成熟した島弧系における現世河川砂堆積物の鉱物組成 Compositional characteristics of Holocene sands in a mature magmatic arc belt : a case study of river sands derived from plutonic and accretionary complexes, northeast Japan

島弧系に由来する砂岩組成は, 火山岩片を中心とする岩片群に富み, 石英粒子が少ないのが特徴とされている.しかし, 日本列島のような成熟した島弧系の河川系には, それぞれの流域に火山岩類の露出が極めて少ないものも多い.筆者らはそのような2つの河川系に注目し, 阿武隅帯を流れる高瀬川と北部北上山地を流れる閉伊川において堆砂の鉱物組成を検討した.前者(高瀬川)では, 石英・正長石・斜長石が主な鉱物組成であり, 深成(花崗)岩片は極粗粒砂〜粗粒砂部分に顕著である.後者(閉伊川)では, 粘板岩やチャートの岩片が多いが, 長石類は極めて少ない.両者の河川系とも流下する距離が短いため, 主要構成鉱物の選択的な摩耗は発生しておらず, 流下に伴った顕著な組成の増加・減少は認められなかった.粒径と鉱物組成のモード比との関係, あるいはQFLダイアグラムと供給源の地質状況との関係, さらにテクトニックセッティングの設定などについても検討を行った.QFLダイアグラムとテクトニックセッティングとの関連性などは, Gazzi-Dickinson測定法によるべきであるが, 中粒〜細粒砂の粒径に限定すれば伝統的測定法によるデータも利用できると考えられるので, 筆者らは後者の測定法によって得られたデータを基にした.テクトニックセッティングを議論する際には, 個々の河川系の堆砂の鉱物組成ではなく, より広域的な見地からの検討が必要であるとの結論を得た.

This paper describes compositional characteristics of the Holocene sands from bar deposits of the Takase and Hei Rivers. The Takase River runs through the Abukuma Plateau, a huge Mesozoic complex of granite and granodiorite, whereas the Hei River flows through the northern Kitakami Massif, an accreted subduction complex. Sand-sized sediments, particularly medium-and fine-grained sands from the Takase River beds are typified by high contents of quartz and feldspar and strikingly low contents of lithic fragments. Medium-grained sands from the Hei River are characterized by an abundance of aphanitic polycrystalline quartz (chert) grains and very low contents of feldspar. Because of the short transport distance, however, there seems to be no remarkable trends of increase or decrease of framework components in both rivers.Abundances of three principal constituents (Q, F and L) are strongly controlled by grain size. Both plutonic lithics and chert grains decrease conspicuously as grain size decreases. Monocrystalline quartz and total feldspar grains are also controlled by grain size, though not showing simple linear trends. However, no medium- and fine-grained sands fall within the "magmatic arc provenance" field of QFL diagram of Dickinson and Suczek (1979), which is characterized by a large amount of volcaniclastic detrital grains. The studied Holocene sands are debouched from rivers through plutons and Mesozoic sedimentary basement rocks to the Pacific Coast of northeast Japan, and are not classified petrographically as magmatic or mature magmatic arc provenance.Order of petrographic models is significant (Ingersoll, 1990), and the studies described herein relate the composition of modern fluvial sand directly to specific source rocks. However, large data sets are needed if we argue regional provenances related to the actual plate-tectonic models. Thus, simple application of modal analyses in ancient sandstones to the QFL tectonic setting of Dickinson and Suczek (1979) should be cautious.