中太平洋贾耽海山玄武岩中单斜辉石成分特征及其地质意义
Petrography and chemical compositions of the clinopyroxene in Jiadan basalts from the Mid-Pacific Mountains and its geological significance
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摘要: 贾耽海山是中太平洋海山群的重要组成部分，其玄武岩中的单斜辉石斑晶对研究洋岛玄武岩岩浆演化和成因具有重要意义。本文通过详细的岩相学观察和系统的矿物化学分析测试，划分了单斜辉石的类型，揭示了中太平洋海山玄武岩的岩浆演化过程。岩相学特征显示中太平洋贾耽海山玄武岩中的单斜辉石可以分为四类，I类单斜辉石具有典型的核-幔-边结构，II类和III类单斜辉石具有核-边结构，IV类单斜辉石显示出均匀无环带的特征。I类单斜辉石核部和III类单斜辉石核部（Mg#值＞85）为循环晶，其他单斜辉石为自生晶。矿物化学分析表明中太平洋贾耽海山玄武岩中的单斜辉石核部具有高Mg#值和Cr含量的特征，从核部-边部，Mg#值和Cr含量逐渐降低，Al2O3、TiO2以及ΣREE含量逐渐升高，指示了同源岩浆不断演化的过程。其中I类和III类单斜辉石核部（Mg#值＞85）结晶温度和压力最高，分别为~1170 ℃和3.50 kbar；I类单斜辉石的幔部、II类单斜辉石核部、III类单斜辉石核部（Mg#值＜85）和IV类单斜辉石的结晶温度和压力较低，分别为~1070 ℃和~0.72 kbar；边部结晶温度和压力最低，为~1030 ℃和~0.40 kbar。综上所述，贾耽海山玄武岩中单斜辉石的形成主要经历了晶粥阶段（11 km）、浅部岩浆房阶段（2.5 km）和岩浆通道阶段（0.4 km）。Abstract: The Jiadan Guyot is an important part of the Mid-Pacific Mountains. The clinopyroxene in its basalts provides valuable insights into the magmatic evolution and formation of oceanic island basalts. This study uses petrographic observations and mineralogical analysis to classify clinopyroxene types and explore basaltic magmatic evolution in the Mid-Pacific Mountains. Clinopyroxene in Jiadan basalts can be divided into four types: Type I exhibits a core-mantle-rim structure; Types II and III have a core-rim structure; and Type IV is homogeneous without zones. The cores of Type I and III clinopyroxene (with Mg# values >85) are antecrysts, while others are autocrysts. Mineralogical analysis reveals that clinopyroxene cores show the highest Mg# values and Cr content, which decrease towards the rim, while Al2O3, TiO2, and ΣREE levels increase. This pattern indicates ongoing evolution of the primitive magma. The highest crystallization temperatures and pressures (~1170 °C and ~3.50 kbar) are observed in the cores of Type I and III clinopyroxene with Mg# >85. Lower temperatures and pressures (~1070 °C and ~0.72 kbar) are found in the mantles of Type I, cores of Type II and III (with Mg# <85), and Type IV clinopyroxene, with further decreases at the rims. In summary, clinopyroxene formation in Jiadan basalts reflects a sequence of magmatic evolution stages, including a deep crystallization stage (~11 km depth), a shallow magma chamber stage (~2.5 km depth), and a magma conduit stage (~0.4 km depth).
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Keywords:
- Zoned clinopyroxene /
- Crystallization conditions /
- Basalt /
- Jiadan Guyot /
- Mid-Pacific Mountains
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