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首页> 外文期刊>Contributions to Mineralogy and Petrology >Petrological and experimental evidence for differentiation of water-rich magmas beneath St. Kitts, Lesser Antilles
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Petrological and experimental evidence for differentiation of water-rich magmas beneath St. Kitts, Lesser Antilles

机译：St. Kitts下面的水富岩岩浆分化的岩石和实验证据

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St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure-temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An= 100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An(97) and An(95), respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (XH2O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950-1025 degrees C, and fO(2) = NNO - 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt Kd(Ca-Na) and dissolved H2O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% -H2O) basaltic andesites that crystallised over a wide pressure range (1.5-6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.
机译：圣基茨在较小的北部安妥尔斯，一种与其岩浆多样性和异常丰富的同源Xenoleiths的俯冲相关的血管内火山弧。我们将Xenoliths，熔体夹杂物和熔岩的地球化学结合在一起，具有高压 - 温度实验，探索圣基茨下面的岩浆区分过程。熔岩系列从玄武岩到流纹岩，具有主要的山脉和玄武岩和玄武岩。由钙磷光锌酶和锥形硅基组成的Xenoliths通常在与富硒和橄榄石的反应关系中，可根据矿物纹理和组合物分为浦项和累积的品种。累积品种，主要由液相阶段的积累形成，包括代表一种或多种岩浆批次代表瞬时固体组合物的合奏; Plutonic品种具有矿物学和纹理，与岩浆糊状物的伸长凝固一致。熔岩和Xenoliths的矿物化学巧妙地不同。例如，具有异常高钙质含量（An = 100）的Plagioclase在一些升高的Xenoliths中发生，而钙化XenoLiths和熔岩的最多钙磷酸酶分别是（97）和（95）。在2.4kbar，950-1025摄氏度和Fo（2）= NNO - 0.6，在ST.Kitts玄武岩中，在4.4kbar，950-1025摄氏度和Fo（2）= NNO - 0.6上进行流体饱和的（XH2O = 1.0,0.66和0.33）进行流体饱和的晶体化实验。到nno + 1.2日志单位。实验再现熔岩液体血液线和许多Xenolith组装，但不能匹配Xenolith和熔岩酚杂交矿物组合物，特别是非常富含富含的Plagioclase。实验确定的Plagioclase-Melt Kd（Ca-Na）与溶解H2O之间的强阳性相关性，以及富含Al富含麦芽糖膜的溶解H2O，表明家长岩浆富含水（> 9wt％-H2O）玄武岩岩石在宽压力范围内（1.5-6 kbar）结晶。实验和天然（熔岩，Xenolith）MAFIC矿物质组合物的比较显示，熔岩中的橄榄石主要是在低压下沉淀的主要晶体，吡喃和尖晶石主要由浮雕糊状物的分解形成的邻烯烯。总体而言，圣基茨Xenoliths和Lavas证明了在跨壳，岩土式糊状体系中的低MgO玄武岩和玄武岩岩浆中的中外分化。将父母的低MgO底部累积为初级的地壳utlaramafic累积，在圣基茨不存在披风的熔体。

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