Magma evolution beneath Bequia, Lesser Antilles, deduced from petrology of lavas and plutonic xenoliths
Magma evolution beneath Bequia, Lesser Antilles, deduced from petrology of lavas and plutonic xenoliths
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根据熔岩和深成捕虏体的岩石学推论，小安的列斯群岛贝基亚岛下方的岩浆演化
DOI：
10.1007/s00410-018-1504-z
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发表时间：
2018
影响因子：
3.5
通讯作者：
Camejo-Harry M
中科院分区：
文献类型：
--
作者：
Camejo-Harry M
喷出和侵入火成岩代表岩浆系统的不同部分，并最终提供有关火山下运作过程的补充信息。为了阐明这些过程，我们已经研究和量化的纹理和矿物成分的深成和堆积包体和熔岩从贝基亚，小安的列斯群岛弧。这两套岩石都含有伊丁斯化的橄榄石、斜长石、单斜辉石和尖晶石的组合，并含有稀有的斜方辉石和钛铁矿。矿物分带是普遍的，但更持久的熔岩比捕虏体。斜长石核和橄榄石分别具有高钙长石（An ≤ 98）和低镁橄榄石（Fo ≤ 84）成分，这意味着从已经分馏的含水镁铁质熔体中结晶。捕虏体的结构从堆晶到正堆晶不等，矿物结晶顺序也各不相同。纹理标准被用来组织捕虏体分为六组。角闪石是捕虏体的一个共同特征，在熔岩中明显缺失，此外还有少量的黑云母和磷灰石。捕虏体的整体成分偏离熔岩下降的液线，支持堆积起源，并通过演化的含水熔体进行不同程度的反应渗透，作为熔体包裹体保存在捕虏体晶体中。熔体包裹体中的挥发性饱和压力表明，在高溶解水含量（高达7.8重量% H2O）的条件下，累积结晶超过162-571 MPa的压力范围内，与其他各种热压估计一致。捕虏体的相组合与已发表的小安的列斯群岛挥发物饱和低镁高铝玄武岩和玄武安山岩在200-1000 MPa压力、950-1050 °C温度和4-7 wt%溶解H2O含量下的实验数据一致。一旦提取中地壳的糊状物，残留熔体上升到更高的水平，并在浅，喷发前的水库进行H2O饱和结晶，形成斑晶和glomocrysts。角闪石在熔岩中的缺失反映了其在低压下的不稳定性，而角闪石在捕虏体中的丰度则证明了其在中地壳分异过程中的重要性。一个复杂的，垂直广泛（6至至少21公里深）的岩浆系统被推断下Bequia。捕虏体代表了上升岩浆中的糊状物碎片。在糊状物中广泛发生的演化熔体，但没有爆发的演化岩浆，在小安的列斯群岛的北方岛屿相反，可能反映了贝基亚岩浆系统的相对不成熟。
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 (An ≤ 98) and low forsterite (Fo ≤ 84) 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 162–571 MPa pressure range under conditions of high dissolved water contents (up to 7.8 wt% 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 200–1000 MPa, temperatures of 950–1050 °C and dissolved H2O contents of 4–7 wt%. 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 21 km 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.
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DOI：
10.1016/j.jvolgeores.2015.02.009
发表时间：
2015-05-15
影响因子：
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作者：
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通讯作者：
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影响因子：
3.9
作者：
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