化过程中，岩浆混合的作用更为重要，酸性端元岩浆遭受角闪石、单斜辉石、石榴石等一些含重稀土矿物的分馏（或残留）。 Kampu、Adatara 和 Funagata 火山并发岩浆组的 LIL-RE-HFS 系统揭示了来源异质性不相容元素地球化学。推断的交代添加剂的化学特征与假设两个压敏脱水反应的两阶段逐步楔形脱水模型（由 Tatsumi 提出，1989）很好地一致，强烈表明板片衍生流体差异渗透到地幔源中是产生各种岩浆的关键过程。