Miocene lamprophyre and felsic volcanics in the outer zone of SW Japan: arc building through multiple sources in an active convergent margin

Subduction-related magmatism is a significant contributor to arc building and continental growth. In the active convergent margin of the SW Japan arc, voluminous volcanic rocks and mafic dykes formed through subduction and back-arc rifting during Miocene. In this study, we investigate a suite of felsic volcanic rocks (rhyolite and dacite) and lamprophyres from central Shikoku Island, which occur in the north-central part of the Chichibu belt and Sanbagawa belt in the Outer zone of the SW Japan arc. Whole-rock geochemistry results show that the rhyolite and dacite samples are peraluminous, belong to the high-K calc-alkaline series, show LREE and LILE enrichment, HREE and HFSE depletion that suggest arc volcanic affinity. The lamprophyre samples belong to alkali lamprophyre, show LREE and HFSE enrichments, and display OIB (oceanic island basalt)-like geochemical features. The ²⁰⁶Pb/²³⁸U weighted mean ages of the zircons from the rhyolite and dacite samples range from 14.39 ± 0.21 Ma to 13.92 ± 0.30 Ma, whereas the lamprophyre shows ²⁰⁶Pb/²³⁸U weighted mean age of 12.21 ± 0.35 Ma. Zircon Lu-Hf isotope data suggest that the rhyolite and dacite were derived from the remelting of the arc crustal rocks and that the lamprophyre was derived from depleted mantle sources and was contaminated by crustal materials. The felsic volcanic rocks (rhyolite and dacite) can be correlated to Miocene (15.5 Ma to 13.5 Ma) magmatism in the Outer Zone of the SW Japan arc. The formation of lamprophyre dykes in the SW Japan arc may be related to the subhorizontal mantle flows beneath the SW Japan arc associated with the opening of the Japan Sea back-arc basin or asthenosphere upwelling through the slab-tear of the Philippine Sea Plate beneath the SW Japan arc. Our study shows that the SW Japan Arc was built by multiple magmatism from diverse sources in an active convergent margin.

俯冲相关岩浆作用是弧构造与大陆生长的重要驱动因素。在西南日本弧（Southwest Japan Arc）的活动汇聚边缘带中，中新世时期因俯冲作用与弧后裂谷作用形成了巨量火山岩与镁铁质岩脉。本研究针对四国岛中部出露于西南日本弧外带秩父带（Chichibu Belt）与三波川带（Sanbagawa Belt）中北部的一套长英质火山岩（流纹岩与英安岩）及煌斑岩开展研究。全岩地球化学分析结果显示，流纹岩与英安岩样品为过铝质，归属高钾钙碱性系列，具有轻稀土元素（Light Rare Earth Element, LREE）与大离子亲石元素（Large Ion Lithophile Element, LILE）富集、重稀土元素（Heavy Rare Earth Element, HREE）与高场强元素（High Field Strength Element, HFSE）亏损的特征，表明其具有弧火山亲缘性。煌斑岩样品属于碱性煌斑岩，兼具轻稀土元素与高场强元素富集特征，并表现出洋岛玄武岩（Oceanic Island Basalt, OIB）型地球化学特征。流纹岩与英安岩样品的锆石²⁰⁶Pb/²³⁸U加权平均年龄区间为14.39±0.21 Ma至13.92±0.30 Ma，而煌斑岩的锆石²⁰⁶Pb/²³⁸U加权平均年龄为12.21±0.35 Ma。锆石Lu-Hf同位素数据表明，流纹岩与英安岩起源于弧地壳岩石的重熔作用，而煌斑岩起源于亏损地幔源区且受到地壳物质的混染。上述长英质火山岩（流纹岩与英安岩）可与西南日本弧外带中新世（15.5 Ma至13.5 Ma）的岩浆作用相对应。西南日本弧内煌斑岩脉的形成可能与菲律宾海板块（Philippine Sea Plate）在西南日本弧下的板片撕裂处引发的软流圈上涌，抑或是与日本海弧后盆地张开相关的西南日本弧下水平地幔流有关。本研究表明，西南日本弧是在活动汇聚边缘背景下，由多源多期岩浆作用共同构建而成。