Silicate-oxide mineral chemistry of mafic-ultramafic rocks as an indicator of the roots of an island arc: The Chilas Complex, Kohistan (Pakistan)

The Chilas Complex is a major lower crustal component of the Cretaceous Kohistan island arc and one of the largest exposed slices of arc magma chamber in the world. Covering more than 8000 km2, it reaches a current tectonic width of around 40 km. It was emplaced at 85 Ma during rifting of the arc soon after the collision of the arc with the Karakoram plate. Over 85% of the Complex comprises homogeneous, olivine‐free gabbronorite and subordinate orthopyroxene–quartz diorite association (MGNA), which contains bodies of up to 30 km2 of ultramafic–mafic–anorthositic association (UMAA) rocks. Primary cumulate textures, igneous layering, and sedimentary structures are well preserved in layered parts of the UMAA in spite of pervasive granulite facies metamorphism. Mineral analyses show that the UMAA is characterized by more magnesian and more aluminous pyroxene and more calcic plagioclase than those in the MGNA. High modal abundances of orthopyroxene, magnetite and ilmenite (in MGNA), general Mg–Fe–Al spatial variations, and an MFA plot of whole‐rock analyses suggest a calc‐alkaline origin for the Complex. Projection of the pyroxene compositions on the Wo–En–Fs face is akin to those of pyroxenes from island arcs gabbros. The presence of highly calcic plagioclase and hornblende in UMAA is indicative of hydrous parental arc magma. The complex may be a product of two‐stage partial melting of a rising mantle diaper. The MGNA rocks represent the earlier phase melting, whereas the UMAA magma resulted from the melting of the same source depleted by the extraction of the earlier melt phase. Some of the massive peridotites in the UMAA may either be cumulates or represent metasomatized and remobilized upper mantle. The Chilas Complex shows similarities with many other (supra)subduction‐related mafic–ultramafic complexes worldwide. コヒスタン(パキスタン),チラス複合岩体:島弧の根源の指標としてのマフィック‐超マフィック岩のケイ酸塩‐酸化鉱物組成 抄録 要旨 チラス複合岩体は白亜紀コヒスタン島弧の主要な下部地殻岩体であり,世界で最も大規模な島弧マグマ溜りの露出体である.8000 km2以上を被い,幅40 kmに達する構造帯をなし,カラコルムプレートと島弧の衝突直後の85 Maに起きた島弧リフティングの際に定置した.複合岩体の85%以上は均質なカンラン石を含まないガブロノーライトとそれに次いで多い斜方輝石石英閃緑岩(MGNA)からなり,30 km2を超える超マフィック‐マフィック‐アノーソサイト複合岩(UMAA)体を含む.岩体全体がグラニュライト相の変成作用を被っているにもかかわらず,初生的な集積岩組織,層状構造,堆積構造がUMAAの層状部によく保存されている. UMAAはMGNAよりもMgとAlに富む輝石とCaに富む斜長石で特徴付けられる.(MGNAの)斜方輝石,磁鉄鉱及びイルメナイトの高いモード,Mg‐Fe‐Alスピネルが示す組成変化傾向,AFM図上の全岩化学組成変化は,複合岩体がカルクアルカリ岩質であることを示唆する.輝石のWo‐En‐Fs組成変化は島弧はんれい岩類のものと類似する.UMAA中のCaに富む斜長石と普通角閃石の存在は親マグマが水に富んでいたことを示す.複合岩体は上昇するマントルダイアピルが二段階の部分融解を経て生じたものであろう.MGNA岩類は初期の融解によって,UMAAはその枯渇した融け残り岩が