Anatomy of the Cretaceous Hobenzan pluton, SW Japan: Internal structure of a small zoned pluton, and its genesis.
Anatomy of the Cretaceous Hobenzan pluton, SW Japan: Internal structure of a small zoned pluton, and its genesis.
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日本西南部白垩纪 Hobenzan 岩体的解剖：小型分区岩体的内部结构及其成因。
DOI：
10.1016/j.lithos.2014.09.002
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发表时间：
2014
期刊：
影响因子：
3.5
通讯作者：
H.
中科院分区：
文献类型：
--
作者：
Imaoka T.;Nakashima K.;Kamei A.;Hayasaka;Y.;Ogita;Y.;Ikawa;T.;Itaya;T.;Takahashi;Y.;Kagami;H.
利用野外、岩相、地球化学、K-Ar和U-Pb年龄等资料，研究了日本西南部白垩系河本赞岩体的内部构造及其形成过程。该岩体呈东西向长形，尺寸约为6.5 × 2.0 km (13 km2)，位于~ 95 Ma的增生杂岩中。岩体中含有早期的闪长岩，但大部分由后期花岗岩组成，呈现出从黑云母-角闪石花岗闪长岩（GD）到角闪石-黑云母花岗岩（HBG）和黑云母花岗岩（BG）的连续分异系列。GD和HBG之间的联系是分级的。该岩体提供了一个简单冷却岩浆体的特殊截面视图。GD在模态和化学成分上没有垂直变化，而HBG则从最底部到岩体顶部呈现分化。HBG初始Sr同位素比值（SrI）由下部向上部逐渐增大。花岗岩的SiO2含量在65 ~ 79 wt.%（无水基），岩浆分异主要受角闪石和斜长石的晶体分选控制。场、元素和Sr-Nd同位素数据与泥质岩和分异结晶（AFC）同化的有限作用一致，同化程度随着分异程度的增加而增加。河本赞岩体在次火山岩浆储层中保留了花岗岩浆演化历史。GD形成了坚硬的海绵，熔体分数从壁向内增加，通过分步结晶形成HBG糊状，加上邻近片岩的小程度同化。一种更演化和富集的低密度熔体通过不完全的晶体熔体分离从HBG的糊状堆积物中分离出来，并在气体驱动的压滤作用下向上移动，如晶洞洞的存在所示，从而形成了岩体顶部的BG。
Field, petrographic, geochemical, and K–Ar and U–Pb age data were used to elucidate the internal structure of the Cretaceous Hobenzan pluton, SW Japan, and the processes which generated that structure. The pluton is elongated E–W with dimensions of about 6.5 × 2.0 km (13 km2), and was emplaced at ~ 95 Ma as a pluton in accretionary complexes. The pluton contains an early tonalite, but most of the body consists of later granitoids that show a continuous differentiation series from biotite–hornblende granodiorite (GD) to hornblende–biotite granite (HBG) and biotite granite (BG). The contacts between the GD and HBG are gradational. The pluton provides an exceptional cross-sectional view of a simple cooling magma body. The GD shows no vertical variations in modal and chemical compositions, whereas the HBG displays differentiation from the lowermost exposure to the top of the pluton. Initial Sr isotope ratios (SrI) in the HBG increase from the lower part to the top of the pluton. The granitoids show continuous compositional variations from 65 to 79 wt.% SiO2(anhydrous basis), and magmatic differentiation was dominantly controlled by crystal fractionation of hornblende and plagioclase. Field, elemental and Sr–Nd isotope data are consistent with limited operation of assimilation with pelitic rocks and fractional crystallization (AFC), in which assimilation increased with higher degrees of differentiation.The Hobenzan pluton retains a history of granitoid magma evolution in a subvolcanic magma reservoir. The GD formed as a rigid sponge, and melt fraction increases inwards from the walls, forming the HBG mush by fractional crystallization, coupled with small degrees of assimilation of adjacent schists. A more evolved and enriched low-density melt segregated from the mushy cumulate of the HBG by incomplete crystal–melt separation, and moved upwards with the assistance of gas-driven filter pressing, as indicated by the presence of miarolitic cavities, thus forming the BG at the roof of the pluton.