萨尔托海豆荚状铬铁矿中显微矿物包体特征及其地质意义探讨
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北京大学地球与空间科学学院

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国家自然科学基金项目(面上项目,重点项目,重大项目)(编号42072077)


Genesis of the Sartohay Podiform Chromitite Based on Microinclusions in Chromite
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School of Earth and Space Sciences, Peking University

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    摘要:

    通过系统研究萨尔托海豆荚状铬铁矿中显微矿物包体的岩石学和地球化学特征,探讨铬铁矿母岩浆的演化过程及铬铁矿矿床成因。铬尖晶石中的包体主要包括橄榄石、顽火辉石、透辉石、角闪石和钠金云母。研究表明,大多数包体由铬尖晶石圈闭的熔体直接结晶形成,橄榄石和辉石包体则是铬尖晶石结晶过程中捕获的捕掳晶。顽火辉石-透辉石包体温压计计算结果显示,平衡压力-温度条件为8.0~21.6 kbar和874~1048 °C。含水矿物包体往往具有较高的Na2O和TiO2含量。铬尖晶石平衡母熔体的Al2O3含量和FeO/MgO比值显示其具有MORB亲缘性。然而,平衡母熔体具有较高的TiO2含量,且铬尖晶石的TiO2含量也较高,表明母熔体成分类似MORB熔体。依据橄榄石包体的化学成分,获得与铬铁矿平衡的原始熔体的MgO含量约为22.7 %。这与矿物包体的高镁特征(橄榄石Fo = 96~98、顽火辉石Mg# = 0.95~0.97、透辉石Mg# = 0.94~0.96)一致。我们认为萨尔托海豆荚状铬铁矿是由原始富Mg含水熔体与由熔体-橄榄岩反应产生的次生拉斑玄武质熔体混合的产物。在熔体混合过程中,铬尖晶石晶体连续生长,并同时圈闭橄榄石和辉石俘虏晶以及熔体,被圈闭的熔体结晶演化并最终形成了我们所观察到的显微硅酸盐矿物包体。

    Abstract:

    Here, we present a petrographic and microanalytical study of microinclusions in chromite from podiform chromitites hosted by the Sartohay ophiolitic mélange in west Junggar, northwestern China, to investigate the parental magma evolution and chromitite genesis. These silicate inclusions comprise olivine, enstatite, diopside, amphibole, and Na-phlogopite. Their morphological characteristics suggest that most inclusions crystallized directly from the captured melt, with a few anhydrous inclusions (olivines and pyroxenes) as solid silicates trapped during the chromite crystallization. Equilibrium pressure–temperature conditions of coexisting enstatite–diopside inclusions are 8.0~21.6 kbar, and 874~1048 °C. The high Na2O and TiO2 contents of hydrous minerals indicate that the parental magma of chromitites was hydrous and enriched in Mg, Na, Ca, and Ti. The calculated Al2O3 content and FeO/MgO ratio of the parental melts in equilibrium with chromite showed MORB affinity. However, the TiO2 values of parental melts and TiO2 contents of chromite evoked parental MORB-like tholeiitic melts. The composition of olivine inclusion was determined, and it was revealed that the primary melts of the Sartohay podiform chromitites had MgO contents of ~22.7 %. This aligns with the observed high magnesian signature in mineral inclusions (Fo = 96~98 in olivine, Mg# = 0.91~0.97 in diopside, and Mg# = 0.92~0.97 in enstatite). We propose that Sartohay podiform chromitites initially formed through the mixing/mingling of primary hydrous Mg-rich melt and the evolved MORB-like melt derived from the melt–peridotite reaction in the upper mantle. In this process, the continuous crystallization of chromite captured micro-silicate mineral inclusions, finally leading to the formation of the Sartohay podiform chromitites.

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  • 收稿日期:2025-01-16
  • 最后修改日期:2025-03-26
  • 录用日期:2025-03-27