Formation of shoshonites from calcalkaline basalt magmas: geochemical and experimental constraints from the type locality
Formation of shoshonites from calcalkaline basalt magmas: geochemical and experimental constraints from the type locality
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DOI：
10.1007/bf00371997
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发表时间：
1987-11
影响因子：
3.5
通讯作者：
J. Meen
中科院分区：
文献类型：
--
作者：
J. Meen
蒙大拿州的独立火山是阿布萨罗卡火山场的主要中心，其中阿布萨罗卡岩，钾玄岩和banakite最初被定义。独立火山的一个岩浆趋势，从高铝拉斑玄武岩通过钾玄岩高钾英安岩，可以模拟观察到的斑晶相（斜长石，紫苏辉石，辉石和磁铁矿）的分离结晶。岩石的微量元素和Sr、Nd同位素组成与该模型相一致。在1atm和10kbar压力下对四种岩石进行的部分熔体实验表明，岩石组成在10kbar压力下比在1atm压力下更接近于无水的液体下降线。下降线涉及斜方辉石的结晶，而不是橄榄石，导致强烈富集K2O，SiO2增加很少。在较低压力下或有水存在时结晶，包括橄榄石，导致SiO2和K2O的富集。玄武岩浆的高压（10 kbar）分离结晶，导致钾玄岩的形成，可能发生在厚地壳的底部（例如，在大陆内部或在非常成熟的弧）。弧相关岩浆的K2O含量和深度之间的关系的贝尼奥夫区的至少一部分可能是由于增厚的地壳对弧后，导致较高的压力的分馏莫霍面水平的商会。
Independence volcano, Montana is a major center of the Absaroka volcanic field, from which absarokite, shoshonite, and banakite were originally defined. One magmatic trend at Independence volcano, from high-alumina tholeiitic basalt through shoshonite to high-K dacite, may be modeled by fractional crystallization of observed phenocryst phases (plagioclase, hypersthene, augite, and magnetite). Trace-element and Sr and Nd isotopic compositions of rocks are consistent with this model.Compositions of partial melts from experiments on four rocks at 1 atm and at 10 kbar demonstrate that rock compositions represent a nearly-anhydrous liquid line of descent at a pressure much closer to 10 kbar than to 1 atm. The line of descent involves crystallization of orthopyroxene, not olivine, resulting in strong enrichment in K2O with little increase in SiO2. Crystallization at either lower pressures or with water present, involving olivine, results in enrichment in both SiO2and K2O.High-pressure (10 kbar) fractional crystallization of basaltic magma, resulting in formation of shoshonites, may occur at the base of thick crust (e.g., in continental interiors or in very mature arcs). At least a portion of the relationship between K2O content of arc-related magmas and depth to the Benioff Zone may be attributed to thickening of crust towards the back-arc, resulting in higher pressures of fractionation in Moho-level chambers.