Cooper George F, Davidson Jon P, Blundy Jon D
1Science Labs, Department of Earth Sciences, Durham University, Durham, DH1 3LE UK.
2School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ UK.
Contrib Mineral Petrol. 2016;171(10):87. doi: 10.1007/s00410-016-1299-8. Epub 2016 Sep 27.
The Lesser Antilles Volcanic Arc is remarkable for the abundance and variety of erupted plutonic xenoliths. These samples provide a window into the deeper crust and record a more protracted crystallisation history than is observed from lavas alone. We present a detailed petrological and in situ geochemical study of xenoliths from Martinique in order to establish their petrogenesis, pre-eruptive storage conditions and their contribution to construction of the sub-volcanic arc crust. The lavas from Martinique are controlled by crystal-liquid differentiation. Amphibole is rarely present in the erupted lavas, but it is a very common component in plutonic xenoliths, allowing us to directly test the involvement of amphibole in the petrogenesis of arc magmas. The plutonic xenoliths provide both textural and geochemical evidence of open system processes and crystal 'cargos'. All xenoliths are plagioclase-bearing, with variable proportions of olivine, spinel, clinopyroxene, orthopyroxene and amphibole, commonly with interstitial melt. In Martinique, the sequence of crystallisation varies in sample type and differs from other islands of the Lesser Antilles arc. The compositional offset between plagioclase (An) and olivine (Fo), suggests crystallisation under high water contents and low pressures from an already fractionated liquid. Texturally, amphibole is either equant (crystallising early in the sequence) or interstitial (crystallising late). Interstitial amphibole is enriched in Ba and LREE compared with early crystallised amphibole and does not follow typical fractionation trends. Modelling of melt compositions indicates that a water-rich, plagioclase-undersaturated reactive melt or fluid percolated through a crystal mush, accompanied by the breakdown of clinopyroxene, and the crystallisation of amphibole. Geothermobarometry estimates and comparisons with experimental studies imply the majority of xenoliths formed in the mid-crust. Martinique cumulate xenoliths are inferred to represent crystal mushes within an open system, through which melt can both percolate and be generated.
小安的列斯火山弧以其大量喷发的深成捕虏体及其多样性而闻名。这些样本为深入了解地壳深处提供了一个窗口，记录了比仅从熔岩中观察到的更为漫长的结晶历史。我们对马提尼克岛的捕虏体进行了详细的岩石学和原位地球化学研究，以确定它们的岩石成因、喷发前的储存条件以及它们对次火山弧地壳构造的贡献。马提尼克岛的熔岩受晶-液分异作用控制。在喷发的熔岩中很少出现角闪石，但它是深成捕虏体中非常常见的成分，这使我们能够直接测试角闪石在弧岩浆岩石成因中的作用。深成捕虏体提供了开放系统过程和晶体“货物”的结构和地球化学证据。所有捕虏体都含有斜长石，橄榄石、尖晶石、单斜辉石、斜方辉石和角闪石的比例各不相同，通常含有间隙熔体。在马提尼克岛，不同类型样本的结晶顺序不同，与小安的列斯弧的其他岛屿也有所不同。斜长石（An）和橄榄石（Fo）之间的成分偏移表明，结晶是在高含水量和低压条件下，从一种已经分异的液体中进行的。在结构上，角闪石要么是等轴状的（在结晶序列早期结晶），要么是间隙状的（在后期结晶）。与早期结晶的角闪石相比，间隙角闪石富含钡和轻稀土元素，并且不遵循典型的分异趋势。熔体成分模拟表明，一种富水、斜长石不饱和的反应性熔体或流体渗透过晶体糊，伴随着单斜辉石的分解和角闪石的结晶。地热压力计估算以及与实验研究的比较表明，大多数捕虏体形成于地壳中部。马提尼克岛堆积捕虏体被推断代表一个开放系统内的晶体糊，熔体既能渗透其中，也能在其中生成。
