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Searching study for a new indicator of partial melting of high-grade metamorphics

高级变质岩部分熔融新指标的寻找研究

基本信息

批准号：
05452085
负责人：
TSUKUI Masashi
金额：
$ 4.42万
依托单位：
Chiba 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 1995
项目状态：
已结题

项目摘要

The purpose of this study is to establish new indicators of partial melting of high-grade metamorphic rocks which formed at temperatures much higher than those of their minimum melting. First, we looked for textures and compositional features of rock-forming minerals that occur commonly in both igneous and metamorphic rocks, and have found the followings.1.The common occurrence of euhedral and calcic plagioclase inclusions in garnet porphyroblasts. Inclusion plagioclase is much more calcic compared to the matrix plagioclase.2.The chemical zoning of garnet and other coexisting cilicate minerals for phosphorus and heavy rare earth elements. Garnet porphyroblasts often consist of phosphorus-poor cores with euhedral and calcic plagioclase inclusions and phosphorus-rich rim with phosphorus mineral inclusions. In addition, garnet often preserves euhedral patterns for heavy rare earth element zonation while major elements zonation patterns are concentric and diffusion controlled. These clearly indicate that phosphorus minerals takes in part of the garnet-forming (partial melting) reaction.Thus, these features are well explained by the following continuous and incongruent melting reaction :sillimanite+biotite+sodic plagioclase+quartz+phosphorous minerals<plus-minus>H_2O=melt+garnet+calcic plagioclase+rutile (or ilmenite)The amount, morphology, internal structure and compositions of zircon in addition to phosphorus minerals such as apatite, monazite, xenotime and may be significant in geochemical modeling, as shown by the SHRIMP dating.Part of these features and insights have been reproduced and examined by high-temperature melting experiments with a HIP apparatus.We also have tried to precisely estimate the temperature, pressure and water content of large-volume felsic magmas erupted from the Aira volcano in Kyushu by experimentally producing phenocryst minerals in them.

本研究的目的是建立新的指标的部分熔融的高级变质岩，形成的温度远高于他们的最低熔融。首先，我们研究了火成岩和变质岩中常见的造岩矿物的结构和成分特征，发现：1.石榴石变斑晶中普遍存在自形和钙质斜长石包裹体。包体中斜长石的钙含量比基质中斜长石的钙含量高得多。2.石榴子石和其他共生硅酸盐矿物中磷和重稀土元素的化学分带。石榴子石变斑晶通常由贫磷核和富磷边缘组成，贫磷核内有自形和钙质斜长石包裹体，富磷边缘有磷矿物包裹体。此外，石榴子石常保持重稀土元素自形分带模式，而主元素分带模式为同心和扩散控制。这表明磷矿物参与了部分石榴子石的形成（部分熔融）反应，因此，这些特征可以用下列连续的、不协调的熔融反应来解释：硅线石+黑云母+钠质斜长石+石英+磷矿物<plus-minus>H_2O=熔体+石榴子石+钙质斜长石+金红石（或钛铁矿）除了磷矿物如磷灰石，独居石，SHRIMP测年表明，这些特征和认识在地球化学模拟中具有重要意义。我们利用HIP装置进行了高温熔融实验，并对其中的一部分进行了再现和验证。此外，我们还尝试通过在九州爱良火山喷发的大量长英质岩浆中提取斑晶矿物，精确估算岩浆的温度、压力和含水量。