THE PETROLOGY OF PLUTONIC BLOCKS AND INCLUSIONS FROM THE LESSER ANTILLES ISLAND ARC
THE PETROLOGY OF PLUTONIC BLOCKS AND INCLUSIONS FROM THE LESSER ANTILLES ISLAND ARC
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DOI：
10.1093/petrology/21.4.743
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发表时间：
1980-01-01
影响因子：
3.9
通讯作者：
WILLS, KJA
中科院分区：
文献类型：
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作者：
ARCULUS, RJ;WILLS, KJA
小安的列斯群岛弧中存在三组主要的深成结核，它们被解释为（a）斑晶簇，（b）变质围岩捕虏体和（c）累积纹理捕虏体。大型堆积块体显示出长英质-镁铁质层状、滑落结构和自侵入特征。大多数标本是顺积体和异积体，只有少量的正积体和叠体。间隙熔渣和玻璃存在于各种样品中。斜长石、角闪石、斜长石、斜方辉石、橄榄石、磁铁矿、黑云母、钛铁矿、石英和磷灰石以不同比例存在于各个块体中。斜长石和角闪石在形态上占主导地位。与北部岛屿常见的两种辉石相比，南部岛屿的斜长石稀有度和角闪石丰度沿弧线表现出显着变化。斜长石的变化范围为An100-36，正长石成分非常低，并且在给定的结晶顺序中通常先于角闪石。只有格林纳达存在不含斜长石的块体。橄榄石仅限于共存斜长石的组合，其钙质高于An89，其成分范围为Fo90-59。单斜辉石主要为钙辉石，单斜辉石、橄榄石、斜长石均与角闪石稳定共存。弧内单斜辉石中的 Ca 含量存在从南到北递减的总体趋势。斜方辉石的范围为 En73–49，最常见于共存斜长石比 An83 钠含量更高的组合中。共存辉石的温度范围为 ⋍800–1050 °C。角闪石组合物包括斜闪石、镁斜长石、镁角闪石和切尔马基角闪石。角闪石的 K 含量从北到南增加，与母岩浆的一般拉斑岩-钙钙碱-碱性变化相同。磁铁矿是主要的尖晶石相，但在格林纳达的一些堆积物中存在铁铬铁矿和铬铁矿磁铁矿，在稀有的圣基茨样品中发现了重石。钛铁矿存在于多个岛屿的区块中；共存的 Fe-Ti 氧化物的温度范围为 710°-950 °C，压力为 15.5 至 10.0 bar。黑云母、石英和磷灰石仅限于演化堆积类型。大多数样品中都发生了来自平衡熔体的间质熔渣/玻璃成分的一些改变，但与喷发熔岩类型的总体相似性是堆积组合的同源关系的重要证据。与相对干燥的层状拉斑玄武岩相比，H2O 的作用对于确定岛弧深成斜长石的钙质性质至关重要。堆积熔岩比较的一些模态和化学特征表明斜长石浮选可能很重要。各种热力学计算表明结晶温度和压力在 850–1050 °C, 4–10 kb 范围内。没有证据表明这些参数存在系统性的沿弧变化。标准角闪石晶体溶液模型的计算结果并不令人满意。累积模式和斑晶模式之间存在一些明显的对比。与熔岩中的稀有性相比，在深成状态下与玄武岩熔体平衡的角闪石的丰度是惊人的。与给定熔体共存的斜长石在深成岩中比斑晶模式更具有斜长石性质。最小二乘分馏试验证明了通过积云相的分馏结晶来关联玄武岩-安山岩-英安岩组的可能性。各个岛屿的累积熔岩套件的微量元素系统学通常也......
Three main groups of plutonic nodules are present in the Lesser Antilles arc and are interpreted as (a) phenocryst clusters, (b) metamorphosed wallrock xenoliths and (c) cumulate textured xenoliths. Large cumulate blocks display felsic-mafic layering, slump structures and auto-intrusive features. The majority of specimens are ad- and heteradcumulates with fewer ortho- and crescumulates. Interstitial scoria and glass are present in a variety of samples. Plagioclase, amphibole, cino- and orthopyroxene, olivine, magnetite, biotite, ilmenite, quartz and apatite are present in various proportions in individual blocks. Plagioclase and amphibole are modally predominant. Significant variation along the arc is displayed in the rarity of orthopyroxene and abundance of amphibole in the southern islands compared with the common presence of two pyroxenes in the northern islands.Plagioclase varies from An100–36with very low orthoclase component, and usually precedes amphibole in a given crystallization sequence. Only on Grenada are plagioclase-free blocks present. Olivine is restricted to assemblages where coexisting plagioclase is more calcic than An89and its composition range is Fo90–59. Clinopyroxene is predominantly calcic augite and cinopyroxene, olivine and plagioclase all coexist stably with amphibole. A general trend of decreasing Ca content in clinopyroxene from south to north in the arc is present.Orthopyroxene ranges from En73–49and is most common in assemblages where the coexisting plagioclase is more sodic than An83Coexisting pyroxenes define temperatures in the range ⋍800–1050 °C. Amphibole compositions include pargasite, magnesiohastingsite, magnesio-hornblende and tschermakitic hornblende. The K content of the amphiboles increases from north to south in the same sense as the general tholeiitic-calcalkalic-alkalic variation of parental magmas.Magnetite is the dominant spinel phase but ferrian chromite and chromian magnetite are present in some Grenada cumulates and pleonaste is found in rare St. Kitts samples. Ilmenite is present in blocks from several islands; coexisting Fe-Ti oxides define a temperature range of 710°-950 °C atof 15.5 to 10.0 bars. Biotite, quartz and apatite are restricted to evolve cumulate types.Some modification of interstitial scoria/glass compositions from equilibrium melts has occurred in the majority of samples, but general similarity with erupted lava types is important evidence for the cognate relationship of the cumulate assemblages. The role of H2O is crucial in determining the calcic nature of island arc plutonic plagioclase when compared with relatively dry, layered tholeiitic plutons. Some modal and chemical features of cumulate-lava comparisons suggest plagioclase flotation may be significant.A variety of thermodynamic calculations indicate temperatures and pressures of crystallization in the range 850–1050 °C, 4–10 kb. No evidence exists for systematic along-arc variations in these parameters. Standard amphibole crystalline solution models give unsatisfactory results forcalculations.Some distinctive contrasts between cumulate and phenocryst modes are present. The abundance of amphibole in equilibrium with basaltic melts in the plutonic situation compared with its rarity in lavas is striking. Plagioclase coexisting with a given melt is more anorthitic in the plutonic than the phenocryst mode.Least squares fractionation tests demonstrate the possibility of relating basalt-andesite-dacite suites by fractional crystallization of the cumulus phases. Trace element systematics of cumulate-lava suites for individual islands also generally …