四川攀西地区层状侵入体中堆晶岩成因的矿物学约束

doi:10.13745/j.esf.sf.2020.5.33

摘要/Abstract

摘要：

火成岩中可以包含多种晶体群这一发现具有重要意义,使得成因矿物学重新成为揭示岩浆系统演化的基本指导思想。但是,这种重要性在许多文献中都没有得到反映,其典型实例就是镁铁质层状侵入体中堆晶岩的成因。争论在于堆晶矿物是循环晶还是母岩浆的液相线相。因此,本文致力于探讨四川攀西地区镁铁质层状侵入体中堆晶岩的形成过程,重申成因矿物学的重要意义。显微镜观察表明,堆晶单斜辉石富含Fe-Ti氧化物出溶叶片(含叶片辉石),表明其形成环境明显不同于与斜长石呈共结关系的单斜辉石(无叶片辉石);无叶片辉石和斜长石中的橄榄石包裹体呈浑圆状,表明了橄榄石与结晶环境间的热力学不平衡。橄榄石与熔体间Fe-Mg分配关系分析表明,根据母岩浆成分推测的橄榄石Fo值远低于岩体中观测橄榄石化学成分变化范围(Fo₆₁-Fo₈₁)的高限,表明至少部分橄榄石不是寄主侵入体的液相线相。橄榄石的Mg^#值(100×Mg/(Mg+Fe))与微量元素(特别是Ni)的相关关系表明存在多种橄榄石晶体群,它们形成于不同的热力学环境中。晶体沉降过程分析表明,寄主岩浆析出的晶体几乎不可能发生快速重力沉降来形成堆晶岩。所有这些证据都表明,形成堆晶岩的矿物主要来自岩浆系统深部不同的岩浆房中,是被岩浆携带输运到终端岩浆房的循环晶。

关键词: 堆晶岩, 橄榄石, 循环晶, 成因矿物学, 攀西地区

Abstract:

It is very important for understanding the igneous processes to discover that several crystal populations can be found in an igneous sample. The discovery makes genetic mineralogy again key to revealing the history of a magmatic system, however, this importance is not reflected in the literature. A typical example is the debate on the origin of cumulates in so-called mafic layered intrusions that wether the cumulated minerals are antecrysts or liquidus of the parent magma. In this contribution, we attempt to clarify the origin of cumulate formation in mafic layered intrusions in the Pan-Xi region, Sichuan Province, and to emphasize the importance of genetic mineralogy. Microscopic observations suggest that clinopyroxene cumulate is rich in Fe-Ti oxide exsolution lamellaes (lamellae-bearing pyroxene), indicating the magmatic condition of crystallizing lamellae-bearing pyroxene is obviously different from crystallizing clinopyroxene (lamellae-free pyroxene), which is co-crystallized with plagioclase. The olivine grains included in lamellae-free pyroxene and plagioclase are neaer-rounded, which suggests thermodynamic disequilibrium between olivine and melt. The Fe-Mg partitioning relation between olivine and melt reveals that liquidus olivine Fo value inferred from the parental magma is much lower than the upper limit of the composition range (Fo₆₁-Fo₈₁) of olivine in cumulate. In other words, the observed olivine grains, partly at least, are not of the liquidus phase of the host intrusion. Correlations between Mg^#[Mg/(Mg+Fe)] and minor elements (especially Ni) also suggest multiple olivine crystal populations formed under different thermodynamic conditions. Crystal settling analysis indicates that crystals precipitating from the host magma may not experience rapid gradational settling and cumulate. All the evidences show that the cumulate-forming mineral crystals are mainly arrived from different chambers at depths of the magmatic system, and they are of antecryst and transferred to the terminal magma chamber. Consequently, the cumulated minerals have larger initial grain radius, which is helpful for their rapid settling to the bottom of the intrusion at an earlier stage to form cumulate.

Key words: cumulate, olivine, antecryst, genetic mineralogy, Pan-Xi region

中图分类号:

P588.1
P571

罗照华. 四川攀西地区层状侵入体中堆晶岩成因的矿物学约束[J]. 地学前缘, 2020, 27(5): 61-69.

LUO Zhaohua. Mineralogical constraints on the formation of cumulates in layered intrusions in the Pan-Xi region, Sichuan Province, China[J]. Earth Science Frontiers, 2020, 27(5): 61-69.

图/表 3

图1 攀西地区镁铁质侵入体的典型岩石类型 a—辉长-辉绿岩,正交偏光;b—同a,单偏光;c—辉长岩,正交偏光;d—堆晶辉橄岩,正交偏光;e—堆晶辉石岩,正交偏光;f—辉绿岩,正交偏光。Ap—磷灰石;Bi—黑云母;Cpx—单斜辉石;Oxide—氧化物;Ol—橄榄石; Q—石英。

Fig.1 Photomicrographs of typical rocks in mafic layered intrusions in the Pan-Xi region

图2 攀西地区火成岩中橄榄石的成分变化 1—低Ti苦橄岩;2—高Ti苦橄岩;3—苦橄岩(未分);4—攀枝花岩体;5—红格岩体;6—白马岩体;7—太和岩体;8—新街岩体;9—平川岩体。投图数据来自文献[12,13,14,15,16]。

Fig.2 Chemical variations of olivine crystals in igneous rocks in the Pan-Xi region

图3 MORB系统的Ni-MgO含量关系硫化物不饱和(实线)、橄榄石/硫化物=100(虚线)、橄榄石/硫化物=10(点划线)的分异趋势据文献[27]重绘。

Fig.3 Concentration relation of Ni-MgO in the MORB model system. Adapted from [27].

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