Igneous activity of young slab subduction and the limit of adakitic magma generation

2019 Fiscal Year Final Research Report

• Project/Area Number: 17K05700
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Petrology/Mineralogy/Economic geology
• Research Institution: Yamagata University
• Principal Investigator: Kagashima Shin-ichi 山形大学, 理学部, 准教授 (70361243)
• Co-Investigator(Kenkyū-buntansha): 高橋 俊郎 新潟大学, 自然科学系, 准教授 (20392946)
• Project Period (FY): 2017-04-01 – 2020-03-31
• Keywords: パタゴニア・バソリス / low Sr initial ratio / 花崗岩 / 部分溶融 / AFCモデル

Outline of Final Research Achievements

The Miocene plutonic rocks of north Chilean Patagonia are divided into three parts : Puerto Cisnes Granites(PCG), Queulat Diorites and Tonalites(QDT) and Queulat Diorites and Porphyries (Parada et al., 2000). We classified samples in this study in five main rock types. These are S-type like rock (Kfs-free) or (Kfs-bearing), tonalite, mafic rock and diorite. The lithology ranges from diorite to Grt-bearing peraluminous granite. Initial Sr and Nd ratios of these rocks are 0.70351 - 0.70412 and 0.51264 - 0.51287, respectively. SrI of metasedimentary rock is 0.70525. The igneous activity of this region was considered from the geochemical characteristics. This study presumed that QDT tonalite was formed by 30% degree of partial melting of the mafic rock. Geochemical modeling shows that the petrogenesis of the S-type like rock (Kfs-bearing) can be explained by assimilation and fractional crystallization between the QDT tonalite magma and the metasedimentary rock.

Free Research Field

地質学，岩石学

Academic Significance and Societal Importance of the Research Achievements

北部パタゴニア地方の花崗岩類の分布は既知データとしてあるが，そのほとんどは地球化学的特徴，年代，成因について不確かであり，現地調査と高精度な化学分析が必要であった．全岩主成分・微量成分組成，希土類元素組成，鉱物化学組成，Sr・Nd同位体組成，ジルコンのU-Pb年代など，パタゴニアバソリスの研究を発展させる上で不可欠なデータを得ることができた．また火山フロントより前弧側に深成岩が分布すること，Sタイプ質花崗岩であるにも関わらず非常に低いSr同位体比をもつことは，この地域が未成熟な活動的大陸縁辺であることを示す．未成熟であるということは，初期地球の大陸地殻形成を解明する手掛かりとなりうる．