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首页> 外文期刊>Contributions to Mineralogy and Petrology >Magma evolution beneath Bequia, Lesser Antilles, deduced from petrology of lavas and plutonic xenoliths
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Magma evolution beneath Bequia, Lesser Antilles, deduced from petrology of lavas and plutonic xenoliths

机译：在牛群中，较小的安妥尔斯（Lafrology）推导出来的岩浆下方的岩浆进化

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摘要

Extrusive and intrusive igneous rocks represent different parts of a magmatic system and ultimately provide complementary information about the processes operating beneath volcanoes. To shed light on such processes, we have examined and quantified the textures and mineral compositions of plutonic and cumulate xenoliths and lavas from Bequia, Lesser Antilles arc. Both suites contain assemblages of iddingsitized olivine, plagioclase, clinopyroxene and spinel with rare orthopyroxene and ilmenite. Mineral zoning is widespread, but more protracted in lavas than xenoliths. Plagioclase cores and olivine have high anorthite (An98) and low forsterite (Fo84) compositions respectively, implying crystallisation from a hydrous mafic melt that was already fractionated. Xenolith textures range from adcumulate to orthocumulate with variable mineral crystallisation sequences. Textural criteria are used to organize the xenoliths into six groups. Amphibole, notably absent from lavas, is a common feature of xenoliths, together with minor biotite and apatite. Bulk compositions of xenoliths deviate from the liquid line of descent of lavas supporting a cumulate origin with varying degrees of reactive infiltration by evolved hydrous melts, preserved as melt inclusions in xenolith crystals. Volatile saturation pressures in melt inclusions indicate cumulate crystallization over a 162-571MPa pressure range under conditions of high dissolved water contents (up to 7.8wt% H2O), consistent with a variety of other thermobarometric estimates. Phase assemblages of xenoliths are consistent with published experimental data on volatile-saturated low-magnesium and high-alumina basalts and basaltic andesite from the Lesser Antilles at pressures of 200-1000MPa, temperatures of 950-1050 degrees C and dissolved H2O contents of 4-7wt%. Once extracted from mid-crustal mushes, residual melts ascend to higher levels and undergo H2O-saturated crystallization in shallow, pre-eruptive reservoirs to form phenocrysts and glomerocrysts. The absence of amphibole from lavas reflects instability at low pressures, whereas its abundance in xenoliths testifies to its importance in mid-crustal differentiation processes. A complex, vertically extensive (6 to at least 21km depth) magmatic system is inferred beneath Bequia. Xenoliths represent fragments of the mush incorporated into ascending magmas. The widespread occurrence of evolved melts in the mush, but the absence of erupted evolved magmas, in contrast to islands in the northern Lesser Antilles, may reflect the relative immaturity of the Bequia magmatic system.
机译：Extruse和Intrame Igage的岩石代表了岩浆系统的不同部分，最终提供有关在火山下方操作的过程的互补信息。为了阐明这种过程，我们已经研究过，并量化了巨振的纹理和矿物质的纹理和矿物质组合物，并从Bequia中抵消了Xenoliths和Lavas。两套套房都含有Iddingsitized Olivine，Plagioclase，Closopyroxene和尖晶石的装配，具有稀有的邻骨质和ilmenite。矿物区分区是普遍的，但在熔岩比Xenoliths延伸更多。 Plagioclase核和橄榄石分别具有高钙磷（AN98）和低叉脂（FO84）组合物，暗示从已经分级的含水乳头熔体中结晶。 Xenolith纹理的范围从患有可变矿物结晶序列的正交。纹理标准用于将Xenoliths组织成六组。特别是来自熔岩的倒话，是Xenoliths的常见特征，与次要的生物烟灰石和磷灰石一起。 Xenoleits的块状组合物偏离熔岩的下降液体，支持累积的血液来源，通过进化的含水熔体变化具有不同程度的反应性渗透，保存为Xenolith晶体中的熔体夹杂物。熔体夹杂物中的挥发性饱和压力表示在高溶解的水含量（高达7.8wt％H 2 O）的条件下，累积结晶在162-571MPa压力范围内（高达7.8wt％H2O），与各种其他热磁化率估计一致。 Xenoliths的相组合与来自较小的安妥尔斯的挥发性饱和低镁和高氧化铝玄武铝和高氧化铝玄武岩和玄武岩的实验数据一致，在200-1000MPa的压力下，950-1050℃的温度，溶解H2O含量为4- 7wt％。一旦从中地壳糊状物中提取，残余熔化就升高到较高水平并在浅喷发储层中经历H 2 O饱和结晶以形成苯基杂交和肾细晶。熔岩的不存在反射在低压下的不稳定性，而其在Xenoliths中的丰度证明了其在中地壳分化过程中的重要性。在Bequia下推断出一种复杂的垂直广泛（6至至少21km深度）岩浆系统。 Xenoliths代表糊状物的碎片，该碎片包含在上升的岩浆中。在糊状物中，在糊状物中的普遍发生发生，但与北方北部安妥尔斯的岛屿相比，没有喷发的岩浆可能反映了Bequia岩浆系统的相对不成熟。

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