Progressive changes in lithological association of the Sanbagawa metamorphic complex, Southwest Japan: Relict clinopyroxene and detrital zircon perspectives
Progressive changes in lithological association of the Sanbagawa metamorphic complex, Southwest Japan: Relict clinopyroxene and detrital zircon perspectives
复制标题
日本西南部三波川变质杂岩岩性组合的渐进变化：遗迹单斜辉石和碎屑锆石前景
DOI：
10.1111/iar.12261
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发表时间：
2018
期刊：
影响因子：
1.5
通讯作者：
Hirata Takafumi
中科院分区：
文献类型：
--
作者：
Endo Shunsuke;Miyazaki Kazuhiro;Danhara Tohru;Iwano Hideki;Hirata Takafumi
四国中部三坝川变质杂岩的主要构造地层单位（白泷单位）以丰富的镁铁质片岩层为特征，显示出与洋中脊玄武岩（MORB）的相似性。这些MORB衍生的片岩层在该单元的南部（结构上较低）域中不存在。相反，零星出现的小变基性岩透镜体，含有残余火成岩矿物（富钛辉石和钾霞石）指示碱性玄武岩岩浆作用是新认识的南部领域。变玄武岩中残留单斜辉石的成分可用于确定原岩玄武岩的构造环境和成因，并给出了三坝川变质杂岩各单元的残留单斜辉石成分。白泷单元的变质等级通常在最高等级带的南侧结构上向上增加，变质分带与岩石地层序列几乎平行。Shirataki单元的原岩组合显示出明显的变化，从南部的低级域（下Shirataki亚单元）由陆源沉积岩（泥岩和砂岩）和少量碱性玄武岩组成，到北方的高级域（上Shirataki亚单元）由陆源和远洋沉积岩和丰富的MORB组成。最年轻的碎屑锆石U-Pb年龄（约95 -90 Ma）表明，这两个领域的晚白垩世沉积年龄在沟槽。俯冲过程中大洋板块地层的逐步剥离可以解释白泷单元岩石组合的变化。
The main tectono‐stratigraphic unit (Shirataki unit) of the Sanbagawa metamorphic complex in central Shikoku is characterized by abundant mafic schist layers that show the mid‐ocean ridge basalt (MORB) affinity. These MORB‐derived schist layers are absent in a southern (structurally lower) domain within the unit. Instead, sporadic occurrences of small metabasite lenses that contain relict igneous minerals (Ti‐rich augite and kaersutite) indicative of alkali basalt magmatism are newly recognized in the southern domain. Compositions of relict clinopyroxene in metabasalt are useful to identify the tectonic setting and origin of the protolith basalt, and those in each unit of the Sanbagawa metamorphic complex are presented. The metamorphic grade of the Shirataki unit generally increases structurally upwards in the southern side of the highest‐grade zone, and metamorphic zonation is subparallel to lithostratigraphic succession. The protolith assemblage of the Shirataki unit shows a distinct change from the southern low‐grade domain (lower Shirataki subunit) composed of terrigenous sedimentary rocks (mudstone and sandstone) with minor alkali basalt to the northern higher‐grade domain (upper Shirataki subunit) consisting of terrigenous and pelagic sedimentary rocks with abundant MORB. The youngest detrital zircon U–Pb ages (ca95–90 Ma) suggest that both domains have Late Cretaceous depositional ages at the trench. Progressive peeling of oceanic plate stratigraphy during subduction can account for the observed change of lithological association in the Shirataki unit.